gear change: estimate new e.t?

[radback]

Hi there,

I've got a 79 trans Am fitted with a 69 428 (overbored to 440) engine, a th350 and a looooong 2.41 posi (even longer as I've got 29" tires instead of the 27" stock ones, so it's like a 2.20!)

Best e.t is 14.5 (on second gear).
Pretty bad for this engine, but of course gear is the problem.

So I intend to put a 3.08 or 3.23 gear.
Any way to guess what kind of e.t to expect with new gear?
Thanks

[ponchopwr70]

What stall converter, cam spec and est hp. I would go with at least 3.55. I would think you would pick up at least a half to full second.

[radback]

What stall converter, cam spec and est hp. I would go with at least 3.55. I would think you would pick up at least a half to full second.

Don't know about the converter or hp (I bought the car with this nearly new engine already fitted), I was told the cam is a Ram Air III spec.

When I said best e.t, it was my fifth run with the car since I got it, and I was with a passenger on a so so track.
I think the car may be able to do a 14 at its best without modifications, so I hope it'll made a low 13 or maybe a hi 12 with a 3.23 gear. The gear should make a big difference doesn't it?
Right now I was just pushing 2nd gear thru quarter mile end wasting 3rd gear.

Sure a 3.55 would be better for runs, but I've to think about highway too (10mpg with the 2.41: I'm affraid what it will be with a 3 series gear even if some people told me I may gain some MPG...)
A 3.23 may be the best choice for me.

[ponchopwr70]

With my 3.55 and a 26" tire Im going through the traps at 5800 in 3rd. If your not getting in third I would expect a big difference. Ill agree with you on the highway, thats why Im going to start installing my gearvendors today. Best of both worlds.

[chicane67]

If you are just interested in quarter mile stuff... I'd drop in some 3.73's and call it a day. But the reality most likely states differently. So below I wrote in some more realistic consideration for a street car. This will give you some cruise mph/kph numbers to think about. I mean... petrol is like $4+ USD a liter over there... and I am thinking that you would want to at least think about milage numbers.

With the TH350 and the 29" tire:

2.41 - at 2500rpm = 84 mph (135 kph) ** Best for land speed records

3.08 - at 2500rpm = 67.6 mph (107.8 kph)

3.23 - at 2500rpm = 66.77 mph (107.5 kph)

3.42 - at 2500rpm = 63 mph (101 kph) *Just about right for both

3.55 - at 2500rpm = 60.7 mph (97 kph) *Just about right for both

3.73 - at 2500rpm = 57.8 mph (93 kph) **Better for drag racing

[radback]

Thanks Chicane,

I think I will stuck with 3.08 or 3.23 for fuel issue, but I really wonder what kind of e.t I will get with this kind of gear instead of my actual 2.41.

any idea? (is there a program to calculate it?)

[68Formula]

http://richmondgear.com/01cartest.html

[radback]

Thanks Amcmike!

I don't know if it's accurate, but this little DOS program is a lot of fun :)

For my car, it shows:
14.5 with a 2.41 (correct)
13.9 3.08
13.8 3.23
13.7 3.42

[68Formula]

Can't say either, except on the stock vehicles (I compared it to my Z28 when it was completely stock), it works. Of course output is only as accurate as the inputs.

Don't forget to use the launch optimization function, so that you're comparing assuming the best launch rpm and traction.

It also estimates the best shift points based on your torque curve.