R&P Ratio for Auto-X???
 

I have a 1967 Pontiac Firebird that I'm setting up for Auto-X. . .I'm in the process up doing some mild suspension, chassis, and brake system upgrades and I just found out what my rear-end ratio is. . .

2.73:1 (41/15)

I'm guessing this would not be a very desirable ratio for Auto-X.

The car is has a 400 Pontiac engine, an automatic (at the moment. . .I have no idea what stall torque converter is in the car) and I'm pretty sure the rear end is an 8.2" 10-bolt (I haven't run back out to verify it's the 8.2" yet, but I'm pretty sure it is. . .it's definitely a 10-bolt). It has squarish axle tube flanges (GM) vs. round axle tube flanges (BOP. . .at least from what I have found online).

I do plan on changing the entire rear end out in the future, but not right now. . .should I change the R&P? If so, recommendations for R&P ratio?

Thanks!

I think transmission gear ratios and wheel/tire diameter should be considered to make this decision.

Don

It's a TH400 so it should be a 2.48:1 first, 1.48:1 second, 1.00:1 third, and 2.07:1 reverse. . .

. . .17" rims with 275/35ZR17 tires in the rear; overall diameter as measured is ~25-25.125
".

R&P Ratio for Auto-X???

do you mean Rack & pinion if so....

why do you want to change to Rack & pinion...

most Rack & pinions add bump steer and limit turning radius on stock frames...

after market frames have the correct geometry designed into them with the Rack & pinion in mind

He means Ring and Pinion. For auto-x you want a combo if engine/trans/rear axle ratio/ and tire that allows you to spend most of the course in one gear. I have seen some people go faster leaving the car in 2nd even though it does not feel faster to them. So, it depends on the combo really.

AHHHH!!!!

Ring and Pinion...that depends on the final transmission ratio and tire diameter and engine max drive-able redline! on the current autocross tracks the larger SCCA tracks are predominantly 2nd gear tracks, the Goodguys tracks are quicker smaller 1st gear tracks....

what I shoot for is the ability to reach 70mph at edge of max redline(7000 rpm) in second gear

7000 RPM. . .WOW!!!

My pretty much stock Pontiac 400 is not gonna go anywhere near 7000 RPM!!!

When I drove my buddies EVO IX we stayed in second gear the entire time. . .same when he drove and when I rode passenger in a Solsitce and BRZ. . .

Gotcha. . .so I need to do some math based on final drive ratio in first and second gear, tire diameter, and engine RPM. . .my goal was to stay in one gear. . .can't wait to ditch the automatic and get a T56 or TR6060!!!

Thanks for the info. . .both of these posts were helpful!!!

Your welcome...

When I talked to the machine shop (specializes in Mopars) about the limitations of the bottom end and the oiling system on mine they told me as long as I stayed under 8500rpm I would be OK. :)

The camshaft and springs are only good for 7600rpm. I use a 7000 chip in the MSD and no worries. Feels weird bumping up against it...