Advice Needed: New Transmission or Plus Sized Wheels for Autox?

[rennch]

Hi,

I'm a competitive autocrosser, and I've got a bit of a dilemma. I love how my car (A 69 Resto-mod RSR with a Euro 3.0, and built suspension) handles on 15" wheels. The problem is, my tranny is from a 72, so I have the 7:3:1 R & P. It's great, and torquey around town, but the gearing is too short for autocross. I find myself in no man's land between 2nd and 3rd every single time I run.

So...my options are, swap my tranny for a taller geared version, or go with 16 or 17" wheels for racing. As I said, it handles brilliantly on 15's, so I hate to go that route, but if Im out of options, I will. Thoughts?

[Flieger]

Go fast enough that 3rd is the right gear, or build the motor to spin faster in second.

[rennch]

Thanks. Anything else you want to add to that?

[Flieger]

I'd go with the 16" wheels if it were me. But the transmission rebuild is the better option from a performance point of view.

[snbush67]

Taller tire on the 15's?

[tobluforu]

See if you can find a 75 with an 8:31, I swapped my 72 tranny for that and never looked back. Love it! You can always switch back to the 72 if you don't like it. I got lucky and found one that was pretty cheap. Good 16 or 17 inch wheels and tires will cost about the same or more than a good used 75 tranny.
Cool car by the way.

[rfloz]

Depends where you are running.

With an 8:31 rear gear and 315/17 rear tires I still hit the stock rev limiter in second gear (65ish mph indicated) at some OCR events (El Toro). San Diego, IIRC (it's been a while), same deal. At Streets of Willow set up and an auto-cross, I hit the rev limiter in 3rd!

So, taller tires alone may not be enough for the larger venues.

[rennch]

That's pretty fast for an autocross Bob! But thanks for the insight!

[Cory M]

can you swap just the ring and pinion?

[rennch]

I'm not sure...transmissions are scary to me.

[Cory M]

I don't know the specifics of the 72 trans, but I know people switch the 8:31 out for the 7:31 to get lower gearing, the "poor man's" regear. I know there are different options depending on mechanical or electric speedo. There is some good info on Pelican if you search. The 7:31 r&p is more rare than the 8:31 so you could probably buy an 8:31 swap it out and cover the parts and some of the install costs with the sale of the 7:31. I was going to swap a 76 trans I had but changed my mind and sold the 7:31 gear (it sold in 1 day).

[Dave at Pelican Parts]

What about tire availability? It sounds like good tires in 15 are becoming harder to find. If you go to 16s or 17s, you might wind up with good choices for longer...

--DD

[rennch]

Dave...all good points. It's just that the car drives SO MUCH better on 15's. There are R Compound tires that I can use as well. Maybe I'll grab a set of 17's from someone and see if they are any faster the next time I run.

[Matt Monson]

Cory,
The 8:31 with allowance for mechanical speedo was a one year only US market thing. It's actually more rare than a 7:31.

[Cory M]

Thanks Matt. I knew they were rare. The one I had wasn't for the mechanical speedo and it was still pretty expensive. So would a later 8:31 fit into the 72 trans?

[Matt Monson]

It will fit but either he loses his speedo drive or he needs to equip it to run an electronic speedo. The other problem becomes his differential. The '72 uses the same differential as a 914. The ring gear offset is wrong for the 8:31. The easiest solution then becomes to find a mid-year (75-77) course spline differential.

At the end of the day it goes out of the shade tree mechanic realm because you're dealing with setting pinion depth and ring gear backlash. The hard stuff that requires $3000 worth of special factory tools to do. Which is why most guys just swap a whole gearbox in.

[Cory M]

thanks for the explanation, seems like a later 915 or a set of wheels would be a lot easier in this case

[rennch]

I'd like to do an A/B test with some 17's while I'm racing to see which ones are faster.

[Patrice911]

Many years ago I was having the same dilemma that you are having. My car is a 73 with a 3.2 Motronic.
Having to go too often in 3rd gear at the AX and top speed limited by the rev limiter on the race track.
Since I wanted to stay on 15” wheel. I change the complete transmission to a newer 915 with 8:31 from the 84-86 era
with the cutout for the Motronic sensor. I had to change the speedo for an electric one, the clutch cable and something with the half shaft.
I sold the original transmission with the 7:31 for almost the same price that I paid for the newer one.
It did exactly what I wanted, less shifting in 3rd at the AX and higher top speed on the race track.
Since last year I don’t do much with the 911 anymore, only street driving. I’m sharing a Miata for track event with my 19 years old son and I started to used the Abarth in AX.
Both car a completely different than the 911 but they are both a lot of fun to drive.

[rennch]

Exactly what I needed Patrice...thanks man!