September 2012 Chevy High Performance Q&A - It Has Come In Twos

Kevin McClelland Jul 27, 2012 0 Comment(s)
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This package will put your Camaro into the high 12-second e.t. range if you can get a hold of the track! We would stay with the 3.42 gears; the 383 will have plenty of slow-speed torque matched with your 700-R4 low First gear ratio. We hope we have answered all your burning crate debate questions. Let us know how it turns out!

Sources: gmperformanceparts.com, hedman.com

Gearing for Torque or Horsepower

Q. If the torque of an engine moves a car, then why does everyone tell you to gear for the horsepower curve? Could you please explain the principles involved, and why? In case it makes a difference, I drag race and do land speed racing. I’m tempted to change to taller rear gears and use the torque band more. What do you think?

I love your column; it’s one of the reasons I subscribe to Chevy High.

Larry Lancaster
Via email

A. Horsepower is just a mathematical equation of torque and rpm. The higher you carry torque in engine speed, the more horsepower you make. As I corrected myself last month, the formula is torque multiplied by rpm divided by 5,252 equals horsepower. With that said, if you have 400 lb-ft at 5,000 rpm and 400 lb-ft at 7,000 rpm, you have much higher horsepower at the 7,000-rpm mark; 400 lb-ft at 5,000 rpm gives you 380.8 hp, and 400 lb-ft at 7,000 rpm produces 533.1 hp.

When someone says to gear for the horsepower curve, they are usually referring to where peak horsepower is, and where the engine drops into the torque curve on gear changes. If you have a wide powerband (and early torque peak and late horsepower peak), you have a very forgiving power curve. This type of power curve will pull vehicles around easily. When you have a narrow powerband, you need to keep the engine “on the pipe”, like a two-stroke motorcycle engine. This is why with high-strung engines you’ll usually find close-ratio manual gearboxes to keep the engine in its powerband.

Racing automatics with high stall-speed converters, you’ll usually want your full stall speed to be a couple of hundred rpm over torque peak. This is especially true when using Powerglide two-speed transmissions. When you launch the car with a Powerglide, the engine sits at stall for a period of time before the engine begins to accelerate. Then again, when you shift into high, the engine falls back into the stall of the converter and must accelerate away again. If the full stall speed is below torque peak, it can be difficult for the engine to accelerate through torque peak.

When you say that you drag and land speed race your car, there is probably nothing farther apart, as for setup. A good friend, Tim Moore, used to do both with his ’67 Camaro. It had a nasty little 350 small-block and a Super T10 trans. When he would drag race, we would swap out the 9-inch third member with 5.43 rear gears and a spool. When he took the car to the dry lakes, we would swap out for a 3.25 third member and a posi. In either application it would go through the traps at 7,000 rpm. At the dragstrip it was going 118 mph, and on the lakebed, in the flying mile, it ran 164 mph. Again, we geared the car to reach horsepower peak at the finish line. This is an example of where it was tuned for the horsepower curve. The toughest gear change was from Third to Fourth on the lakebed. The Super T10 was a close-ratio gearbox, and the little Mouse would just barely pull away from the gear change.

The point is there won’t be one gearset that will be optimal for both land speed competition and drag racing. Either pick one type of racing or prepare yourself to get your hands dirty between events. Hope this has helped. Have fun! CHP

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