justeno_heat

I just was wondering why it is that 4 cylinders can rev higher than 6's or 8's can. Because I was having a decusion about that earlier and need specs about it.

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WILDACEX187

lighter internals maybe.

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IGSDann

Lighter components, less sucking/squishing to do... Though a Ferrari's V12 can wind up quick, but those are lower displacement.

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Ritz S14

Less rotational mass. Usually shorter stroke.

Shin_Kudo

I don't think it's necessarily true, by the way, I just think that it's more common to rev higher.

I mean, take the new M5, it's a V10 and redlines above 8.

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Ritz S14

Yeah, but it's only a 5litre. Being a 10 cylinder... it has baby pistons. = less rotational mass..and I bet it has a pretty short stroke.

IGSDann

That's why the RB20 revs like crazy.. 0.33 liters per cylinder compared to a KA at 0.6

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Andrew Bohan

since it has more cylinders, it doesn't NEED to rev as high in order to have the same firing rate as a 4-cylinder.

MakotoS13

silly kids. does anybody know the redline on a C5R? suckaz fuggin tall.

V8's only need to achieve the same efficiency as a 4 banger in race applications. 4 bangers, on the other hand dont usually make power till the upper half of the powerband.

V8's rule, there is no replacement for displacement.

Ritz S14

Sure there is. They're called turbos and supercharges.. Isn't there a 300hp Wrx Sti or something? An N/A 2.4L S2000 puts out what 240hp? That's 4 cylinders, small displacement, and a good amount of power.

OptionZero

Horsepower = Torque x RPM (divided by ...5252?)

Fewer cyl = less displacement = less torque (in general)

But moma...I still want to make power!

Thus, higher RPM become necessary to still make decent power.

Either way you make power, but having a larger power band is far easier to achieve with more displacement (forced induction or otherwise)

Rennen

It's all a matter of undersquare vs oversquare engines. (I suggest most of you search google for those terms )

Oversquare engines produce more horsepower at higher revs compared to undersquare engines that produce more torque at lower revs. And *most* V8s are larger(heavier) cars, they benefit from more torque, and are designed with undersquare engines. Lighter V8 sportscars like vettes can take advantage of an undersquare engine.

-Matt

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sykikchimp

Exactly.. how an engine revs has nothing to do with how many cylinders it has.

It has to do with bore vs. stroke, internal balance and harmonics, and valvetrain design.

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kandyflip445

And rotational mass as stated earlier. Take a look at that one video from a Porche GT. That thing revs like a freakin motorcycle.

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atom

It's not quite that simple. As stated a million times the rotating mass and harmonics have a LOT to do with it as well. If I have a huge engine for some sort of industrial application and it is oversquare, it still ain't gonna rev high because of the mass. The design of the engine has a lot to do with it as well (ie B18C is undersquare yet it still revs high with very little modification).

Anyways, it's more of the application than anything else. Auto makers put 4 bangers in smaller, lighter passenger cars and v8's in big passenger cars. They use the V8 because they need the torque down low (also why a lot of passenger V8's are also undersquare) to get those giant land yachts moving.

wootwoot

You guys forgetting head design and where the designer wants the powerband? Not all 4 cylinders rev high

Var

so there are many factors, not just one, that determine how high an engine revs. Now another question would be which design has the highest possible potential for highest revs as far as the bottom end is concerned. Inline, flat, V?

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citizen

electric!!! lol

Var

lose

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nismo2491

really I don't think inline/flat/v has much to do with it.
and I'm as much a turbo/sc fan as the next guy, but the whole no displacement for displacement does come into effect here because you are in theory actually raising the displacement. its called modified displacement or something like that. think of it this way, at 1 bar you are running at 100% volumetric efficiency. at 2 bar you are running at 200% volumetric efficiency therefore in theory doubling the displacement because you are putting in twice as much air, my exact numbers may be off but you get the point.
KEvin

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ledzeppelin240

You are not changing the displacement. No matter how big of a turbo you use for the KA it is still a 2.4l unless otherwise modified. At 12PSI of boost the KA24E can flow 391CFM @6000RPM. A 305 can flow 450CFM at 6000RPM. The 4cyl has half the cylinders but with a turbo it can flow close to the same CFM. The efficiency of the engine has alot to do with what it can and can't handle.

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nismo2491

here's this though: at normal atmospheric pressure with no vacuum your motor would displace 2.4L if you did complete revolutions. at the same time if you had 2x the atmospheric pressure you would still displace 2.4L worth of air. but if you put that air in a big expandable airtight back, and then put the air in the normal pressure (14.7psi/1bar) it would then become 4.4L
you didn't change the physical displacement but you changed the theoretical displacement (think of it this way, theoretical displacement = volumetric efficency x physical displacement, at 2x atmosphere you should be at 200% volumetric efficiency)
KEvin

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sepulchral

enough of that shit, thats y turbo rocks

ZK

Doesn't this belong in tech?

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nismo2491

good analogy for this: you have a helium tank that when at the highest possible pressure is enough to fill up say 100 balloons. but if you only fill the tank to 50% pressure it will fill half as many baloons. now you didn't change the physical volume of the tank, but you did change the volume of the air in it at normal atmospheric pressure, and sice displacement is how much the displacement is how much the motor moves......
now actual displacement is figured out by ((bore*pi)*stroke)=displacement

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