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Old 05-05-2001, 12:42 AM   #1
lenjiay
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Question wrx horsepower to wheel??

Hi,
I know that the new wrx produces 227 bhp to the crank, but how much actually goes to the wheel?? Are there any 4 wheel drive dynos out there?

Jeff
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Old 05-05-2001, 06:22 PM   #2
John2.5RS
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It should lose around 23% to drivetrain and paracitic losses. Do the math. I'm too tired.
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Old 05-05-2001, 08:12 PM   #3
dandbest
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I tought that 2 wheel drive cars lost 23%. From what I understand, it's a lot more with all wheel drive cars. For example, I think it was SCC or someone that got 98 horses to the wheels of a 99RS that has 165hp at the crank. To me, that sounds more like 40.6% (165-98=67/165=40.6) I could be wrong... but thats what the math says...

Danny
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Old 05-05-2001, 09:00 PM   #4
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4wd/AWD cars DO NOT LOSE MORE POWER THAN ANY OTHER layout. However they do have a nasty problem of needing more power to accelerate, due to a larger rotating mass. People confuse this with power loss to the wheels, because the chassis dyno notices the difference in rotating mass. F=MA, so given that the car puts out the same F, but has more M, A will be lower. Since the Dynojet measures A, it will show less "power". If you used a dyno that holds at a given RPM then increases the load until the car is at WOT, at that RPM without accleration or decleration you will get the same HP to the tires.

Also if the front and rear rollers are not lnked so that they rotate at the same rate you can have problems where the center diff eats power trying to get the slow roller up to speed. This doesn't mimic a car on the road very well does it?

My biggest piece of advice, is forget the damn dyno numbers. Does it matter if your car dynos to 98 or 398HP if it has the same on road performance? A piece of paper claiming that you car is fast, isn't worth much if you loose all your races.
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Old 05-05-2001, 09:13 PM   #5
dandbest
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XT6Wagon, I agree with almost everything you say, but figure this one out- my gf has an 01VW Jetta VR6 auto, that according to VW has 174 hp. I have an 01RS stick with an intake and a cat-back exaust and she beats the crap out of me EVERY SINGLE TIME. Now, I don't wanna hear "you cant drive" cause believe me, I CAN drive. How would you explain that?

Danny
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Old 05-06-2001, 01:47 AM   #6
XT6Wagon
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So, Does you intake and exaust make power? Not all do, and itmight suprise you that a car can be slower with MORE power, depending on where that extra power is in the RPM band, and what you give up for it. One more question are you shifitng to late? I seem to recall that the 01's have no power past 5,500 or so. Shifting at the redline might not be the fastest way to run.

Also I did say that AWD cars take more HP to acclerate at the same rate. They don't LOOSE HP, but they need more than, say a FWD car.

The extra 30-40 lbs of rotating mass makes a bit of a difference....
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Old 05-06-2001, 05:30 AM   #7
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When I think of power loss due to drivetrain, I'm thinking of two things. Rotating weight and friction. You may not think of extra rotating wieght as "lost" power, but I do. Technically you are correct, which is why pullys and light flywheels don't really produce more HP, they just reduce the rotating weight. Extra rotating wieght affects the car just like making the car much heavier. It takes more HP to accelerate a much heaveir car.

what about extra losses to friction? All that extra rotatiing weight rotates on something right?

More stuff means more friction and thats where the whole thing becomes hard to calculate. If that extra rotating weight were the only issue then it would be easy to calculate into the equation, and it is not, which is why drivetrain loss to the wheels is a subject of debate. It is a fixed loss or it a percentage? Is it a little of both?

Most dyno systems measure the force of the car as it is used to accelerate some specific mass at WOT, such as the drums on a dynoject, and I think that is the best way to measure the power of the car, becuase it comes closest to measuring what the car actually does driving down the road. (Your doing the same thing while driving, but the mass is the car itself.)

Converting wheel numbers to CRANK or doing the oposite is meaningless, becuase...

1. Different dynos measure different and give different numbers.

2. There really is no magical formula for how much HP or torque is lost to the wheels.

Back to the original question...Is there a 4 wheel dyno out there? I suggest you check out the Road Dyno system, which I'm currently playing with. Check out my website for more info.

Dave www.i-awd.com
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Old 05-06-2001, 08:08 PM   #8
dandbest
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XT6Wagon, I have Larry's intake, and a 2.25" exaust. From what I can feel, the car is definetly NOT slower than it was when it was stock. It just might be the extra noise thats making me say this, but, I think that it's a bit faster. Also, I shift at 6000RPM. I do that cause it lands me in "better" RPM in the next gear. In any case, I've tried racing her pretty much EVERY single possible way, and she WALKS all over me. If this means anything, I did smoke an SI a few weeks ago, so....

Danny
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Old 05-07-2001, 05:13 AM   #9
micah
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The SCC article specifically said that the 95 hp number was for comparison only. If the RS only got 95 hp to the wheels it would not turn in the 0-60 times that it does.

IF ANYONE ELSE EVER MENTIONS THE 95 HP NUMBER I'M GOING TO CLUB THIS BABY SEAL.

<rant off>
mbs
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Old 05-07-2001, 05:52 AM   #10
Turboy
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TORQUE:
RS - 166 ft lbs@4000
GLX - 181 ft lbs@3200
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Old 05-07-2001, 06:17 AM   #11
fast si
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xt6wagon: How can 4wd/awd cars not lose more power to the drivetrain? each additional differential will require a certain amount of power to operate.
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Old 05-07-2001, 06:24 AM   #12
fast si
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turboy: don't forget these figures also:

GLX:3153 lbs
RS:2820 lbs

The jetta needs that extra torque.
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Old 05-07-2001, 06:33 AM   #13
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yeah XT... i always enjoy reading what you post, but this time i am confused.
as the chemical energy is converted to mechanical energy, energy losses are involved (darn u friction! give me my perpetual motion) anyway, the more wheels you have to turn and the more mechanical components introduced to achieve this = more energy losses ( or energy per second... power). i am a bit rusty on the physics so please correct me if i am wrong about anything
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Old 05-07-2001, 07:56 AM   #14
NebScoob
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Even with a frictionless drivetrain, the AWD layout will require more HP to accelerate the car because of the extra rotating mass, which is what I think was the main argument. Which one is bigger is the question, losses due to friction or the kind of "loss" that occurs from all the extra rotating wieght?

I'm guessing the main factor would be the weight and NOT extra friction.
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Old 05-07-2001, 08:49 AM   #15
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XT6: I think you mean that AWD car shows smaller figures in the DynoJet. Not quite sure if I understod what you said. In 2wd it multiplies (should at least) every wheels force (as plotted in the function of rpm) by two. In four wheel system it multiplies it by four. So assuming that every wheel has the same J (meaning the ability to resists the rotation) the dynos should have the same figures. The loss can be calculated. I don't have any knowledge of the systems which are used on dynoing but on the basis of simple fysics, comparing AWD cars power to 2wd shouldn't be problem. The integral over the hp curve from launch rpm to peak rpm should be quite good measurement for total efficiency. In real world many things makes above mentioned comparation inexact (like switching gears). Some new Audis BTW has gear box converted with cone shaped wheel. Sounds very nice. The computer can count the best agency. Not troghoutly cleared my mind but it might have the chance to use the motor at the peak hp. Maybe forgot to mention something...
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Old 05-07-2001, 09:34 AM   #16
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Um... so the answer to the original question, I'm guessing, is that nobody has actually put a WRX on a 4-wheel dyno yet

Jordan
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Old 05-07-2001, 09:52 AM   #17
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Guy's this isn't the 70's. The mecahnical losses due to extra diffs, and ujoints and what not are VERY minimal. More to the point which has more losses, one diff running 200HP through it, or two running 100 each? Should be the same given a equal effiency.
Same with frictional losses, as they should be minimal and about the same as a 2wd.

People confuse the DYNO reading lower due to extra driveline mass, with POWER LOSS. Do you confuse adding mass to the body as a power loss when you go to the strip? NO, the car is slower, but the reason is not lower power.

Several people I know do not use chassis dynos at all, they use engine dynos and drag strips. That way they can evaluate the motor for power, and then the car/motor combination. They don't have to sort out what is an artifact of the dyno and what is a real result.
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Old 05-07-2001, 02:46 PM   #18
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Simply put, I find it hard to believe that when power is routed through the center diff a small amount of power(possibly even 2-3%) isn't lost, and duplicated by the front and rear diffs. I am sure that whenever power is routed, or split, a slight loss occurs. I am not saying that 165-95 would happen, but surely a 2.5 rs with the driveshaft disconnected is losing less power to the drivetrain. There is less drivetrain. Less moving parts, less friction. Less friction, More power.


And I am sure that not much has changed in the way of drivetrain losses in recent years, unless we are talking about high dollar racing applications.
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