Best Gearbox selection? Gurus please advise!

Well,

I've bitten down and decided to buy a 5-speed box for my car after the untimely demise of the 4-speed manual.

What would be the best choice of box for my 1976 911S Coupe? (175bhp, 2288lbs - approx). I am after a car with stronger performance, but not too concerned with fuel economy or limiting the top speed. It currently runs @ about 2,700 rpm at 100 kmh (58 mph)in fourth, but I haven't done the math and don't know what kind of diff ratio I've got. I guess that gets changed along with the box...

I can get my hands on a good G50 5-speed from an 89 Carerra, or should I go for a 915 from an SC or something? What would be best? I know that the G50s are much smoother boxes. It probably isn't a big deal to install but I'm after a 5-speed for sure.

What the heck are "Airport" gears? If they're good, I'd be keen to get them too!


Any help is much appreciated!

Adam



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Adam Chaplin
adamchaplin@optushome.com.au
1976 911S Coupe

Adam,

I'm quite certain that I have discussed or recommended airport gears more than a dozen times over the last 16 months or so, but only 5 threads show up on a current search!

In any case, the explanation in the following thread should give you a good start on the topic:

http://www.pelicanparts.com/ultimate/Forum3/HTML/007403.html

A G-50 conversion is rather expensive, and an SC 915 is not geared any differently than your car, overall! A '72-'73 915 transaxle offers the best gearing to give a boost in acceleration!

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Warren Hall
1973 911S Targa

Thanks for the input, Warren!

I'm reading thru a few old posts and I've found some info on the G50, but I'm probably looking more for the budget upgrade. I'd entertain the 915/12 if I could find out some more info on it.

I'm cooking up a gear/diff/tyre ratio sheet for Excel that works out curves, road speeds @ revs etc.

This sounds strange, but I've got the service book stored in the car and that's not stored at home at the moment (I'm keeping it at a relative's place while I can use their garage!).

I don't have a clue what kind of final drive I've got or any of the existing 915 4-speed ratios. Is there a website that will tell me where to get 915/12 & 915 (four speed) RoW ratios??

This info would really help. I would love to compare it to the "Airport" gearset that you mentioned in the above link! It looks good so far on my spreadsheet, (even with it's wildly guessed diff ratio).

Thanks again

Adam,

You probably have a 915/49 trans with 8:31 (3.875:1) ring & pinion, AZ-11:35 (3.182) 1st gear, KW-20:32 (1.6) 2nd gear, PR-25:27 (1.08) 3rd gear, and SN-28:23 (0.821) 4th gear.

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Warren Hall
1973 911S Targa

Warren..

A friend who previously owned the car told me that his mechanic offered to "add" another gear to the existing gearbox. Am I able to simply remove the box, tear it apart and rebuild, but change a few gears over upping the number by one?

I certainly hope so! I think this sounds eminently feasable, given that they're both "915" boxes. I don't want the extra cost involved with going to G50, and the 915 will easily cope with my lowly power output.

Please tell me this is possible!

Adam