Differences between dyno´s

[Alstrup]

Hello.
We often talk about hp and torque and we do it in loose terms, because we hardly ever menthion under which conditions we measure. Now, I have more than once "complained" over data coming from especially Mustang Dyno´s because the numbers did not compute with anything that I knew at least, and was therefore not usable for much except that car that day. However, I recently ran into a fella that knew something about Mustang dyno´s, and he explained a few things to me which made the whoile thing make much more sense.

Normally we compare everything to and with a water brake dyno using either a SAE60 or SAE 77 (SAE J1349) calibration and a 300 rpm/second climb because those are sort of the industry standard. (Some use 500 rpm/second) BUT, when the Mustang dyno´s come from factory they are calibrated to resemble a large electric dyno. That results in a totally different set of numbers. We did some math back and forth for me to grasp just how much difference there is, so we took a dyno run that I recently made on a customer´s mild cammed 1776 engine which made peak of 116 hp and 111 lbs torque. These numbers are converted from Din to SAE 60 to have an easy conversion to the Mustang calibration. (1,7 hp correction) Here´s the catch. On a Mustang dyno, with the standard Mustang calibration that same engine would have shown 84 hp and 84 lbs torque with peaks about 250 rpm lower than with a SAE60 calibration. (This is flywheel hp. If the dyno is not set up to measure that and only measures wheel hp the number would be even lower, like around 70 hp.)
The Mustang dyno´s can easily be calibrated to SAE J1349. It is simply to go into the program and set it up. No big deal, but apparently a good deal of Mustang dyno owners do not understand how or don´t bother to do it.

This may be somewhat nerdy information, but at least it explained to me why the Mustangs often shows numbers that we could not relate to, until now
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[ratsnotmice]

Can you clarify this a little more, are you saying that some are not using a correction at all?

[ratsnotmice]

Ok, maybe I am understanding maybe not. Here's what I know:

In the US, most performance/aftermarket shops doing dyno testing still use the J607(STP) correction.

The J607 correction will generally yield numbers 4-5% higher than J1349.

I've found that's where alot of the discrepancies happen.

[ratsnotmice]

[Alstrup]

Yes, it can soon be a little confusing. Here is a comparison chart that may help understasnding it a little better:
http://www.bigdynodatabase.com/DynoCF.php
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[vwracerdave]

Comparing dyno numbers from your shop to mine is not very useful. Weather has a big effect on engine performance, nobody includes that with their dyno numbers. On a chassis dyno you can add 10 PSI to the rear tires and get a whole new set of numbers. Your dyno is your tool to get the most out of your engine. Always tune your engine on the same dyno, and don't try making tuning changes by visiting others shops because their dyno will have a little bit of calibration differences. The only accurate way to compare your engine to mine is to line them up at the track and race them.

Then there is always the "Orange County Correction Factor".
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2010 Sportsman ET Champion - Mid-America Dragway - Arkansas City, KS
1997 Sportsman ET Champion - Thunder Valley Raceway Park - Noble ,OK

[Alstrup]

Dave, for max power, yes, but then you still have weight, tyre pressure, and driving abilities into the factor. Also, a straight line comparison does not tell much about driveability of the engine, AFR at part load, tip in issues etc. As for weather calibration. Our measurements are always calibrated for temperature and humidity. Din, EØS and SAE 1349/1394 always are, - or at least should be, or they do not meet the calibration standards.
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[jpaull]

Great Info Alstrup,

For some that are not understanding, The Mustang Chassis dyno is reading 27% less then Alstrups Chassis dyno. Thats huge! I have heard 13% between Mustang dynos calibrated correctly vs not, so that is a incredible difference that you seen.

The Sae vs Din differences will usually be only 6% or less, so this is not the difference he is seeing. My total correction factor is usually only 1.04 which is only 4% more then reading the torque straight off the machine(Engine dyno). This is one advantage of a engine dyno vs chassis dyno, you dont have those "extra" correction factors that might account for larger differences like the Mustang Chassis dyno differences.

The calibration Alstrup is refering to is the way the chassis dyno arrives at its initial horsepower/torque reading before the weather/altitude factors are even calculated.

[k@rlos]

I take dyno figures with a pinch of salt, quite frankly I don’t give a shit what the figures say as they’ve gone up & down every time and I only use 1 place. I Only use the dyno to get my jetting spot on and See what timing the engine likes the most. My bus does great burnouts and catches a lot cars out at the lights so I consider that fast/powerful for a Street driven camper. Too many variables too take a rolling road seriously I think.

Should be used as a tuning tool, nothing more imo.

[jpaull]

"Should" be used as a tuning tool. If it is used that way, of course the tool works and thats good. This is for 1 person, comparing only their engines progress against itself and not weighing the number as a benchmark in relation to any other engine.
Except so many people dont. They get a dyno number, brag about it to all their friends, put it on their car show plaque on their car like its gospel and brag on the internet about it. But so many of these "types" never make a pass down the track. As Keeton and Dave said, their point was that so many brag with a high number end up turning a lower quarter mile time. The number means very little if its not even the correct number.

Alstrups point aside from quarter mile times is the value of how much and when the torque/hp happens. This is valueable. But if its not the right number, its not valueable. Comparing 1 engine to another to know how well you did has value. so it IS important to have the correct number, this is part of the value of the "tool" in bench-marking progress.

[ratsnotmice]

[Alstrup]

Yes. - What Gkeeton is referring to between chassis dyno´s are basicly exactly what I´m talking about, only the other way around. If you don´t calibrate your dyno to a recognized, or at least known specification it is hard to compare numbers etc. (and btw. its not my dyno, it belongs to Westspeed)
Wrt repeatability. If the software is calibrated correct and you measure in EØS, or SAE 1349/1394 and the engine in operating temperature, the engine software should show the same power within 1% or less cold or warm, dry or humid.
I have seen cars go on different dynos within 4 weeks of each other, 2 Dynomet and one Bosch Optima and the same car shows 169-171 hp and 215-217 Nm torque. That you can do if the software is calibrated.
I had a friends car in for tune up (which I wrote about in another thread, which after tune up showed the same numbers almost 5 years after the first dyno session within 0,7 hp. If that is not good repeatability I do not know what is.

IMHO the part with showing who´s Willie is the largest is sort of a biproduct. On the other hand we all like to bragg The real gain is to figure out which does what. Dave is right on that.
I do consider myself lucky since I have access to both an engine dyno, chassis dyno and an axle hub dyno. We use them a lot. The engine dyno is almost always 100% booked. When we get cars in with a problem it is always interesting to see where it is at and where it is when we are done with it. For instance, about 2 months ago we had a 512 BB in which had been to another shop where they couldnt fix the problem of a poor running engine. 2½ days later it spun like a kitten, had again incredible throttle response and finally resumed its factory hp We have fixed engines that ran like a bat out of you know where at WOT, but like crap almost everywhere else, wobbly fuel curves etc etc.We once had a hot rodded Silver shadow in which wouldnt pick up well. A couple of days later it was chasing the bat. (That was actually a fun job) It is a very powerfull tool if you know how to use it (and read/understand the gauges)

Anyway, that was not the message. The message was that at least I have now found out why the Mustang dyno´s often show odd numbers compared to what would be expected.
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[Alstrup]

[ratsnotmice]

There is also some different ideas about the efficiency factor included in weather correction. Some say it should be included others say no.

Then there is the subject about how to factor drivetrain inertia and frictional losses.

The original SAE J1349 document was only produced for engine dyno applications run in steady state and with pretty tight enviromental controls in place. There are strict limitations on how much correction is allowed for the test to be valid according to the document.

In the aftermarket having access to raw numbers is important; load cell output and calibration, roll speed and acceleration, parasitic losses, roll inertia mass equivalent, data filtering type and speed, weather station data and calibration, etc..

Or we could just drive on and make a couple hits and have high fives all around.

[ratsnotmice]

[Alstrup]

[ratsnotmice]

[ratsnotmice]

[Alstrup]

SAE J1394 is the latest correction to the standard (I think) 2004.
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