Denis54

Can anyone tell me what ratios can be used for 2nd, 3rd and 4th gear in a SC 915 transmission to be used exclusively on the street?

Also I would appreciate if I could get a rough estimation of what each one of them cost.

Steve Weiner-Rennsport Systems

Hi Denis54:

There are quite a few ratios that could be used here. Which one you want depend on what size rear tires you have, whether your engine is stock or not, and how fast you wish to go. You really should never leave a big RPM gap between 4th and 5th.

Custom gears usually run around $ 525/each.

Bill Verburg

Why would you want to change street ratios?

Denis54

I asked the question because in some previous posts, Steve has said:

"Hands-down, the best thing you can do to increase the acceleration of the car is to install a close-ratio gearset. Nothing you can practically do even comes close to what close-ratio gears do. Its like adding 50 HP to the car"

My car has 225-50-16 tires. For the time being, it is stock but I am looking at various upgrade possibilities. I will likely add SSI with 964 cams. I doubt I will do anything to P&C. Top speed is not a priority. I use the car for short fun trips that rarely last more than 30-45 minutes

Bill Verburg

For a race car yes, by all means change the gearing, for a car thats mostly A/Xed and DE yes change the gears, for a car that isn't driven on freeways much ok.

What happens is the revs get a bit buzzy w/ a track short stack of gears.

You would feel an immediate improvement by just using 245/45x16 tires which are a bit shorter than what you now have.

Did you see this thread
and this diagram comparing a short and std gearbox


red is the short box blue is the stock US Carrera
*note that the short box raised second
*both have thw same 3rd
*as a result the short box has 2,3,4,5 much closer together so that fine distinctions of gearing on a track can be made, you will never be far from the ratio you need/want
*67mph w/ short gears &245/45x16 is ~3500rpm, and w/ stock ~3000rpm

Steve Weiner-Rennsport Systems

Hi Bill:

LOL,..For sure, cloase-ratio gears do the very same things for street cars, as they do for race cars. Only the ratios will be different,...

The stock gears in all of these cars since 1974 has been selected to meet the Federally-mandated CAFE (corporate average fuel economy) standards and this has seriously compromised overall acceleration. As long as one is willing to forego the need to travel at 150+ MPH, one can change the 2nd through 5th gear ratios to achieve much improved acceleration and this applies as much to a 2.0 T as a 3.8 litre RS engine.

Race cars usually get a "full active" transmission where 1st gear is raised and incorporated into the gearstack to make it a useful ratio on the track, and not just for leaving the pit area.

FWIW,....just the act of installing a good close-ratio gearstack into a stock Carrera, 964 or 993 makes a HUGE difference!

Bill Verburg

As I said as long as you don't mind the extra revs on the highway it's fine.

My car does not need any lower gearing even in double o/d 5th with 275/40s, 1st can be like being on ice. Now if I could only cram a 6th in there....It would be nice to have the middle gears closer together.

Steve Weiner-Rennsport Systems

One can usually find a good compromise,.....

I just spoke with a person this morning with a '71 911 that had a '95 993 motor and 6-speed G/50 in there; street & track car.

Where there's a will, there is a way,....

Denis54

Steve, would the following made sense for a car used for short drives like I described?

1st: 11-35 (stock) or a ratio of: 3.18
2nd: 15-30 or a ratio of: 2.00
3rd: 21-31 or a ratio of: 1.48
4th:24-27 or a ratio of: 1.13
5th: 28-23 (stock) or a ratio of: 0.82

I think would not mind the larger gap between 4th and 5th as I normally use 1st through 4th for fun drives and 5th as a highway gear when I travel on the highway.

Steve Weiner-Rennsport Systems

Hi Denis:

That looks OK to me but I'd likely use the 28/24 5th.

Remember, air resistance increase with the square of the speed and it requires more force to accelerate, the faster you go. With that in mind, I do prefer to keep the taller gears closer together and this make passing easier, too.

Having a jump from 4th to 5th (and vice-versa) can make it challenging to pass when the RPM change is wide,.....

Bill Verburg

Here's what it looks like w/ the gears Steve suggested

Bill Verburg

Steve, Here's what my car looks like with a 993 G50/31 6spd


biggest diff seems to be that my 3rd splits the differnce between the 6spds 3rd & 4th

I can close up the difference by going to a shorter rear tire, but that makes 1st gear worse.

You'll excuse my enthusiasim here, but I find this stuff fascinating.

blue is 915/67 '84 euro 915
red is G50/31 '95 993RS

Steve Weiner-Rennsport Systems

Hi Bill:


Lookin' good!

I too, don't think shorter tires (or shorter R&P's) are a viable solution in the quest for more performance,....

Your enthusiasm for this subject is well founded and deserved,.....I use a DOS-based software package made by Bill Mitchell that works VERY well that shows all splits and graphs/overlays anything you want.

chavez45

Reference

Hey Fellas,
Let me add anther quick question. This is my first 911. I bought with a lot of mods. Paxton S/C, Sport Exhaust, double H custom gated shifter, ??? closer gears, I don't know all the ratios. I haven't logged through them to see. I am pretty sure I am running a 7:31 ring/pin with 28:23 5th. I love the acceleration playing on the street but I also commute 60 miles interstate one way daily. Averaging about 660 miles a week. I need better fuel mileage since I am not getting but 16-17. I was looking at a 29:21 or 29:22 to drop the Rev's and saves some gas. I an usually able to run 90 at 4300 rpm all the way down the interstate.