Hi everybody. I've read a lot of people say that your cam/heads (powerband) needs to be matched with your gearing for optimal performance. Can someone explain what that actually means?

I know with automatics and toque converters it is important that the thing stalls when you get to the power band and then you only get 4 speeds (I think), so gearing would be more important. But I have a T56 (and planning to go with 4.10 rear), so what do I have to consider in this case?

I am exploring my engine options (another post) and I just want to understand a little better how the drivetrain will affect whatever want to put in my engine.

I chose 4.10's because I would be redline in 4th gear at about 107 mph with the tires I have right now. If I by some chance I make enough power that I redline before the finish line then I can go with a taller tire which is supposed to be better for traction anyhow. It also depends on what your goal for the car is.

so the idea is to be close to read line by the end of the 1/4?

I am not really looking to build this car for drag racing. I would probably only take it to the strip to tune it and such? Ultimate goal of this thing is to take it road racing and some around the city cruising.

I went with 4.10 because I figured it'll give me deeper gear for launches and at the same time because T56 6th gear is .5 it will still be very highway friendly. Also deeper gear would provide slightly more torque in lower gears which I am hoping would compensate for engine where power curve was shifted up slightly.

Does this make sense? I am looking at an engine to go to something like 6,500 rpm

The goal is to be ~100-200 rpm above peak hp as you cross the 1/4mi finish.

With 4.10's your going to rip through the rpms very fast, so your going to need to have an engine that makes its power at a higher rpm.

Use this little program
http://www.jason.fletcher.net/tech/d...ds/cartest.zip

simulate a camaro with 4.10's and the t56 gearing and note the gear and rpm it hits the 1320 (1/4 mile) mark at. The choose your cam that peaks out just below that rpm.


Then go on kazaa and get desktop dyno and simulate an engine with different cam profiles until you find something that matches up with your gearing.

Typically, with an engine geared towards racing the powerband is high in the RPM range and relatively narrow. You use tall gears to keep the engine in the powerband throughout the 1/4 mile, or around a road course.

A car is a system, and all of the components in that system need to work together to make it perform optimally. A car with a 6,500 RPM max engine speed will make most of its power from about 3,000 RPM up. The powerband (HP) is probably between about 3,800 RPM and 6,200 RPM if the engine is designed properly. This engine would need about a 9.5:1 - 10.5:1 compression ratio, and heads capable of flowing enough air & fuel in this RPM range. Your carb needs to be sized to this RPM range, and the cam needs to be designed for this RPM range. Your headers need to be sized to this RPM range. Then you need to consider stall speed for the converter, which will require that you take into account the power characteristics of the engine, the weight of the car, the rear end gear, and the tire size and type. That sounds like a lot to consider, and it is, but to create a car that is best suited to its intended purpose those are the things you need to consider.

BTW, 4.10s are a good choice for a T56 car with a 6,000+ RPM motor.

Hey, neat program, the cartest one is pretty spiffy. A bit slow on the times, I thought. I would hope a 1990 GTA could do the 1/4 in a bit faster then 16 seconds.

It takes some fiddling to get really accurate. For one the third gens are 3800 lbs in that program. I measured mine to be just under 3100 with no driver and no gas.

[edit]

i just ran the gta it ran a 15.2. Thats not far off with 2.77 gears
Drop the weight a few hundred pounds and it mid 14's. 3670 seems to much.