Does a Longer Stroke Make More Torque?

Added: 2026-05-09

[00:00:00]Welcome to the Psych World podcast. I'm Mark Hoyer. I'm the editor-inchief. I'm with Kevin Cameron, our tech Kevin Cameron, our technical editor. Thanks for joining us. Um, if you've never been with us, we've been doing this a while.

[00:00:13]We're on our we're close to about 130 episodes. We have a very big back catalog. You can find us on YouTube, which is probably where you got us, but on Spotify. And we also have a Patreon, Cyber World Podcast. Patreon. Uh, there's a link in the description. If you join us over there, you get everything commercial free plus bonus content. That means more podcasts. We do some short form stuff. Um, and as I like to say, we've hit topics such as the excellence of butter and there was a lot to talk about and it was motorsports related.

[00:00:44]>> Yes.

[00:00:45]>> Anyway, thanks for um thanks for being with us. Uh, today we're going to talk about primarily the effects of an engine's uh board to stroke ratio. We'll probably talk about connecting rods because I have props. Here's a connecting rod and uh maybe a little bit of connecting rod surface treatment and rod ratio, but um >> carefully note the darkening of the small end of that rod.

[00:01:09]>> Yes. Yes, indeed.

[00:01:11]>> That's where the heat is.

[00:01:13]>> Yeah, we'll get uh well, let's get underway. We we I guess you know, if you've never joined us, and even if you have um what are we working against?

[00:01:23]We're trying to work against the mythologies of mechanics and things that people say and believe that maybe aren't necessarily rooted in fact. Uh such as one that we're going to talk about here is do long strokes make more torque? And while there are reasons that long strokes and small bores might be torquy, they're not for some of the mythological reasons, sorry, that we that we sometimes uh talk about. So, go ahead, Kevin. Tell us what's up, man.

[00:01:55]>> Well, uh I've been hearing this thing about uh Harley's having being so torquy, which they are because they have to be to start a heavy cruiser or tour from rest without a lot of revving up and embarrassing clutch slipping. You want to just glide powerfully away. And the engines that Harley has built, their two valve engines have produced strong bottom torque uh for this purpose. And I want to emphasize for this purpose because there are no statements like do long strokes make big torque.

[00:02:43]All there are is engineering a device for a specific purpose.

[00:02:50]And in the case of the Harley with its tendency towards long strokes, it's it's slowly going away up there on Juno Avenue, but uh they keep it around.

[00:03:03]And there's a good reason for wanting good bottom torque. Now for that for their type of motorcycle, one of the things that is not often discussed is oh some people are going to call it intake mock index but I want to call it limits of intake flow.

[00:03:29]If you have uh let's say we have four valves, most engines do now, including the big tours at last. Uh and we're going to try to operate this engine at various RPM. And we notice that as the engine revs up, uh the torque has been pretty level.

[00:03:56]recent designs generate really strong wide torque. So, it's more like a torque plateau than a torque curve.

[00:04:06]But we notice that as the engine revs up more that the torque begins to fall.

[00:04:12]And the rising horsepower carries on for a while, but then it too softens and hooks over and it begins to make less power as you rev it up. The reason for this, principal reason for this is that as the speed of air flow through the intake ports rises towards half of the speed of sound, uh, it encounters losses that fix it. So that applying more suction on the engine side will not pull more air through the port. And that's why the torque falls off. And uh good old Harry Ricardo, one of the fathers of the internal combustion engine, called this wire drawing. And what that meant was that um what he meant by it was that the harder you pull on the intake port, the less dense the air becomes because there's a resistance to flow.

[00:05:16]So when the English were developing their wonderful racing singles, AJS, uh, Veliset, Norton, um, etc. >> Do they get to take a drink if you say Veliset is what I want to >> Oh, we hadn't thought of that.

[00:05:37]>> And if I say TZ750, does that count?

[00:05:40]Sir, go ahead.

[00:05:42]>> All right. Um, when engine developers reached this point, they knew that their engine was not going to be able to operate at another 500 RPM higher, but they're thinking to themselves, well, the guys at the competing manufacturers are sure going to show up at the man TT with more power than they had last year, and we have to do even better.

[00:06:11]So what did they do? They couldn't get more air flow through the valve and intake pipe because they had streamlined them as best they could. Um, good old Harry West got into the airflow business 1926 or so. So he was his services were for sale. he was in business to improve air flow and a lot of people took him up on it. And so what you had to do at this point is you had to increase the size of the intake valve and the intake port in order to move engine operation up by a few hundred RPM.

[00:06:56]Oh, well, we put a bigger intake in last year and we were playing around with it and we saw that the edge of the intake valve is pretty close to the cylinder wall and the uh this valve isn't going to flow any more air because it's in the way and the cylinder wall is in the way. What we need is a larger bore. Not a lot larger, but enough larger to accommodate a bigger intake port. And that's why if you look at these various designs, they slowly got larger bores and shorter strokes. Now, it's it's common today to say, "Oh, well, uh, you got to have a large bore and a short stroke because a short stroke lets your engine uh, rev up more." And you put those big valves in to make sure you get the the good stuff in and the burnt stuff out in the shorter time available as you raise the revs. That's approximately true.

[00:08:04]But the English weren't going to try to go to the ultimate in one jump. What was the ultimate? Well, during the period when they were running Formula 1 engines to 20,000 RPM, the largest number bore divided by stroke that I saw was 2.25.

[00:08:35]which means that that the stroke is is just going away. It was down in the 30s uh millimeters and the the bore was tremendous.

[00:08:48]So, uh, the English didn't run into these problems that we're going to discuss here because they were making the progress that was possible for them because at the same time they were starting out with engines with iron heads and cylinders which didn't cool very well. H maybe we'll make a bronze head. Many of you will have heard of bronze head versions of various engines such as Vincent's racing 500 single with the bronze head.

[00:09:21]>> Cool.

[00:09:21]>> The 40. Yeah, 40. The 40 Triumphs. There was a bronze head that they used 500 twins.

[00:09:29]>> Y and >> so they're changing that. Oils are improving. Fuels are improving.

[00:09:37]So you can't really put your finger on any one thing, but there is this trend throughout somewhat larger bores to allow larger valves whose airflow will serve the engine at a somewhat higher RPM.

[00:09:54]And racers are conservative. They have to be. Their experience guides them to conservatism. Because when you decide, I'm going to try out all my crazy ideas at once, usually you have a long period of fixing problems. So, it's best, like the old-timer said, don't bring a new bike to Daytona.

[00:10:20]Bring a bike that is worked out that you understand perfectly. Anyway, the the big thought here is that the intake port has limits, has flow limits.

[00:10:35]If you try to rush the air through it too fast by revving up an engine without making the valves bigger, then you're going to run into that point where the torque curve starts to, oh, I'm tired.

[00:10:49]And the horsepower curve also softens and starts back down. That's why uh Harley's are all done. I'm talking about the two valve designs now. They're all done at 4,800 to 5,000 RPM. Why?

[00:11:07]They don't need any more than that. They want them to pull strongly at 3500 RPM to accelerate up on ramps and for passing.

[00:11:19]And to be fair, they're using a large displacement to give you that >> sure tremendous displacement.

[00:11:25]>> I mean, >> but their valves are quite limited and they basically choke the engine out. It wheezes out at 4,800 or 5,000. That was accepted for years. That was the thing.

[00:11:41]That was okay. Everyone agreed.

[00:11:44]But now we get to this thing, do long strokes equal high torque?

[00:11:52]People have explained it to me as if I were stupid time and again they said, "Well, come on, man. It just stands to reason." I mean, it's like, you know, you're trying to bust loose some some big bolt, you put an extension bar, you increase the stroke, gives you more leverage. So, uh, what you're doing is you're letting the the force on the piston act on the crankshaft at a larger radius and thus it has to make more torque.

[00:12:23]>> Well, I'm I'm the same size piston.

[00:12:26]>> Yes.

[00:12:27]>> On the torque wrench. That's the problem. If we're building a 1,00 C engine or a 74 inch engine or whatever it happens to be and we decide to increase the displacement, we could increase the stroke uh and just leave it at that. All we've increased is the stroke. Then we have to agree there's more leverage. The piston is the same size as before. So there's the same force acting on a longer lever.

[00:13:00]So the torque goes up. But in fact, if you're building a 74 and you increase the stroke, it won't be a 74 unless you also make the piston smaller.

[00:13:12]So if you're building into a class, if you do the arithmetic on this, you will find that the area of the piston decreases exactly as much as the stroke increases.

[00:13:28]That is the force acting on the piston is reduced by the same amount that the stroke is increased. So there is no increase in torque. Now at this point another idea needs to be introduced and that is that the word torque means different things to different people in the laboratory um in the old dino shack uh with the terrible noises coming out uh torque means how much twisting force in pounds feet is the engine generating at XRPM.

[00:14:11]It is a physical quantity which you can measure. Hey guys, gather around. Look at this. What's it say? And they all say the same. Yeah. 62 uh pounds feet.

[00:14:25]You write it down. But when people are saying, "I bought I've been years. I swore I was never going to do it, but I bought a Harley." Laughter all around.

[00:14:36]Well, you know, I've had it with breaking my back on some stretched out sport bike, and I'm just going to kick back and enjoy myself. Yeah, okay. Yeah, sure.

[00:14:48]But, uh, man, it sure is torquy.

[00:14:54]Now, that's a different meaning of the word torque. What he means is that almost regal without regard to how fast the engine is turning, if he turns the throttle, the bike will accelerate in a forceful way. That is torquy. A torquy engine has a broad torque curve that is strong enough to accelerate the motorcycle.

[00:15:24]And long stroke engines tend to be torquy for a solid physical reason.

[00:15:36]If the stroke is long, the bore is small and so are the valves which means that their best uh their region in which their velocity does the best job of filling the cylinder with none of that fade at the end of the curve.

[00:15:58]Those small valves are going to give this engine a broad and strong torque.

[00:16:06]And if you look at uh valve timings for Harley-Davidsons and other motors of the type that pull big heavy bikes, you'll find that they are like Volkswagen uh valve timings. The intakes open pretty close to top dead center and they close pretty close to bottom center.

[00:16:33]And that short timing does not carry on towards higher RPM very well.

[00:16:41]But there's no reason why these bikes should pretend to be road warriors when in fact they are touring machines.

[00:16:52]Signing off at 5,000 is just fine.

[00:16:57]And so it's always well if people start talking about torque to get it clear what are we talking about something that can be measured on a dynamometer or are we talking about a feeling we get when we ride the bicycle and when people say that's a torquy bike I really like it. It's easy to ride etc. And that uh in a two- valve engine used to mean that the intake valve is delivering its best flow sort of right in the middle of the engine's operating range so that it pulls strongly off the bottom. When it goes up the on-ramp, it passes traffic competently.

[00:17:50]The valves could be made bigger by changing the bore stroke ratio so that the bore is bigger with more room for valves. But then the best operating point for those bigger ports would be up at higher RPM.

[00:18:09]Why did I buy a tour bike? Why not buy a 600 Super Sport and go touring at 10,000 RPM?

[00:18:20]because you don't like the vibration.

[00:18:23]Because you don't like the racket and the feeling, this thing is wearing out so fast I may not make Denver tonight.

[00:18:34]So a long stroke creates making the stroke longer increases piston shaking force because piston shaking force is directly proportional to stroke.

[00:18:49]But piston shaking force is also proportional to the square of RPM.

[00:18:56]So what people want in these big torquy bikes is smoothness. And that's why they give them an overdrive ratio so that when you get up to cruising speed and shift into sixth, the engines down, they're just purring away or the rubber mounts are doing their good job. or the balancers. And it's a pleasant ride.

[00:19:21]>> You know, I just rode my wife's uh my wife's Sportster today, XR1200X.

[00:19:26]So, the enhanced brakes and adjustable suspension and uh you know, it's the old aircooled parallel uh parallel push rods, 3 and 12 in bore or um 88.9, and the stroke is 96.8 mm or 3.812. So, getting close to a 4 in stroke. What's neat about the XR is it has that Sportster torque so that at 2500 rubber mount Sportster 2500 it feels great like you right off the bottom it's filling well but they did a lot of hot rod work to it now when I went on the press launch for that bike I asked the engine guy said oh you probably talk this is 200 8 when I went on the press launch and I asked the engine guy I'm like well you know you guys probably talk to Bule you know get a little pointers There's there was a very distinct Juul has nothing to do with this motorcycle. I mean very very definitive. So they hotted up their Sportster engine to for the sporty. You know the XR was the the naked kind of a street tracker in inspired sporty bike.

[00:20:34]Pulled nice. It it tapers off, you know, as it gets up to seven, but it is really broad and it has very great bottom end torque and and keeps pulling. It's a really nice combination, but it is two valve and it's exactly what you're talking about, Kevin, even in the sportiest version of the Sportster.

[00:20:55]>> Well, this is this is why the the big tour rigs of have are shifting to four valves as follows.

[00:21:06]Uh if your riders are saying, you know, um I think my bike could use a little more punch for passing in uh on-ramp because um I got it loaded pretty good and my life partner is no featherweight either and we just we just think it would go better with a little more punch. Well, it's easy to get more horsepower from a two valve engine. Just leave the intake valve open longer after bottom center. And then at higher speeds when the intake air is going fast enough that when the piston is rising and the intake valve is still a bit open, the rushing air overcomes the tendency of the piston to push the air back up the intake pipe and the valve closes and it traps a good solid charge.

[00:22:06]But by keeping the intake open longer at low speed, the piston's going to push it back out because the intake velocity isn't high enough. So, uh, this is how Jerry Branch was able to get 100 horsepower from a 74 um, 40 years ago by revving it up to 7600.

[00:22:33]Try touring with that.

[00:22:36]So, uh, how are they going to get more power? Well, one thing is if you switch to four valves, you can put in quite a lot of more valve area with two intakes than with one great big one. And the two intakes can open and close more quickly because they're lighter.

[00:22:57]the two valves weigh less than the one previous valve because of the old squared cube rule. Um the weight is in the volume in the valve which is usually the cube of dimension and the area of the valve which is what's delivering air flow is the square. So what they did was they said, "We know that with four valves we can run short timing, the timing that our customers love, and yet because the intake valves have greater area, we can deliver a lot of power up high that wasn't there before because the single valve was wheezing out."

[00:23:49]And oh, but if you make those big ports, then they're going to have a low flow velocity. Uh uh uh because we're opening close to top center and closing close to bottom center, which means we have to get the air in there quickly, which means velocity. So you've created intake area, but now you're going to use it uh at a high speed for a short duration.

[00:24:20]So you end up with the kind of punch that people want at 3500 and which keeps on pulling.

[00:24:29]I'm not going to let that Goldwing guy embarrass me.

[00:24:34]So um they all end up with a similar solution because that's physics you know people people look out to the limits of the universe and the molecules the electrons the protons all seem to be obeying the same physical laws I don't like the word as we have here and they're 14 billion years in the past so pretty reassuring. I like the idea that the electrons will behave when I get to my hotel room just as I expect them to.

[00:25:12]So, uh there was a uh a grand old southern boy motorcycle tuner called Mississippi round man Larry Warl. He's no longer with us, but he had a great deal of experience, two-stroke and four- stroke.

[00:25:36]And he enjoyed his work.

[00:25:40]And on some bus ride somewhere, he described to me three engines that they had built at American Honda during their um entry to AMA Superbike beginning in 1980.

[00:25:55]and he said what we found was the fastest combustion occurred in the engine with the smallest bore.

[00:26:05]Now, one way to look at this would be to say, well, the distance from the spark plug in the center of the cylinder out to the cylinder wall is shorter. So, yeah, you'd expect it.

[00:26:18]But there's another effect going on and that is that as the piston comes up and compresses the charge and stops at top dead center, the smaller the bore, the taller the space above the piston.

[00:26:35]And if there's plenty of room above the piston, whatever motion you've given the air, whether it's circular swirl or if it's tumble motion is going to be able to persist longer in a more open chamber than it will if the bore is larger and the stroke is shorter, which means that there's hardly any gap between the piston and the cylinder head. And this is the main reason why the larger you make the bore in general, the earlier you have to ignite the charge.

[00:27:14]And in some of these really racy engines with extreme bore stroke ratios like 2:1, they were timing the spark at 60° before top center or more.

[00:27:31]And once I happened to be in the office of uh Ducati's, he's the CEO now, but back then he was a jack of all trades. He did whatever they whatever needed doing. And I asked him um how does uh why do they keep pursuing larger bore and shorter stroke as they do in as they were doing in Formula 1 when when this early ignition timing means a longer period of heat loss from the burning charge to the metal inside the engine and he said, "Oh, it's very simple." He said, "Yes, the losses are serious, but so are the gains and the gains are greater than the losses. So that's why they go that way."

[00:28:28]But uh when the when Dorna created Moto GP, their four-stroke class, their first one, they put a limit on bore at 81 millimeters for uh well now it's a th00and cc's.

[00:28:49]Uh it was 81 by 48.5.

[00:28:53]So, not 2:1, more like 1.75 or so because they didn't want an RPM race with everyone furiously building one prototype after another. When BMW got into Formula 1 years ago, they built more than 100 test engines with different characteristics, different bores and strokes and evaluated them in detail. Do you think that costs money?

[00:29:30]I do think so. So they were trying to avoid that and uh that was the reason because you're you're trying to push RPM up by having an enormous bore with gigantic valves in it and a little tiny stroke and you've created an engine that is not entirely practical.

[00:29:58]Now, uh, racing class of course doesn't have to be entirely practical, but it it shouldn't be laughable, should it?

[00:30:09]So, anyway, this was this has been a problem. Now, another thing that comes up quite frequently in engines with larger bores and shorter strokes is a terrible compromise, which we all hate. We want we want a set of principles that will guide us uneringly to the goal of winning races or having an excellent street bike or whatever the goal is.

[00:30:38]But what they found was um Steve Johnson was building these uh FCR 750 superbike engines and he found that if he pushed the compression rate ratio up, the combustion chamber became wafer thin and the motion of the intake flow coming into the cylinder at hundreds of feet per second to create a a tumble motion to store that energy to turn it into flame accelerating turbulence at top center.

[00:31:22]By the time the piston got to top center, all that material was crowded into such a tight combustion chamber that it had lost most of its energy. So they had to advance the timing and advance it. Oh, here's the maximum. Yeah, we got 58°.

[00:31:43]And of course, uh, lots of heat going into engine oil and coolant, disappearing. We paid for it, but we're getting nothing from it.

[00:31:54]So, they began to build different engines for different racetracks. For a short racetrack with a lot of accelerations off of the several corners, they would build a high compression engine that had lousy top end because of this loss of turbulence.

[00:32:14]And then for the high-speed track, they would lower the compression to open up the chamber so that they could store intake charge motion and turn it into turbulence.

[00:32:27]but they'd lost some acceleration because of the lower compression ratio.

[00:32:33]And this has been uh a problem in Formula 1 and Moto GP and in any form of um engine development that moves in the direction of larger bore and shorter stroke.

[00:32:48]And those people at that time were really annoyed at at what was happening because they couldn't they couldn't make an engine that would perform well on all types of circuits.

[00:33:08]So the curse of compromise.

[00:33:12]>> Well the long timings too. We have seen many applications of more than one spark plug. So you light the flame in two areas and h half the distance it needs to cover. There's a aftermarket Jaguar, you know, XK cylinder head that has completely redone flow. It looks like a Jaguar head, but it has twin plugs and it's a, you know, it's a fresh piece and the torque, the torque numbers are astronomically higher. And like many of the engines that Kevin's talking about in the old days, the two valves, they were getting hemispherical u heads, you know, the Hemi. Oh, well, actually, what you want is the wedge, but the the Hemi, you know, the Hemi, you're doming the piston. Um, I don't have my do piston, but you're doming the piston. And imagine, you know, why do you make a tunnel to go through the mountain? Well, it's less steep and it's a shorter trip. So, the straight line under the Hemi is a shorter line. So that's why we do flat, you know, flat pistons because the the flame travel just goes this way instead of over the mountain to the other side. So if you light it over here, it's got to go all the way over here. And that's why we get detonation and you know the problems.

[00:34:22]And with the with the Hemi, if you have the Hemi and that's what you're sticking with, and you put two plugs in, you're just having the distance that the flame has to travel to meet itself. You're shortening the time of heating that Kevin's talking. And what he means in the most base sense about heat going into the oil and heat going into the coolant is you're making the heat and all that energy should be pushing the piston down, not heating the other parts promptly, right away, >> right? Not not dwelling in there and growing and getting hot and you know.

[00:34:56]So, uh, good old Keith Duckworth, after spending two years trying to get one of his, uh, little fours to burn in a short time using squish to generate turbulence and failing, he decided to take another direction and it was four valves. And then with four valves, you think to yourself, well, we can't really offset the ports and have rotary swirl. So, what what can we do? And he came up with the idea of tumble.

[00:35:36]And uh he made the valve angle uh included angle between the exhausts on one side of the head and the intakes on the other 32° so that the combustion chamber was nearly flat. Now, here's something that doesn't get much um conversation, and that is that a true Hemi with the valve stems at 90° to each other has twice the surface area of a flat disc of the same diameter as the bore.

[00:36:19]Twice. Which means twice the exposure to heat from the hot combustion chamber.

[00:36:27]Twice. And this is why all those West Coast tuners um I'm most familiar with the late Kenny Austinine wanted yearned inexpressibly to put the equivalent of Norton's export twin cylinder head with its valves at 58° onto the 650 Triumph which had 90° and a deep uh VU surface area hemi combustion chamber.

[00:37:03]Now are all those people who praise the hemi chamber uh full of uh undesirable matter?

[00:37:13]No.

[00:37:14]Because it turns out that in terms of air flow in cubic feet per minute versus uh the diame the area of the valve head in square inches was better in a two- valve Hemi head than in any four- valve pent roof head.

[00:37:40]Better.

[00:37:42]And for a long time from 1923 right on through the 50s, it was the fervent belief of the faithful that the Hemi head rules and people who had worked at Triumph knew about the overheating problems with their engines. Because if you have 90° valve angle in a deep hem chamber, how are you going to get to 10:1 compression ratio?

[00:38:17]>> I'll tell you, >> by filling the chamber with the giant lump of aluminum >> mandial pistons. I bought my 58 Triumph Trophy years ago and uh it had some problems and it I decided to do the top end and I pulled the uh I pulled the barrel off and I pulled the head off and what did I find? Bondial high compression pistons. They were 10 or 11 to one and they had a huge wedge on the top to go up and fill the chamber. And what else does that do? Blocks the flow.

[00:38:48]>> Yes.

[00:38:48]>> So it was uh and it also had a tremendous amount of clearance. Uh the running clearance for that was 10 thou 11 thou. I measured it and I'm like well this must be wrong. And then later on no in fact because it was getting so hot.

[00:39:04]getting so hot because the pressure of the piston increases its surface area and that hot combustion gas >> just beats and pounds. Those molecules are so many little Joe Lewis's and all that beating and pounding by molecules is temperature.

[00:39:26]And in a gas it's it's the speeding, zipping, zooming velocity of the molecules. And in solid materials, it's the furious vibration of atoms around their places in the in the crystal lattice. Sort of like the individual in society yearning to be free. I can just vibrate, but I can't get loose.

[00:39:51]Um, but in the case of these hot running pistons, they could get loose because the pistons could melt.

[00:39:59]So Keith Duckworth did us all a favor by squashing the combustion chamber down to a disc and the valve included angle kept getting narrower and narrower. Kawasaki got it down to 25° and I'm speaking to one of their engineers and he said, "You know what?" He said, "We'd like the valves, the valve included angle to be zero. We'd like all the valve stems standing up straight like they do in a in a General Motors diesel, but we can't because the drive sprockets on the end of the cam shaft, they won't go any closer together.

[00:40:40]Sounds like an excuse, but it makes a good story.

[00:40:45]So that's where we've ended up is with larger bores than strokes because in engines that are supposed to make a lot of power, you need valves big enough to flow the air at a at an intake velocity that is in the comfort zone of air.

[00:41:06]One of the things that happens in intake ports is that shocks form. Remember the P47 Thunderbolt? The aircraft that GIS called the Jug? Oh, do >> it was just a big old milk bottle that was crammed through the air by an R2800 18 cylinder radial and it began to get sonic flow on various parts of its robust body because the air was having to do what Mark was just talking about, climb over the hill going the long way.

[00:41:44]The air in order to keep up was reaching the speed of sound in various places and shock waves formed and carried away energy. Oh, let's eat this. And well, we have a bird feeder out here. Squirrels and jays and all kinds of all the living creatures come as if there was no food anywhere.

[00:42:06]Um, so then some bad things happened.

[00:42:15]Everyone was making the change to four valves.

[00:42:19]Everyone was making the change to liquid coolant.

[00:42:23]And uh at one point, I think 1990, Kawasaki decided that with their 750, which was the basis for the Superbike, that they were going to shorten the stroke from 51.5 to 47.3 or something.

[00:42:41]Rob Mazi told me it took us it took me he said took me two years to equal the power of the longer stroke engine.

[00:42:53]And in 1988 Suzuki shortened their stroke from high 40s to mid4s and they couldn't get it to work right.

[00:43:05]Could not.

[00:43:07]And so what did they do after a year of suffering? They went back to the old longer stroke.

[00:43:17]So something was happening. You know, this is supposed to be a smooth process of progressive engineering. We're going to shorten the stroke, make the bore bigger, put in bigger valves. We're >> never closer to perfection. Yep.

[00:43:31]>> Yeah. We're we're just uh you know, we're like the faithful praying in a church of our own making and then uh being swept away by an avalanche. Wait a minute, this isn't supposed to happen.

[00:43:46]Um, so I I was uh at Ducati one time and talking with Cladio Demenali and I said I see that at least two of the Japanese companies tried to build a Duckworth engine and couldn't they couldn't make it work. They when they shortened the stroke one more time, everything ran out of gas and it was not an improvement.

[00:44:15]Well, he said, I can't commentate comment on what they do because I'm not part of their organization, but I can tell you what we do.

[00:44:28]and he said, "We vary the intake port downdraft angle."

[00:44:37]Uh, the old Triumphs twins had horizontal intake ports pretty close to it.

[00:44:44]So, that's zero downdraft. Um, Norton singles had 12 degrees for a while and I think they went up to 20 somewhere like that. And you can see looking at them, you can see that the carburetor is sloping down. So that was one of Dominical's variables and the other one was intake port diameter which controls intake velocity.

[00:45:12]Now the downdraft angle determines whether the energy of the intake flow which as I've said can be several hundred feet per second can be used to generate this uh barrel motion which we now call tumble by reducing the downdraft angle. So the flow goes straight to the other cylinder wall, turns down, hits the piston, and creates this tumble motion. Or we can stand the port up more steeply and fill the cylinder more completely and to hell with uh tumble. So it's a compromise. How much of this do we need to go with how much of that?

[00:46:11]And by controlling the intake velocity, uh they were deciding how much energy to invest in all of this energy storage bank because that's what swirl which the British invented in 1922 with the offset intake port >> or tumble >> which Duckworth invented in 1967.

[00:46:36]two ways to store intake energy to be transformed as the piston approaches top dead center into random turbulence. And there are some wonderful visualizations of what this turbulence looks like.

[00:46:51]It looks like cotton candy that is being pressed flat. You know that kind that's wiggly wiggly.

[00:46:59]Very deeply wrinkled flame fronts. Now, the flame doesn't go crackle crackle from the spark plug out to the cylinder wall. Instead, what's happening is that violent motion shreds the original flame kernel into pieces that are burning and carries them in random directions to all parts of the chamber so that the resulting area of flame is so great that the chamber burns in a reasonable length of time. the the if you turn that into a a flame velocity, which you can do with simple arithmetic, how far is it from the spark plug to the cylinder wall, how fast is the engine turning, etc. You find that the velocity is from uh 50 to 200 ft per second flame velocity. People refer to this as an explosion.

[00:47:59]If you were trying to do surface mining using that kind of an explosive, you would wait a long time for your paycheck because that is not an explosion. But if you get serious and get to high explosives like RDX or PETN, what is the flame velocity? Not rather I should say reaction velocity. Because what's happening is the molecules contain both the fuel and the oxidizer.

[00:48:31]And if you jostle it hard enough, they fly into each other's arms and are burned up in an instant. 30,000 ft per second.

[00:48:47]>> So >> speed of sound is 1125 feet per second.

[00:48:51]That's the speed.

[00:48:51]>> Yes.

[00:48:52]>> So like nothing.

[00:48:55]So, um, just one of those things. Anyway, uh, how did they know they had got the amount? How did Ducati know that they had the amount of turbulence that they desired?

[00:49:08]Because they put an animometer like device. You can see them on the tops of all the uh, tractor trailer rigs at the Drag Race Nationals. They're up there whirling around telling them the wind speed.

[00:49:24]They all need to know it.

[00:49:27]Anyway, this thing is inside the cylinder and if it if they have through experience and they got plenty of experience because during the time they ran V twins in World Superbike, they changed the boring stroke like every 10 minutes. It just seemed like they were up, down, and around. So, and every time they made those engines different, they remained notably powerful.

[00:49:56]They didn't always win the championship, but they always came close. So the animometer in the cylinder nowadays it would be done with you know digital simulation that told them how far away they were from the condition that they had found by previous experience to be successful.

[00:50:19]And this leaves us with the question, did the Japanese manufacturers just look at Duckworth DFV and say, "Oh, 32 degrees of downdraft angle. We can do better than that.

[00:50:34]Here's 50."

[00:50:36]Um, and end up off the map or at least near the edge. So that uh one manufacturer had to go back to the previous longer stroke and the other took them two years to equal the performance of the previous longer stroke.

[00:50:59]And here's another example.

[00:51:02]Um, I'm giving these examples so that to disabuse us all of the pretends principle that shorter strokes, bigger bores, bigger valves, higher RPM always win. How did Kawasaki with a the longest stroke in world superbike 55 mm win six championships in a row?

[00:51:35]It could be on the one hand you could say, well they they got to know their engine really well and the other companies were constantly changing things and well Ducati got away with changing things.

[00:51:49]So, uh, to me it looks as though that was Kawasaki's compromise where old Steve Johnson was having to build his FCR750 one way for a track that emphasized acceleration and changed the combustion chamber shape for a high-speed track that uh, Ducati employed the scientific method to devise uh combustion systems that worked.

[00:52:25]I recommend it. Experiment, thoughtful consideration of results.

[00:52:33]Yeah, it reminds me of see of looking for the sweet spot all the time. And we had a podcast about sweet running engines and what makes an engine work and not work. And I don't know, uh, you were talking about longer stroke, the GSXR100, the K5, the 2005, that era had a longer stroke.

[00:52:55]And there there was something, there is something really great about that era engine. And I don't know that it, you know, is it going to make the peak power that the current 1000's making in a full race application? Maybe, maybe not. I'm a street guy mostly. You know, I like my Norton Commando. Why?

[00:53:13]because it lights at 28°, meaning it's has a short ignition timing. So, it's mixing well. The chamber's a good shape, >> heat loss >> and reduced heat loss, and it just feels good. It's 45 I think it I think I downed it at 45 footbs at the rear wheel and uh it's wonderful indeed. You know, it isn't as much torque as a Sportster 1200, but it it's wonderful and sweet.

[00:53:39]And when you talk about optimizing the intake angle and getting great filling and uh rapid combustion and quick mixing, it reminds me of of looking for those sweet spots and how hard, you know, companies are.

[00:53:58]I don't know. You talked about downdraft. I think back to the FBZ series, the Yamahas where they just kept going more and more and they were represented. The FC um the phaser, the phaser had this representation of of of intake angle on it and this like sort of scoop-l like thing and it's just coming ramming it down into into the engine, you know. And um it's interesting everything we try where where where was the data you know what I I want to see into this process. I want to see into Ducati's process and I want to go back and and find the exact moment at Yamaha where somebody absolutely insisted we do a five valve and absolutely insisted that we make the intake incredibly steep because it's just going to go blam. It's not doesn't have to go around the corner and get into the intake. It's just going to go like this and shoot straight in, you know.

[00:54:57]>> Yep.

[00:54:58]>> Well, this was another of uh Duckworth's comments. Duckworth thought he was pretty cool guy. Uh he said um in this world people divide in roughly into two camps. the dull and diligent and the clever but lazy.

[00:55:21]I happen to be clever and diligent.

[00:55:26]And he made direct statements such as I can get more air into a cylinder with my finger than those people across the English Channel. He was probably speaking of Matra and their V12s. Then they can get into their cylinders with all their flow measuring equipment.

[00:55:49]Basically, he was saying, I have the experience to know what works and these fellows over there are masters of what ought to work.

[00:56:02]So, uh I want to just append something about the the Hemi uh combustion chamber and the great flow that it can produce, which is that uh going back to the old business of putting um a cylinder head and on the flow bench and blowing air through open exhaust valves and out the port. and you set it up so you have 12 inches of pressure, water pressure difference across the port and it's blowing a certain number of CFM.

[00:56:36]And you take a piece of paper and roll it up into a cone shape, cut it off so that it just barely fits in the hole and the flow goes up 30%.

[00:56:50]How can that be?

[00:56:52]Well, Berni observed, he did not enact a law called Berni's law. He observed that moving air has a lower pressure than stationary air. And there's a good reason for that. In stationary air, the molecules are going in all different directions. They're not all going in one direction. When you when you put the air into motion, a lot of that random motion now becomes organized motion. So there's less pressure anyway.

[00:57:26]Uh how could the flow go up 30% just by sticking some a paper cone in the hole?

[00:57:34]Goes up because the low pressure in the jet coming out of the jet of air coming out of the exhaust port is being pinched from all sides by the atmosphere.

[00:57:46]It's throttling it down.

[00:57:49]And when you put the cone on there, it causes it allows the air to slow down and recover its pressure without being pinched by the surrounding stationary air.

[00:58:03]Well, the the hemi shape of the combustion chamber with the intake valve in it, when the flow comes out of the valve, it can immediately attach to that curved shape and travel a distance and make a gradual transition from high velocity to pressure. It is diffused.

[00:58:25]The head surface is acting as a diffuser which the rolled up paper cone is also a diffuser.

[00:58:33]So this is this is um a wonderful thing about that particular deal is the high specific flow that you can get if your valve is surrounded by this nice curvature. So the flow attaches to it.

[00:58:49]So um there's Always more to say, but there isn't always more time. What are we going to do?

[00:59:00]>> Well, I just wanted to point out Kevin's um fun with buckets and uh get your hose and get your car wash bucket or your garden bucket. Don't use the garden bucket to wash your car, but uh take your bucket and get your hose and pretend that that's the intake port and um shoot it at the wall and watch it swirl around. And that's why the intake port is offset. You know, the hose is coming in at an angle and goes and you get your swirl and then you can aim it in to the water and you can watch the thing just roll and roll, you know. So, just do that. Make these observations. Kevin's always um he's always been fond of putting cream into your tea or your coffee and pouring it in and watching it tumble and and visualizing that as uh as the mixture getting itself. Uh, every time I pour the white liquid into the dark liquid.

[00:59:56]>> And I think I I I think a manner of thinking is what this is, Kevin, is a manner of thinking is constantly analogizing your obsession, which is internal combustion.

[01:00:09]Analogizing it to everything else in life.

[01:00:11]>> Treatment.

[01:00:13]>> Well, of course, don't we all? We all need treatment. The treatment is more horsepower.

[01:00:18]>> Yes. I got a fever and the only cure is more horsepower.

[01:00:22]>> That's right. It doesn't come in a pill.

[01:00:26]Anyway, um the bore stroke thing can get you into trouble. You can go so far towards big bore, short stroke that you can't burn the chamber. And this was another thing that Duckworth said. He said those people there are people in this business i.e. Formula 1 who can do a wonderful job of filling the cylinder, but they can't burn it once they've captured it.

[01:00:56]They don't know what it takes to burn it. And what it takes is vigorous random turbulence that can distribute the flame throughout the chamber in such a way that it quickly eats up all the unburned charge. The pressure goes up, the piston goes down.

[01:01:15]If all goes well, a good result.

[01:01:21]So that's how it goes.

[01:01:27]Um actually understand the things that you're doing. Don't just say this looks a lot like that, so it ought to work like that. Instead, you have to understand why the example works. And that's where Ducati's scientific method what intake velocity what uh tumble RPM measure the things find out what works and then in your new design you'll be able to start out with something that is close at least rather than just I like that intake downdraft angle that looks Yeah, I guess when you're Ducati and uh you're a big company, you can invest in all of your instruments and and do all all of those things.

[01:02:20]>> They showed me the rig. They showed me the rig. It was just a lot of junk, but it was a rig that worked. They got numbers for them so that they could say, "Ah, now we know what it takes."

[01:02:34]>> So, when you're a backyard guy like me, uh everything is by analogy. everything is by saying, "Well, that works over there, so I'm going to try this here."

[01:02:43]If I see that exhaust system and somebody messed with that, and I might look at the cam timings and say, "What's the port like?" And how big should those primary tubes be anyways? Is bigger always better? Not always. In fact, you know, if you watch enough videos on YouTube, you may learn something or you may learn the wrong thing. Who knows?

[01:03:05]One of one of the uh strong uh tuning guys uh got a Bule and they looked at the head. He looked at the headers on it. Those things are way too big. That can't work at all. So, they made a complete other set of headers and lost 16 horsepower because they didn't like the look of it.

[01:03:32]Well, it's nice to if wetting your finger and holding it up to see which way and how fast the wind blows is enough, but it isn't always enough.

[01:03:43]And so often Ducatti have improvised.

[01:03:47]For example, they have their chassis beater, which is a dreadful machine. It has an electric motor driving a rotating lump, and it beats the living daylights out of a motorcycle's suspension.

[01:04:02]You clamp the motorcycle in place, turn on the machine, and go to lunch and come back and everything is broken.

[01:04:09]You need to know if that's going to happen on the street.

[01:04:13]>> Well, yeah. No, I saw it at Arillia. I saw a similar rig. They were uh doing a chassis test. It was essentially um it was like a treadmill that somebody screwed a 4x4 onto the belt.

[01:04:26]>> Oh, that would go. Yeah. and and it just kept coming around and that thing had wires all over it and it was just getting the daylights beat out of it. It was awesome. It's a fun tour. That was uh what was that around 2003 to that was part of the trip to Magello with the um the MLE uh VWIN the uh factory version we rode around Magello and as Italians often do they secretly put transponders on the uh bikes. No one knew, but they were um they were giving awards that night to uh the people who set the fastest laps and so forth. And luckily I luckily I beat one of my colleagues by you know 07 or 0017.

[01:05:11]So I never let him live it down.

[01:05:16]Well, I think what you said um um the key to all this is not some magic formula like long strokes make big torque or ultra riv make killer power.

[01:05:26]It is careful designed aimed at achieving specific results.

[01:05:30]>> Aimed at a specific result tailored to what you're trying to achieve.

[01:05:36]And that's different from just saying we're going to make boo here. Watch this. Hold my beer.

[01:05:44]Um, well, that's that's a a formula, too.

[01:05:51]So, we're breaking our own rule here.

[01:05:53]But, uh, you get it. You get the the gist of it, I'm sure. I hope.

[01:06:00]>> Oh, I'm I'm into the gist, Kevin. Of course, that's why I'm here. Well, thanks for listening, folks. Join us on Patreon. That's where we do it all commercial free and we do some uh extra extra short form stuff. We will, you know, I promised we would talk about rods. We'll make that into a short form and we'll throw that up on Patreon and I'll get my collection of connecting rods together. I don't have an HBAM curillo rod. You know, something beautiful CP curillo. Now, I don't have one of those because I installed all of those.

[01:06:31]>> Okay, Kevin's Kevin's going to bring one. I will I will find my um Vansenheind drag bike gigantic connecting rod that they gave to us to photograph in the studio. I'll bring in those valves, too. And and we'll have uh regular tiny valves and big valves and connecting rods and we'll do a show and tell on uh on that one, Kevin. How's that sound?

[01:06:52]>> Live it up.

[01:06:53]>> It'll be fun.

[01:06:57]>> The rod show.

[01:06:58]>> The rod show.

[01:06:59]>> Join us.

[01:07:00]>> The rod show. Join us. We'll even Maybe I'll get a scale. I don't have a Do you have a titanium rod?

[01:07:07]>> No, I don't.

[01:07:08]>> I don't I don't have a titanium rod. We used to have one in the collection and it disappeared. But uh well, we can just do the math. We can measure it. What we'll do, Kevin, is we'll weigh a steel rod and then we'll just calculate the percentage mass of titanium off of it and then we'll know.

[01:07:26]But the dimensions might be different because the dimensions have to be different because the material is different. See, we're already doing the podcast.

[01:07:34]>> Yes.

[01:07:34]>> Let's just go have lunch. Thanks everyone. We'll see you next time. Yep.