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Discussion Starter #1
I was thinking about drivetrain losses, and whats being thrown around here just doesn't seem right. I hear a common number of 25% for AWD as drivetrain loss. That seems enormously high to me and here's why...

Lets take stock horsepower numbers of a 2nd Gen 3000gt:
320hp * 760 watt/hp = 243,200 watts

Now thats a lot of power. Now take 25% of that...
243,200 * .25 = 60,800 watts.

That is an enourmous amount of power going somewhere....mostly into heat if you guys are correct about the 25% drivetrain loss. So 60KW of power is going into heat.

Now lets say that you drive at maximum horsepower for 1 hour.
We have...
60,800 joules/sec * 3600 seconds/hour = 218,880,000 joules

Thats 218.8 Megajoules that is dissappaited in the drivetrain. Where is all this energy going? I can't belive it would go into heat, or the differential would melt down. I just don't think theres that much drivetrain loss.

BTW, the rest of the horsepower goes into losses of tires on the road (very small percentage) and wind resistance which goes on the velocity squared, or it takes 4 times as much power to go twice as fast.

What are your thoughts on this? I know alot of you say that its not possible to move a 4000 pound car a quarter mile in 13.5 seconds with 320 hp, but torque curves have a lot to do with it. If you take the area under the torque curve for a vr4, you're going to find it has more area than a naturally asperated car with same maxium torque. Area under the torque curve should be a pretty good measure of 1/4 mile time.
 

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All anyone of us really have to go on are speculations and educated guesses.
In terms of hp loss thru the drivetrain, it is common practice to ASSUME 15-18% loss for a rear wheel drive car, and in the neighborhood of 25% for an AWD car.
Most of this assumption is based on dyno runs for stock vehicles, and the comparison of rated hp at the flywheel to actual dyno hp at the wheels.
I do not know of anyone who has actually removed one of our AWD engines, dynoed it on a stand, and then re-installed it and dynoed it for whp.
From the information I have seen, I believe that we lose about 80 hp or so thru the AWD platform. It could be higher or lower, but it should be safe to assume that approximate figure.
Personally, I do not believe that if a stock car loses say, 25%, or about 80hp as an example, thru the drivetrain, that it can be assumed that if it that makes 600 flywheel hp it will lose 150hp, or 25% of 600.
It just does not make sense that the car would lose an additional 70 hp.
 

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Subaru wrx dynos 147hp to the ground.
thats like 33% drivetrain loss and or their
engine output is overated.

Needless to say I'm happy with with 26% driveline loss.

Or maybe I'm not as I tried to improve this loss
with alum flywheel/underdrive pully.
 

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Discussion Starter #6
Well thats what I'm trying to say. No way is subaru loosing 33% to gears. Its got to be that the engine output is overrated. Theres just too much power that has to go somewhere.
 

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I am no expert. If I was the car manufacturer. I would built and cool ass AWD drivetrain for take off .. once we have traction. I will add a switch to make AWD disable to fwd at certain speed .. not sure that is possible or not.. but would that make more power?? Well.. that is my BS lol :)
 

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I assume you are digging through your thermodynamics practice. It will be easier if you calculate this for an instant rather than over time as you have introduced in your original note (all you need is the 60Kw assumption to solve it.) I give you as homework the following:

Heat: energy required to maintain a body (or aggregrate components) of about 300 lbs at an average of about 20 degrees above ambient temperature (when running). Extra credit if you apply an insulation value for the heat transfer from material to air.

Rotational mass: energy required to maintain all the components spinning (even without friction.) Use about 80 foot pounds for the wheels and about 50 foot pounds for the balance of the spining equipment. Extra credit if you determine the energy required to bring this to mass to rotation instead of steady state.

Vibration: energy required to vibrate some of this stuff, let's say about 5 pounds at about 1,000 hz.

Assuming you answer does not quite match your 60Kw assumption ... which of the above is the largest player in the consumption and what would be the easiest cause/effect item to change to determine the answer (hint: delta from ambient.)
 

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I think awd cars drivetrainloss depends on
the% of power to front and back so a 50/50 split
would be the worse while 60/40 would be better
like in a 3000gt. Or in a diablo their awd splits
20/80.
 

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Brian A. Bochenek said:
All anyone of us really have to go on are speculations and educated guesses.
Nah. The brake dynos show drivetrain loss as does the CarTest2000 program. HP drain by the drivetrain starts out "low" and increases with speed in each gear. "loss" is about the same in each gear, something like 100 HP at top speed in the gear and maybe half that at lower speeds in a gear - assuming we are talking about WOT situations, not cruising. Drivetrain power "loss" is independent of HP basically.

Why do people keep bringing this up? Forget the percentage thing. Forget the fixed number thing. Figure ~100-110 Hp loss at *max speed* in a gear at WOT - for any engine ouput. Cruising situations are of course enturely different and nobody really cares about power then anyway. :)
 

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Jeff I believe I seen this number in my manual..I
may be wrong but I'll go check.:cool:
 

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The manuals and promotional literature are wrong. Read my web page above.
 

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Discussion Starter #14
I guess I was wrong...

Ok David Walden, I'll bite. I'm actually a computer engineer and not a mechanical engineer / physicist, but I'll break out my physics books and do a more accurate analysis per your "homework." I was just trying to simplify things by breaking them down into energy dissapated by heat alone. Let me work on that at work tomorrow and I'll post by my results.

I do still think its quite a bit of energy going somewhere whether more of its going to vibration than I orginally suspected.

Just for everyone elses info, lets see how much energy we got here.

Lets bring joules to calories. 1 joule = 1/4.19 calories.
60,800 joules = 14,510 calories,
and 1 calorie is the amount of energy to raise 1 gram of water 1 degree celcius. So we have a differential equal to 300 lbs = 136,000 grams. So that means every second we raise 300 lbs of water (or I'm using the specific heat of water to be close enough to steel) by
14,510 / 136,000 = .1
.1 degrees. That means in 3,600 seconds, the water would be raised 360 degress...
Thats not bad at all, so I stand corrected. With no temperature gradiant to drop heat to the air, in 1 hour our 300 lbs of water would be only 360 degrees.

Ok. I guess I just needed to prove it to myself. That makes sense. I'm not going to go through any advanced thermodynamics.
 

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Jeff, I agree with you.
The reason for my statement is that no one has actually dynoed one of our engines stock, out of the car, and there has been quite a bit of speculation in previous posts about the truth to manufacturers hp ratings.
I should have mentioned in my original reply that we should focus less on loss, because we can't change it cost-effectively thru the drivetrain, and focus on output, specifically whp.
I think it is fun to hypothesize about what a car makes at the flywheel, because it's simply a bigger, more fun number, as it is for most enthusiasts when talking cars.
Anytime I open my hood, people always ask how much power the ENGINE is making.
I tell them what it dynoed at the wheels, and almost inevitably the conversation turns to "so, what's that at the flywheel?"
This from even seasoned car nuts, who understand chassis dynos.
I know what it is going to take for my car to run the way I want, so I'll keep tuning for whp.
 
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