Hey all,
I was looking at the MCS dyno charts under the Reading List section. I was hoping somebody would be able to explain to me how to make sense of a dyno chart. What can you tell about a car by glancing at the hp and torque charts.

I know for example when you read a car specs, they say xx hp at xx rpm. Are hp charts usually a sloping line? Are torque charts usually shaped like a bell? if so what does the "apex" signify?

thx for edumacating me!

 

Andy at Ross-tech is your man, I think, for chart reading.

There are two kinds of dynos--inertia and brake. Inertia dynos have a big drum of a known mass, and they measure how quickly your car can spin it up. From this they can calculate the torque (torque is twisting force, which is of course translated to acceleration.)

Brake dynos measure the same thing, but use a brake (some kind of steady resistance.) Much more useful for tuning as you can hold it an rpm.

(Two notes: 1.) car makers measure the engine in a jig and give you a number right from the crankshaft vs. the wheels. Their numbers will always be higher. 2.) Dynos compensate for temp, altitude & humidity, and adjust the numbers accordingly.)

Torque is as close a measure as you can get to the accelerative force. So a torque curve should show you where in the RPM range you have the most oomph. In terms of what you feel, torque is the best indicator. Neglecting wind and friction losses (and in a single gear), the same amount of forque will buy you the same acceleration rate anywhere in the curve.

Horsepower is a measure of work. Pushing 2,000 lbs down the road at 10mph is more work than pushing it at 5mph. (HP = torque * RPM / 5252.)

Side note: gearing acts like a multiplier. 1st gear will accelerate with greater force than 2nd, etc.

What you want, then, is to figure out where in the RPM range you're interested in making power. If your daily drive has you at 2-4,000 RPMs then focus on the torque curve in that area. If you're going on ralleys on the week-end, and want to hold 3rd gear to redline, then the upper end of the curve deserves your attention.

I always focus on torque, but Horsepower is useful for accentuating differences. Particularly if you're racing, where horsepower in the upper ranges (the flexibility to hold one gear another 500 rpm) is important.

I think the February issue of Evo has a big piece on torque vs. horsepower. It has a bugatti on the cover.

HTH,

Jeff

 

Thanks Jeff, very informative post