undrway

Nope, sensor inserted in tailpipe downstream of the cat.

john s

A healthly cat will store oxygen, causing both phase & amplitute errors

captainbilly

This is the first time I have seen a real explaination of how an inertia dyno works and it clearly explains why the numbers obtained from one are only comparable between cars with identical rotating parts (wheels, tires, brakes, trannys, etc.

Bottom line is that you cannot compare the HP or torque numbers from a A4 to an S4 or TT or A6 or M3 etc. etc. It also expains why I have seen so many posts that seem to say that heavy wheels rob HP. The wheels of course don't have any effect on HP but the dyno will record lower numbers on the car with the heavy wheels because they will take longer to accelerate. The next time you hear someone say that he knows his Z28 has more HP than your chipped S4 because the dyno says so, you need to know what kind of dyno the two cars were tested on. If the comparison is on a inertia dyno for one or both cars then we can't know how the cars actually compare.

It also occurs to me that the great difference in power losses from the flywheel to the wheels may in fact only be a funtion of the testing method and not actual. What I mean is that if the dyno is an inertia type then all the rotational inertia of the entire drivetrain would have to somehow be calculated and subtracted from that of the dyno to get actual HP readings. Without doing that calculation the numbers on an inertia dyno are only comparable between two identical cars (drivetrains are identical) on identical design dynos (same rotational inertia).

Brett / APR

It seems so simple but it is so rarely understood. The dyno manufacturer is not going to point this fact out and let everyone know it's products weaknesses. The gear used during a run is also particularly important in maintaining accuracy. If you run a car in third gear on an inertial dyno and then run the car in fourth gear you will get different results. The reason is that although the engine speeds will be similar, you will have to accelerate the rotating assembly (wheels/tires/etc.) between a higher speed difference (say 50-90MPH instead of 35-65.) This requires more energy. This energy is sapped from the engine and never reaches the dynamometer at all.

Someone posted earlier that a a dyno was only good for tuning and bragging rights. Well, if we were talking about an inertial dynamometer I would take that one step further and say that it is only good for bragging rights. An inertial dynamometer can be handy in seeing the immediate difference of a bolt-on part (assuming the test is run carefully and other variables are carefully maintained.) This is about the limit of an inertial dynamometer's capabilities with regards to actual tuning.

An engine dynamometer is the obvious first choice for serious engine tuning. However, this is costly and, in many cases, simply not possible due to various constraints. Fortunately, it looks like there is a good second choice now on the market.

Keep in mind that besides understanding the ins and outs of the particular dynamometer being used it is also critical to maintain careful procedures to ensure accuracy. Intake charge temperature, ambient temperature, humidity, altitude (or barometric pressure), and oxygen content are all critical. I mentioned oxygen content because I have seen dynamometer readings taken in very poorly ventilated areas. What the people performing the readings were not aware of was that they were destroying the accuracy of their tests by not having an adequate supply of fresh air. The engine was lowering the oxygen content of the air as it consumed it and then it was trying to inhale the same air.

Brett
APR

joe@vwvortex

because you don't know the baseline hp each individual engine may be making. 6hp is well within the range of differences in stock motors. I've seen plenty of VW's dyno'd and some were better than others. I said it before and i'll say it again, anything other than comparing the numbers of your car stock, to your car modified is worthless when it comes to comparing mods. Also IMO - I doubt ESP would make any difference on a dyno as it's more concerned with lateral movements and the car being tied down would eliminate that.

Mach V Dan

At risk of repeating myself, we didn't set this up to be "standard." It wasn't a chip comparison test, it wasn't a "determine the exact crank horsepower of your car" thing. It was simply a "put your car on the dyno" day. Since we did it with other cars, under the same conditions, I'd say comparisons among those cars are probably fairly accurate (with the exception of FWD and AWD of course). As I said before, the farther you get from that exact situation, the less accurate any comparison will be. Other dynos measure differently, can be set up differently, for all I know they have different levels of maintenance and are greased differently. There are certainly different brands of interial dynos. Then there are load dynos that use a different method altogether. I think I put plenty of disclaimers in my original post, although the indignant posts have flown anyway.

I don't think the gear choice matters a whole lot. The torque/hp measurements are at the wheels. (Okay, with the exception of inertial issues, as mentioned above.) The dyno simply measures how fast the intertial wheels change speed. RPMs are taken directly off the engine. So you get torque (measured by the rollers) and engine speed. That's all. Gearing shouldn't matter, other than that lower gears would have faster runs, and yes, drivetrain intertia would come in to play more. (For the record, the Dynojet user's manual doesn't say you should make your runs in any given gear.)

Fourth gear is a little impractical in our case because the runs get really LOOOOONNNG, and the wheel speeds are very high. The dyno is set up to resist as if the car weighs 3200 pounds. Accelerating up through the RPM range in fourth takes a while, as it would in real life. It's pretty hard on the engine, running at 6000 RPMs for an extended period, especially since the fan in front of the dyno doesn't blow air at 100 mph. Third is more practical.

As for the drivetrain loss thing, if you think 10% isn't good, that's fine. I just used the 10% as a rough guess; I'm not claiming any actual numbers at the crank, just what we observed at the wheels. And the 10%, in this case, seemed to fit the stock FWD cars we saw. On the other hand, if we assume the A8 was at full rated power (it seemed powerful enough), its drivetrain loss (for this situation, for this dyno, on this day...) would be more like 25% or so, instead of 20%. (Factory SAE net horsepower at the crank is 300hp for that year's 4.2.)

Look, all you smart people can pick apart what we did all day long. "Wrong dyno." "Wrong gear." "Wrong oxygen sensor placement." Fine, fine, fine. At YOUR dyno session, you'll have the chance to do it "the right way."

--Dan<ul><li><a href="http://www.machvw.com">Mach V Motorsports</a></li></ul>

Mach V Dan

Sounds like a cool setup, and my understanding is that the price is right, too. Will you guys have one soon, Brett?

I have to admit, I'd have to get used to the idea of jacking up the car and unbolting all the wheels for a dyno run. But it's portable! Heck, you could have a house call dyno service!

See the <a href="http://www.dynapack.com/index.html">Dynapack web site</a>

--Dan<ul><li><a href="http://www.machvw.com">Mach V Motorsports</a></li></ul>

Mach V Dan

<center><img src="http://store5.yimg.com/I/machv_1628_1038839"></center><p>The graph is from my modded '91 Mitsu. Eclipse AWD. :^)

I'm almost there on the peak hp part, but the "flat curve of torque" is indeed a dream...

--Dan<ul><li><a href="http://www.machvw.com">Mach V Motorsports</a></li></ul>

Larrytt

ESP has zero bearing on the results.

6hp is still 6hp on identical cars. I was trying to point that out in comparing it to the two 180TTQ's with different chips and different exhausts yet they show identical numbers! That disturbed me more than the 6hp difference between the two 225TTQ's.

See what I am talking about?

Rave'nGTi

just wanting to appologise for one, I'm not trying to come off as an *** like some people think I am...

I've just been working with dyno's (dynojet especially) for several years now... from race cars to street cars, honda's to dragsters, powerglides to 6 speeds... and it makes for very interesting discusion.

from a friend in detroit that runs detroit speedworks...

"We always want to run in whatever gear is closest to 1:1 in order to reduce the effect of transmission ratio on torque output and gear reduction losses. We do this all the time just as another way to be more consistent and accurate."

thats pretty much all I'm trying to say lol... so gearing does matter... it affects the torque reading and therefore the hp you get.

we actually discussed gear ratio's last night... the 225 would be 5th gear and the 180 would be 4th. I dont think it should take that long for a TT to get from 2500 to 6grand in any gear... why would it be run at 6000 for an extended time though?