Some motorcycle engines have extra magic. What is that magic and what design elements make the difference between being a good engine and a great one? Technical Editor Kevin Cameron and Editor-in-Chief Mark Hoyer discuss the key features of motorcycle engines that influence power, response, and efficiency, and most importantly: Make us happy!
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Listen on Spotify: https://open.spotify.com/show/6CLI74xvMBFLDOC1tQaCOQ
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00:00 Welcome to the Cycleworld podcast. I'm Mark Hoyer, the editor. I'm with Kevin Cameron, our technical editor.
00:07 Today's episode is examining what secrets make an engine so great.
00:14 What are the qualities of a design that make an engine happy and fun?
00:19 There are a lot of great engines, and there are a lot of less great engines.
00:23 Ones that don't maybe move us as much emotionally or even also literally.
00:30 I'm going to start off by asking Kevin Cameron, because he may have thought about this before.
00:40 Okay, well, presumably the things that make an engine happy are also the things that make it seem sweet to us.
00:51 Yes.
00:53 You had suggested, and I agree that the Norton Commando is a good place to start.
01:01 It was designed just after the war by Bert Hopwood in 1947.
01:07 Once he had finished this still highly respected design, he was fired by Norton.
01:15 But the ways of business are exceedingly strange.
01:22 But there is an engine that burns in a very short timing.
01:29 And the faster that an engine burns its charge, the less heat it loses to cooling.
01:38 Because you've built a fire inside the combustion chamber.
01:42 And this engine is able to be timed at 28 degrees before top center.
01:49 And lots of British twins are timed at 36.
01:54 And there are imported engines which are timed at 45 and more.
02:00 I can tell you a VeloCet 500 spec is something like 38 degrees.
02:05 Domed chamber, domed piston.
02:09 We can't do that with modern gas, but that's what the spec would call for is 38 degrees.
02:15 We should explain what we're talking about a little bit.
02:18 I think so that we, you know, for people who haven't thought about ignition timing and don't know what 28 degrees before top dead center means.
02:27 Good idea. Good idea.
02:29 You know, I'm going to kick you off with you're lighting the fire with the spark.
02:36 And the speed of the combustion doesn't vary that much, correct?
02:42 That's correct.
02:43 Whatever the RPM is, the time that it takes the charge to burn can change a little because of turbulence and other things at higher RPM.
02:51 But generally speaking, you light it with the spark and then you have a period of time where it's burning, not exploding.
02:58 It's burning and it's rising the pressure.
03:01 And ignition timing is lighting the fire, getting the pressure to rise and having the crankshaft to be in a position where lighting at 28 degrees before top dead center.
03:11 The crankshaft is rotating.
03:13 The pressure is rising and then you're getting to what you want is the optimal angle.
03:20 Correct, Kevin?
03:21 Yes.
03:22 Go ahead.
03:23 Of the crankshaft when the piston begins to really move downward.
03:28 And we know that work is force times distance.
03:32 So we've got force in the form of peak combustion pressure at about 10 or 12 degrees after top dead center.
03:42 And we're going to have distance now because the piston is starting to move.
03:48 So that's why whatever the velocity of the flame spreading is, we have to ignite the charge such that peak pressure is reached at that point, 10 or 12 degrees after top dead center.
04:03 If we have a slow burning combustion chamber, we might have to light 45 degrees before top center.
04:10 If we have an exemplary fast burning chamber, we could light it off later at 28 degrees.
04:18 And people have objected, well, if you're igniting it before top dead center, doesn't that mean that you're doing negative work?
04:28 It's actually not very serious because for many degrees after the spark passes, you can hardly detect a pressure rise.
04:39 And the reason for that is the flame kernel is small.
04:42 It's doubling in size rapidly, but twice of something small is still small.
04:50 So initially, we're not getting a lot of volume burned.
04:55 So the pressure rise is fairly gradual.
04:58 And you said something very important about the pistons really starting to move.
05:03 And so that's another thing, it's another fundamental we should think of is the piston stops twice.
05:10 And when the crank is very near top dead center, there's a long swing where the piston is just going to do, to do, to do, to do, to do, to do.
05:19 Hardly any motion.
05:20 Hardly any motion.
05:21 And it's when the crank pin is at 90 degrees that you get the highest velocity of the piston moving down.
05:27 And then when it hits bottom, it stops again.
05:30 And that's, you know, that light components and all the other things are totally different discussion.
05:35 So we'll go back to timing, 28 degrees on a Norton Commando, which you set it with a strobe and you get 28 degrees at whatever thousand RPM when it's all in.
05:45 And all in means maximum advance.
05:48 And when you're at idle, the reason you would set it at 10 degrees before top dead center is because the engine is spinning more slowly and you're lighting the fire.
05:56 It's taking longer for the crank to go and you're, you're lighting it so that you get maximum power at idle, so to speak.
06:06 You're getting the most efficient use of the pressure at the idle RPM.
06:09 And that's why timing changes over the rev range.
06:13 It's why it advances.
06:14 It's why you light it sooner.
06:15 And also, if you've ever owned a kickstart motorcycle with an advanced lever on the handlebar, if you fail to retard it and the cylinder volume is 500 CCs, you can have a painful right leg.
06:31 Indeed.
06:32 Because the thing will kick back.
06:35 Yep.
06:36 Anyway.
06:37 Even with an automatic timing device, it can still happen.
06:43 Yes, it can.
06:44 So the ingredients, the ingredients of sweet running.
06:49 Yes.
06:50 Another one.
06:52 Aside from we've just discussed rapid combustion.
06:57 And the advantage of that is that exposes cool metal to hot gas for a shorter time.
07:04 High flowing intake port.
07:06 We obviously want as little obstruction to filling the cylinder as possible.
07:11 The Norton has a nice, straight, smooth, well-designed intake port.
07:18 Another criterion is as high a velocity as you can achieve without choking.
07:27 Because the old theory of let's hog out the intake ports, which is a 1950s concept, trips over its own feet when you're in the mid-range and you want to pass and you turn the throttle and almost nothing happens.
07:46 Because there's not enough velocity in the intake pipe to keep air flowing into the engine past bottom dead center.
07:56 We leave the intake valves open for a bit after bottom center because we want to let the intake flow, which has just been raised to a high velocity of several hundred feet per second.
08:08 We don't want to slam the door in its face at bottom center.
08:12 We leave the valves open for a little while and it keeps coasting in.
08:17 It will coast in longer the higher the intake velocity.
08:21 Another thing we need is an open combustion chamber.
08:27 One that has room for energy storage.
08:33 Where do we get the energy?
08:35 We get it from the intake process.
08:38 The intake is rushing into the cylinder at several hundred feet a second.
08:42 In the commando, we've offset the intake port so that instead of approaching the cylinder on a diameter, it approaches it on a tangent.
08:53 This causes that rapid intake flow to create swirl around the cylinder axis.
09:00 It becomes an air flywheel.
09:03 It stores energy for when the piston shoves all that gas up to the cylinder head into a tiny volume.
09:14 So that there will be enough energy to become turbulence because turbulence is essential to combustion speed.
09:22 If you take a mixture, a proper mixture of fuel vapor and air, 14.5 to 1, and in a plastic tube, I saw this experiment done.
09:35 You hold your lighter up in front of it and it goes "Brrrp!"
09:41 It burns at like one foot per second.
09:44 You couldn't have an engine that burned that slowly.
09:47 In order to get a realistic combustion speed of somewhere in the vicinity of 50 to 150 feet per second, you need turbulence.
09:58 Turbulence shreds the flame kernel and takes the pieces to all parts of the combustion chamber,
10:07 much like Paul Revere carrying the message to all the outlying towns around Boston.
10:13 That turbulence tends to decay if there are projections in the combustion chamber that interfere with it.
10:25 The worst of these is the tall piston dome.
10:29 Because when that dome goes up into the chamber, the space for the combustion is like the peel of half an orange.
10:39 It's thin and it has tremendous area.
10:43 We would like a flat-top piston and we would like a shallow combustion chamber.
10:49 The analogy I've always used is you're on one side of a mountain and you want to get to the other side of a mountain.
10:59 The most efficient way is to make a tunnel, is basically to flatten the mountain.
11:03 If the flame has to go over the mountain, no matter where you light it, it has to do all of this movement.
11:10 It's terrible. That's why we like that kind of a chamber.
11:16 Also, another kind of obstruction that can exist is very deep valve cutouts.
11:22 If our engine has a lot of valve overlap, it means that at top center, at the end of the exhaust stroke, both valves are open quite a bit.
11:31 If they hit the piston, we don't have an engine.
11:36 Because squish does decay, we like to give it a kick right in the middle of combustion.
11:43 The way we do that is with what's called squish.
11:47 Part of the piston is made to come very close to the cylinder head at top center.
11:52 By close, we mean 27 thousandths of an inch or about a thumbnail's worth.
12:00 What that does is the mixture between those two surfaces as the piston glides to a stop is squished out from in between.
12:11 It creates inward flowing jets that give the mixture a last moment stirring.
12:19 Another point that we'd like to avoid is large surface area.
12:27 Triumph, when Edward Turner designed the Speed Twin back in '36, '37, chose the fashionable hemispherical chamber at that time, a true hemisphere.
12:41 The two valve stems were at 90 degrees to each other.
12:46 That deep chamber has twice the surface area of a disc, the diameter of the bore.
12:54 All that surface area means extra heat going into the cylinder head, extra heat going into the piston dome.
13:02 We'd like to flatten the dome and make the combustion chamber shallower.
13:09 Of course, necessary for any engine to have good torque is to have a high compression ratio.
13:16 Because the pressure at peak combustion is roughly seven times the pressure at the end of compression stroke.
13:27 So lots of us have done compression testing.
13:31 We know approximately what good compression is.
13:34 It's somewhere between 140 and 200 and something psi.
13:38 If you multiply 200 psi times seven, you get 1400 psi.
13:43 You'd rather have that than you would, say, those post-war engines had a seven to one compression ratio.
13:51 Seven times seven is 490 pounds, much less torque.
13:57 So in order to have this high compression ratio, we have to squeeze the chamber volume down, which leaves less room for turbulence and creates more rapid decay of the turbulence.
14:14 So some engines, a particular example was Yamaha's FCR 750 with five valves.
14:22 You could either have exceptional acceleration.
14:27 Always pick it on the five valve.
14:29 Yes. You could either have, oh, it had its points, you know, it had very light valves and it could reach good RPM.
14:37 So as we know, every design is a compromise.
14:41 In their case, if they raised the compression ratio until the motorcycle accelerated well, there was so little space for turbulence that combustion became slow and therefore the engine did not top end well.
14:58 If they reduce the compression to get the top end back, it didn't accelerate well.
15:03 They're trading intake area, right? They're trading intake area for a chamber shape.
15:11 Yeah.
15:12 Basically, I mean.
15:13 Just a compromise.
15:15 Because, I mean, since we, yeah, what we're talking about.
15:18 So all of this is adding up to like we're asking the question, what is the secret element of an engine that makes it sweet?
15:30 And it's all of the things.
15:32 I mean, it's everything that the engine is designed like having a good bore and stroke ratio and, you know, having the diameter cylinder, blah, blah.
15:40 But it's really the core element is truly the combustion chamber shape.
15:46 It sure is.
15:47 That's the beginning of everything in a combustion engine that we.
15:52 That's the personality right there.
15:54 Yeah, that we know and love.
15:56 And so we're just sitting here talking about a Norton Commando designed in 19, the engine and the intakes designed in 1947.
16:03 And I'm wearing the Norton Commando shirt.
16:05 This is my Norton Commando.
16:07 And you can go down in our bio or description and you can buy one of these shirts.
16:13 So we also have Cygo World shirts. So just a little pitch for the merch here.
16:17 I wore this in honor of the program.
16:20 We're talking about a really old engine.
16:24 That has a spectacular torque quality, people, a lot of torque about Norton, you know, it was a really great running engine.
16:30 The XR 750 dirt tracker has a notoriously high flow port and a great combustion chamber.
16:38 And it's still the, you know, there was a guy racing one down in Daytona, XR 750 cutting fast laps, you know, still to this day.
16:48 And that's the magic.
16:50 I think you have a two valve head.
16:52 You talked about, you know, the valves being 45 degrees, you know, the valve included angle.
17:00 A lot of that times that was to get flow out of a two valve setup.
17:04 Because in the early days, they regarded the intake and exhaust ports as plumbing.
17:10 They didn't regard them as high velocity ducts.
17:14 So the Norton was tremendously progressive at the time that Hopwood designed it.
17:20 Because the Italians at the same time were giving their engines 90 degree valve included angles.
17:29 100 degrees, even 110 degrees.
17:33 And so for Hopwood to swing it back to 57 or 58 degrees made the chamber much shallower.
17:41 And Harley Davidson, when they evolved the iron Harley in two steps into the aluminum XR,
17:50 they started with a 90 degree combustion, a hemispherical combustion chamber and 90 degree valve angle.
17:56 And they brought the valves together, made the chamber shallower, brought the piston dome down.
18:02 And eventually had the winningest engine in dirt track ever.
18:07 Yeah. Was that Zylstra or O'Brien?
18:11 Zylstra and O'Brien were the design team.
18:14 O'Brien would stand over him saying things like, "I want an inch of goddamn aluminum on top of that combustion chamber."
18:24 And Zylstra would say mildly, "Well, how about three quarters of an inch?"
18:29 You know, a lot of aluminum is heavy.
18:32 Yeah. Well, all right.
18:34 And Dick O'Brien. Yeah. So we're talking about Dick O'Brien, who was the legendary, you know, tuner, manager.
18:41 1957 to what? '84?
18:43 Yeah. And I actually interviewed him after he "retired" and moved to Florida.
18:50 And he was doing like NASCAR cylinder heads.
18:53 And I called him to interview him about, you know, his work and then work in the past and so forth.
18:59 And just as you say, profanity was the punctuation of every sentence.
19:05 And they're like, "Still sending me these goddamn NASCAR parts."
19:09 You know, and they would just ship them to him and he'd do some porting work and work on the chamber.
19:14 So he's really the guy who laid all that stuff down.
19:17 And that evolution of the chamber is heading to a truth.
19:22 Now, we're talking about two valve cylinder heads, which can be magical, obviously.
19:28 They have some real advantages that you can't get any other way.
19:32 Yeah.
19:34 One of them is that if when the flow comes out from under a valve, if you can attach the flow to a surface,
19:42 you will get a better flow coefficient, maybe as much as 15%.
19:48 And with a HEMI, which is enclosing to a degree, that curving surface invites the flow coming out from under the valve to attach to it.
19:58 But in a pentroof head, which is like just a bent piece of paper, the valves are in a flat surface,
20:06 except for the Suzuki's twin swirl combustion chamber, which attempted to put little HEMIs around each valve.
20:15 Good try, guys.
20:18 But if you go on a flow bench and compare specific flow, that is, flow per square inch of valve head area,
20:26 you will find that the Norton Commando is still superior in specific flow.
20:35 But in these more modern four valve engines, they've made the bore bigger so they could make the valves bigger.
20:42 So the four valve wins just on brute force valve area.
20:48 So again, it's a compromise.
20:50 There's the sweet, lovely attached flow situation in the HEMI head,
20:59 and you'd love to be able to achieve that in a four valve, but can't have everything.
21:05 Yeah.
21:06 Well, you've got two valves.
21:08 They're lighter.
21:09 You can spin it faster.
21:11 You're making the bore bigger and doing all the things and getting the valves open quicker, all that.
21:16 And you're just filling, as you say, it's brutal.
21:19 You're just filling the crap out of the cylinder out of sheer force and area.
21:23 And it is beautiful.
21:25 As you were talking about the attached flow, I was going back to the reading that I've done in the past by David Byzard.
21:35 And he's a flow expert engine guy.
21:38 If you've never seen a David Byzard book, you know, he did a ton of stuff.
21:42 His big hit was Mini Cooper, kind of Mini Cooper performance engines and carburation and porting.
21:50 He's our hero.
21:51 He's our hero.
21:52 Get those books.
21:53 I mean, the analysis that he does, it can cause headaches.
21:59 But that's sometimes when you're trying to learn things, you have to read stuff over and over and suffer the headache.
22:04 And then the lights go on and you're like, holy cow, that's awesome.
22:07 And, you know, I've seen him talk about like the small block Chevy, bathtub, combustion chamber kind of shape.
22:14 And the way he's porting, the way he talks about porting those cylinders is putting, as it's coming into the cylinder,
22:22 it's hogging it out a little bit and putting a bowl, getting the area around the guide opened up a little bit
22:30 and then getting a curve around the bowl, allowing the air to flow and just hit that curve and go in, as you say, tangentially.
22:40 So you have the cylinder and it's coming in over here and he's opening the area to unshroud the valve in the chamber
22:47 so that there's room for the air to come in smoothly and to get into the chamber and start doing that swirl that you're talking about.
22:54 And, you know, yay two valves, like, hey, that two valve Ducati back there runs spectacular, really fun bike,
23:01 but does not rev to 17,000 RPM and does not make the most amount of power that you could make out of a 900cc twin.
23:12 It's got great running qualities and it's beautiful and all the things.
23:16 I mean, we don't need 17,000 RPM out of an XR750 or a dirt tracker.
23:21 It doesn't necessarily need to be a four valve head.
23:24 It can be because you have some flexibility, I think, in tuning as you would with, you know, an Indian Scout these days, the 750.
23:32 You certainly could do some things with that that you couldn't do with two valves.
23:36 But let's talk about the, let's talk about what, how do you call that? The before BC and before Cosworth and after Cosworth.
23:48 Well, you know, you look in any, almost any car engine today and 99% of motorcycle engines,
23:58 and you'll see four valves at a narrow angle with a relatively low compact combustion chamber.
24:06 And you will also see intake ports that do not any longer.
24:10 Here's the cylinder axis. They don't come in like this on a Triumph, on a BSA,
24:16 on any traditional motorcycle, you have a 90 degree angle because the gas tank is right here.
24:22 How are we going to get the carburetor under the gas tank if we have a down draft intake port?
24:27 But modern bikes, it's not a gas tank.
24:32 It's an air box. And the gas tank has slid back, in some cases, under the rider's seat.
24:40 So that it's this anatomical looking thing. And what looks like the gas tank is just a cover over the air box,
24:48 which is a piece of Samsonite luggage with latches to hold it shut.
24:53 So all the motorcycle designers are cringing at you right now for disparaging their definitely not a good brand.
25:05 Well, what Duckworth found was that by varying the down draft angle of the intake port,
25:17 he could get various effects. If he made the down draft angle very steep,
25:23 he was using the velocity energy of the incoming air to fill the cylinder.
25:32 If he brought it down somewhat, he was using part of the energy to come across the head,
25:40 down the far cylinder wall, back across the piston, and up to create a loop that was a different kind of swirl.
25:49 And at first he called it barrel motion. Today we call it tumble.
25:54 Because if you have a cylinder in front of you, the motion is like this. It's tumbling along.
26:01 That's just in pausing momentarily. Duckworth is the Wirth in Cosworth.
26:09 Yes. Keith Duckworth and what? Michael Koston, I think.
26:15 So he discovered a way to store intake energy in a four valve engine.
26:24 I suppose you could offset the ports, but evidently nobody chose to do it that way.
26:30 And you'll notice if you look at an actual Cosworth engine, the down draft angle is not extreme.
26:39 He's trying to create that tumble motion. On some of the earlier Japanese designs,
26:46 the intake down draft angle is steeper. They're concentrating on filling the cylinder.
26:52 Cosworth himself in his book says it's a split. Pick one.
26:59 You know, where do you want the end? How do you wish to use the energy that you have?
27:04 And those engines, the valves are very smoothly fared into the combustion chamber so that they aren't sticking up to create turbulence, damping sharp edges.
27:17 The crown of the piston has almost not a mark on it.
27:21 And if it weren't for the fact that Formula One went bigger and bigger and bigger cylinder bore and shorter and shorter stroke until they got
27:33 the bore became 2.3 times what the stroke was.
27:39 They had such an enormous combustion chamber, so thin that the combustion was slowed way down.
27:46 And when I asked Claudio de Manicali at Ducati about this, he said, yes, it's true.
27:52 We lose something on the slow combustion, but we gain it back because that way we can have enough valve area
28:01 to fill the cylinder at some monstrous twin turning 12,000 RPM.
28:07 Always a miracle. I mean, when Paul Smart, the late Paul Smart, stood in front of Ducati's famous glass van,
28:17 its walls were made of glass, and he said, they're telling me this thing revs to 9,800.
28:24 Is that even possible? And this, of course, is a man who grew up with Norton's.
28:29 6,800 was good. 7,200 was shaky. And here this thing's turning 9,800 in 1972.
28:38 So that was another story.
28:43 But Massimo Bori went to Cosworth and he spent some time there.
28:49 He came back. He wrote a technical paper on what would happen if we combined Cosworth combustion chamber
28:57 with Ducati's desmodromic valve operation. And of course, we know what happens.
29:03 MotoGP world champions. Bordi was at the company a long time ago.
29:08 He was the bridge, let's call him the bridge to the modern era from Taglioni.
29:16 Very much so.
29:18 And I got to throw a quick shout out to Gigi Mengele, who was a very, very practical engineer.
29:27 Bruno Di Prato knew him. Bruno Di Prato is our European contributor, and he worked at Ducati.
29:32 And Dr. Taglioni was the best man at Bruno Di Prato's wedding. So they were friends.
29:39 They knew each other. And Bruno has great respect for Gigi Mengele, and as should we all,
29:46 because he really made the four valve cylinder head, the elegant mechanical execution that it turned out to be.
29:53 Like, you'll curse it if you ever try to adjust the valves on an older four valve.
30:00 But it is elegant and it works beautifully. And that combination, you know, we spoke recently with Stefano Fontoni
30:10 about the Superquadro Mono, the single cylinder that's powering the 698 Hypermotard.
30:18 With a 116 millimeter bore.
30:20 Gigantic bore, lots of valve area, and an intake port angle that is not extreme downdraft.
30:32 You know, it looks like it's sort of in the 25 degree range, and it's doing exactly what you said that Cosworth did,
30:39 you know, ages ago. And Fontoni, again, pregnant comment during our interview with him was,
30:44 we have some different ideas about airflow. And, you know, here we are.
30:49 And I mean, you know, on the four cylinder bikes, they're getting MotoGP championships,
30:53 and they're accelerating, they're clearly accelerating well, and they're clearly top-ending well.
30:59 And it's a beautiful adaptation of Formula One technology that has made its way into all of our lives without our,
31:12 many of us without our knowing it, Kevin's of course knowing it.
31:15 It just turned out to be good at everything.
31:18 Yeah.
31:20 And so that's a wonderful thing. And I think that Duckworth just kept plugging away,
31:27 trying to make engines burn quickly, and stumbled across this.
31:33 He didn't sit back and get a brain fever from thinking about it.
31:38 He just did the work. And this is what came out of it.
31:42 I think that's wonderful.
31:44 Yeah.
31:45 Theory should, theory emerges from practice.
31:48 Yeah. Well, that's really, I mean, isn't that development in a nutshell?
31:53 I mean.
31:54 It is. It is. Yes. The science, the science guys come in later and they say,
31:59 "Oh, you see, the reason this works is, oh, well, it's good to know."
32:03 Because the people who've been, for example, when Ducati were first doing their large bore twins,
32:11 they studied in-cylinder charge motion with an anemometer inside the cylinder,
32:18 just the way Harry Ricardo did, between the two world wars.
32:23 What's an anemometer? Help us out here, Kevin.
32:26 You've seen them on the weather stations, the things with the little cups that whirl around.
32:31 They measure wind speed.
32:33 Well, what Harry Ricardo and Domenic Colli wanted to measure was the rotation speed of the turbulence or the axial or tumble motion in the combustion chamber.
32:47 And I suspect a lot of that now is done with CFD, computer fluid dynamics.
32:55 But as recently as 20 years ago, they were using this anemometer.
33:01 But in any case, what Claudio said was, we work with two principal variables and we discuss both of them here.
33:10 One is the intake downdraft angle. Up favors cylinder filling. Down favors turbulence generation.
33:18 The second variable is the diameter of the intake port.
33:21 If you make it smaller, you might lose something on top, but you'll gain it in the mid-range
33:27 because it will have such high velocity in the mid-range that it will make better use of the cam timing that you have.
33:34 And again and again, there have been engines in development that have lacked a bit of mid-range.
33:43 And during their R&D, they've sent the head to the machine shop.
33:49 They've bored it and sleeved it back a sixteenth of an inch.
33:53 Even a small amount sometimes is useful. And they have a fatter mid-range.
33:59 Now, why do we care about a fatter mid-range?
34:01 Because the highways of this great nation and the circuits of all racing championships are not straightaways.
34:13 So the engine has to pull across a range of RPM.
34:17 And if we've got an engine that is slanted towards peak RPM, like the wonderful sport bikes that we enjoyed for 20 years,
34:29 then you're going to have a weak mid-range.
34:32 And you're going to find yourself riding around town getting beat in roll-ons by sportsters, 20-year-old sportsters,
34:42 because they don't have anything but mid-range.
34:44 Yeah, I mean, I think the point on a racetrack, I mean, what we're talking about is balancing a high-flowing intake port
34:53 with an intake port that also has high velocity at lower, especially at lower RPM in mid-range.
35:02 Because on a racetrack, they're not made of straightaways, you would have two opportunities at a racetrack,
35:11 maybe two decent straights typically on a racetrack where you could take advantage of a lot of top end.
35:19 But if it's got 14 turns and you can pick up a tenth on every corner exit, you've got a winner, right?
35:27 That's why you go for torque.
35:29 It's more controllable by the rider and you have more opportunities to take advantage of it.
35:36 I talked to an endurance race team in the 600 class, I think it was Wira, they were racing.
35:42 And the guy said, "Yeah, these people, they build these engines for this kind of racing that have all this top end
35:49 and they don't have any flexibility in passing."
35:52 We build a fat mid-range, we try to keep top end, they're going for that balance,
35:57 but they had options on the racetrack because the engine had sweet torque.
36:03 I remember the drag racers who used to put 750 heads on 1100s because the ports were automatically smaller
36:14 and it helps the acceleration.
36:18 But you do have to choose what you're trying to build.
36:22 You could build such a top end motor for Bonneville, you could never reach the top end
36:27 because it's like a step ladder that only has two rungs and they're all at the top.
36:33 How do I climb?
36:36 - Well, the other issue at Bonneville is traction.
36:39 - Oh, sure, it's 40% or something like that.
36:41 - So you're on salt.
36:43 It's sort of a basic thing that you tend not to think about.
36:47 You think, "Well, I'm going to turn up the boost and I'm going to go fast at Bonneville."
36:51 But Bonneville is a salt flat and there isn't a ton of traction.
36:54 All the people that I've talked about, you get to a point where aero is really smashing you back.
37:00 Aero is the thing slowing you down because it's going up, up, up, the drag, the coefficient.
37:05 - Like 500 pounds of drag. A lot.
37:08 - Yeah, and so you're getting up to that point and the reason that you need not just top end
37:13 is you have to be able to control.
37:16 You have to feel the amount of slip and you're going to get the last few miles an hour on motorcycles anyway
37:21 by controlling that and feeling the traction and being able to feed in the torque that you need.
37:26 Now, it is a top end experience, but there's a band that they're working with
37:30 and you have to take advantage of that to get the speed to get the record.
37:35 It's not just brute force.
37:37 - Yep. When it comes to squish, a classic two-stroke cylinder head
37:43 consists of a golf ball shaped combustion chamber in the middle, which is lovely.
37:48 It's open like no four-stroke chamber can be.
37:52 And the squish is a ring all around the cylinder head that can be made to come very close to the piston.
37:59 And so you can generate as much squish as you like.
38:02 Probably, well, I won't say that.
38:07 The thing though is that with the two-stroke engine,
38:12 because it has the open combustion chamber plus squish in such large measure,
38:20 on top end, it is normal for a high performance two-stroke to be ignited at 15 degrees before top center.
38:28 - Amazing. - Because it burns so fast.
38:31 And some of those crazy Europeans running very high compression
38:35 were trying to light them as late as 10 degrees before top center.
38:39 That's because the two-stroke cylinder head is a kind of an example that a four-stroke builder might look at and say,
38:46 "Well, I may not personally prefer two-strokes, but that's a pretty nice head."
38:54 - Well, yeah. I mean, the advantage of a two-stroke having its intake valving and pumping
38:59 and all of that not happening on the cylinder head gives you the ultimate freedom there.
39:05 You don't need pockets. - Carbon. Yep.
39:07 - You don't need pockets in the valve. You can have any kind of piston top you want.
39:12 And then, yeah, beautiful circle. It's usually kind of wedged at the end.
39:17 And then you have that, say, golf ball size right in the middle,
39:20 getting everything stuffed right around the spark plug.
39:23 - Then it can really whirl in there. - Yeah, and away it goes.
39:28 - When it comes to high intake velocity,
39:31 what people have done is they have tried to make ports as small as possible and still supply the air
39:40 because the aim of having high velocity even in the mid-range is, one,
39:47 to have the energy to accelerate combustion with turbulence,
39:52 and two, to keep air rushing into the cylinder after bottom dead center.
39:57 So people have worked and worked and worked on those small but high velocity ports
40:04 until they've begun to reach the speed of sound.
40:08 Because what happens in the cylinder, we naturally think without thinking,
40:13 that when the piston starts down, the air follows it.
40:17 The air has inertia. It can't follow it.
40:20 The piston is accelerating downward at something like several thousand Gs.
40:29 So the piston first, in the first half of the stroke, roughly, it pulls in that vacuum.
40:36 And the air is kind of going, "Do you feel something happening?"
40:40 And, "Yeah, I noticed it myself."
40:44 And it starts to get down the port.
40:48 And so the filling actually occurs in the second half of the intake stroke,
40:53 and the air really gets moving at the end.
40:57 And that's why it is necessary to close the intake ports after bottom dead center.
41:04 And this is why Screaming Eagle is such an easy business to be in
41:08 because all you have to do is not give them a 180-degree intake camshaft.
41:13 Opens at top center, closes at bottom center.
41:15 "Wait, what if we let it-- kept it open 30 degrees after bottom dead center?"
41:20 No, no.
41:21 Well, you have to remember it's a compromise.
41:25 And in this case, their problem is, how do we get this substantial motorcycle
41:30 moving away from the stoplight with possibly a substantial wife
41:35 and substantial supplies in the carrier boxes?
41:39 Standing start at an uphill stoplight, no embarrassment.
41:43 You need bottom torque. "Shut it," said the engine.
41:47 Right, and that's the way America has ridden.
41:51 It's the way America has driven.
41:53 I mean, that's why there's big-block Chevys and big-block Fords.
41:58 We turn at 2,500. Our peak torque is--
42:01 Definitely. My mother got into third degree, third gear in our Kaiser.
42:06 As soon as she could, I could hear it knocking, but that's the way they drove.
42:10 Yeah, it was-- and I mean, you know, a big twin.
42:17 What a street engine it is.
42:19 It's making power exactly where the bulk of customers would want it.
42:25 And even though the Goldwing 6 is a very different way to achieve that,
42:31 it achieves the very same thing.
42:33 Absolutely.
42:34 Tremendous torque at 900 RPM.
42:37 When I saw that, I just thought, "There's a forehead smacker."
42:41 Yeah, when we-- yeah, that is--
42:44 and, you know, if we're talking about the secret of sweet running engines
42:47 for a street bike, a touring bike like that,
42:50 you have displacement, knockout flow, great velocity, spectacular filling.
42:58 And what you get on a Goldwing is you have a flat six,
43:02 so you've got six fairly over-square cylinders,
43:06 and then you have lots of intake valve or lots of valve area,
43:11 and it's tuned to make 108 foot-pounds at something around 1,200 RPM.
43:21 It's more than the average electric motor.
43:23 It's just spectacular.
43:25 It really is.
43:26 For that purpose-- and it revs well, too.
43:28 I mean, that's the other thing about the Goldwing.
43:30 They're not taking it up to 10,000, but it revs well.
43:33 It revs freely.
43:34 It feels good.
43:35 And it's the product-- again, it goes back to Cosworth.
43:41 We have-- really, that chamber changed everything.
43:44 So when we're talking about the ingredients of a sweet running engine,
43:48 it is high flow married balanced with-- compromised with high velocity.
43:56 Yep.
43:57 And then, as you pointed out, that open combustion chamber that allows--
44:00 you know, it doesn't have peaks.
44:02 It's like a nice flat piston.
44:03 It's got a--
44:04 It's not going to be sticking up into it.
44:06 Yep.
44:07 Nice tight shape with squish, which is usually a ring around the outside
44:10 of the cylinder where the piston, as you say, gets 27,000ths away.
44:14 So the piston's coming up, and it's squishing everything in the middle
44:18 and lighting it up.
44:19 And that's why on a Norton Commando, when you're in third gear
44:24 and you shift from second and you go to third and you drop the RPM back
44:28 to, say, 3,000 or so, and you roll in, it just surges forward.
44:33 And it is not an astronomical amount of torque in the global sense.
44:40 Yep.
44:41 But it makes a beautiful amount of torque, and it has a surge to it,
44:47 and you're getting fast combustion and velocity working all together.
44:51 And it's filling the crap out of the cylinder and taking advantage of--
44:57 it's taking advantage of the tumble and energy to make the combustion happy.
45:02 And that is how we get a happy motorcycle.
45:04 So this was not meant to be a Norton Commando podcast.
45:08 I don't think it was.
45:10 But it is an example.
45:12 It was an exemplary thing of its time.
45:14 It was before its time.
45:17 It, I think, represented the two-valve idea exceptionally well
45:22 until four valves said, "Hey," and Cosworth said, "Hey, look at what we can do,"
45:30 and now we have tumble, which is the same concept.
45:33 And we got everything that we get out of modern engines because of that change
45:38 from two valves to four and the optimization of that flow.
45:42 So that is the secret of a good running engine and something that is sweet,
45:47 that feels good to ride, and so many engines are doing that now.
45:51 So many--I mean, we are--
45:53 The message got around.
45:55 We are exceptionally lucky to be experiencing products.
45:59 All these manufacturers buy each other's product and they test the daylights out of it.
46:04 And I'm sure that some people have come back and wondered,
46:07 "How am I going to write this report without making our stuff look like junk?"
46:13 Yeah, that's all.
46:15 Oh, boy, we could do a whole podcast on--
46:18 We fixed the problem that wasn't a problem.
46:21 Yeah.
46:22 I've been at press launches where they say that, like there's a lean surge and--
46:26 Oh, yeah.
46:27 It wasn't really a problem, but we fixed it.
46:31 Yeah, it's pretty solid.
46:33 If you like what we're doing, please like and subscribe.
46:37 We are on Spotify.
46:39 If you're listening on Spotify, it would help out to give us a shout-out, like it, rate it.
46:45 If you're not, even if you're not, go over to Spotify and try and kick us up.
46:50 Listen while you're driving or whatever.
46:53 We appreciate all the comments and support.
46:55 Again, throw some topics in the comments.
46:57 Last week's podcast was about the Yamaha R1, and there's been some great feedback,
47:03 and people really enjoyed talking about that product.
47:06 We'll do more specific product things.
47:09 Anyway, we appreciate your comments and support there.
47:13 Next week, we're going to tackle oil.
47:17 Oil.
47:18 What oil do I put in my bike?
47:21 We're going to try and make that a real humdinger for you.
47:24 Okay?
47:25 Thank you.
47:26 Thanks for listening.
47:27 We really appreciate it.
47:28 Thanks, as ever, Kevin.
47:30 You've taught me--
47:31 It's been fun.
47:32 Yeah, you've taught me stuff.
47:34 You've taught me things about motorcycling, specific things, specific design elements,
47:39 and so forth.
47:40 One of the things I've learned from reading your work and talking to you all these years
47:43 is also a curiosity and disciplined or changed manner of thinking, thinking about the problem,
47:52 like asking yourself, "What's really going on?"
47:54 An example that I use is turning the throttle on a cold motorcycle.
47:59 Oh, okay.
48:00 Opening the throttle, because at a closed throttle, there's a high vacuum, and it would
48:06 be vapor pressure, the vapor pressure in that high vacuum is allowing the fuel to atomize
48:12 better.
48:13 When you open the throttle, the pressure drops suddenly, and you get globules, and that's
48:18 why it bogs when it's cold.
48:20 I would never, ever have thought about that if we hadn't had the manner of thinking discussions
48:28 all these years, and your books, and talking to you.
48:31 Those guys at Honda thought about it so much, and I'm sure the other manufacturers have
48:36 done the same.
48:37 They just haven't written SAE papers about it.
48:40 They have written mathematical models of what's going on in the intake port.
48:45 Some of the fuel is delivered as a vapor, fully evaporated.
48:49 Some in the form of droplets.
48:51 What is the size range?
48:53 Some is wall wash.
48:55 When you suddenly move the throttle, everything changes, just as you said.
48:59 In a cold motorcycle, it goes, "Hah!"
49:03 Like somebody had given it a good one in the guts.
49:07 Yeah.
49:08 Well, let's say a cold carbureted motorcycle.
49:10 Yes.
49:11 Things have changed.
49:12 We don't have to rely on signals so much.
49:14 It's a blessing.
49:16 We'll talk about signals some other time.
49:18 Signal, oh yes.
49:20 Let's hear it for signal.
49:22 Let's hear it for signal.
49:23 Well, thanks for listening, everyone, and we'll catch you next time.
49:26 time.
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