20 replies to this topic

[ciaoduc]

Is there a formula for figuring out the horsepower/torque gained by reducing rotating mass? I know it's all the rage in import tuning to put light weight flywheels, con rods, etc... I was just wondering if I were to put TI rods, multi-plate clutch or drilled crankshaft in my Ford (Chevy, Mazda, Ducati, etc...) how could I figure out if the gains I would see from the loss in rotating mass would outweigh the loss of weight in my pocket book.

[Chevy II Nova]

The gains are not tremendous, and there are probably too many variables to have a clear formula for it. With the flywheel the desired effect is usually throttle response rather than power.

[Greg Locock]

Well, let's make it easy. THERE IS NO (significant) CHANGE IN THE POWER OUTPUT OF THE ENGINE, STEADY STATE.

What you are doing is reducing the rotational inertia, and due to a rather neat effect to do with geared systems, it can have a big effect on your acceleration.

The rule is that rotational inertias in a geared system have an effect proportional to the SQUARE of the gear ratio, so if you have 3:1 first gear and 4:1 in your diff you will see a reduction in rotational inertia 144 times greater than if you had reduced the inertia of the wheel by the same amount.

For various reasons it is easier to do a power sum, so we get

power at wheels=rate of change of kinetic energy+rate of change of rotational energy-rolling resistance*v-airdrag*v

rateofchangeofKE=d/dt (1/2*m*v^2)=1/2*m*2*v*a=m*v*a

rateofchangeofRE=d/dt(1/2*I1*w^2+1/2*I2*w^2*AR^2+1/2*I3*w^2*AR^2*GR^2)

w*rollingradius=v

hence

rateofchangeofRE=1/radius^2*d/dt(1/2*I1*v^2+1/2*I2*v^2*AR^2+1/2*I3*v^2*AR^2*GR^2)

rateofchangeofRE=v*a/radius^2*(I1+AR^2*I2+AR^2*GR^2*I3)

airdrag=1/2*airdensity*Cd*A*v^2

rollingresistance=Cr*m*g

Cr is about 0.015

I1 is the rotational inertia of the wheels and halfshafts, I2 is the propshaft and output shaft of the gearbox and I3 is everything that rotates at engine speed. The I of a uniform cylinder is mass*radius^2/2

AR is the axle ratio and GR is the gear ratio.

OK, I think those are all the equations you need to get a solution for the change in acceleration.

so at a given speed

change in acceleration=-1/(m*v+(v/radius^2)*(I1+AR^2*I2+AR^2*GR^2*I3))+1/(m*v+(v/radius^2)*(I1+AR^2*I2+AR^2*GR^2*I4))


where I4 is the new rotational inertia of the engine, and I3 is the old one.

Somehow I doubt that helps!

[Chevy II Nova]

I was about to say that too. I swear.

[ciaoduc]

WOW! Where did you come up with all of that Greg? Is it out of some book?

This question was post in hopes of ending a friendly argument. My friend was saying that these changes will actually increase horsepower whereas I was suggesting that they just make the power that's there easier to access (ie. better throttle response). However, a dyno would certainly record higher figures since the power is more readily available...or would it just shift the curve? Acceleration would be improved too.
Also, what difference would installing a flat plain crank (with corresponding cam change) make?
Am I on the right BB for this? Has this already been discussed elsewhere? I couldn't find it.

[Greg Locock]

I was about half way through writing that lot when I realised it was going to get very ugly, very quickly.

OK, here is the useful little nugget:

for a change in the rotational inertia of the flywheel "I" the reduction in mass you have to accelerate at the contact patch is (take a deep breath)


(1/rollingradius^2)*(AR^2*GR^2*I))

So, if your flywheel inertia drops from say 0.2 kg m^2 to 0.1 and radius =.4 gives

1/.16*144*.1

or the equivalent of almost 90 kg less linear inertia. So in a 1 tonne car you might see an improvement of 5% or so in acceleration in first gear. The acceleration in first gear is already heavily constrained by the rotational inertia of the powertrain, so its effective inertia is much higher than 1 tonne. You can use the same equations to find out how much, 25% is the number I remember.

You are right, there is no appreciable difference in dyno power, just transient responses.

[WPT]

A change in the mass moment of inertia of the rotating parts will only affect the hp to the rear wheel(s) when the machine is accelerating (angular velocity is increasing). The magnitude of the hp increase is greater in the lower gears where the acceleration is greater. For a 70's era Ducati 750 V-twin, ~1hp gain could be had be removing the 5lb flywheel. This for an engine which made ~55hp at the rear wheel. WPT

[ciaoduc]

Would the more even firing nature of a flat plain crank have any effect on the horsepower/torque of an otherwise identical engine. I'm guessing that it would only increase if one were to take advantage of the exhaust scaveging (sp?).

[Wuzak]

Originally posted by ciaoduc
Would the more even firing nature of a flat plain crank have any effect on the horsepower/torque of an otherwise identical engine. I'm guessing that it would only increase if one were to take advantage of the exhaust scaveging (sp?).

I don't think that a flat plane crank gives more even firing than a "normal" one.

In the case of a V8, exhaust scavenging is said to be the most important reason.

The side effect is more vibrations.

[jpf]

The flat-plane crank should also be lighter overall, because it does away with the full counterweights of a cross-crank. So a flat plane crank is itself a significant reduction in rotating mass, which I thought was a/the primary reason for their use. Of course, I learned almost everything I know about this from the AutoZine Technical School ...
http://autozine.kyul.../smooth4.htm#V8

[McGuire]

Not disputing anything said here in the least, as it is all right on. Rather, I am arguing for the special case here and there. If a reduction in rotating inertia is sufficient, and enough to produce a meaningful reduction in frictional loadings, then the bearings, fluid pumps etc. can be reduced to suit, resulting in an incremental net power gain...at which point the cycle starts over. This has been a focus of development in racing series where an artificial rev limit is imposed, for example in the IRL. However, the gain is not in the reduction of mass itself but in the secondary benefits, which can take some mighty fine whittling to exploit.

This is off-topic as it relates more to reciprocating rather than rotating mass, but it is sometimes found (especially in 2-valve pushrod layouts) that valve train inertia imposes a ceiling on max rpm below the volumetric capability of the engine. If the float, bounce, etc. can be corrected or controlled, the engine will make more power at its new, higher operating limit. I was once involved in a program with a large pushrod V8 which used one hulking 60mm intake valve. Lightening the valve and calming the harmonics a bit created virtually a whole new engine.

[McGuire]

Quite right about 90 vs. 180 degree cranks: both fire at even 90 degree intervals.

Looking over autozine in the past, I have found some technical info that was very good, and some that was...not.

[Scoots]

Originally posted by ciaoduc
WOW! Where did you come up with all of that Greg? Is it out of some book?

This question was post in hopes of ending a friendly argument. My friend was saying that these changes will actually increase horsepower whereas I was suggesting that they just make the power that's there easier to access (ie. better throttle response). However, a dyno would certainly record higher figures since the power is more readily available...or would it just shift the curve? Acceleration would be improved too.
Also, what difference would installing a flat plain crank (with corresponding cam change) make?
Am I on the right BB for this? Has this already been discussed elsewhere? I couldn't find it.

I would say you win your friendly argument based on a possible narrow set of rules for the discussion.

Lightening the flywheel, clutch, driveshaft, axles, wheels, brakes, will show as a hp increase on a wheel dyno, but the engine is not actually producing more power.

[D-Type]

Correct me if I'm wrong.
I always understood that you provided a flywheel on an engine so that its inertia ironed out the impulses of the power strokes. Lightening the flywheel (or any part of the rotating mass) improves the accelaration of an engine at the expense of rougher running and vibration and the associated ill effects, and also produces a corresponding reduction in engine braking on overrun.

[WPT]

Did not notice any difference in running of a Ducati 750 V-twin with/without its 5lb flywheel. Lightening the flywheel will cause an increase in engine braking, not a decrease. WPT

[D-Type]

Surely with a lighter flywheel the engine will have less inertia so if you lift off or change down there is less engine to bring up to speed and hence less braking effect.

[Scoots]

Originally posted by D-Type
Surely with a lighter flywheel the engine will have less inertia so if you lift off or change down there is less engine to bring up to speed and hence less braking effect.

But, there is less mass keeping it spinning ... I think it's most likely a case-by-case thing.

[WPT]

A flywheel has angular momentum of I*w, where I is the mass moment of inertia about the rotational axis and w is the angular velocity in rads/sec. The energy stored in a flywheel is 1/2*I*w^2. You are correct in saying that lightening a flywheel will cause the engine to free rev quicker, but this is not "engine braking", unless you do not blip the throttle on downshifts to match revs, which is not recommennded because it causes high wear in the clutch. WPT

[D-Type]

Deliberately dumb question (to clarify thinking)

If lightening the flywheel improves acceleration and power, why not remove it completely?

[alexbiker]

Originally posted by D-Type
Deliberately dumb question (to clarify thinking)

If lightening the flywheel improves acceleration and power, why not remove it completely?

Some of the latest racing and performance engines are flywheel-less by design. However, most car engines require a flywheel to keep the engine spinning at idle, and would stall if it were removed. Flywheel masses have been steadily dropping in the quest for responsiveness.

Alex

[DoS]

Originally posted by alexbiker


Some of the latest racing and performance engines are flywheel-less by design. However, most car engines require a flywheel to keep the engine spinning at idle, and would stall if it were removed. Flywheel masses have been steadily dropping in the quest for responsiveness.

Alex

i am no engine expert but what you say makes sense. First thing i notice on an engine with a heavy flywheel is that it free-revs kind of slow (that in production cars where nowdays all of them come with relatively heavy flywheels) and when i actually drive that car the engine will keep spinning even in very low revs (much lower than idle) with first gear and little to no throttle open. In contrast, lighter flywheel engines on the same car will stop spinning at higher revs.