pd'83SC

I am in the planning stages of doing a 3.6 conversion on my '83 SC and I am going to have the 915 gearbox "rebuilt" as well. I would like to take full advantage of the healthy increase in torque of the 3.6 over the 3.0, so I am contemplating gearing changes and the installation of a LSD while the gearbox is apart. Any experiences and opinions on this topic are welcome. Thanks!

PD

Bill Verburg

As with everything else, the answer is, it depends. If track use is the sole or primary use then go with a 7:31 fd and gear internally for the tracks you will be on the most. Keep in mind that the 7:31 has a rep for being weaker than the more usual 8:31(When mine was rebuilt I turned down the 7:31 for that reason) Also keep in mind the rear tires that you will use. Most of us seem to be using 255/40x17 thru 275/40x17, all are taller than the more normal 245/45x16 and provide correspondingly taller final drive ratios. For mostly interstate driving I am very pleased with 275/40x17, 8:31 and a tallish 1.31 overdriven 5th. The 3.6 is very torquey in its standard state and can pull the higher gearing in the lighter chassis easily.

Others, I'm sure Jack will Jump in here, are inbetween the 2 cited extremes. With an 8:31 and lowered internal ratios particularly 2,3,4 in combination with the 275/40x17 rear tires.

pd'83SC

thanks Bill....I am planning to keep the 8:31, and car will be about 75% road, 25% track. Rear tires will most likely be 255/45-16.

JackOlsen

If it's mostly a street car, I'd suggest getting the limited slip and leaving the gearing alone. Changing to new gears is very expensive.

A very worthwhile upgrade, if you're going to be tracking the car, is to get a cooler installed. I put a temp gauge on my 7:31 box (before it died), and the tranny would sometimes run hotter than the engine (as much as 260F). (Making things worse, gear oil brakes down at a significantly lower temperature than engine oil.) In street driving, I've never seen the tranny get very hot. But on the track, I think this is a must, especially for pre G50 boxes.

<a href="http://forums.pelicanparts.com/showthread.php?s=&threadid=66461" target="_blank">Here's a link</a> to some of the details on what I did when I switched to an 8:31 915. Like Bill said, I'm running 275/40 tires on 10x17 rear wheels. With my current gearing, redline in 5th is 141 mph (135 mph on 245x45/16's). This puts a lot of that 3.6 torque right where I can use it.

JackOlsen

Oh, and also look into Seine System's gate shift kit. It's half the price of Robotek's, and a better design, to boot. Wouldn't want to ruin that new powerplant with a blown shift.

You're also going to need better oil cooling for the new motor.

Gratuitous action shot:

pd'83SC

...thanks for the input Jack. After plotting the stock gearing on the gear chart for an 8:31 ring & pinion, and using the peak torque and horsepower ratings for the 3.6, the stock gears appear right in the ballpark, except maybe a wee bit taller first (14:37 instead of 11:35) might be more useful and will give less rpm drop at the change (1800 rpm vs. 2600 rpm). I realize that this is the most expensive gearset to change and that plotting on paper is not "real world". Also, do you think the taller first would have adverse effects on clutch wear?
...thanks again guys!
PD

JackOlsen

I run a stock first on an 8:31. And to be honest, I don't notice a significant difference from the stock first on the old (shorter) 7:31. First gear is overpowered anyway -- and launches from a stop are a very expensive (and disappointing) hobby with any 911.

<a href="http://members.rennlist.com/jackolsen/tranny.mov" target="_blank">2-3-4-5</a> is there the fun is, IMO.

What are you using the car for?

pd'83SC

...car will be mostly a road car, but if I finish it before I am too old, I will do the occasional DE event (Carolina, Road Atlanta, Roebling). I have removed all "non-essentials" from the car and plan to go the "lightweight" route..RUF-type bumpers (B&B cooler up front), RS tail, 7/9 x 16 Fuchs...

JackOlsen

Then I'd say a stock 8:31 box is the smartest way to go. The cost of fine tuning the gearing is pretty high, especially when the factory has already done it. By adding the 3.6, you're correcting for the way they made the gearing taller than ideal in order to meet Swiss noise regulations.

Gearing for a specific type of track (in my case the 2-3 mile road-course tracks on the west coast) means getting the ratios as close as possible for the actual speeds you'll be traveling. My redline in fifth, with 275/40 tires, is 141 mph (with a stock fifth and those tires the figure jumps to 165). This makes sense for what I use it for, but you might want the bragging rights that a stock fifth would give you. Also, if you're looking to impress the world a little between stoplight, my ratios are not the best way.

pd'83SC

...thanks again Jack. I'll post results when I am up and running.

PD