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Walt Fricke Walt Fricke is offline
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Join Date: May 2004
Location: Boulder, Colorado
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Here is a Dynojet rear wheel torque and HP curve chart for an SC with a stock '82 engine and '74 headers and straight pipes. Other SCs in similar configuration are about the same, depending partly on cam timing - mine was set toward the torque end and my peak torque was higher than that of various buddies. And their peak HP was above mine. I took the car to the dyno about five years ago. Good thing, too, as my glasspack muffler was nicely loud but cost me buckets of power - I thought I had messed up the engine rebuild until I put something better on.

Engines "start to run out of breath" at their torque peak. But the beneficial effects of RPM increases on horsepower outweigh that until the horsepower peak. And you keep going beyond that so that your rear wheel thrust in the higher gear will equal that in the old lower gear right at the shift. Doing it any other way will cost you, as you can see from the thrust/speed/gear graphs.

In lower gears the car will still accelerate well into the 7,000s because torque and power have hardly fallen to zero up there. I used to run my 2.7 up to 7,200 at one point on one track because I could save a shift - slow corner to slow corner, with not enough space in between to make shifting up worth it. That car's rev limiter was easy to tweak so this could be done and I didn't have to sit on the rev limiter or back out. But that would be silly on a long straight.

I make peak torque on this engine to be somewhere around 3,800 rpm, but the curve is nicely sort of flat around there. Peak horsepower is around 5,500. With stock gears and 245/45-16s, the second to third shift is best at 6,250 rpm according to the formula, and the rest of the shifts are about 6,200 (the stock first is so low you just run up to redline and shift - that's the best you can do). This keeps you on the fattest part of the HP curve and gives the best accelleration.

Were I to plug in 7/31 with 8/31 gears "short" box numbers, I'd find that I'd be shifting at a lower RPM, and the drop between gears would be less (I'd start at a higher RPM in the new, higher, gear). Thus at both the start and end of 3d and 4th, and the end of 2d and the start of 5th, I'd be at a higher horsepower (and rear wheel thrust). So I'd accelerate faster. Unless we are going for top speed records as in Bonneville (slightly different considerations there), I'll do better with the short box than the stock box.

So gears matter for performance, if acceleration is the measure of it. Of course they don't make the engine more powerful, but they get the most that it has to give. Isn't that what dd is saying?

As to camgrinder's experiment, the way to compare apples to apples is to do this with the same car. The practicality of this depends on what gears you have and what speed you start at. Skipping third is going to have you revving way beyond where the power is in second, and starting out way below where it is in 4th. Which means you'll be putting down a whole lot less thrust for a long time as you slowly wind out. Starting in 3d (say the start speed is 30 mph?) will be kind of slow for a while, but until you get up to 6200 rpm (or so) you will be way ahead of the game, I posit, for that speed range where you are lugging in 4th. Since acceleration falls off so much with speed, you should be better off sacrificing at the low end. Put another way, you'll be screwed up in both gears part of the time one way, but only in one gear the other, and not where it counts as much. That's why the books recommend setting your gearing so you use it in equal speed blocks - 60-80, 80-100, 100-120, that kind of thing.

Now when that '78 gets its 9.7/1 CR CIS pistons, good exhaust, and 964 cams, his torque curve is not only going to be a bit higher (due to the CR boost), it is going to shift to the right with the better breathing, which will make a large difference in horsepower and performance, I predict. (My 2.7 race motor has a torque peak at 6,000, and a HP peak at 7,100, and has 50 more RWHP than the 3.0 even though it has 5 ft/lbs less peak torque). Add the planned weight loss and he'll run rings around us stockers even without shorter gears. Now a tricky question would be to compare the effect of gears only versus engine only in terms of straight acceleration, which I think is what he has in mind.

Walt Fricke (it is us track guys who rate seat time ahead of everything else)
Old 09-28-2005, 06:59 PM
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