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I was trying to understand more about torque when I was comparing two engines, totally different engines.

one engine ( Straigh 4 ) that produces 149 ft-lb at the maximum HP given (237HP@8300) the other ( Straight 6 ) that produces 254 ft-lb at the maximum HP given (276HP@4200)

what strikes me is the I6 engine produces around 41% more torque than the I4. considering that it has 50% more cylinders which in this case 6 versus 4. and the RPM at which it delivers this torque is 50% less in the I6 compared to the I4.

So I was wondering, is there something I am missing? lets say if the I4 engine was redesigned with added cylinders, say 6 cylinders (2 more cylinders) would it operate at the same RPM? e.g.8300? or will that figure drop?

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Caveat

(Before proceeding, let's get one thing out of the way)

There's no such thing as a free lunch. Adding cylinders alone will not make more horses.

The engine either has to be more efficient or it has to be fed more air and fuel. This makes it difficult to compare things like torque and power.

The reason why this matters is that you have to hold everything else constant in order to be able to see the effect of changing one aspect of the engine's design by itself.

This is not as straightforward as one might think, since changing one variable can have an effect on several key parameters.


So what happens when more cylinders are added?

Assuming cylinders of the same dimension are added, for the same air-fuel flow rate:

  • Increased rotational mass

    This isn't just the weight of the pistons themselves. The engine now has more connecting rods, crank pins, cams and valves to rotate. Because there is more mass to hurl around, the engine is expected to operate at a lower engine speed.

  • Increased engine displacement

    This doesn't guarantee more torque or power though, since:

    a. Volumetric efficiency may have changed for better or worse depending on speed. This is where playing with things like valve timing and cam profile can have a significant impact.

    b. there is less air-fuel mixture to go around per cylinder, so there is a trade-off between the number of cylinders firing and the amount of go-juice in per cylinder.

  • Longer crankshaft

    This contributes to higher rotational mass, which results in lower engine speed, but there is another effect that could impact the rare oddball engine. A longer shaft has a reduced stiffness, which lowers the maximum speed which the engine can be safely run at (look up "rotordynamics"). Most engine speed limits are set well below the rotordynamic-safe limit though.

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The Max RPM of a engine is dependant on a ton of factors namely:

  • Mass of component:

If your components are heavy like the ones required in diesel engines then obviously the speed at which it can rotate for each stroke will be less compared to a lightweight design.

  • Stroke:

Simple Short stroke = Shorter time = Fast rpm(Superbike)

  • Valvetrain:

If your intake and exhaust valves are not able to open and close fast then certainly your RPM will not be high, performance valves(triple spring) can substantially increase max RPM.This is why OHV config engine have low RPM even as low as 3500 which the OHC design eliminates.

  • Displacement:

Higher Displacement engine will have heavier components , heavier reciprocating mass and thus lower RPMs. A 3.0 liter V8 will have better RPM than a 5.9 liter V8(old chevy engines)

To answer your question: Yes Addition of 2 more cylinders will drop the Rpm but the amount is determined by the mass of the piston and lot of other factors. Also based on the above factors at which Rpm it will produce the same amount of torque will also change.

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The RPM at which an engine produces its maximum torque is mostly dictated by the design of the intake and exhaust runners, as well as the camshaft profile. It's not really to do with how many cylinders it has.

(Although obviously more cylinders usually means a larger total capacity which translates to more fuel/air being burned and hence more power).

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    Disagree. Adding more cylinders will have a significant impact on the RPM range. More cylinders --> more rotating mass --> lower rotating speed. – Zaid Sep 29 '15 at 11:25
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    Well yes and no. Piston size impacts RPM more than the length (and weight) of the crank. Which is why performance engines all have cylinders in the 500cc range. E.g. All Audi petrol engines for instance have roughly the same red line (I think around 7000RPM), from their lowly 4-cylinder 2.0 liter, right up to the 10-cylinder 5.2 liter. what they all have in common is a roughly identical cylinder capacity of 500cc. I'm not saying you're wrong, just that piston size is a much bigger factor than rotational mass. – Captain Kenpachi Sep 29 '15 at 11:43

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