While studying flow data sheets, cfm-per-square-inch velocity charts, and obscure flow window area information is fun, the whole reason for doing all this research is to pick the best cylinder head and camshaft combination to ultimately make a bunch of power. It seems crazy, but a lot of people have put a ton of time into this avocation. So it makes sense to look at a few engine combinations to see how these cylinder heads, cams, compression, induction, and exhaust systems all combine to make power.
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The previous chapter dealt with matching cams and heads, and these two components have the greatest impact on the power curve, but that assumes that the rest of the engine is properly configured. We’ve used this small sampling of engines here mainly because we know the dyno numbers are accurate and that they’ve not been “enhanced” in search of hero horsepower numbers that unfortunately dominate bench racing sessions and Internet chat room discussions on small-block Chevy engines.
It’s also important to point out that these engines are not introduced here as the ultimate combinations for torque or horsepower. In fact, in every one of these examples, we could go back and change the cam, heads, carburetor, or some other significant component and perhaps gain a little power. One point worth mentioning is that often these attempts at making more power merely cost the designer power at some other RPM point. Generally, a bigger cam will push the torque curve up, which does a great job of improving peak horsepower, but this comes at the cost of a significant loss of torque below peak torque. That is the compromise of engine development. Ideally, what we’d like to do is make a change that pumps the entire torque curve up over the previous combination with no penalty anywhere. When you can do that, you’re a hero, but it happens less frequently.
More realistically, making changes to the power curve is a little more sophisticated than just shooting for more peak horsepower. Sure, big horsepower numbers are important and even make for great bragging rights. But if we are talking about street engines here, then we should also be realistic enough to acknowledge that the average street car is probably not set up to take advantage of an effort aimed at only increasing peak horsepower. What does this mean? Let’s take a closer look at the overall power curve and what it means to a typical street-driven car.
Let’s take the situation of a stout street engine, making an honest 500 hp at 6,500 rpm. A dedicated drag racer who is interested in building the quickest automatic possible would immediately plug in a 5,000 rpm stall speed converter and a 4.56:1 gear with a 29-inch tall rear tire to spin this engine at about 6,600 rpm through the lights. This combination in a 3,700-lb car would push it to 10.99 at 121 mph. Taking a more conservative approach to the same car with a taller 3.73:1 rear gear, 26-inch tall tires, and a more streetable 3,000-rpm stall speed converter slows this combination down to a 11.43 at around 119 mph. Note that we did not change the power curve, but changed the gears and made the car more conservative for the street and also used more of the engine’s torque curve. Note that the trap speed changed much less than the ET. The biggest change was the launch RPM with a higher stall speed converter to take advantage of launching the car at its peak torque. This clearly makes the car more ETefficient since the MPH only changed 1 mph compared to an ET differential of over 0.40 second. Another big change was the deeper rear gear ratio that both launched the car harder and also carried more RPM through the lights. The deeper gear mostly compensated for the taller rear tire, but this also plants more tire on the track for a better 60-foot time.
The deeper geared combination spends more time in the RPM band between 5,500 and 6,500 rpm, while the taller 3.73:1-gear combo spends the majority of its time between 5,000 and 5,500 rpm. The point here is that if you’re unwilling to drive a street car with 4.10:1 or perhaps 4.56:1 gears, it doesn’t make much sense to build an engine where you rely on the power it makes above 5,500 rpm since your engine will spend very little time in this power band. Turning this situation around, the smart engine builder would then work on enhancing the power his engine makes within the RPM band where the car will spend a majority of its time. It just makes sense to do it that way.
What this really comes down to is whether you are going to build an engine and then configure the car around the engine, or build an engine that will take maximum advantage of the combination the car has at the time. With most street cars not optimally geared or equipped with high stall speed converters, this means enhancing the torque curve in the 4,000 to 5,000 rpm area where this power has a dramatic affect on vehicle acceleration. If you think about it, this is why even a mild 150- hp nitrous system has such a dramatic impact on acceleration. That 150-hp at peak horsepower is really only used at the top of first and second gears because the car clears the lights in third gear at a much lower RPM. This jump in horsepower helps, but the reality is that the major torque gain created by the nitrous (along the lines of 200 ft lbs of torque at 4,000 rpm) is the major reason the car accelerates so strongly and pulls off those one-second ET improvements. It’s not the horsepower gain, but the massive infusion of torque that is the reason for the quicker ET and speed. It’s that simple.
Don’t get us wrong; horsepower still plays an important part in a quick ET and an impressive trap speed. The torque versus horsepower controversy will no doubt rage on long after these pages have turned to dust, but we contend that maximizing overall useable power always results in a quicker car with all other variables being the same. In other words, if we have two engines that make the same peak horsepower at the same RPM, but one makes more torque than the other, the more powerful torque curve engine will out-accelerate its opponent every time. And as long as we can stick those tires to the pavement on our street-driven car, we’ll always choose to give up a little bit of top-end power for a bigger boost in torque right around peak torque. Torque and traction will beat that other guy to the next stoplight every time. You can bank on that.
Written by Chris Petris and Posted with Permission of CarTechBooks