dynosoar1

Hey guys…..looking to add a little pep to the 83 SC and am considering pulling the transaxle and doing an LSD and swapping gears 2/3/4 for a close ratio setup. Current setup is totally stock and am wondering if anyone has done this and has recommendations and if the bang for the buck is worth it over the stock setup.

All input welcome 🙏

KevinGross

Sure, it's been done a bazillion times, given all the available ratios. You don't mention your application, assume it's just street, so things to consider:

• You want a nice progression of ratios.
• You should probably consider adjusting 5th gear as well, to keep it from feeling like a boat anchor but considering also highway cruising.

If you're driving street only, you'll probably never notice whether you have an LSD in the transmission and may wish to consider the value.

Sample gear chart below, one I did a few weeks ago for a customer. This is his "as-is" set-up and may not be where he ends up. Because his focus is race, we also have to think about terminal velocity, corner entry/exit RPM, that sort of thing. Cheers,

dynosoar1

Thanks…..I was playing around with gear charts last night and thinking based on my area (hilly) I would lean towards shorter gearing than what you listed above….toyed with numbers at a shift point of 5000, 5500 and 6600 to get an idea of speeds for each. Was thinking stock first gear (3.1818), 2.059 second, 1.348 third, 1.125 fourth and stock fifth gear (0.7894).

The LSD is preferable for some of the roads I drive…..could probably get a TBD but I may eventually get it out to the track a few times and would like to have it if so. I’d hate to regret not doing it and I think the little bit of push in the front the LSD may give when not at a track is liveable (let me know if you think or know differently).

The biggest thing I’m wondering is will these changes really wake up the feeling when driving the car….like really feel like a major difference? Bang for buck again. Transmission works fine in stock form now so would only do if it made it worlds better.

Bill Verburg

No idea what you have but for a street car stock 915/44 is pretty decent unless the engine has been heavily amped up



Here's a 3.2 Carrera modified w/ short 2 & 3


One of the best i've ever seen w/ a stockish 3.0, even better w/ a motor that revs to the low to mid 7s


A survey for most stock 915s,, rear tires make a big difference too, pretty cheap to mount some 225/45s on 8x16, or 245/45 on 9x16, 15s even better




here's a friend's, he left 5 long for cruise




This one was set up for WGI w/ a heavily modded motor, note the rpms

dynosoar1

This is what I was considering…



I did this last night with the thinking that for most of my driving I would be in 2 and 3 but would still feel an improved acceleration from stock?

Bill….. how does your friend like his set up? He has the 1.429 in 3 whereas I have the 1.38 in my thoughts.

Bill Verburg

JMO but a little taller second wound probably be better

here's another way of looking at it, this is the thrust curves for a stock US 3.2 Carrera stock and w/ shorter gears 2-4, blue is the short gears

Objectionable thing here 1st is too low giving that bug gap to 2nd, you raised 1 making that better at the cost of acceleration in 1 and w/ stock gearing the tail of a lower gears crossed the next gear curve, this is throwing away acceleration

The shorter blue curves just kiss the next curve in 3 & 4, that's what you want

Spyerx

This is what we did on mine. 911/01 box Revs to 8k. Speeds are based on the cn36 tire (Avon is similar) I don’t find the ratios too short and it puts 2-4 in the canyon fun zone with the race pattern 901. Shortened up the stack and tightened the ratios just a bit.

lit really fits the profile of my motor, very happy.

dynosoar1

so keeping first gear stock…..means the gap between 1st and 2nd can only be closed by going with something in the 2.00 range or more correct? But you say err on the the taller side?

Bill Verburg

You will never be able to fully close the 1 2 gap w/ a useful 5 speed

For track use we often raise 1 a little to make it more useful on pit road and the occasional really tight corner, for instance my 993 w/ a Cup/RSR trans has a 3.154 1 compared to a street 3.818

The taller a gear set is the wider the useful range of speeds at the cost of some acceleration,

In 915s 1 is a very expensive gear to change because you need to buy a whole new main shaft

what ever 1 you use you are going to look for ~30+/-mph range in 2-5 tending to a little less as the gears get higher for a street trans and ~25+/- tending to a little less in higher gears for a track trans

get much smaller and the gears become increasingly narrow in useful range unless the rev limit is correspondingly raised to counter the narrowing useful range

here are 2 examples on 2-5 short gearing, the first is a track only box the second is a track oriented box that could be used on the street though cruise is heavily degraded



and lastly what the factory did for a dual purpose trans compared to a stock street trans
top is street
next is std SC/RS
3rd is Rally/track SC/RS

Xxyion

oops wrong thread

TheTorch

Bill do you share that spreadsheet or can I get you to run my ratios on it? I have close 2,3,4 and while I think I can reverse engineer the sheet, would save me time... thx

1975 915/40:

1st: 3.18 (11/35) -- stock i believe ?
2nd: 2.19 (16/35)
3rd: 1.58 (19/30)
4th: 1.17 (24/28)
5th: 0.82 (28/23) -- stock ?

Bill Verburg

24.5" OD rear tires, 6800rpm shift points

TheTorch

Bill,
Thanks so much. If I'm reading it right, it's darn close to you second track setup, and explains why I definitely spend more effort shifting among 2,3,4 relative to my friends 915 stock ratio.
If I ever go back into the box I think I might be happier with a more street friendly 2,3,4...

Thoughts welcome.