Drivetrain loss

[Bob]

Well it has to have a well engineered drive train for that much power, so somehow I don't think its going to lose 300hp.

It also has to be massive to handle that much power and retain reliability (I'm assuming owners of a million dollar car demand that it doesn't break every 350 miles), and its all wheel drive. So chances are it loses quite a bit of power.

[Egg Nog]

Yeah, I definitely wouldn't be surprised to see that a Veyron had 700hp at the wheels.

I'd be extremely surprised if it produced near 800hp.


Edit: To some of you who seem a bit confused, please let it be known that there is no "drivetrain loss constant". All drivetrain combinations have different losses. Anytime someone mentions an estimated drivetrain loss, it's from a percentage based on what is similar or common, and never exact without factual proof.

[Bob]

Drivetrain loss is probably also not constant. If you think about it, spinning at a fixed rpm against some external force would not require any increase in momentum of the moving parts, so the loss would be entirely from friction. While accelerating and especially with throttle response drivetrain inertia is a big factor. However since peak loss on a dyno or at high speed is mostly due to friction, you would think that at lower rpms the loss would be much less.
(power lost = Force of friction x distance of rubbing parts ; distance would be lower at lower rpm)

[Egg Nog]

Drivetrain loss is probably also not constant. If you think about it, spinning at a fixed rpm against some external force would not require any increase in momentum of the moving parts, so the loss would be entirely from friction. While accelerating and especially with throttle response drivetrain inertia is a big factor. However since peak loss on a dyno or at high speed is mostly due to friction, you would think that at lower rpms the loss would be much less.
(power lost = Force of friction x distance of rubbing parts ; distance would be lower at lower rpm)

Right, good point - I guess it's a fairly conceptual claim. I guess what we need to do now is redifine what we've been doing as comparing drivetrain loss during peak power output

[KnifeEdge_2K1]

the guy's right, losses from friction are pretty much constant (they vary a bit but not so much that you'll keep the same % loss at all times)

another thing is technically you do lose more power when you add more power ... let me explain

when you add more power you increase the rate of acceleration of the car, but since you're accellerating faster, more power/torque is used to accelerate the rotating parts of the car (flywheel, pistons, wheels/tires are biggest factors), this energy isn't measurable on a chassis dyno, which is why drivetrain losses follow a trend of pretty much constant %

on a hub dyno where the rate of acceleration is kept constant, drivetrain losses become fixed, and a 20hp nitrous shot would actually show up as a 20hp increase for example

the percentage loss thing i think makes more sense since in real life when you're really pushin it (not that you should) you're accelerating as quickly as possible, meaning you're not going to have a constant rate of acceleration, meaning the more power you add, the more you lose as well

hope my extremely confusing post was helpful ^^

[mehrshadvr4]

her is right.you get the percentage when you calculate the lost power from drive trane.for example a car has 100 crank HP.then you get 75 whp that's 25% drivetrain loose.the percentage will drop when you make more power,but not that much.you won't loose only 15% if you make 300 crank HP from same car,becouse if you keep making power you'll reach 0% drivetrain loos,and that's impossible.so it's possible you loos 23% min of the crank hp to the wheels if you make 300bhp.

[revetec]

Drivetrain losses are usually no more than 15% of the flywheel power for FWD vehicles and no more than 17% for RWD ones. A good guide is to deduct 10% of the flywheel figure plus another 10 bhp for FWD, and 12% plus 10 bhp for RWD.

One thing to note is that when taking full drive train losses into consideration as a corrective formulae for a dynamometer, is that it depends on what gear you are in. If you have a 1:1 ratio such as a rear wheel drive 4th gear and the dyno test was carried out in that gear, usually the total drive train losses will be less due to the drive being direct from the imput shaft to the output shaft of the transmission. In this situation it will show greater power at the flywheel than the engine actually has. It is more acurate to perform the dyno test is this situation in 3rd gear, using actual known drive train losses.

A good resource for drive train losses information is http://www.pumaracing.co.uk/trans.htm