Re-gear tansmission - 915?

[bornrich]

I'm considering regearing my transmission (915) in my 81 SC.
The suggestion by my tech is to change the ratios in 2nd, 3rd and 4th and leave 1st and 5th alone. The idea is to shorten the rpm drop between the first four gears and leave 5th for comfortable highway cruising. I need to rebuild my trany anyway so this is a consideration. I track my car about 4-6 time a year and use it a a semi daily driver.

Has anyone else done this mod?

I know the more popular change is to change the ring and pinion but that while added tothe low speed acceleration it will make cruising on the highway really buzzy.

[Bill Verburg]

Here is what you have now,



They are really not half bad for a stock transmission.

What is the reason you feel the need to regear?

[bornrich]

To get a little quicker acceleration in the lower gears with out the handicap of a buzzy 5th on the freeway. I want my cake and eat it too. :-)

[Bill Verburg]

oK here is one example I have seen used


Not something I would want but it does fit the bill for what you asked about.

[Jack Olsen]

Using 5th as an overdrive creates a bigger sacrifice than you might think. On most tracks, it's going to sock you in the gut just at the point where things are getting fun.

3-4-5 is where you want your gears tight on a track. Closing the gaps on 1-2-3 is more fun for stoplight launches. There really isn't a compromise solution. Stock gearing is probably the closest there is to a have-your-cake-and-eat-it gearbox -- and it's also by far the cheapest.

In my case, I put up with the high RPMs for freeway cruising and a modest top speed (136 mph). But my car is very well set up for west coast tracks, and a lot of fun between 36 and 136 mph. I have a tall 2nd, stock 3rd, short 4th and short 5th.

[Bill Verburg]

I agree w/ Jack 100%, here is what he is talking about.

[tsuter]

Its a nice exercise until you look at the cost of those new gears and what that adds to a stock rebuild.....but hey people are spending $25,000 on rebuilds we are now learning......and proud of it.

[bornrich]

I'm well aware of the costs involved. I just investigating what others may have done. The feedback from Jack and Bill is enlightening and well worth considering. I may still do the re-gearing but at least now I have more inforamtion to base my decision on. Thank you for sharing your experiences.

[bornrich]

One more thing, while I have done quite a few trackdays at Norcal tracks. I have not gotten to the point wher I've gotten to use 5th at any of the track.s So that had an effect on my inquiry. If I onlu use 2-3-4th at the track then 5th could be saved for the highway. Maybe I will get faster so I will be using 5th and the point will be moot.
My goal at trackdays are to have fun and get faster, I haven't reached the car limits by a long shot, but I have fun.

[rdane]

Originally posted by Jack Olsen
and a lot of fun between 36 and 136 mph. I have a tall 2nd, stock 3rd, short 4th and short 5th.

Mine is a street car for the most part with similar to what Jack has done. I used a short 5th for a couple of years and finally went back to the stock 5th which makes more sense for most of us.

But you have to realize we both have 3.6 power as well in lwt cars.

The long 2nd and short 3/4/5 was really fun with a 3.0 liter engine. Just loud in the cabin on the freeway.

[Wil Ferch]

There was an article in Pano a few years back.... I think it was titled " Designer Gear box"...or somesuch. Very close in principle to what is being proposed here. The Pano author ( ???? Alan Caldwell ????) said he'd gladly give up top speed for tighter spacing of all 5 gears...maybe he jinxed-in a tall 5th too, IIRC.......

I think the $ problem is that 1st gear is part of the mainshaft....no?....and as such is the first sterp to financial ruin. All the other gears are separately replacable, I gather. I don't know for certain as I haven't personally taken down g'boxes....

- Wil

[Jack Olsen]

Bill, thanks for those charts! Is it possible to do them with a 25.7 wheel diameter and a 6850 redline?

[gtu935]

I would recommend choosing a top speed in 5th that is liveable (usually a 1:1 5th grear ratio and work down from there. Look at that gear chart and then choose your compromises from there. Remember that you want the rpm rev drops to be less at the higher speeds. ie 4th and 5th as the above gear chart choices reflect.

[RLJ]

I have a friend Gordon Ledbetter. Worked at Garrretsons, for Gamroth and is a noted west coast 915 specialist. He built my tranny! I am lucky to have friends like Grodon.

His answer. 83 SC 915 tranny with factory LSD.

First gear: stock 11:35
Second: 17:34 down three
Third: 21:29 down two
Fourth: 25:26 down one
Fifth: stock 38:30

With a short strok 3.2 at two hundred ninty eight horsepower and 2350 # , alloy flywheel and clutch package on the "street" this set up "ROCKS" the car revs like an electric motor


Randy Jones
1971 911 "Iris"

[coloradoporsche]

I may be stating the obvious here, but re-gearing your transmission will likely bump you up a class if you ever decide to race the car. It may impact re-sale, too, for good or bad.

[rickdm]

When I bought my car the PO had put in a taller second gear with all the others stock. He did this mostly for his DE use. On the tracks I frequent (MSR in Fort Worth and TWS in College Station)where you spend 90% of your time in 2nd-3rd it is pretty nice. However on the new section at MSR they have added a slow hairpin leading into an uphill section and that tall second gear just does not give me enough grunt, Also on the street I really notice the 1st-2nd rev drop. However I think the gains in other parts of the track make up for those shortcomings. Not something I would undertake, but I am glad it is there.

rickdm

[Bill Verburg]

Jack, here you go. In the interest of better accuracy I used 811 revs/mi as being the better measure for 315/35 or 275/40 x17 tires. That works out to an average loaded dynamic radius of 24.9". That is the # I used in this chart because that is what the car sees. If you want I can redo w/ the static unloaded radius which you requested.

[Bill Verburg]

When thinking about a re-gear there are several different strategies and points to consider.
The task is considerably more difficult if you want a dual purpose vehicle.

you need to

• have a reasonable top speed, another way of saying you need a reasonable freeway gear.
• you don't want to have to shift in a corner or over/under rev thru a corner
• you want progressively closer gears w/ smaller drops as you go up thru the gears because of aero effects
• you want a reasonable first for clutch life if nothing else
• there are going to be compromises that have to be accepted

For most people most of the time stock gearing up thru the SC series was pretty decent.

The Carreras started getting a little long(for some purposes) and by the time 993 got here, US gearing was abysmal for sporting purposes(great for gas mileage though)

It is expensive but can be very rewarding in many cases, it all depends on the uses and compomises you can accept.

[Jack Olsen]

Cool. Thanks. That matches up with my GPS numbers.

[Bill Verburg]

Isn't math wonderfull