Cobra Dyno Techniques...

[Poison_S]

I have always thought the only way to get accurate HP and TQ number was to dyno a car in fourth gear where the gear ratio was 1:1, which is true for our car.

However, I just found out that my dyno numbers were obtained from running the car in third gear, where the ratio is 1.3:1.

Here is a link to my previous post about the results, including a image of the dyno printout.

My Dyno Results in THIRD GEAR!!!

Interpreting DYNO Data

 

[Poison_S]

ttt

 

[fakesnake.com]

How many RPM's did they take it up to? Were your exhaust baffles in or out?

 

[Poison_S]

FakeSnake,

Thanks for the reply. The shop took the engine up to the shut-off point (6500 rpm) and baffles in.

I have been doing some Internet research this morning and I talked to Jason at DynoJet. I found out that in order to get accurate HP/TQ numbers, you need to run the car in a gear that has a ratio of 1:1. For our cars, as with most manuals, that is fourth gear.

When I confronted the shop on this, they said the car could have been ran in 1st or 2nd gear! And the results would not be affected.

From my basic understanding, this seems ludicrous.

Anyone support this reasoning?

 

[fakesnake.com]

I have never taken notice when I have had my car at dyno place.
I've never seen a place dyno it in 1st or 2nd either.

 

[slow lane]

If they are correcting for both the final drive (rear end) ratio and the transmission gear ratios, it won't make any difference whether they do it in 1st, 2nd, 3rd or 4th.

Not all cars have a gear that's 1:1 in the transmission, and no vehicles have 1:1 final drive ratios, so both ratios need to be factored in.

A dyno run in 1st gear on a dyno jet would last a second and involve nothing but wheelspin.

 

[Got Incon?]

Heres the skinny.

The faster your car accelerates, the more torque and hp that will be lost by adding rotational momentum to your cars rotating parts.

So you will read the most hp and torque if your vehicle is not accelerating any. Obviously, a dynojet cannot do this. But by dynoing in a numerically lower gear, your vehicle won't accelerate the rollers as fast, and you will read more hp/tq over a pull in first gear (even if there wasn't wheelspin)

Motorcycles usually dyno in all gears, and you can look at their hp/tq versus speed and you will see that they rise on a slope through the first three gears then start to flatten out as less energy is being put into the rotational momentum.

As such, I do not know if dynojet software takes this into account or not. Call them and ask.


See on the road, part of your energy is asorbed by friction (both aero and mech), part is asorbed by spinning up the moving parts of your car, and the rest goes towards the vehicle kinetic energy. A dynojet only measures this last part, hence to get your true hp/tq, you have to know the other two factors. The unfortunate part of a dynojet is that the load it places on the car is not equivalent to the the actual load the car would place on itself if it was rolling. It's well known turbo cars will usually not spool to full boost for this reason on dynojets, and measured hp will be down from actual. (And god knows the tuning can get screwed up if this isn't taken into account)

If someone has trouble understanding this and wants to call BS you can set up an experiment yourself.

Take two surfaces, one a oiled steel surface, another a dry rubbery surface, and roll a bowling ball down them both at the same angle.

Guess which will reach the bottom first and why thats so.

 

[slow lane]

But given that your torque curve is well below where it should be, you should walk through all of inputs into the dyno with the guy that ran your car.

Need:
1 - Transmission Ratio (if he ran it in 3rd, should be 1.3)
2 - Final Drive Ratio (3.55)
3 - Tire Diameter

These three inputs plus the drivetrain efficiency will get you to flywheel torque from the torque applied to the dyno. What the dyno chart should be is the flywheel torque * drivetrain efficiency. If he adjusted for third gear (1.3) but had the final drive ratio at 3.27 (old Cobras / Mustang GTs) your torque peak stock would hit 360 (about right) and your horsepower (385 a bit high unless you already had the CAI installed).

 

[Got Incon?]

The problem is while you must know the drivetrain ratios to get back to engine RPM, this doesn't mean that they compensate for the above mentioned rotational momentum effect. I don't know if they do or don't, you'd have to talk to an engineer at dynojet.

I'm going to guess that they don't, because every car has a different rotational inertia.

 

[slow lane]

Also;

horsepower = torque * ( RPM / 5253 )

torque = dyno torque * ( tire diameter (inches) / 24 )
--------------------------------------------------------
( final drive ratio * transmission ratio)

 

[slow lane]

The rotational momentum issue is a wierd one.

My dynos have been on a SuperFlow eddy current dyno which applies both inertial load and an eddy current load to approximate a fixed rate of acceleration. This makes the steps between points on the dyno roughly equal and the load on the engine closer to real world driving (but still somewhat less).

The wierd part is that the eddy current dynos consistently read lower torque (and thus horsepower) numbers even though theoretically less torque is being taken up by accellerating the rotational mass of the drivetrain.

Anyone with an explanation???

 

[Got Incon?]

The equation you show for torque is missing a factor of 2 in the denominator

That or instead of measuring tire diameter in inches you need to measure tire radius.

I would guess the differences in measurements is purely due to the correction factors the different companies use for their dynos. But yes, its generally true that the eddy current dynos show less hp than a dynojet.

 

[Elapidae]

Originally posted by Poison_S
I have always thought the only way to get accurate HP and TQ number was to dyno a car in fourth gear where the gear ratio was 1:1, which is true for our car.

However, I just found out that my dyno numbers were obtained from running the car in third gear, where the ratio is 1.3:1.
:gt:

Can anyone tell me how to calculate what my true numbers should be? I imagine it is not a linear relationship. :shrug:

Also, would this be grounds to have the performance shop that did the dyno in the wrong gear, redo it at no charge?

:cuss:

I thought my TQ numbers were low and this would explain that!!
:fm:

Here is a link to my previous post about the results, including a image of the dyno printout.

My Dyno Results in THIRD FRIGGIN GEAR!!!

Are your "Stock" and "After" pulls both done in 3rd gear?

I would want it done in 4th if it were me. Just takes out one possible calculation error IMHO :dw:

 

[Poison_S]

GotIcon and SlowLane, thanks for the time in responding. I believe that I now have a better understanding of what is involved in the dyno measurement process. :read:
:dw:

I will take this up with the performance shop and report back to the forum any findings. I already have an appointment to have the car re-dynoed. At this point, they disagree with the 1:1 ratio arguement, but say they will humor me with another test in fourth gear.

If the results are the same (+/- 1%), I owe them for another run.
If the results vary more, they will take this as a lesson learned and the pull is free.

Stay tuned...

 

[Poison_S]

Well, the arguement has been settled, the question has been answered, the jury has a verdict...
:uh oh:
There IS a difference dyno testing a s/c Cobra mustang in third gear versus fourth gear.

Testing in Third Gear: 387.7 rwHP, 351.6 rwTQ
Testing in Fourth Gear: 396.2 rwHP, 372.2 rwTQ
Differences: 8.5 rwHP and 20.6 rwTQ!!!

Same dyno, same guy doing the testing and weather conditions nearly identical.

See for yourself, here is the dyno printout to show it:

 

[Elapidae]

WOW!!!

It also makes the dyno chart HUGE

 

[Bizarro]

FWIW....

I have had my car dynoed three different times....at three different locations....
ALL have dynoed in 4th gear....
and 95% of everyone I have ever spoken with who have had their cars dynoed, have had it dynoed in 4th gear.