Thread: AWD WHP vs 2WD WHP??????

140hp x 0.85=119whp divided 0.85=140hp

So, if you take the weight of the cars you can get bhp/lb, or is there a different formula for that?

HP / Weight = Ratio
300 / 2700 = 0.11. That's .11 HP per lb

Or


Weight / HP = Weight to HP Ratio
2700 / 300 = 9. 9:1, which translates to 9lbs per HP

The Ariel Atom uses the second method: http://www.arielatom.com/specs/atom-3-specs

can someone explain to me how this works and who comes up with these % losses in the first palce?..so take 2 skylines both AWD same gearbox same tyres wheels everything only thing that changes is the engine output.. one has 300WHPHP the other 1000WHP

so you 300WHP is gona be making 375HP at the fly accorsing to the 25% figure yet the 1000BHP one is supossed to be making 1250HP at the flywheel.. suddenly the drivetrain causes so much friction that is takes nearly the same amount of power to turn as the standard car?

if i have a drivetrain i can turn the input shaft with my hand (me replacing the engine) and it will take a certain amount of energy to do that.. now if someone who is fitter than me and stronger (a more powerfull engine) does the same surely he isnt going to use more engergy than i am to turn it at the same speed. the gearbox doesnt suddenly develope more friction...

surly a drivetrain loss should be a fixed figure.. IE it takes X amount of energy (HP) to move this drivetrain at this speed so thats what it looses..

maybe im missing somthing but i dont get it..

also 250BHP loss is 186Kw!! thats like saying the drive train is disapating enough energy to power nearly 10 of these bad boys.. eBay.ie: 20kW Gas Fired portable Space/Blow Heater **SAVE ?50*** (item 250518704445 end time 22-Nov-09 08:42:10 GMT)

thats alot of heat to try and move...

I never liked that idea either, and engineers agree the percentage is a rough esitmate used at stock or stock-like power levels. Once you really start getting up there in power, the percentage thing doesn't work very well any more. Like you said, the 1,000 WHP Skyline is now dissipating 250 HP to heat through the drive train? I don't think so. That would make the drive train start to glow red after a bit of racing.

The parasitic loss through the drive train however does go up with power creation. Apparently the more power you push through the transmission and differential (anything with fluid) the sheering forces multiply acting on the fluid and more heat is created (power lost) than when you're at lower power levels. Makes sense why very powerful cars need transmission coolers, etc.

I explain my theory on the real math in post #7: http://www.sr20-forum.com/turbo/25129-awd-whp-vs-2wd-whp.html#post323273


Correct. But that's assuming you're both putting in the same amount of power. If you turn it as fast as you can, you will encounter a certain amount of loss through the drive train. The stronger guy will accelerate the drive train faster, but will lose more power through the drive train. You don't lose the power until you try to use it.


Se my rebuttal above, and post #7.



You're on to something, and I believe post #7 has the answer for you.

After you're done reading post #7 you'll be able to follow me here.

Stock Skyline: 280 HP with assumed 25% drive train loss gives a 70 HP loss and 210 WHP.
Figure maybe transmission + transfer case + 2 differentials account for 50 HP loss and the other 20 HP is due to the drive shafts/wheels, etc.
That 20 HP loss stays constant*. The 50 HP loss due to the fluid filled cases is 17% of the original flywheel output. So with the 1,000 HP skyline you figure 17% of that which is 170 HP is lost to the fluid filled cases and then the same 20 HP is lost to the non-dynamic stuff which gives a loss total of 190 HP instead of 250 HP.

That's just a wild guess, and 190 HP loss seems like a lot still, but you get the idea.


*I haven't studied up on my rotational inertia. You just very well might encounter more resistance when trying to rotate an object about an axis the faster you try to rotate it. I just haven't looked that hard into it. It might not be constant. I guess a good question to ask would be, does the drive shaft get hotter the faster you spin it (ignoring air resistance) because it saps more and more power from the source? Or does it stay the same temperature. Because we know the fluid filled cases certainly get hotter the more power you put through them.

*homer simpson* DOH!!!!!!




lmao...apparently i also didnt eat my wheaties!!!

i missed pos 7 ill have a read in the morning im off to bed now

Here is an interesting article to read, it's a little off topic but talks about the losses.

Dyno tests confirm Nissan GT-R drivetrain loss, power ratings - MotorAuthority


I'm more with the percentage staying the same as power increases, instead a constant hp number, but this post brings an interesting post:
FerrariChat.com - View Single Post - 1520WHP Supra

yeah i dont think the percentage thing works very well either. Although if you look at an average

Our cars stock dyno between 115-120whp depending on dyno conditions

Thats about 20-25whp less than rated at the flywheel.

Ive seen stock new honda civic si's rated 200hp at the crank dyno in the 172-178 whp range Thats a loss of about 22-28whp So that right there in itself is proving the percentage thing wrong. I think by drivetrain its more of a hp loss by whether or not its FWD, RWD, or AWD and also dependant on the type of drivetrain and the wheels and tires being used, and how big of brakes are on the car.

But for fwd cars you can expect a loss between 20-30whp on average. Again this can change based on wheel and tire weight and brake size and weight, drivetrain component weights. Again all depending.

RWD of course your going to lose a bit more. Ive seen stock SR20 swapped 240's dyno in the high 160 to low 170whp range so they obviously lose a bit more power.