Anyone know how to calculate thr rpm through the traps with a 5.38 gear and a turbo 400 trans?

 

Not without the tire diameter you will be using.

 

TH400, TH350, Powerglide all have the same high gear ratio of 1:1 so tranny choice means nothing. MPH will vary depending on the amount of HP the engine produces.

The formula is:
rpm = (168 x gear ratio x mph) / tire radius

 

And you also need to add ~5% for converter slip which is partially compensated by tire growth (if you are usin a traditional bias ply slick).

Just for example, if you trap at 130mph and use a 28" tire, with a 5.38 you would need ~8,300rpm. With a 31" tires, that drops to ~7,500.

Rich

 

Estimating the Vega's race weight of 2800 pounds, the quoted 490 hp will only get 126 mph. If the race weight is down to 2500 pounds then 131 mph. If the car's that light, why are you even bothering with a big heavy TH400? Go with a powerglide.

 

Steven, I just picked the car up. I didn't have a choice as to which trans it came with. It has 32x14" Goodyear slicks and the fellow I got the car from said it ran mid 6's in the 1/8th.Dyno numbers I got from him too. I run 1/4 mile tracks and was wondering how tight it would wind to going thru the traps
Thanks guys

 

Mid 6's in the 1/8 should be a low 10 second car. A top speed of 130 mph would be a good guess at that ET.

guessing at mph

rpm = (168 x 5.38 x 130) / 16
rpm = 7343.7

Since all torque converters will slip, that finish line rpm should be more in the 7700 - 7900 range. There's also the possibility that your mph be be closer to 126 - 128 which will change all the numbers.

 

That's tighter than I would like to run. Maybe I should swap out to a 4.56
Thanks for the help

 

I use 4.56 gears with a 29" tire. I do my 1-2 shift at 6500 and cross the finish line at 7200 (powerglide). I'd like to go to a 4.71 gear for my new engine. My converter stalls at 5700 on the transbrake.