Here is a very interesting read that I got from one of my books on big block Chevy's. It was originally written by Ed Staffel.
Torque is an expression of physical force acting on a lever over time and distance. Horsepower is not a force, it is a method of measurement that is arrived at by comparing the force of torque and in our case, its relationship to the RPMs an engine turns. The classic formula for measuring torque and RPM and then expressing this measurement as horsepower is HP = (Torque x RPM) / 5252. The number 5252 is a constant in the formula that relates to time and distance and so horsepower and torque measurements will always be equal at 5252 RPMs. Look at the plot curves on a dyno sheet and the two plot lines showing horsepower and torque will always cross each other at 5252 RPM and be equal at that point. When an engine is on the dyno, the dyno actually determines how much torque is being made and then a calculation is done to determine horsepower at that given RPM.
It happens that when an engine reaches its peak torque and then starts to fall off to a lower torque figure, horsepower can continue to climb upwards because the RPMs can continue to go up. On a motor this multiplying effect of the RPMs can still act positively on the horsepower side of the equation depending on how quickly or the rate at which the torque curve falls lower after peak torque.
So what, you say? You already know all of this. My point is when you build the motor, what you are trying most to attain is torque and large amounts of torque over the broadest, flattest range you can achieve in your circumstances. If you can do this, horsepower will take care of itself. Remember that most motors do not spend all of their time at peak torque or peak horsepower. It is possible to keep a motor close to peak power through the proper selection of transmission and rear gear ratios or if you have a boat. At the low end and middle of the engine's operating RPM range, keeping cylinder pressures as high as possible will accelerate the vehicle quickly. At the high end of the engine's operating range if you can keep the torque from dropping off too quickly after torque peak and help the engine to hang on longer in terms of torque production, horsepower will continue to climb even though torque is declining. This is generally due to better breathing heads, valve sizes, induction systems and longer duration camshafts that operate well at higher RPMs.
Let me give you an example. We have two engines and at peak torque they are both making 600ft lbs at 4500 RPM. At this point both motors are putting out 514 horsepower. As the RPMs continue to climb and torque begins to fall lower, we find that at 5500 RPM motor "A" is now putting out 580ft lbs and is making 607 horsepower. The other "B" has dropped lower and is making 520ft lbs, which means at 5500 RPM it is making 544 horsepower.
We then take both motors to 6500 RPM. Torque is still falling off and motor "A" is now making 540ft lbs which at 6500 RPM comes out to 668 horsepower. Motor "B" has fallen to 475ft lbs and is making 588 hp. At 7500 RPM torque has fallen further still. The "A" engine is making 480ft lbs and 685 horsepower. "B" is down to 400ft lbs, and horsepower has fallen to 571 horsepower and is headed down, not up.
Motor "A" Motor "B"
RPM Torque/HP Torque/HP
4500 600 514 600 514
5500 580 607 520 544
6500 540 668 475 588
7500 480 685 400 571
At 7500 RPMs motor "A" is making 114 more horsepower than motor "B", but it didn't do this because of horsepower. Motor "A" did this because the torque curve dropped off at a slower rate than motor "B". At all RPM points above 4500 RPM motor "A" had more torque than motor "B".
It's TORQUE that you should be looking for when you build an engine. Horsepower is an interesting afterthought.
