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

[rennch]

@patrice, what was your wheel and tire setup for Autox?

[safe]

I have a 4-speed 915 with 8:31. I really like the 2nd gear, great in auto-x, it has about a 15 mph advantage to a 5 speed 7:31 at 6000 rpm.
Unfortunately 2nd, 3rd and 4th are a bit to far apart... On a quick course I would like a lower 3rd.

[Patrice911]

My car is RS style, stock 73 fender in the front and Carrera flare in the back. Wheel are Fuchs 7” front and 8” rear. Tire are 225/50 or 45 (depending of the tire brand used) /15 on all 4 corner.
Same tire for AX or track.

[tobluforu]

Originally Posted by 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.

I guess I got real lucky, I posted a WTB and had one in a month.

[Turbodon1]

Don't know if you're interested but I have a 915 close ratio (all five gears custom) with a tall first, gateshift, Quaiffe, WEVO reinforcements, PHII side cover, new carrier bearings. New dogs/synchros on third and fourth. 7:31 rp etc. etc. Ran only ten minutes before old 2.7 gave up.
I just installed a 930 box to handle more hp from a new engine and removed this box rather than damaging it.

[trekkor]

Install a shorter 3rd gear set and run the 15's.

[Cajundaddy]

I'd go with 17s for the better tire selection and run 225/245 to good effect.

[rennch]

I can run 245/315 if I want to. @turbodon...what are you asking for it?

[Turbodon1]

Correction, second gear is stock. 1st -5th ratios are respectively: 16:35, 19:32, 22:29, 25:27, 26:25. First gear was a little tall as I run 27.5" tall rears. For typical 25" dia. tires this should be perfect. Also, the bellhousing was machined by WEVO to upgrade the guide tube. It has almost every upgrade available. My dilemma is that I have over $10k invested in parts alone. It is true that brakes and tires are cheaper than gears. A note on changing any single gear; when the box is apart the job always seems to become something like "well, while we're here you might as well change..." Then $$$ later you find yourself asking whether anything was improved. I don't have a price yet but I would guess that having this box duplicated would cost even more.

[trekkor]

I don't agree with that.

I blew up my third gear.
We torn the trans down, swapped in a new gear set and buttoned it up in an afternoon.
Didn't cost anything except the gasket/seal set from our host.

Find a short third gear and handle this.

[trekkor]

Sounds like you have a 901 gear box, right?

[Turbodon1]

I'm not a computer guy but if you go to "Instant-G.com" and enter tire sizes with my ratios for the gears you will see how it looks. But yes, it is a 915 box. First gear is integral with the mainshaft. Unlike the 901 it can't be changed unless you machine off the old gear and weld on a new one. Then re-heat treat the whole shaft.

[Cory M]

Originally Posted by Turbodon1

I'm not a computer guy but if you go to "Instant-G.com" and enter tire sizes with my ratios for the gears you will see how it looks. But yes, it is a 915 box. First gear is integral with the mainshaft. Unlike the 901 it can't be changed unless you machine off the old gear and weld on a new one. Then re-heat treat the whole shaft.

there are aftermarket splined shafts that allow you to use different 1st gears, not cheap but machining and welding isn't the only option anymore

[Turbodon1]

"If I had known then what I know now life would have been easier" Thanks for the info.

[trekkor]

Sounds like you need a short third gear, right?

[Turbodon1]

If emptyo installs a short third gear he will be left with a car with a huge gap between third and fourth

[trekkor]

Not really a big deal unless he tracks it, too.

Our cars are fast enough in 2nd gear you could skip third altogether and shift into 4th at 55mph.

[Turbodon1]

These are the ratios. Tire size is for example. I would think these ratios would work well for autocross where torque is as important as horsepower.

[trekkor]

Do you have a link to that work sheet?

My gear set in my 901 is a -C-F-Ka-S-X. 7k rev limit.
( Ka is non existing stock gear, as it's a flipped ZD 5th installed as a short 3rd. 22:31 )

[Turbodon1]

enter "Instant-G.com" and scroll down for the charts. I had trouble opening the document. Once opened you can plot different ratios. It's fun to try different numbers and see what they result in. I found out that a RUF 930 five speed with my tall tires would top out at 220 mph. Thats way faster than I, or the car, would go. I saw a post with a reference to late actor Paul Newman; something like "driving 180 is not as fun as taking a 30 mph corner at 29.99mph"