Joe (zyxt)

Okay, I've been mulling this over for a bit, as it seems people come up with different numbers all the time from their dyno runs. Of course you have the argument of exactly how much driveline loss there is with any given car, but that's not what I'm after in this post. I'm only interested in the *correct* way to do the math, provided you know the percentage loss.

Here's my take on it:
Let's say in one example you have a car with a 20% power loss from engine to rear wheels. (No particular car, just an example.) Since the power is being generated at the engine, and then being *lost* on its way to the wheels, I would assume the formula would be like this:
(engine power) * (.80) = (wheel power)
So, if you do a dyno run and find the wheel power, you would divide the wheel power by .80.

Does this sound about right?

CMH

X=wheel hp, 80/100 x X/?. Which is just about the same way your doing it but without the decimal point.

zain

If you approximate your driveline losses as a constant percentage, then you are assuming that the torque needed to overcome the driveline friction is constant with RPM.

Which is an OK approximation. If you had a dyno plot of the engine on a stand, you could then take the torque line and move every point on it down 20% (or whatever you think your driveline losses are). Same for the HP line (as HP is just torque*RPM/5252) to get what the net car would be.

Conversely, you could take a wheel-dyno plot and move the lines up by the right fraction. Divide by 0.8 in this example.

But, there is more in going from the wheel dyno to the shaft. You also have to scale for whatever gear you were in in the dyno run, and the final gear ratio. If you want to calculate shaft HP, you need to take out the "lever arm" of these gear ratios. Most reputable wheel dyno run plot already take this into account, however.

Also in real mechanical systems, friction isn't constant with velocity of parts. This is especially true if there are any viscous coupling type parts. Mecifully, the audis still have the old Torsen gizmo that is all gears. So the approximation ought to be close. But if you have an auto or tip car, then I'm pretty sure your torque converter has a non linear drive line loss effect. This would mean needing to move your torque line around by a different fraction depending on RPM at that point on the curve, and then recomputing the HP curve.

Of course since we all can't agree on what the basic driveline loss is for our cars, there's no point in trying to get the non-linear terms right. I've heard anyplace from 15% to 45% driveline losses for the quattro. Sometimes I think people like to believe there are large losses because then they can multiply/divide and conclude they have a nice big engine output due to their mods.

I'd love to see a stock car, tuned to basic specs, with my exact drivetrain on a dyno. Then we could do your math to get the right scale factor for the drive-line. All I ever get to see are non-stock cars, and guesses about hwo to go from dyno to shaft.

Joe (zyxt)

I completely agree. The losses are probably non-linear through different speeds, however you are right that there's just no use arguing over that when people can't even agree on a linear value.

However, a while back BOOSTD posted a dyno run he did over at APR on their 4-wheel dyno. They did a stock, unmodified A4 to get a baseline, which is rated by Audi to put out about 150 hp. It was getting about 118 at the wheels on it's best run. So, that gives you about a 22% loss.

I don't have the link to the post handy, but I'm sure it can be found on the search engine relatively easily. Found it last night in just a few minutes.

zain

22% huh. I wonder what it would be with a tiptronic tranny?

Mike O.

...because driveline loss is NOT a percentage - it is a fixed torque figure for each RPM point.

This has been bantered about on the E30SIG site numerous times. The bottom line is that they have dynoed both stock and modified engines first on a dyno stand (engine only) and then on a driveline dyno. The result: the tranny and rear end consume the same torque (equateable to hp in conversion) at a given RPM regardless of the engine.

Logic further dictates that unless you have made modifications to the drivetrain itself, it will take the same torque to get it to spin 1000 rpm - regardless of power source (electric motor, stock engine, modified engine, guy on a stationary bike, etc.).

Sorry,
Mike O.

zain

Take the torque curve and move the line down by a fixed value (offset) as opposed to a percentage (gain), to go from shaft HP/torque to wheel.

That kind of makes sense. If you believe that the loss in the driveline is static friction and not dynamic viscous loading.

But some of it has to by dynamic loading. The moment of intertia of the spinning components dictates that if I go from 0-6000 RPM in 5 seconds, it would take more torque to do the same thing in 4 seconds. Here the driveline loss is not a constant with RPM, because it has a rate-of-change-in-RPM term.

Do the math. Four (quattro) 36 lb wheel/tires spun from zero to 60 MPH in equivalent wheel RPM takes 28 ftlbs of torque. Which would be seen as driveline loss. If you took 12 seconds to do it, you would only take half that to this loss.

If you slowly climb through the RPM on your dyno the above effect would not be seen I think.

The loss has a constant term for static friction, an RPM dependant term for viscous friction and a delta-RPM term for rotational components.

People say percentage. Or constant. It's just an appoximation. The dynamic system in the time domain is:

TsubA = C + C(omega) + C(omega-dot)


- Z

smallTTs

Comparing rearwheel dyno data to flywheel dyno data is a real can of worms. Losses vary not only due to driveline effiencies, inertia, traction/wheel slippage, but also such mundane things as tire pressure. By the way, that's an interesting way to fudge chassis dyno data.

Taking one percentage and applying it to all data from a given car is shaky, at best. Applying the same percentage to another car is less than shaky.

We witnesed a test at Superflow where they ran a big-block Chevy (~800 hp) engine on one of their engine dynos one day, then installed the engine back into it's Nova drag-car chassis and ran it on their AutoDyn chassis dyno the next day. They had fairly good corrections for the drag slicks and powerglide trans/convertor losses from many runs. When the curves were overlaid, with the empirically derived correction factor applied they agreed within 2%! We were all impressed.

The AutoDyn, in my opinion, is much more sophisticated (and more costly) than the Dynojet chassis dyno, and Superflow's software (WinDyn) is outstanding. I was impressed with the correlation. In another test, they ran a 450hp rated Viper on the AutoDyn chassis dyno, and in 5th gear, it pulled a corrected 450 +/- about 5 hp. at 203 dyno mph. Of course, the engine got very hot, even with mega fans blowing air on the car.

I'm new to this particular forum, but my friend LarryTT asked me to visit. My background is relevant. You don't have to believe what I said above, but I think Mike O. and zain have a good grasp of the topic; believe them.

Apples don't always equal oranges, unfortunately.

My highly-opinionated $.02.

zain

Mike is right that there is a constant friction term. And if you start to include dynamic acceleration terms and viscous friction, and tire deformation, well it does become a science fair.

Most plausable tests of chassis/wheel dynos of stock cars have produced 20-25% less peak HP at the wheels than stock. Depending on vintage of A4 (the test I've seen are A4) then that's 20-30 HP to run the quattro driveline.

If we doubled the HP of the engine, would it take double the HP to run the drive line? (and let's just stay at the peak HP point for sake of discussion)

Mike would say no. Fixed losses. Fixed torque, they fixed HP loss at the RPM at peak. And to first order, he's probably close.

Unless the 0-60 time went down a lot, the torque needed to spin up all the moving parts isn't going to increase much. Doubling your HP doesn't exactly half our 0-60 time as we all know. Losses due to viscous terms and intertia will increase as a percentage, but relative to achieved acceleration rate (like 0-60 time ratios) not shaft HP.

So until your mods start making significant changes to your 0-60 times, saying 20-30 HP to run the driveline (at peak) (fixed) isn't a bad approximation.

I'd love to see some solid data to back up how much torque/hp it takes to run the quattro driveline however.

smallTTs

pay enough attention to temp/humidity/baro correction factors. Accurate inlet air temps are very important for accurate correction, and these can vary considerably during chassis dyno pulls. I have seen chassis dyno runs where the air temp wasn't measured even under the hood.

Engine dyno rooms are much more controlled environments and inlet air temp is measured at the carb or MAF. IMO, this is how all chassis runs should be monitored. With charge coolers, it's also important, IMO, to try to keep as close as possible to the 77F or so inlet temp which the OEM engine dyno folks correct to.

Additionally, manufacturers' dyno numbers are taken at steady-state (step) conditions. Ideally, we should dyno an engine in an acceleration mode which matches the chassis run. That would be something like 300 to 500 rpm/sec.

Between inlet air conditions and acceleration rate, you could easily 'misplace' 20-30 hp. Unless you have a chassis dyno capable of step-rpm runs (like the AutoDyn with eddy current absorbers), and accurately measure air conditions, correlation to anything other than runs on the same dyno with the same car is just a guess.

At the extreme end are race shops like Hendrick Motorsports (Jeff Gordon, Terry Labonte, etc) where they are looking for 2-3 hp changes on 750+ hp engines. A typical engine dyno repeatability is usually given at 1-2%. That's as much as 15 hp.
To get 1/4 to 1/2% repeatability takes an extremely **** approach. At a seminar given by Hendrick MS' dyno chief, we learned that they not only calibrate the dyno with weights before and after a test series (instead of monthly), but they control inlet air temp and even fuel temp as it enters the carb to a few degrees F. That blew my mind. I believed him when he said they really can see 2-3 hp changes. Geeesh!

Audi knows exactly how much the driveline losses are, but unless you have an inside contact, geting those #s isn't practical. I, too would like to know. Comparing Torsen to Haldex would also be interesting.

My $.02