R&P Ratio for Auto-X???

[atomicjoe23]

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!

[dhutton]

Originally Posted by atomicjoe23

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

[atomicjoe23]

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
".

[Rod P]

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

[68Cuda]

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.

[Rod P]

Originally Posted by 68Cuda

He means Ring and Pinion.

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

[atomicjoe23]

Originally Posted by Rod P

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. . .

Originally Posted by 68Cuda

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.

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!!!

[68Cuda]

Originally Posted by atomicjoe23

7000 RPM. . .WOW!!!

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

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...