April 2012 Chevy High Performance Q&A

Kevin McClelland Feb 22, 2012 0 Comment(s)

With regard to inflated dyno numbers, my operator said his machine is very tight. If I wanted higher numbers I should go to a shop, [a name he gave me], and I can expect much higher readings. Isn’t horsepower and torque the same no matter where you go? Horsepower and torque are mathematical formulas with set parameters—there shouldn’t be a difference.

I am curious on my cam choice. I purchased a solid roller cam from Crower (PN CA-56171) with 230/238 degrees duration at 0.050-inch tappet lift, and 0.340/0.355-inch lobe lift. I used 1.6:1 rocker arms. This is set on top of 210 AFR heads with a 10.6 COMP ratio. The intake is an Edelbrock Victor with a Holley 750-cfm mechanical secondary double-pumper. It has 13/4-inch headers, and the car is approx 3,500 pounds. The majority of my driving will be below 5,000 rpm. Would changing lifts or duration have helped with more torque and horsepower? My dyno numbers are as follows: at 2,700 rpm, 498 lb-ft, to 5,100 rpm, 494 lb-ft; max torque was 528 lb-ft at 3,700 rpm. Max horsepower of 483 occurred between 5,300 and 5,500 rpm.

H. LeRoy Thomas
Chico, CA

Let’s go in order with your questions. Most engine dynos use a water brake to apply load to the engine to either control the acceleration of the engine or hold the engine speed at a fixed rpm. Whichever style of testing, there is a strain gauge attached to the brake that reads out the amount of torque applied to the brake. This measurement is expressed in lb-ft. To derive a horsepower number from this, you must do a little math. The formula for horsepower is: torque × 5,252 ÷ engine speed = horsepower. The 5,252 is the constant in the equation, and this is why horsepower and torque always cross at 5,252 rpm on a dyno test.

Next, you can manipulate the dyno results in many ways on both engine and chassis dynos. The strain gauge must be calibrated to a specific amount of load. In 20 years of running an engine dyno, we’d always calibrate the dyno near the peak torque of the engine that was going to be tested. If it was a performance small-block, we would use the proper amount of weight to equal around 430 lb-ft of torque. For a pump-gas big-block race engine, it would be calibrated in the 700 lb-ft range. This would calibrate the strain gauge closest to the projected power output of the test engine. Let’s say you’re calibrating the dyno to run your engine. You could hang around 500 pounds on the calibration arm and tell the dyno it was 530 lb-ft! This would give you a fudge factor of 30 lb-ft. This is just one way you can fool the numbers. The easiest way to play with the numbers is to enter the incorrect atmospheric conditions into the computer for its correction factor. If you put in a low barometric pressure reading, or a high inlet air temperature, this will kick up the power with the correction factor.

Finally, have you driven your Chevelle yet? The horsepower and torque numbers sound really nice, especially the torque curve, its width, and where the peaks occur. Don’t worry about the hero horsepower numbers. You hit the nail on the head when you said that 98 percent of the time the car will be driven below 5,000 rpm. Yes, you could have gone with more duration on your camshaft and picked up the power well over 500 hp, but you’d have given up that really nice, fat torque curve. Remember, torque is what moves the car. Horsepower is just how many times you hit the crankshaft per minute.


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