915 Gearing Question

Jimmy18E · 08-31-2009, 07:42 PM

Guys,

Need advice on a 915 LTD Slip(magnesium case) tranny I have available to me for my build. It will be going on a Twin Plug, 10:5 to 1 built 2.5 in a 71 911.

The Tranny is completely just rebuilt with a 8:31 R&P, new LTD Slip as well as new 1-5 syncros. This is what I know/have about the tranny.
1st Gear 3.18 - 11/35
2nd " " 2.0 - 15/30
3rd " " 1.42 - 21/30
4th " " 1.08 - 25/27
5th " " .82 - 28/23

The print out says (at 7500 RPM) 1st 43.5 MPH, 2nd 69.1 MPH, 3rd 96.6 MPH, 4th 128.0 MPH and 5th 168.3PH.

Great mechanic and shop, trust the guy completely...... Want to know if this tranny is a little too tall in the gearing for this motor or will be a good choice??

Car is sub 2300 lbs and will be a weekend (some DE) back road car, 5th is taller for any highway travel.

Thanks for any input on the above tranny choice.

Jimmy

ix0ifan · 09-01-2009, 12:00 AM

What size tyres have been used in the calcs ? and do you have the rev drop data which will be a good guide to how it will drive.

I suspect the gap to 5th will be a bit on the large side.

Jimmy18E · 09-01-2009, 01:36 AM

IxOifan,

The info was based on 24 inch tires..... The rev drops were pointed out to me as good................

5th is tall because I was looking for a highway gear but will be in 1-4 99% of the time.

Based on the info provided, what do you think as far as you can tell.

I really appriciate the help.

Jimmy

ix0ifan · 09-01-2009, 09:40 AM

Seem to be as follows:

1st - 2nd 2780rpm

2nd - 3rd 2140 rpm

3rd - 4th 1800 rpm

4th - 5th 1800rpm

I guess the question must be how strongly it will pull from 4700rpm, which will depend on where the torque curve is on the proposed motor.

What cams are you planning as this may have some impact?

I think it does seem a bit tall for my liking and I would consider either a 7:31 or 8:35 Final Drive.

This would give:

1st 37 mph

2nd 60 mph with a 2420 rpm drop

3rd 83mph with an 1860 rpm drop

4th 111 mph with a 1560 rpm drop

5th 144 mph with a 1565 rpm drop

Cruising at 80mph would only be 4166 rpm

If you stick to the 8:31 cruising at 80mph will be 3600 which would be a bit like a modern 6 speed Turbo diesel and would probably need a down shift for any accel.

I would try to match the gear charts to the anticipated power curve if possible.

Eagledriver · 09-01-2009, 04:35 PM

First gear doesn't matter except to start an autocross. I like the tall 5th, there are very few tracks that need fifth gear. If it were me I'd try a taller 2nd and a shorter 4th. A 26/26 is a common 4th and will pull harder for your long straights. An 18/32 2nd will give you a nice pull out of slower corners and give a lower drop going into 3rd. Both of these are available stock gears, although the 18/32 is a little rare.

-Andy

Bill Verburg · Bill Verburg's Garage

here's what it looks like

For dual purpose it's ok, though, depending on usage a lower top could be used, for a pure track box it leaves a lot to be desired

Jimmy18E · 09-04-2009, 12:18 PM

Bill,

Thank you for working that info out, great stuff. I can go with a newly rebuilt stock 915/LTD slip/stock gear ratios with a 7:31 R&P if I would be better off? This car will be 95% of the time used only for fast weekend back road driving with other P car owners.

I was hopeful this tranny would be a good set up for my use because of the deal I can get on it due to a trade in the making.

Thanks Again,

Jimmy

2.70Racer · 09-04-2009, 03:50 PM

A 7:31 tranaxle is a better choice for a 2.5 engine for the use you describe.
It will be quicker on most dedicated road race tracks.
If you plan on parking lot autocrosses the 8:31 is better with it's higher first and second gear.
I cut 2 seconds off my track time with my 7:31.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Jimmy18E

Bill,

Thank you for working that info out, great stuff. I can go with a newly rebuilt stock 915/LTD slip/stock gear ratios with a 7:31 R&P if I would be better off? This car will be 95% of the time used only for fast weekend back road driving with other P car owners.

I was hopeful this tranny would be a good set up for my use because of the deal I can get on it due to a trade in the making.

Thanks Again,

Jimmy

You'll need to be more specific as which 915, there were ~10 or so w/ 7:31.

as to which is better, it depends. for a track trans ideally you will be maxed out in a gear at the braking point at the end of each straight. It's impossible to do w/ a single box that is used at multiple tracks so the next best is to make the fewest compromises that hurt.