Rnm

iTrader: (4)

Exactly my thoughts.

dubbleugly01

iTrader: (24)

what!? hp and torque are mathmatically related, you can't affect one without affecting the other. Changing gears won't raise torque and leave hp alone.

Dyno'ing in a higher gear will allow for quicker spool vs. rpm on the dyno, just like hitting full boost in 5th gear comes alot earlier in the rpm band than hitting full boost in 2nd gear when you're on the street. That's where the torque increase comes from when dyno'ing in higher gears, but hp goes up linearly with torque.

- HP = torque * RPM / 5252
- torque = HP * 5252 / RPM
- torque = HP at 5252 RPM

Pull your dyno sheet and verify for yourself.

Also, the 5 sp and 6 sp Evo's are closer to 1:1 in fourth than in third. Third is close to 1.4 and fourth is close to 1.1 in both trannies.

Rnm

iTrader: (4)

I'm not disagreeing with you, but I do have to point out something that I experienced when I dyno'd my IX earlier this year. When I had just a flash and mbc, I dyno'd the car on a dynojet and the temps weren't ideal at all considering it was 93 degrees in the shop and wasn't well ventilated. Anyways, the first pull was done in 4th gear and I netted 270whp/318wtq, however the next run was in 3rd gear and that netted 300whp/310wtq so its interesting to hear another person say you can't affect one without the other regarding changing gears, unless in that 4th gear pull I had timing pulled since it was really long.

jbrennen

iTrader: (1)

First, your 270 whp and your 318 wtq are in fact... unrelated. I'm assuming those are peak numbers, and the HP peak and the torque peak are probably separated by 2500-3000 RPM -- at least they are on my car. Comparing the car's power output (measured as wtq) at 4000 RPM to its power output (measured as whp) at 6500 RPM really is comparing apples and oranges.

However, at a particular RPM, let's say 4000 RPM, whp and wtq are mathematically related, and it's independent of gearing. If you have 200 whp at 4000 RPM, you will have 263 wtq at 4000 RPM. Doesn't matter what gear you use.


If you run back to back dyno runs in 3rd gear and then 4th gear, and if heat soak is ruled out or eliminated as an issue, you would expect to see similar results to what you see. At low RPMs, the gearing in 4th gear is more efficient, and the parasitic losses associated with high speed rotation of the driveshaft, axles, rotational mass, sidewall deflection, etc., are not yet large. (You'll also typically have maximum boost sooner -- at a lower engine speed -- in a turbocharged motor when you run a longer gear.) But as the RPMs increase, and you're spinning all of those parts almost 40% faster in 4th gear than you were in 3rd gear, you start using up a significant part of your power just spinning mass around and deflecting your tires' sidewalls, etc. It's certainly easy to believe that spinning all of that stuff 40% faster could use up 10% of your available power (from 300 down to 270). So seeing 4th gear give a better peak torque number (near the boost peak) and a lower peak HP number (near redline) is not at all unexpected. In fact, if you didn't see that behavior it would be surprising.


I suspect that the reason that dyno runs are usually done in 3rd is for three basic reasons. First, a pull takes less time, and dyno time is money. Second, 3rd gear will often give better peak HP numbers because there are less drivetrain losses. And third, it's safer to run a car up to 80 mph on a dyno than up to 110 mph on a dyno.

KevinD

iTrader: (56)

it is true though, hp and torque are directly related.

look at the dyno sheet for those two runs. look where the peak torque is on the RPM scale, and then run those two numbers through the HP calcuation and i bet with 100% certainty that is where the HP is on that curve. then look at the peak HP for that curve and calculate where the torque is... see a trend?

the reason your two dyno runs show such different HP values is because in the 4th gear pull where you only made 270hp but 318 ft-b of torque, your peak torque was below 5252 rpm (thus the reason your torque is higher then the HP), and as the rpm's increased over that your torque was falling off really bad.

however in the 3rd gear pull, it looks like you have higher hp, which means the torque was not falling off nearly as bad after 5252 rpm.

KevinD

iTrader: (56)

thats because it is not actually Wheel Torque.... it is actually the torque at the crank before the gear reduction. the chassis dyno measures HP, not torque. it calculates torque based of the power applied to the rollers, and uses the engine speed to calculate torque. that then makes the dyno plot independent of gear, gear ratio, final drive ratio, tire size, and wheel size, tire deflection, etc etc etc...

i hate when people say wtq because it is not a value that carries any meaning. i have well over 2000 ft-lb of torque at the wheels in first gear on my car.

KevinD

iTrader: (56)

and last, running it in 3rd gear takes long enough that the latency of the wideband the tuner is tuning with will give values of lambda near the actual RPM the event happened at.

trying to tune in 1st gear is near impossible because there is to much lag on the sensor to get accurate readings for tuning.

jbrennen

iTrader: (1)

It's also difficult (impossible?) to achieve full boost in 1st gear -- there's just not enough load on the motor, and the RPMs blow right through the torque peak before the turbo even spools up.

f-dub

iTrader: (5)

You input the gear ratio into the dyno computer and it adjusts for it. Otherwise you would see a great deal more torque in 3rd gear than 4th.

shiv@vishnu

iTrader: (20)

FWIW, I now dyno both 5 speeds and 6 speeds in 3rd gear. This is because they are nearly equal in ratio. 4th gear in the 6 speed tranny is much taller than the 3rd gear in the 5 speed. If you use to high of a gear to dyno cars, they various types of losse (frictional, inertial, thermal, etc,.) start to get screwy and obscure data. Dynoing both cars in 3rd gear tends to hold all the individual loss factors similar-- allowing for direct car-to-car comparisons.

-shiv

boomn29

iTrader: (47)

+1.

MR's and STi's are supposed to dyno in 5th.

shiv@vishnu

iTrader: (20)

Could someone explain why it is so important to dyno in a 1:1 gear?

-shiv

vwjeff

iTrader: (13)

It does, cause more load is put on the car. People that dyno in 3rd are stupid.
A very good friend of mine owns a dyno just down the street from my house and we ALWAYS dyno cars in 4th. He a SCT tuner, and a dynojet owner. He runs the cars in 4th for the load, to get better readings of the knock, maf, and wideband readings.

ApexVIII

iTrader: (1)

because it is a direct drive. the would be no increase in torque due to gearing . its the amount of torque the motor makes

Borti

Not true.

STi's 5th gear tops out at 137 mph. The heatsoak from that would cause unreliable numbers.