Need Some Gear Ratio Tech

[SNEAKY]

what kinda rear gear ratio are you guys running on the turbo cars, and why?


i have a friend with a turbo car, and is talking about switching to a higher numerical gear cause he mainly runs local 1/8 mile tracks. it is a 347 with a PT88, with a glide and a Trans brake.


he currently is running 3.55 and is talking about going to 4.10-4.30's.



BTW this is a mustang

thanx in advance

 

[UNGN]

Our gear ratio's don't really translate to other cars. Our stock motor's HP peak is 4400-4600 RPM. Even with ported heads, we can't rev our cranks much above 5800 RPM without Fear of driving over it.

We go with the highest gear ratio that will allow us to run the mph we can go in the distance specified. With the stock block, for some reason that always turns out to be 3.42:1.

 

[SNEAKY]

Originally posted by UNGN
Our gear ratio's don't really translate to other cars. Our stock motor's HP peak is 4400-4600 RPM. Even with ported heads, we can't rev our cranks much above 5800 RPM without Fear of driving over it.

We go with the highest gear ratio that will allow us to run the mph we can go in the distance specified. With the stock block, for some reason that always turns out to be 3.42:1.

his reasoning is, that he will pick up in the 1/8.

the only time we take it to the 1/4 is for FunFordWeekend and when we go to Noble to stamp some buick butt


i thought that with a turbo motor, it was about how much load you put on the motor, and how long you stayed in the torque curve.

this motor probly makes in excess of 1000ft lbs of torque, and is spun to about 6900

 

[Ormand]

The rear gear used on a Turbo Buick is right for a turbo Buick. You can't apply that to a different engine/car/transmission/tire combo. If you are talking about a SBC with a powerglide, it is a whole different animal.

 

[KEVINS]

They are correct. I think it depends on the tranny, converter, tires, etc b/c he may be able to set the ENTIRE car up to run great in the 1/8 but it may not make it to the end of the 1/4 with decent power with a simple gear change.
For instance: More gear with less stall may help ease the car out of the hole w/o blowing the tires off and may help control boost. He should worry about too much gear b/c the motor may spin too quickly and not build enough "usable" boost and the torque curve may vary enough to hurt performance.

My guess is with a turbo car the best 1/8 mile car is a great running 1/4 mile car. If you think about it can he 60' better with more gear in the car now? If not then I would not run too much gear.. He may be better off running different height tires instead of re-gearing it.

I dunno..
ks

 

[420]

Here's a gear ratio question for you - If a Buick steps up to a 30" tire, would a 3.73 gear be in order? Assume it's a 6500 rpm capable 109 block, and a 140 ish trap car.

 

[KEVINS]

If it's a 1/4 miler then I would use this equation to see if the trap RPM's are what I want at the finish line:

RPM = (336 X MPH X GEAR RATIO) / TIRE DIA

This has always been within 200 rpm for us if the converter is good and I always subtract 1" from the "Advertised" tire height and the #'s are much closer.

(336 x 140 x 3.73) / 30 = 5848 rpm

or subtracting the 1" from tire diameter:

(336 x 140 x 3.73) / 29 = 6050 rpm

ks

 

[420]

Good formula, Kevins What's the relevance of 336?

 

[KEVINS]

Originally posted by TransAm WS6 420
Good formula, Kevins What's the relevance of 336?

Just a Units Conversion frome feet/inches/miles.

ks