Iggs

Can someone please help, and explain this to me: Drivetrain efficiencies should be in the 80% range (20% loss). If the flywheel horsepower of a stock 2.7T, is 250 hp, and the drivetrain losses are 20%, then that means that 250 hp x 20% = 50 hp are dissipated by the drivetrain.

I read on GIAC website that stock 2.7T makes only 165 HP at the wheels, Hey!!!!! That is a huge power loss, can that be really true (the dyno says so!)

with their chip HP will be around 310 at the flywheel and 208 at the wheels, I thought it should be around 200 Stock!!

so what am I missing??

The car certainly feels faster the 165HP at the wheels.

captainbilly

If the drivetrain is really absorbing 85hp that would mean that there would be about 64kw of heat generated. That's 265 amps of 240vac power, more than enough to heat a large house in North Dakota. I'm not saying it's impossible because I haven't run any calcs on what how hot the tranny and differentials would get under these conditions but it seems unlikely. My guess is that engine output is generally overstated so that when the car gets on a wheel dyno the numbers come out low so the difference is attributed to drivetrain losses. It also occurs to me that some of the loss may come from the tire/dyno interface.

Iggs

Are you saying that either 2.7T engine does no truly produce 250 flywheel hp, or that the dyno is bad?

This is Giac published charts, I would think they know what they are doing..

Could it be the quattro that is is dissipating some horsies???

Hmm... these numbers still bother me.<ul><li><a href="http://www.giacusa.com/">http://www.giacusa.com/</a</li></ul>

Bollinger

They know a lot, they don't know everything.

myfirstimport

Think of it as a dog sled. 4 equal medium sized dogs will out perform 2 large dogs.

SloopJohnB

Well, a 1965 Mustang 289 hi po usually pulls about 150 hp at the rear wheels. Granted it's lighter, but what the hey.

One other point...once the drivetrain tax has been paid, usually horspower increases go right to the wheels. So if you go from 250 to 310 you should go from 165 to 225hp at the wheels.

DWC_2.7T

Alternator, A/C compressor, power steering, air pump...it all adds up.

Augie

When you consider what an Audi engine must conquer on a four-wheel dyno, compared to two-wheel-drive cars, that 165 hp may, in fact, be right on.

In theoretical order of importance:

First, consider that the Quattro system demands that power be transmitted thru three differentials, compared to a single differential on a 2wd vehicle. This alone will account for a significant horsepower loss.

Second, the rolling resistance of all four tires is part of the loss that the dyno "sees", instead of around half that for the average car.

Third, consider the added frictional resistance of the second drive axle's parts, and the added resistance of all related driven parts, including halfshaft joints, wheel bearings, etc.

Lastly, consider the rotational inertia of all the extra rotating parts that must be accelerated on an awd vehicle while on an awd dyno. That would include the wheels and tires, of course, plus brakes, axles, one more drive shaft, two more halfshafts, the frammis and gzordin wheels (OK, I lied about those two), and every other damned thing that needs to be accelerated. My guess is that rotational inertia is *not* last on the list, but I'm not fanatical on the order. :-)

Anecdotally, did you know when you put a GM "Bow Tie" aluminum driveshaft on one of those V8 Camabirds, you typically get 3-4 additional horsepower on a Dynojet? Every last bit of this "extra" power comes from the reduced rotating inertia of the lighter driveshaft. Compared to many other drivetrain parts, the driveshaft is not a hugely consequential inertia item, and yet the additional power is always there, when measured back to back. Consider all those other rotating items, and you can see where rotating inertia may, in fact, be number one or two on the list of what's stealing 85 horsepower out of 250.

Out on the road, of course, every car must accelerate all four wheels, etc. They just don't have to do it on the dyno. Depending on your point of view, either Quattros are being penalized on the dyno, or 2wd cars are being given a break. In actuality, 2wd cars really *are* being given a break, compared to real-world road conditions.

That's why 2.7T Audis aren't moving chicanes out on the road. The only penalty they have to put up with is the three differentials, and they seem to do that quite well, thank you.


As a by the way, the gentleman who said something about engine accessories being a part of the problem isn't strictly correct. Engine accessories are also part of the equation when engine power is being measured - at least under current SAE Net or DIN rules.

Audiablo

I discovered this first-hand while driving on a lake of glare ice. I assumed that under such low-friction conditions, I could launch the car in 3rd gear and everything would just spin up really fast. Nope, dumb idea. What I got was "Fragrant Clutch No. 5".

I came to realize that the engine not only has to work to move the car forward, but also get hundreds of pounds of drivetrain to overcome their rotational inertia as well. Yeah, I'm sure the available friction even on the ice was a factor, but I bet if you put the car on a hoist and popped it in 3rd and stomped on it, the clutch still couldn't hold tight to bring up the speed of the powertrain as fast as the engine can blast out its power.

DrHandball

Auto manufacturers are supposed to be rating their engines as SAE net. This means with accessories. I don't know if that includes AC and power steering but it does include water pump and alternator. It would include the fan but most cars today are using electric fans to cool the radiator.

As far as AC losses are concerned, I read somewhere not too long ago that when running the AC you consume somewhere around 10-15 HP.