Merc63

Mechanical Engineer: Does less reciprocating mass in an engines internals equal more power produced by the engine?

I read a post in another thread, a member claimed that a P31 should be a quicker car because of the lighter forged internals.

So... I started reading about lighter internals and their effect on the engine/car and can't find a straight answer.

Anyone with a degree know the answer? I think there are so many variables it is difficult to figure out without a strong physics background.

At first I was thinking that the less rotating mass wouldn't produce anymore power because you are using the same amount of air and fuel which produces that power via combustion. Given that everything on the engine is exactly the same but we lighten the internals.

Then I started reading about it and thinking if this engine makes 500HP/500TQ and we lessen the weight of material it has to spin then I'd think that would free up some lost power??? BUT THEN.. if that was the case... when you lighten a car from 4000lbs to 3500lbs, it doesn't increase HP or TQ, but the car will accelerate faster... So essentially the car hasn't gained any power, it just takes less force to move the car..

So now I am just confused.

CELLryuu

lets look at it like a fly wheel. you lessen the weight and the engine spins up the red line faster but it is easier to slow the momentum of the spin. increase the weight and it spins up slower but is harder to slow down its momentum (mass in motion tends to stay in motion).

they are both making (relatively) the same power but they are achieving it in slightly different ways. the power difference for a few pounds is going to be a pixel on a dyno-meter.

now, if we take a 100% increase on internal moving components there are all kinds of issues. friction and gravity will show there ugly heads and power will be noticeably effected. Its an exponential scale so calibrating the components for there intended purpose is key.

in the case of the p31. ive had both. my non p31 had a eurocharged tune. to be honest. i cant tell a difference in the way either performs even though the p31 had 6-7 pounds less internal weight. the main benefit of the p31 internals is that they are forged and therefore stronger.

hope this helps.

cyberorth

I think in the case of 156 engine, the tuning is what determines the power. The engine is built well within the tolerance of the horsepower it can make. The question then is whether with the same EXACT tune will the non p31 be making less power and I think the answer would be a yes...how much? maybe very little.

Rsquad911

If you take the F1 approach.... Lighter is always better.

In the P31's case.. Lighter and stronger is always better =O)

chuthis

Here's my guess...

Engines waste energy to move their internal parts around. An engine with lighter internals would require less energy to complete a cycle than one with heavy parts therefore wasting less energy and allowing it to have a higher overall output.

DuaneC63

That's correct. (There is also a point of demishing return if the reciprocating mass gets too light and is unable to handle the power output, hence forged reciprocating parts). The old school trick was to balance and blueprint the engine and knife edge the crank. With modern CNC equipment this become less of a issue as reciprocation parts are manufactured within very tight tolerances. This allows manufacturers to use smaller displacement engines to produce more HP with less emissions and better gas milage.

Tolip

In general, it takes force to accelerate a mass, and energy to apply that force.
If there is less mass to accelerate, it will require less energy, and more will be available to push the car.

Merc63

Ok so the less weight of the internals would "free" up some energy that normally would be lost. So in a sense the car is not going to make more power, but it will be more efficient with the power its producing?

msheredy

What I've been taught is that a lighter reciprocating mass will indeed make more horsepower but may suffer from a loss of torque since it takes momentum to move a vehicle.

dodger63

Hey merc you going to edit your post on bb?

Merc63

HUH??? What's BB?? I dont post anywhere else.

GermanCars

This has been discussed on this forum ever since Mercedes talked about the higher horsepower C63 engine linked to lighter weight SLS engine internals.

My understanding, and please correct me if I am wrong, is that torque (and calculated horsepower) is measured at specific RPM points, with no regard to ACCELERATION RATE. If this is the case, lighter weight components will have no impact on measured horsepower. Does a light flywheel impact dyno measurements? Surely one of the many people working with dynometers on this forum should be able answer this.

Not that one should disregard the value of light weight reciprocating engine components. Rotating mass will impact ACCELERATION RATES. More important is enhanced durability, due to lower mass and resulting lower stress transferred to connecting rods, bearings, etc.

The SLS engine components are a great addition to you track addicts who intend to operate their engine at high RPM over extended periods of time. And it may show in a very very minor improvement in 0-to-60 or 1/4 E.T. I would however be very surprised if you would see it on a dyno.

What do all you dyno tuners have to say?

dodger63

My bad, sorry I got you confused with someone else, talking about the same subject

dtohio

imo rotating weight (crankshaft) does not change hp, it does add inertia and slows acceleration. reciproctating weight(pistons) changes hp as it takes energy to start and stop the recip mass. I dont know where that energy goes, I think it has to be heat loss as in entropy.Except when using flywheel momentum ,stored energy, on a start, lighter parts would give faster acceleration and less hp loss.