bigturboz28

If running a turbo car on a dynojet (vs. mustang) why not dyno it in 5th gear if it is not loaded properly in 4th? From what I understand the dyno doesn't know what rear end gear you have, it just checks how fast you spin the weight and your RPM's.

Why won't this work (dyno in 5th to load turbo better)?

ABeasst

iTrader: (5)

5th puts a LOT of load on the car and can cause detonation at least on the street. plus you'd be skewing the dyno since you arent in a 1:1 gear and would be toping out high

dynocar

That is a good idea, providing that the 5th gr can handle the extra power and the tires and dyno can handle the extra MPH. Doing this would better simulate the extra load that the engine would see on the street. If this is an inertia only Dynojet, it simulates a 2500 Lb load on the car with no provision for simulating air resistance as MPH increases. This would prevent normal turbo spool-up and not expose real detonation that shoud be tuned out. ABeasst states, "you'd be skewing the dyno", I think that it needs to be skewed if it does not provide the proper load for tuning purposes. Actually, aren't we un-skewing it?

MIGHTYMOUSE

iTrader: (59)

you can use a boost controller to adjust to whatever psi you would like to make.

bigturboz28

I have 4.11 gears, SO, if I run in 5th it is like running in 4th with a 3.08 gear in the back.

4.11 rear end gear in 5th final ratio -> ~3.04
3.08 rear end gear in 4th final ratio -> ~3.08

Any thoughts why this will not work to help load up the car properly on a dyno and get the tune more accurate?

dynocar

You got it, this will better emulate real world engine loads for better mapping of fuel and spark on an inertia dyno. I would rather error on too much load while tuning, you may race someone up a hill.

kbracing96

iTrader: (25)

Peak numbers won't be as high as you would have more drive train loss, as you are not running 1:1 though the trans in 5th gear causing more friction with gears turning at different speeds..

dynocar

We are discussing the best tuning proceedures, however, less important, on some combos 5th or even 6th can produce more measured RWHP. A higher trans gear reduces the engine/drivetrain inertia losses due to their lessor rate of acceleration which sometimes offsets the slight loss in a transmission's efficiancy. But if the rear gear and transmission gear combo generates too high of a testing MPH, the additional tires, gears and bearing frictional losses negate the lessor rate of acceleration gains. So peak HP/TQ measurement gear selection vary, often unpredictably.

bigturboz28

Should be going to tune again on Tuesday, i'll post the differences.

engineermike

We found the opposite to be true. Dyno'd my last car in 1st, 2nd, then Drive. The power and torque numbers were progressvely higher. The way it works is the drivetrain weight absorbs less power when it rev's slower in the higher gears.

kbracing96

iTrader: (25)

I would agree that you should make more power in drive (1:1) then in 1st or 2nd, for 2 reasons. 1, you are loading the turbo more in D and 2, there is less drive train loss being in D as all the gears in the trans a spinning a the same speed. When you go in to over drive, (5-6 in an M6 car) then the gears are not spinning at the same speed any more and you will have MORE friction and less hp making it to the rear wheels. It could be possible that the increases load on the turbo, would help to increase hp and make up for the increase in friction in the trans. It would be a more accurate measure of hp to use a load baring dyno, like a mustang dyno to load the car in 3rd or 4th (which ever is 1:1).

engineermike

1. That was with my NA combintation, not turbocharged.
2. Efficiency of a gearset has little to do with ratio, be it reducing or increasing.

When you use an inertial dyno, a certain amount of power is put into accelerating the crankshaft, damper, converter, transmission gears, driveshaft, and wheels. The slower all of these accelerate, the more power can be put into accelerating the drum. So, with higher gears, the rwhp reading will only go up to a point of diminishing returns.

With my old combo, the numbers were:
1st: 260 rwhp
2nd: 315 rwhp
3rd: 335 rwhp

I'm sure if I had dyno'd it in OD, it would've been somewhere around 340 rwhp.