Need help from dyno/auto math experts

[s13satch]

I am having friendly argument with one of my professors about calculating horsepower at the wheels. I dyno'd my 240sx last semester and got peak reading of 102 hp and 163 lb/ft at the wheels. Do these numbers seem odd? I think so. A factory 240sx makes approximately 125 hp and 135 lb/ft to the wheels. It seemed funny to me that my car makes 28 lb/ft more than stock, but 23 hp less.

I know all about how a dyno works because I've been trained to use them. I figured there was an error in the software for the measured torque to respective horsepower calculation. To test my theory, I took several of my data points and calculated the horsepower by hand using: HP= (torque x rpm) / 5252. Using this equation, I got results that average about 35% higher than the dyno calculations. My new peak wheel horsepower was 137, which seems to make a lot more sense.

Now comes the argument. My professor feels that the equation I used is not applicable because it was designed for converting measured torque at the flywheel. I agree that the equation is meant for that purpose, but think that it shouldn't matter. If that equation finds a car to have 20% more hp than torque at the flywheel, would that car not still have the same 20% more ratio at the wheels? He believes that the dyno is correct and that I would need a horsepower equation which contained the rpm signal from the roller sensor on the dyno (not the engine rpm), my final drive ratio, and my wheel diameter. I do not know of any such equation.

[InsanityInc]

You're both partially right. The equation to get horsepower is ALWAYS torque*rpm/5252. If you **** up your tire/roller/final drive ratio, your torque will look crazy (do a dyno in first gear and assume the ratio is 1, for example, and your rwtq will read to be ~4x what it should be).

However, your horsepower will always be the same no matter what, because since it is measuring at the wheels your gained wheel torque from gearing will be proportionally exactly opposite from your loss in wheel rpm.

The problem is, if they input something wrong in the dyno, the torque readings will be wrong. So basically if they screwed up you really cant trust any of the data, except possibly the horsepower.

[s13satch]

You're both partially right. The equation to get horsepower is ALWAYS torque*rpm/5252. If you **** up your tire/roller/final drive ratio, your torque will look crazy (do a dyno in first gear and assume the ratio is 1, for example, and your rwtq will read to be ~4x what it should be).

However, your horsepower will always be the same no matter what, because since it is measuring at the wheels your gained wheel torque from gearing will be proportionally exactly opposite from your loss in wheel rpm.

The problem is, if they input something wrong in the dyno, the torque readings will be wrong. So basically if they screwed up you really cant trust any of the data, except possibly the horsepower.

As torque changes in different gears, so to does the hp. I have seen this as several people in my class did dyno runs in multiple gears. The horsepower increased with the torque. I follow your train of thought, but that is just not what is recorded.

I do not think the information used to calibrate the dyno to read torque was entered incorrectly. The only data which needed to be entered into the system was the vehicle weight and hp needed to maintain 50 mph, both of which where taken out of a federal guidebook.
Modified by s13satch at 9:10 PM 1/30/2006