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Old 01-12-2018, 09:12 AM
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Location: Gilroy, CA
Cobra Make, Engine: SPF 2291, Whipple Blown & Injected 4V ModMotor
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Mike, olddog and a few others seem to catch the gist of this.

If we use his dyno graph and select the data for the white and blue curves the white curve intersects the Y-axis at 100 ft/lbs of TQ at 8100 rpm. If you use the formula for HP, the HP at that point in the pull should have been 100*8100/5252 or 154 HP. When you check the HP trace it is showing 154 HP.

Torque is a proxy for engine volumetric efficiency. As engine volumetric efficiency begins to nose over with increasing rpm so to will torque — in absolute lockstep. Your transmission gears selection for acceleration (drag racing) should be chosen to straddle the peak horsepower point on the engine's power curve to capture the greatest area under the curve if you want to optimize the vehicle performance.

For a road race application you want to select gear ratios that complement the vehicle's exit speeds from the various turns on the road course in such a manner as to again capture the greatest area under the curve. Once you are exiting the turn, not in the turn, you are once again in what amounts to a drag race and it is area under the curve and torque applied to the tires that accelerate the vehicle.

Because drag racing is a different type of contest than road racing you can make some generalizations that tend to be pretty close to the mark. One is that torque essentially accelerates the car and is a primary contributor to elapsed time. The other is that quarter mile trap speed is a pretty good proxy for horsepower.

For every day drivers torque at the axle is what we are most frequently presented with and associate with performance. When you have a lazy acceleration you can crutch the car's performance by going to a taller rear gear, i.e. 3.31 to a 3.73. The taller rear axle ratio mechanically leverages the engine torque to provide more twisting force (torque) at the axle/tire for increased vehicle acceleration. The ratio change has no effect whatsoever on engine power only torque — you experience it's effects as increased acceleration or an increase in the driver seat of the pants dyno numbers.

When we get away from all the fancy words this is actually pretty simple stuff that, surprisingly most of us already have a fairly good intuitive feel for. Don't set aside / ignore your personal experience for urban legend. You will do yourself a disservice.



Ed
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Last edited by eschaider; 01-12-2018 at 04:57 PM.. Reason: Spelling & Grammar
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