ReverendDexter SuperDork
Nov. 23, 2010 3:55 p.m.

So, a few myths that I'm curious about the physics behind.

One, bore vs stroke and how that contributes to engine behavior. My understanding is that a larger bore:stroke ratio makes for a more "rev-happy" motor. Is that true? If so, why? Is it because you end up with a lower piston speed for the same rpm? Wouldn't piston velocity be just as important, though, and with the larger bore meaning a bigger (and thus heavier) piston, wouldn't the piston velocity equal out?

What about engine layouts? The mythos around me is that inline motors produce more torque than an equivalent v-motor. Is that remotely true? If so, why? I know that Inline motors have more main journals per cylinder which supposedly makes for a stronger shortblock, is the added torque just due to engine manufacturers taking advantage of that? Along the same lines, the mythos is that a V-engine will rev "better" than an inline, is that due to the shorter crank being less susceptible to [insert appropriate physics term that basically means crank wobble]?

Gar. I need to take a proper engine physics class...

curtis73 HalfDork
Nov. 23, 2010 5:10 p.m.

There is a lot of wives-tale type information out there, but as an engine builder I can offer my two cents:

Bore vs Stroke is sort of a moot point. Given two identical combinations of parts, cam timing events, intake and exhaust velocity and mass flow, bore and stroke won't make any appreciable difference. This exact thing was proven on a dyno many times, but the one particular example I recall was setting using a Buick 455 (huge bore, short stroke) and and Olds 455 (small bore, long stroke). Everything was matched as closely as possible. Both made nearly identical torque and horsepower curves with peaks at the same RPM.

It was long thought that long strokes increased torque because of the greater leverage on the crank. That would be true if the engine were providing torque at 0 RPM, but since the torque in an engine is expressed as a function of power, motion is required. The bottom line is that there are 455 cubes worth of displacement available to provide force to the crankshaft.

Where you find truth in the wives tales is that (for a given displacement) longer strokes lend themselves to low-rpm torque... not directly because of the longer stroke, but because making a long stroke engine rev is an engineering hurdle. Its not that you can't, its rather that it takes a considerable amount of energy to overcome the inertia of the heavy, long legged rotating parts and high piston speeds. It also takes more exotic (expensive) materials to keep things together. If you think of the piston speeds and velocity changes in that long stroke Olds compared to the short stroke Buick, it stands to reason that it takes a lot more energy (and strength of parts) in the Olds to spin it to 6500 RPM. In a nutshell, its not that long strokes make more torque, its just that the physics make it a wiser choice to engineer it for lower RPM.

The other main reason for long stroke/small bore engines disliking revs is due to valve size. Small bores limit the amount of flow area you can achieve, therefore limiting RPM.

For this reason, I tend to chose large bore, short stroke engines for several reasons. Mainly, they can be equally competent stump-pullers as their long-stroke brothers, but if plans change in the future I'm just a cam and head swap away from bigger power.

There are reasons for chosing long stroke, however. Depending on chamber design they tend to be less likely to detonate and the smaller surface area of the cylinders means that parasitic friction is often reduced.

The argument for inline versus Vee and all of the vee angles possible is mostly due to reliability and harmonics. Higher RPM harmonics can not only kill reliability, but power as well. If the engine starts wasting energy on moving the engine around on the mounts, its losing it at the crank. Again, though its a trade off. Inline 6s are typically built with 7 main bearings which makes them very strong and reliable, but the additional friction and inherent harmonics of the longer crankshaft make them less suitable for higher RPM use. The 60* V6 is a much more stable platform for higher RPM but typically only has 4 main bearings and a crankshaft that holds two rods between mains instead of one.

Working at a transmission shop, I see the problems of the inline six vibes all the time. The Cummins diesel wreaks havoc on transmissions. Since it pulses every 120 degrees (and since they are rather violent pulses) it makes transmissions vibrate apart. A good example is the NV5600 transmission that was available behind both the Cummins and the Duramax. Behind a Cummins I see a lot of them around 150-160k. Since the Duramax came out I have seen one in my shop.

Jensenman SuperDork
Nov. 23, 2010 7:48 p.m.

Another aspect of the bore/stroke ratio: a larger bore allows the use of larger valves, meaning better flow. But this happens mostly at higher RPM, therefore you have to spin the engine faster to move enough air to make big HP. Yamaha's 5 valve Genesis and YZF engines are perfect examples of how this works.

Woops: Curtis already covered this. I doff my brain bucket to the maestro.

Streetwiseguy HalfDork
Nov. 23, 2010 8:07 p.m.

I don't think there is much more that needs to be added to this thread. Good essay.

tuna55 Dork
Nov. 23, 2010 8:43 p.m.
Streetwiseguy wrote: I don't think there is much more that needs to be added to this thread. Good essay.

Indeed. Thanks for that.

Spinout007 Dork
Nov. 23, 2010 9:24 p.m.

Ive ridden in an Olds 455 @ 7500ish rpm ( I couldn't keep an eye on exact rpm as I was to busy looking for an OS handle ) truly a beast of a motor. Nice choice for an example.

Strike_Zero Reader
Nov. 23, 2010 9:35 p.m.

Awesome read Curtis. . . .

AquaHusky New Reader
Nov. 24, 2010 9:05 a.m.

I have always thought that I-6s were the perfect engine because of their even firing. Smooth and you can make them in any size because of the balance and harmonics, which is why we have dual plane V8 cranks, balance and loading. Never would have thought that they would be violent enough to smash transmissions. although, the 2 extra pulses would make power application smoother per given RPM.

ReverendDexter SuperDork
Nov. 24, 2010 10:17 a.m.

Awesome, thanks Curtis.

pres589 HalfDork
Nov. 24, 2010 10:26 a.m.

You really want to think about firing order and crankpin offsets affecting power delivery, research the Honda motorcycles that were "Big Bang" some seasons and "Screamers" on others, or even the same depending on the team running them.

ReverendDexter SuperDork
Nov. 24, 2010 10:32 a.m.

I thought the "Big Bang" motors were a Yamaha design, not Honda.

Ranger50 Reader
Nov. 24, 2010 10:51 a.m.

Look at a 5.4 from Ford. It is a tiny bore motor with a LONG stroke and a damn tall deck height, 3.55" bore, 4.145" stroke, and 10" deck height.

In 4v config, you can EASILY turn 8k and still have power up there. Put the 2v heads on them and I wouldn't put them in a gokart. Plus with the charger Ford and Shelby puts on them, the 4v's, 800hp possible without porting, valve size changes, cam changes, etc.

Then on the converse, the 5.0 from 87-up in Mustangs. 300ft/lbs of torque from a 3" stroke motor.

IMO, depending on what I have to build, the whole argument is, "It depends".

Brian

pres589 HalfDork
Nov. 24, 2010 11:01 a.m.

In reply to ReverendDexter:

Per Wikipedia's table;

112° V4 'Big bang' (2-stroke) 180° 1990 Honda NSR500

90° V4 'Screamer' (2-stroke) 180° 1984 Honda NSR500

There's more variability with the Honda stuff but this is a good example.

p.s. V4 for life yo.

pilotbraden Reader
Nov. 24, 2010 11:06 a.m.

Yamaha R1 uses a cross plane crank putting each piston 90 degrees from the next. This gives a firing interval of 270-180-90-180 degrees.

A big bang fires more than 1 cylinder at a time.

ReverendDexter SuperDork
Nov. 24, 2010 11:07 a.m.

Did Yamaha (somewhat) recently do a Big Bang I4, then?

EDIT: Interesting, they call it a Big Bang motor, but I was also under the impression that a big bang motor fired two cylinders at once: Hell for Leather: 2009 Yamaha R1

pilotbraden Reader
Nov. 24, 2010 11:17 a.m.

I do not know of a big bang street bike. In MotoGP Yamaha has run big bang I-4, Ducati a big bang V-4, Honda big bang I-4. I do not know about Suzuki. I am almost certain that Kawasaki runs a screamer only

Racedreamer New Reader
Nov. 24, 2010 2:37 p.m.

Thanks Curtis - Great Info.

curtis73 HalfDork
Nov. 24, 2010 3:12 p.m.

Thanks for the kind words, folks.

Ranger50 wrote: Look at a 5.4 from Ford. It is a tiny bore motor with a LONG stroke and a damn tall deck height, 3.55" bore, 4.145" stroke, and 10" deck height. In 4v config, you can EASILY turn 8k and still have power up there. Put the 2v heads on them and I wouldn't put them in a gokart. Plus with the charger Ford and Shelby puts on them, the 4v's, 800hp possible without porting, valve size changes, cam changes, etc. Then on the converse, the 5.0 from 87-up in Mustangs. 300ft/lbs of torque from a 3" stroke motor. IMO, depending on what I have to build, the whole argument is, "It depends". Brian

Very good point. The more valves you have, the greater surface area of the chamber can be used for flow. That, combined with some pretty awesome new port designs, have eliminated many of the flow limitations with small bores

It does of course come with the trade-off of less area available to soak heat into the water - good for retaining energy in the combustion, but harder to engineer valves and timing curves to stave off detonation and keep emissions in check.

But again, not worse or better, just different. And most of the new technology used makes the trade-offs much less of a factor... especially compared to my 455 example :)

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