Tuning / performance update

[Pullstarter]

A constant percentage would seem plausible, the more power you make the more friction you create? I understand where you're coming from though John. Where's an engineer when you need one?

[Guy_H]

Having just done almost 70 dyno runs on Saturday (with Mainline, the manufacturers of the Dyno's) they tell us that the drivetrain loss will change & they will never try to estimate it at all.

The data they gave us off the dyno (raw data) shows the front to rear split, and how the torque & power shifted (with drivetrain losses) all over on Haldex cars. Nobody can explain.

Anyway, the drivetrain loss went down as everything (oil) was up to operating temps. Then the drivetrain loss went up as the tyres got hot & sticky.

Its a tough one, a nice simple formula would be good!

[Pullstarter]

Thanks for the info Guy, seems like a riddle never to be solved then?

[clip]

A constant percentage would seem plausible, the more power you make the more friction you create? I understand where you're coming from though John. Where's an engineer when you need one?

I know, I wish I was smarter! I don't know if this helps, but this is from a VW forum that I just found after doing what I hate doing (googling that is ), but only because this stuff interests me. Anyway, might explain it some more - might not either. I'm even more confused now especially with Guy's real world input Anyway, here it is:


The whole concept of a fixed percentage drivetrain loss in evaluating chassis dyno tests is one of the most absurd things i've ever heard of in my experience as a powertrain engineer, and is the motivation behind this post.

First, a primer: energy (or in the case relevant to this discussion -- power) cannot be created or destroyed. Simple enough? Where then, does that power that is transmitted from the crank, to the wheels, and ultimately to the road, go? Most of it goes to friction and therefore heat.

Second, an illustration: For argument's sake, take a stock 90HP TDI, and we'll arbitrarily say that it has a 15% driveline loss. That means that the engine would be developing about 104HP (90/1.15) at the crank. The loss through the drivetrain was 14HP (104-90). Now, you do a bunch of engine mods without touching the drivetrain, and you now measure, say, 135HP at the wheels. Adding the customary 15% to refer back to the crank, you get 155HP, but the loss through the drivetrain is now 20HP, a difference of 6HP, WHEN NOTHING HAS BEEN TOUCHED THERE!

Do you now see the absurdity of this concept?

Firstly, may I submit that 2WD vehicles with manual transmissions have very good mechanical efficiencies, as evidenced by the fact that 2 quarts of non-pressurized, non-circulating oil is sufficient to keep the entire transmission cool and lubricated. In fact, to attach a number to it, manual transmissions are usually over 90% efficient, and many over 95%. That implies a loss through the transmission of between 5.3-11%. Even the best automatic transmissions with lock-up TCs achieve between 80-85% efficiencies.

Secondly, may I submit that contrary to popular (mis)conception, flywheel weights, rim weights/diameters and tire type (should) have very little contribution to the HP numbers on a rolling road dyno. Heavy flywheels and rims act as inertial dampers but do not destroy or create energy, nor transform it to heat, as would have to happen to if it is to result in a greater or lesser HP value on the dyno. Tires will shed energy in the form of heat by the simple contact with the ground and also though the flexing of the treads and sidewalls, but this amount is negligable in the scheme of things that it is generally ignored unless you are an engineer for an OEM, race car team or tire manufacturer. More on inertia in a moment.

Thirdly, I hope the above underscores that an accurate measurement of drivetrain loss cannot be overgeneralized. For one, it is not constant across the entire measurement range within a given run. In fact, friction increases roughly linearly with speed. In automotive engineering speak, this is quantified by a parameter called the FMEP (friction mean effective pressure), and although it's is not called that, it is manifested in many engine graphs you may read without even realising it. Frictional losses are different at 2000RPM to 4000 RPM, etc., etc. You cannot, therefore, equate the drivetrain loss of a car whose engine is turning at 8000RPM at the maximum rated power to one turning at 4000RPM, because on the basis of the RPM alone, frictional losses at 8000RPM are roughly double that at 4000RPM.

That said, yes, it's true: gear selection when performing a rolling road dyno DOES have an impact on HP, but it is not usually borne in dyno results, because the difference is small and within the inevitable variation from test-to-test and also measurement error.

Further, engine/driveline design considerations mean that there is a wide variance in frictional losses between different cars; the comparison of mechanical efficiencies between manual- and automatic transmissions have already been discussed above. Cars with AWD, automatic trannies, and large-displacement/many-cylinder engines will tend to have higher frictional losses than small-displacement, 2WD, manuals.

Finally, The importance of "motored" or coast-down tests in a dyno evaluation is important and needs to be stresssed, because that is what accounts for your true frictional losses and balances the inertial "ledger sheet" of the different driveline components, including the wheels and tires. The energy that is absorbed in the form of inertia in the flywheel/wheels/tires, etc. is accounted for ("given back," to oversimplify) in this coast-down, and when doing a street (i.e. butt) dyno, also accounts for the very important aerodynamic drag.

[Sharkie]

Thats just confusing clip .....

No matter how you try to explain it, and I'm sure somebody can exactly .... on average its just a constant % ..... varies from run to run and same model car to same model car as the conditions change .... even the same car over a few runs as the conditions change ....

but the best way of looing at it quickly without frying your brain is just a constant % ....

Put 10 stock Polo GTI's on a dyno under the same conditions and they average 89kw atw (23% loss) ..... chip those 10 and dyno them again under the same conditions and they'd average 123kw (again 23% loss).

Simon's car just showed it again, 22% loss constant. Don't ask me how it works ... just trust in the fact it does .... more or less .... .... I'd bet $$$ on it that my car would do pretty much the same ....

A rough % is not exactly correct all of the time but it works most of the time ....

[Pullstarter]

You brain me hurt!!

[clip]

Nope, you're all wrong and I'm right - because I read it on the internet! Here's another few snipits:

There was a huge discussion years ago on the QLD WRX YAHOO GROUPS, where a guy callled MARIO and his GTR did all the testing to prove the theory... if it was a % loss as everyone makes it out to be, the heat generated in the loss would be enough to melt the gears and shafts of a car... some of the old heads on this forum will know of the discussion with MARIO.

....I can remember all of the above clearly...Mario had 3 or 4 engines in that car and every one of them went on an engine dyno first, then into the car and re-dynoed. From memory there was a consistant 70kw loss difference no matter what the power level of the engines. (aka GTR700)

_______________________________________________

When we began to contemplate flywheel horsepower figures for our different combinations, we bantered about some concepts that can be deceiving. Most enthusiasts have been exposed to the idea that flywheel and rear-wheel horsepower can be equated by factoring in a given percentage for drivetrain loss - the drag that occurs from all the items between the flywheel and the rear tires. You may have seen factors such as 15 percent for stick-shift cars and 25 percent for automatics, applied by dividing rear-wheel horsepower by either 0.85 for stick-shifts or 0.75 for automatics.

Now take a time-out and consider the following. Our original baseline indicated 195 hp at the rear wheels, which when divided by 0.85 equates to 229 flywheel horsepower, and implies that the drivetrain is absorbing some 34 horsepower. On the other hand, our combination of blower and traditional bolt-ons netted nearly 340 hp on the Blood Enterprises dyno, which when divided by 0.85, equates to 400 flywheel horses, and implication that the drivetrain is now absorbing 60 hp.

Nothing has changed between the flywheel and the rear wheels on our '93 LX, so does it make sense to figure the drivetrain is now absorbing nearly twice as much power? Such a concept just doesn't jibe in our little brains, so we asked a couple of people in the biz what they thought. Lee Bender of C&L Performance and Paul Svinicki of Paul's High Performance are both well versed in evaluating Mustangs on the dyno, and they both agreed that extrapolating drivertrain horsepower loss via percentages is flawed. Lee believes that the stick Mustangs experience roughly a 35hp loss through the drivetrain, whether they make 200 hp or 400 hp. He did explain that ultra-high-powered vehicles - typically race cars - can be and exception to this rule, but that's a topic for another time. Interestingly, a 35hp loss for stick-shifted drivetrains is strikingly similar to the difference between Ford's horsepower ratings and the rear-wheel numbers we've observed on dynos across the nation. Hmmm...

[jimbomort]

Thats just confusing clip .....

No matter how you try to explain it, and I'm sure somebody can exactly .... on average its just a constant % .....

Sharkie I dont think its that confusing - the explanation Clip posted from the web seemed the best explanation so far and I dont see anything there that conflicted with Guys real world experience.

As per Clips clip "energy (or power) cannot be created or destroyed....." 'so any extra has to be dissipated as heat or the like'. Thats a principle of physics and well tested.

Therefore cant see how you can have a fixed % loss, without a great deal more heat or the like being generated as engine power is increased. Sure more power is likely to generate a bit more heat in the engine, stress in the drivetrain etc, so can see that power loss mauy not be a strictly fixed kW amount either, but nothing like the extra suggested by a fixed %.

That would mean double the power, you double the power losses, which without being a physicist, would mean something like double the losses in frictional heat and the like.

That just doesn't make sense to me.

[Sharkie]

I'm happy either way .... actually would prefer it to be a fixed loss .... but its been shown in many many tests not to work exactly according to that theory ....

Don't really care as dyno's in the words of Jasn78 here are lying machines .... its black magic ....

[clip]

.... its black magic ....

yep, it's really hard to get any definitive proof either way. In the end, for our purposes with engine modding, it's all about the % gains made from the mod, not the actual figures - they're not to be trusted.

If you really want to test how fast your car is, put someone in that really knows how to drive, that's worth about a 30% gain in power I reckon.