UrS4

I have a newbie question on a subject that has been baffling me for a while now.

Mustang GT stock crank hp is 300.
Put it on a dyno and on average a 5 spd manual will crank out 265whp.
Okay, that is a 12% drivetrain loss. Makes sense.

You slap on some bolt on and now the dyno spins to 315whp. Awesome!

So what is your crank hp number?

Do you still have the 12% drivetrain loss? So your crank hp is now 358?

Wouldn't it actually be 350 hp? It was only 35 hp when the car was stock yielding the 12% loss. Whyare your drivetrain losses a percentage of your whp? Doesn't it take the same amount of power to spin your accessories and drivetrainafter you added LTs, CAI+tune, etc.

Lets expand this into higher numbers, say from a stock GT500. Stock crank number is 500hp. A 15% loss is 425whp. Now you do CAI+Tune, LTs, X-pipe, pulley swap and now you are at 500whp so do you still only have a 75 hp drivetrain loss or do you now have a 88 hp loss because of the 15%?

You and I have seen plenty of people say that the drivetrain % loss is static. Whatever your whp number is just figure 12-15% loss.

I would argue that if it took only 35 hp to spin everything that that number would stay static and not the % of whp.

I understand that going from NA to FI can change you drivetrain loss as it take hp to turn a blower but once you have that new number why would it change after a tune or LTs?

Thanks for setting me straight.

marcuskeeler

Think of it in a different way.

Irrespective of the power of the motor, it would take, let's say, 50hp to get the wheels to run at 75mph. So you see, the motive force doesn't even enter the equation.
The mods you do which affect the driveline loss would be limited to stuff like heavier/lighter wheels, gearbox/clutch swaps, drive shaft swaps and rear end gears.

intrinsic losses on the motor itself, blower drive, under drive pulleys etc, aren't classed as driveline losses. They are losses experienced incrank HP losses.

99flhr

[quote

You and I have seen plenty of people say that the drivetrain % loss is static. Whatever your whp number is just figure 12-15% loss.

I would argue that if it took only 35 hp to spin everything that that number would stay static and not the % of whp.

[/quote]

Here`s a simple way to look at it,
V6 & V8 Mustang, essentially the same car, with the same built in "losses" Yet when you add the extra 100hp of the GT you still lose 12 to 15 percent, same as the six.

If your theory were correct, the percentage of loss would be much lower for the eight.

marcuskeeler

Who says you still lose 10-15% when you add extra power ? That's funny It'sa basic approximation is all. Don't get fixated on the percentage too much.

But with more power going into a fixed loss drivetrain, the percentage WILL be lower.

It's a basic concept. Take it to the MAXIMUM Extreme.....

Mustang, 300hp. Say a loss of about 40hp lost through the drivetrain hence 14%

install the new procharger super/turbo/nitrous charger and get 1,000,000 hp ([8D]). So, by your reckoning the drivetrain suddenly now takes 140,000 HP to turn the wheels??? [&:]

99flhr

I agree with your "approximation", these are not hard and fast rules.

Anybody know what rwp is on a Mustang 4.0?

Had a BMW Z-3 2.8, these are available with engines rated between 170 & 315 hp. When dyno`d all modelsexihibited roughly the same% ofloss.

acascianelli

~165whp / ~200wtq

UrS4

So the driveline losses are a static number generated when the car is bone stock. Things that will decrease this number is lightening the components that make up the driveline, correct? Like driveshafts, brake rotors, wheels, etc. And when you add power you are not decreasing the driveline losses but mathmatically you are reducing the percentage lost vs whp, correct?

Iunderstand it is an approximation, I guess the only way to know is take Fords word about the crank hp, even though they seem to sand bag sometimes, and then have a perfectly calibrated and operated dyno to get an accurate %.

I just thought it was funny when people put bigger blowers, pullies, etc. and had doubled their whp and when asked what their crank hp, some would respond sayin well 15% loss so crank number would be grossly overrated.

marcuskeeler

Yes, that's exactly as I see it.

Ithink the generic 15% driveline loss has become the accepted figure for an average stock vehicle. The bigger the car the more powerful the motor +more weighty the components = the higher the losses. So it kind of averages out.

The only wayI see a driveline making more losses by having extrapower pushed through it would be from the components being stressed, like gears running tighter under the stress etc. Not much though I'm sure.

MrSandman

It's closer to 185 rhp, I believe.. unless a CAI and tune gave me 33 Hp.