Chassis dyno...

I know that hp at the wheels is typically 20-25% lower than at the crank due to driveline losses... What about torque? Does the drive line also consume 20-25% of the torque?


I'm thinking that it would have to since the torque is a function of the RPM, but then again, 5000 rpm on the dyno chart (engine speed) is not 5000 rpm of tire speed due to gear reduction and tire sizes.

Horsepower is a mathematical formula,torque is an actual measurement.
Dynos measure the torque,so the loss of torque has to be there. The 20-25% numbers are a big argument among different manufacturers.
Hope this helps.
Keith

My car was chassis dynoed at 460 hp at 6300 but was still climbing, i now run the limiter at 6500.
Car is 3200 race ready, i reckon my flywheel hp is somewhere close to 600, which would equate to around 25%.
Best tuning tool around.
Mick

I'm assuming you're about to put your car on a chassis dyno...don't put your car on it just to see what kind of power it is making. It has been discussed here many times, but the dyno is just a tool. Don't get discouraged if it only says you're making 300 hp, but don't get excited if it says you're making 700 either.
Same car, same day, on 2 different dynos can yield very different results.

yes I know about dynos and how much they can change from brand to brand, shop to shop, operator to operator, even day to day on the same dyno.

shop in town was doing 3 basline pulls for $30, figured with my new 5.9L in the dakota, might as well go see what the torque curve looks like, find out where the best place to shift it ought to be, find out if I'm really running out of head flow above 5,000 rpm and the power is dropping WAY off, etc.

but, since it's spitting out some rear wheel power numbers, and it seems the vast majority of folks out there talk about crank hp when talking about a certain combo, since that seems to be the "universal standard" that takes out gear ratios, autos vs sticks, etc. on power output.

I know that there's no good way to take a chassis dyno and calculate flywheel power, and vice versa, since every vehicle combo will be different, and that the only way to know either number for sure, is to dyno it!

that all being said, I know that Hp is a calculation of Torque vs RPM, and that at 5252 every dyno graph hp/tq curve crosses. but was just curious as to whether or not a chassis dyno uses different formula's to calculate the Hp/Tq, since there's gear reduction/torque multiplication in the rear differential and tire size diameter, and that the torque the chassis dyno is seeing is rear wheel torque, not flywheel torque...and how that correlates to flywheel torque.


ok...now that the dyno pulls are done, I made 285hp and 325 torque. does that mean I'm in the 375 hp ballpark at the flywheel, and 400 ft lbs of torque ballpark at the flywheel? or is my motor making 375/325?

You would apply the correction % (whatever you decide to use) to both the TQ and HP numbers. As was said, HP is an equation derived from TQ and RPM, so HP cannot go +20% unless TQ does.

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You would apply the correction % (whatever you decide to use) to both the TQ and HP numbers. As was said, HP is an equation derived from TQ and RPM, so HP cannot go +20% unless TQ does.

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fair enough. thanks!