Gearing selection questions...

Jeff Alton · 05-14-2006, 08:10 PM

As many of you know my car in too many pieces to count. The motor is being rebuilt to 3.4 with big HP (hoping close to 285-300hp) I have a stock 915 (84) with LSD and Wevo side cover.

My question is......

I am considering putting short gears in this car. Should I do it now while the trans is partialy apart (for R/P setup)?

Should I wait until I have a dyno plot of the motor? Do it now or pull it again later?

If I do it now, how can I figure out what gears to put in it. (2, 3, 4, adn 5)?

Cheers

jluetjen · jluetjen's Garage

You can do the gears before you have your torque curve mapped out. If your engine is going to have a wide flat torque curve, you most likely will not gain a whole lot by a close ratio box. But if your engine is going to have a strong torque curve over a relatively narrow engine speed range, you will want to pick your gear so that you stay within the desired range. This is where having the torque curve mapped out will help you.

Just figure out what you want your top speed to be, what first gear your going to be using and then space out the intermediate gears in between. Ideally you want the gears to get closer (smaller drop in rev's when you shift) as you get into the higher gears. If this is for track racing, you can go a step further figure out what your speeds will be for the key corners and pick your gears to ensure that you are not shifting mid-corner.

klaucke · 05-15-2006, 07:55 AM

I would talk to rdane if you are looking at gearing choices, he has a short box 915 in his car w/ a 3.6 Vario. Also, Bill V. has posted gearing charts before that might be helpful, maybe he will chime in. Like John said, knowing your torque curve is obviously key in gearing choices.

This thread might be helpful:
3.6 transplant gear box selection?

Jeff Alton · 05-15-2006, 11:44 AM

Thanks, I may just re gear it in the winter. I guess I would be better off seeing the torque curve before hand.

Cheers

rdane · rdane's Garage

I've had similar boxes in my 3.4 and tried 3 different gear sets in the 3.6 before I was happy. Currently what I like the best is a longer 2nd, short 3 and 4 with a stock 5th for the freeway. Same box with a short 5th (equals the stock 4th) was really fun at Pacific raceways. But I was 4K at 80 mph in 5th. Too rough on long drives.

The stock box really takes alot away from these engines IMO. That said I won my class at Bogus with a stock '81 box....but I wasn't liking it

High hp, peaky 3.4 would be a hoot with a short gear stack and a 130/135 top end on 17" tires.

Bill Verburg · Bill Verburg's Garage

Assuming a stock US Carrera gearbox, here is what you have now. As the others mentioned the torque curve and gearbox should match, but generally keeping the revs between say 4800 and 6800 for a streetable 911 is going to be pretty good. A sportier gearbox will have increasingly small drops as you go up through the gears.