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      01-26-2009, 12:18 PM   #23
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The only advantage over the Dynojet that the Dynapack or Mustang Dyno has is the ability to do load based tuning. Since none of us have the ability to tune our own cars then that is negated. I'm sorry PencilGeek, but the Dynojet is still the industry standard, and I find them the easiest to compare dyno plots among multiple shops and cars provided the SAE correction information is correct. Anyone that gets dynoed on a Dynojet should get the files from the shop and you can download the Dynojet dyno viewing program and plot the runs yourself. You can view what was input for the SAE correction as well.
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      01-26-2009, 12:54 PM   #24
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The closer to the drive shafts one gets when making the measurement, one can claim the more "accurate" the measurement is in terms of what your engine/drivetrain is producing at the hubs. When you take the wheels off line and measure the torque at the hubs, you are bypassing the variance associated with the wheels (and their inertia effects if you are doing an acceleration run), and traction issues (a function of tire and roller compounds and surfaces, acceleration rates and so on), etc. Plus, a chassis dyno can actually vary the load on the engine to pinpoint its max output at the hubs at a given rpm precisely (if you do a steady-state test). To the best of my knowledge, roller dynos cannot do this--at least the ones commonly used. I think that is what Pencilgeek means by inference. My understanding is that the roller dynos can only do acceleration runs, and different acceleration rates should yield significant variance, so unless you are using the exact same dyno with the same acceleration rate, the results are not be comparable. SAE correction factors don't have anything to do with any of this. A steady state test on a chassis dyno, corrected for atmospheric conditions via SAE guidelines, on the same care on different days should yield almost exactly the same results.

Look at these results:

http://www.rri.se/index.php?DN=29

They've done a sedan and a coupe, different model years, 7 months apart. Exactly the same result (after SAE correction). 373 bhp at the hubs, steady state. I don't think you'll see that kind of consistency with roller dynos.

It looks like PGs numbers are lower although he did use a chassis dyno as well, but I think that is because he did acceleration tests, which will, by definition, result in a rotational inertia related decrease.
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      01-26-2009, 02:34 PM   #25
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Quote:
Originally Posted by lucid View Post
The closer to the drive shafts one gets when making the measurement, one can claim the more "accurate" the measurement is in terms of what your engine/drivetrain is producing at the hubs. When you take the wheels off line and measure the torque at the hubs, you are bypassing the variance associated with the wheels (and their inertia effects if you are doing an acceleration run), and traction issues (a function of tire and roller compounds and surfaces, acceleration rates and so on), etc.
That is all true as we are measuring the power to the wheels not to the hubs on a Dynojet roller dyno. Changing out your wheels can change the numbers. I don't think any of us are making enough power to create traction issues. The bottom line is be knowledgeable about your car, the various dynos, and the shops in your town. Pick the best one and run your car stock and then after each part you purchase to really know what your car is doing. The Dynojet is still the most popular, but they all work. I would also make sure you go to a dyno that has the A/F probe so you can see what your A/F ratio is doing on the dyno which could explain poor results.
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      01-26-2009, 03:25 PM   #26
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Quote:
Originally Posted by PencilGeek View Post
Well, if you're looking for accurate results, I wouldn't use a roller dyno -- as they basically guess at horsepower based on the physics formula: F=M*A (Force = Mass * Acceleration). So they basically look at how fast you accelerate and the mass of the car, and basically guess -- or infer -- how much horsepower you've got. Once you throw in the inertial effects of the wheels, traction to the rollers, load put on the car by the straps, and the fact that I'll bet NOBODY actually weighs their car before the dyno -- you basically end up with useless results (with respect to accuracy).

So if you're looking for accuracy, the only choice you've got (short of pulling your motor out and putting it on a motor dyno), is to use a dyno that attaches directly to your hubs, places a hydraulic load on the motor, and measures torque exactly like a motor dyno does. Basically, you need a Dynapack or Rototest. I don't know which will be easier to find in Greece.

Now that I've said this, I predict the next 20 posts will be from people who swear by roller dynos, and will come up with all kinds of convoluted reasons why they're "good enough." Since you said you want accuracy, there's very little they can offer to give you what you want.

Click on my "Dyno Runs" link in my signature for a complete description and list of many tests I've already run on the M3.
very helpfull, many thanks!
      01-26-2009, 08:41 PM   #27
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Quote:
Originally Posted by King Tut View Post
That is all true as we are measuring the power to the wheels not to the hubs on a Dynojet roller dyno. Changing out your wheels can change the numbers. I don't think any of us are making enough power to create traction issues. The bottom line is be knowledgeable about your car, the various dynos, and the shops in your town. Pick the best one and run your car stock and then after each part you purchase to really know what your car is doing. The Dynojet is still the most popular, but they all work. I would also make sure you go to a dyno that has the A/F probe so you can see what your A/F ratio is doing on the dyno which could explain poor results.
The thing is they need to run exactly the same acceleration parameters for your results to be somewhat comparable. Also, traction is always an issue. It may not be a major issue, but there is no way of telling how much power is being lost/not being transmitted at the tire-roller interface by staring at it, and a 3-4% difference is significant variance for instance.
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      01-26-2009, 09:16 PM   #28
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Where are you getting this 3 to 4% number from? How did this turn into a me defending Dynojet thread?
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      01-26-2009, 10:02 PM   #29
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Quote:
Originally Posted by King Tut View Post
Where are you getting this 3 to 4% number from? How did this turn into a me defending Dynojet thread?
I put those numbers out as an example. I have no idea what the losses are exactly, but there are losses at the interface for sure, which will be higher during an acceleration test, and I don't think they are negligible. Can't help you with the second point.

It boils down to how accurate you want to be when comparing setups, and what you are willing to do to ensure similar test conditions. All I am saying is that measuring torque at the hubs at steady state will yield the most consistent figures in terms of engine output.

If you want to measure the power the car is putting down on the road, things become fuzzy because the accelerartion parameters really affect the outcome.
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      01-26-2009, 10:45 PM   #30
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      01-27-2009, 12:02 AM   #31
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see http://nceuro.org/forums/viewtopic.php?t=12563 for more details on this particular build.
      01-27-2009, 09:29 AM   #32
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I definitely await your next set of dyno runs with the AA parts installed.
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      01-27-2009, 09:40 AM   #33
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Quote:
Originally Posted by PencilGeek View Post
Well, if you're looking for accurate results, I wouldn't use a roller dyno -- as they basically guess at horsepower based on the physics formula: F=M*A (Force = Mass * Acceleration). So they basically look at how fast you accelerate and the mass of the car, and basically guess -- or infer -- how much horsepower you've got. Once you throw in the inertial effects of the wheels, traction to the rollers, load put on the car by the straps, and the fact that I'll bet NOBODY actually weighs their car before the dyno -- you basically end up with useless results (with respect to accuracy).

So if you're looking for accuracy, the only choice you've got (short of pulling your motor out and putting it on a motor dyno), is to use a dyno that attaches directly to your hubs, places a hydraulic load on the motor, and measures torque exactly like a motor dyno does. Basically, you need a Dynapack or Rototest. I don't know which will be easier to find in Greece.

Now that I've said this, I predict the next 20 posts will be from people who swear by roller dynos, and will come up with all kinds of convoluted reasons why they're "good enough." Since you said you want accuracy, there's very little they can offer to give you what you want.

Click on my "Dyno Runs" link in my signature for a complete description and list of many tests I've already run on the M3.
From my point of view, you've done some really good work here. Stuff that's of clear value to both M3 owners and general motorheads alike. The information regarding temp sensor placement is especially important.

That said, you've been getting a little too much hyperbole from Dynapack.

Not to denigrate them, since I agree with you that their method offers better repeatability, but there are essentially only two differences between how a Dynapack does it compared with how chassis roller dynos do it, and only one of them is significant.

The insignificant difference is the rotating inertia of the drive wheels and tires. Granted, we're looking at probably 100 pounds or greater here (in total), but with a typical dyno run being done in, say, fourth gear, they don't spin up very quickly (thus minimizing their effect), and although they will in fact reduce the resulting power and torque readings, it won't be by much. More importantly, their effect is completely repeatable, run to run, so when you're attempting to measure differences due to modifications, it's a non issue.

The other difference is more meaningful, in my opinion. It's the rolling resistance of the tires that introduces accuracy problems. Granted, all tires have rolling resistance, so the observed numbers will be reduced further from those observed with a Dynapack. But that's not the issue. If the results are repeatable, so what. You're still getting good numbers when measuring before and after modifications, as mentioned.

The issue is that the results are less repeatable.

A tire's rolling resistance will vary based on temperature, inflation pressure, and how big the guy was who cinched your car down over the rollers - and the differences can be significant. Tire temperature is less significant if you're doing two or three passes, because after the first pass they're already very warm, but inflation pressure matters a fair bit, and what the tire load is can be pretty critical. What you need is enough cinching so that tire load minimizes wheel slip, but not any more than that. This will tend to vary shop to shop, operator to operator, and perhaps even day to day, based on the guy's mood.

So those of you who are taking your prized possession to the dyno shop, pay attention.

Back on the Dynapack thing (and making sure you pay attention to the vehicle's setup parameters on the chassis dyno), it won't do to say that the Dynopack readings are more accurate than chassis dynos. Granted, they'll be higher (and closer to the engine's output at the clutch), but chassis dyno guys can (and will) say that their method is more accurate, since it measures power delivered to the road - and last anybody knew, nobody races on a dyno.

By the way, vehicle mass is immaterial on an inertial chassis dyno. The only thing that matters is how quickly the vehicle can accelerate the rollers (a known mass), and what the equivalent road speed is at that point. Horsepower is directly calculated this way, and torque is a complete unknown - unless you hook up a tach lead, in which case the dyno software can also calculate torque for you. This is opposite to how a Dynapack (or in fact an engine dyno) does it, in that they measure torque, and from that they calculate power based on rpm. Neither method is "better" - just different.

Lastly, I don't know what features are part of the latest Dynojet products in particular, but their are plenty of chassis dynos that can exert a variable load, thereby calulating power at rpm steps with no acceleration, thus eliminating rotational inertia from the results - if that's important to the client. Furthermore, most chassis dynos can provide coast-down figures, giving the client a good feel for drivetrain friction and inertia.

I mean no offense with this post, and have healthy respect for both Dynapack and for what you've posted. Just wanted to set the record straight.

Bruce
      01-31-2009, 01:09 AM   #34
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I am not going to get into an argument about dyno's, but I have some fresh pictures of our project car. Here is a link to our project page: http://nceuro.org/forums/viewtopic.php?t=12563
Be sure to see the last page.

We just installed the full active autowerke signature exhaust without cats! It sounds great!
Today we did the Active Short Shift Kit, AA Mufflers, AA X-Pipe, AA Straight pipes, and upgraded AA software.

We have successfully gone 30 miles since the install with NO check engine lights(thanks to some tricks up our sleeves). The car is running great and sounds fantastic.

Tomorrow we plan to install the H&R sport springs, 12mm spacers, AA Pulleys, and hopefully vinyl the roof black again and put on the carbon fiber trunk spoiler.

We will keep you all posted. Here are a couple pics from the day.





      01-31-2009, 09:54 AM   #35
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Pulleys, and hopefully vinyl the roof black again and put on the carbon fiber trunk spoiler.
Please post lots of pics of the car with the black roof. I've been debating doing this on my car and I would really like to see how it looks on another white E90 before committing. Thanks.
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      06-05-2010, 06:27 AM   #36
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480bhp 350ft-lbs torque stock m3

I had my car test an a uk car show and it came out at 450bhp and 350ft-lbs torque..........Any clues...?
      06-05-2010, 08:36 AM   #37
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Quote:
Originally Posted by PencilGeek View Post
Well, if you're looking for accurate results, I wouldn't use a roller dyno -- as they basically guess at horsepower based on the physics formula: F=M*A (Force = Mass * Acceleration). So they basically look at how fast you accelerate and the mass of the car, and basically guess -- or infer -- how much horsepower you've got. Once you throw in the inertial effects of the wheels, traction to the rollers, load put on the car by the straps, and the fact that I'll bet NOBODY actually weighs their car before the dyno -- you basically end up with useless results (with respect to accuracy).

So if you're looking for accuracy, the only choice you've got (short of pulling your motor out and putting it on a motor dyno), is to use a dyno that attaches directly to your hubs, places a hydraulic load on the motor, and measures torque exactly like a motor dyno does. Basically, you need a Dynapack or Rototest. I don't know which will be easier to find in Greece.

Now that I've said this, I predict the next 20 posts will be from people who swear by roller dynos, and will come up with all kinds of convoluted reasons why they're "good enough." Since you said you want accuracy, there's very little they can offer to give you what you want.

Click on my "Dyno Runs" link in my signature for a complete description and list of many tests I've already run on the M3.
Well preached! I knew there was a lot of factors to consider getting a car dyno'd.
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