Steep gears/Smooth idle

[samandw]

Hey,

I have a '79 Cutlass that I plan on upgrading over the next few years to do both autocrossing and drag-racing duty. Part of my plan includes a 6-speed manual trans conversion (T56). My question is this, conventional wisdom says that when you run steep gears you need a hot cam to go with it aka lopey idle. I'm thinking about making use of the .5 overdrive on the T56 by installing steep rear-gears (like 4.56-5.47) and still maintaning sedate freeway RPM. I'd also like a smooth idle and decent economy.

Anyone know why I can't build an engine that makes loads of low-end torque, put it in front of steep gears with slick's and 60' the heck out of it? I know I'd have to learn how to shift REALLY fast, but that doesn't seem like a big problem Any thoughts on why this is a bad idea?

 

[UNGN]

Gears that steep are dumb, unless you are planning to run 30-32" tall tires.

Even with a 6 speed, you still are only going to use 4 speeds in the 1/4 mile. Pick a rearend ratio that puts you at your HP peak crossing the traps in 4th gear in the 1/4. If you build an engine with lots of low end torque, you are looking at a HP peak around 5500 RPM.

With a 26-28" tire, you are looking for a rear gear in the 3.55 - 4.11 range.

 

[samandw]

I know that's what the conventional wisdom is, but I'd like to know WHY.

It still seems to me that lots of torque multiplied by a large ratio should yield stellar initial accelleration. Since I don't care about trap on this car, just ET, the 1st 60' makes or break a run (as it usually does).
Can someone explain to me why steep gears and low-end cams aren't compatible? Does the rotating inertia of the engine start to play a significant part?

5.47 gears and a T56 would put 5500 RPM and 5th gear at 104 mph with a 275-50R15, about where I expect to trap. . .

 

[UNGN]

Originally posted by samandw
5.47 gears and a T56 would put 5500 RPM and 5th gear at 104 mph with a 275-50R15, about where I expect to trap. . .

You can only accelerate as fast as the tires will hold. Whether you have 5.13 gears or 3.55's gears. Super stock guys run steep gears because they run really tall tires. Steep gears with small tires means you have to shift more. If you manage to hook in first, the 1-2 shift may unload the tires result in wheelspin.

Shifting into 5th in the 1/4 mile is also dumb. The 5th gear in a T56 is .74 which is even worse that the .77 drop of the 3-4 shift. In the mile drags, I get to speed shift into 5th (I have a .81 drop to 5th) Nothing sucks more.

Last year at the mile drags, one of the C5 Corvette guys missed 5th but found 3rd... to the tune of 3 bent valves.

I'd be shocked if the car was even .2 seconds quicker in the 1/4 mile with street tires, but build it like you want and don't let me stop you.

 

[samandw]

Good point on the 4-5 ratio drop, I hadn't thought about that.

You said I might get .2 on street tires, and I'd have to agree with you, but I do plan on bolting on slicks or drag radials when I drag race it, you think that gears would make a bigger difference (than .2) with a really sticky drag tire?

Thanks for the input