turbo_pwr

I know it's pretty much accepted that we lose 20% from Crank to Wheels on a dyno. I talked to a pretty well established Vette Shop and the owner stated that he had seen Vettes that lost anywhere from 15~25% depending on the tranny/gearing. What I forgot to ask him was about Torque. Do you lose 20% in Torque as well? How does that translate on a dyno?

Gernby

Dynos ONLY measure RW torque. RWHP is calculated from the torque. So the loss IS a loss in torque.

Gernby

I think the S2000 has so much drivetrain loss due to small gears, secondary reduction gear, and the 4.10 final drive.

turbo_pwr

[QUOTE]Originally posted by gernby
[B]Dynos ONLY measure RW torque.

ultimate lurker

You can't really use percentages to calculate drivetrain losses. It simply isn't accurate as not all losses scale with hp. In reality, part of the losses are relatively fixed, part of them are variable. But for looking at gains on the dyno, you're better off just assuming a fixed loss - that way you know you won't overestimate the gains from modifications/tuning.

Here are some things to remember about drivetrain losses:

1. The longer a run takes (to a point) the more hp the car will show. Why? Because you're accelerating the engine at a slower rate. This means that it takes less energy to accelerate the crank and flywheel. This leaves extra energy to accelerate the dyno. The converse of this is true as the faster the run (for a given rpm range) the more energy required to accelerate the various parts of the car.

....However...if the run takes too long, and you don't have sufficient cooling airflow, the dyno can actually show less hp due to heat soak, etc.

2. For a given drivetrain configuration, the tranny size/differential/axles/wheels will pretty much determine the losses. For example, losses on most FWD, manual transmission, moderate to high hp engines will tend to land in the 25-30 hp range (on a dynojet). This pretty much holds true for Honda B-series, H-series, Nissan SR20DE, etc. Smaller transmissioned and powered cars (say Honda D-series) will lose something closer to 15 hp. For a RWD 200-300 hp car, losses tend to land in the 40-50 hp range. Smaller, lowered powered cars tend to lose something more like 25 hp.

UL

Gernby

UL,

When you dyno S2000s, what gear do you use? My last dyno was done in 5th gear, but when our local S2000 club Dyno Day, we used 4th gear. Isn't the correct gear whichever one has the closest to a 1:1 ratio?

turbo_pwr

You guys aren't really answering my question or you are and I am missing it. I just want to know if the same thinking/theory applies in terms of torque.

If it is a given that car "A" loses 20% hp from crank to wheels does it lose the same in terms of torque? Or is torque independent?

turbo_pwr

gernby, I would think it would be 4th since 5th and 6th are Overdrive gears, aren't they? I could be wrong. I think it should be 4th. Even though people have used 3rd which is way further away from a 1:1 situation.

UL what do stock S2000s dyno in terms of torque? Also, I believe I read somewhere where you felt that all things being equal that your dyno reads higher then a dynojet. Am I correct? If so what do you feel the difference is?

Gernby

An "over drive" is any gear that is less than a 1:1 ratio. 5th gear is just below that (.97:1 I think). The further you get above or below 1:1 contributes to the drivetrain loss. I know for sure that an S2000 will produce lower dyno numbers in 3rd gear than it will in 4th gear.

Reading UL's post, I was a bit confused myself, but I think he was saying that drivetrain losses are more "flat". I took it to be like a fee, instead of a tax. For example, you pay a $15 fee whether you buy $10 worth of stuff, or $100 worth of stuff. A corvette's drivetrain sucks up 45 HP (13% of 350 HP), and an S2000's drivetrain sucks up 45 HP (18% of 240 HP).

Gernby

The dynapack attaches to the hubs of the car, so the rolling resistance of the tires, and weight of the wheels do no add to the drivetrain loss. This provides numbers that are closer to what they would be at the crank.