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915 Gearing changes

I am considering changing the gear ratios in my 915 this winter. I have a hot rod 3.0 that makes most of its power from 4,500 rpm to about 7,500 rpm. I would like to get the gears closer together so it doesn't fall off the pipe, so to speak. Who has done this and what ratios did you use?

Lindy
Old 10-22-2010, 06:02 AM
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Do a search for Bill Verburg and you'll find some very nice gear charting. Others (Jack Olsen comes to mind) with short gears have chimed in with their versions as well. Check out Jack's videos and you can see how his gears keep him on full boil when he's at the Willow big track.

You need to give us a bit more info on yours:

Ring & pinion?
Tire diameter?

For reference I have short gears in my '79 racecar with a '84 915. Stock 8:31 R/P, 275/40-17 rear tires.

1st = stock 3.18
2nd = long 1.68
3rd = shorter 1.33
4th = 1.125
5th = shorter 1:1




I think the choice made for my gears was a fair compromise (I didn't select them, previous owner had them installed) for track use. Longer 2nd lets me use 2nd for medium-slow tight corners and not run out of gear. In really tight corners a shorter 2nd would be better, as I experienced with my '87 G50. The tight 3-4-5 spacing is nice for longer corners as I stay within the powerband pretty well.

Still there's always going to be a compromise somewhere, as no gearset is going to be good for every track. That's why some people have multiple gearboxes spare gears, to set up their trans for each track. Makes a big difference IMO.

I can really see the benefit of my semi-short gearbox as compared to my stock G50. I used to short shift 2-3 some places and also be in 3rd thru some corners because 2nd was too short to carry the whole turn. Now my gears are better balanced for the tracks I run and I don't find myself griping about gear selection much at all anymore.
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Last edited by KTL; 10-22-2010 at 07:18 AM..
Old 10-22-2010, 07:10 AM
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I have a stock '78 915 with stock R&P. I'm running the same on the rear as you, 275/40/17.

Thanks for the reply.

Lindy
Old 10-22-2010, 07:20 AM
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Is this a street car or a track car?
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Old 10-22-2010, 08:55 AM
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Old 10-22-2010, 10:01 AM
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One consideration when regearing a street car is highway driving. If you look at the example of KTL's ratios, what he runs for a 5th gear in his track car is the stock 4th gear ratio in many street cars. Running that 5th gear at 75mph on the highway for an hour or two during a Sunday drive might not be particularly pleasant for you, and it will definitely kill your gas mileage.

The other thing to look at is cost versus what sort of retuns you see for the changes. These days, new 915 gears are $725 or so. If you get 4 new gears, that's as much as a full rebuild usually costs. Good used gears in the special ratios are becoming harder to find and seem to get more expenisve every day. An old racer's trick is to find the 3rd gear out of a 4spd box and use it as 4th in the 5spd 915. That's the ratio KTL runs as his 4th.

Beyond that, you need to figure out where you want to see the acceleration gains if you are budget limited. For some guys, they want to improve their 0-60 times or 0-90 times and will just go with changing 2 and 3 and leave 4th and 5th unchanged. Or you can do it rally style and put the stock 3rd in 4th, but leave 5th stock with a huge RPM drop and next to no accleration when you use it, but you've still got a highway gear.

Another approach if you like barn storming and canyon carving is to go with a taller 2nd gear (like in KTL's box), as well as a slightly taller 3rd, making the whole stack, once you leave 1st, tighter.

What I am getting at is there's about twenty different ways to spin this thing. A little better idea of what you are doing with the car and what you are interested in will help people to make it more specific to you. If you tell me you're canyon carving my advice will be very different than in you tell me you are taking the car to the Friday night 1/4 mile event versus if you're just gonna do some DE/trackday playing with it. And knowing how much you are willing to spend to make it happen helps. Working around stock ratios on a budget is a bit different than just going all new and having all the available ratios to work with.
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Old 10-22-2010, 10:18 AM
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For all I know Matt, you may have made the recommendation for my gear set. The car's receipt folder came with a gear chart from Carquip!
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'79 911SC-based 3.0L 930S/934/965/911R looking thing NASA GTS-3
Enginerded by Red Snail Racing, MotoDelta Motorsports, G2 Performance, Elbowmotions
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Old 10-22-2010, 10:58 AM
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Thanks Matt and Kevin, the information really helps. My car is a street racer if I can use that term here without getting blasted. I will use it for DE, Auto-X and blasting around out in the country (no canyons here). I would like a tighter box that doesn't make such a big jump between 2, 3 and 4. I am inclined to leave 5 alone as I would like to be able to drive down the highway without spinning a main bearing. First also seems to be OK at the moment. 0-60 is important but not the ultimate goal. I'm looking for a compromise that's a little better than what I have.

I won't pay $725 a set for gears. I'll look at the forum and wait until I see a reasonable deal in the classifieds; now there is a pretty good deal on stock gear sets for $200 a set. Thanks again for your reply, it's a huge help.

Lindy
Old 10-22-2010, 12:07 PM
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The factory did a great job when they built the SCRS

here is a workup of an SCRS stock trans w/ 7500rpm redline and 275/40 x17 tires


and here is the optional SCRS trans, again w/ 7500rpm redline and 275/40 x17 tires


The stock one would make for a nice dual use trans, that's what I have in my '76C3

the optional is a track or rally only trans.

the primary issue w/ going short is that you will need to shift a lot more, sometimes in an awkward spot, to avoid that I sometimes have to short shift to the next gear which negates the short box advantages
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Old 10-22-2010, 12:14 PM
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Bill,

Thanks for the info; glad to see another sailor among us all. I sail a Laser in the Masters class around the country when not tinkering with the 911. With the SCRS, how much difference is there between that gear stack and stock? To be clear, how much different is the car to drive with the SCRS box?

Lindy
Old 10-22-2010, 12:30 PM
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Quote:
Originally Posted by lindy 911 View Post
Bill,

Thanks for the info; glad to see another sailor among us all. I sail a Laser in the Masters class around the country when not tinkering with the 911. With the SCRS, how much difference is there between that gear stack and stock? To be clear, how much different is the car to drive with the SCRS box?

Lindy
what transmission do you have now?
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Old 10-22-2010, 12:54 PM
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915 from a '78 SC
Old 10-22-2010, 01:19 PM
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Here's what you have now


the difference is that acceleration is better w/ shorter gears but you need to shift more, shorter gearing thrives on revs, having a 7500rpm redline is good, and will go a ways to reduce the amount of shifting that would be necessary w/ less available revs
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Old 10-22-2010, 01:32 PM
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915 gear set

I have a 911 track car with an 84 915 and a 3.6. I shift around 6700 and the rpm drop is about 1000 to 1200 rpm. My first go around had a shorter third. Later I decided to go to a stock third and a taller 2nd. I like this much better. Here is my current setup (had the tranny apart today)

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Old 10-22-2010, 02:14 PM
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