Gear selection? Jack...or any tranny guys?

[k9handler]

Hello all,
I am going to be doing the ol' 3.6 swap next month and I must say that I don't want the 915 tranny to hold the car back.
Today I hit a long open (unlimited) section of the Autobahn and let her rip for a high speed run. The car accelerates very well in the power band and manages to stay in it through the shift and keeps pushing all the way to 6k rpm. But the car will hit 105 mph in 4th gear, while 5th takes the car on to the 130-135 mph range at 5500 rpm. I would like to get a little higher top end speed without giving up the bottom end tourque and the ability to stay in the power band through the shifts. What should I do?

Thanks

[Jack Olsen]

Put in a taller 5th, if you expect to be in Germany for a while.

This chart shows the different gears available, with the ones my tranny has indicated on the left, and stock (although I'm not sure for what year) on the right. As you can see, you could get close to 180, if you wanted.

[k9handler]

Thanks Jack...the chart is great!
It does raise some questions though. My plan is to have a very street worthy 911, and to have fun and compete at the local Autox. Now I know the 5th gear has nothing to do with the autox scene (1st - 3rd) is all I have seen used...but on the autobahn I want to be able to run with the big dogs! I notice that you have a lower 5th...is that for the larger tracks that you run at? In reguards to shift points and the chart, what points on the chart to you go by? I understand how the taller gear will give me the higher top speed...but will the 3.6 have tourque once in the higher range, or should I just stick with the stock gearing? Thanks.

[pbs911]

Don't forget the extra torque that will be placed on the tranny. If you are going to go to the trouble of switching a gear you might as well have a spray bar put in, a cooler and reinforced side plate. Of course, this is not based on my personal experiece, just what has sunk into my thick head from this board.

[k9handler]

Spray bar? Side plate? Ok...I am not aware of these uprades! The cooler I have seen and was not real sure of how they mount...is the connection already on the 915 tranny?

[Jack Olsen]

Stock gearing is by far the cheapest, especially if your 915 isn't in need of a rebuild already.

My transmission is optimized for track driving. 1st and 3rd are stock, but 2nd is bumped up and 4th and 5th are pushed down. The goal is to have as little space between the track-used gears (2nd-5th) as possible, so that the car can be kept in the best part of the powerband (5500-6500 rpm) between the speeds of 60-140, which are what most tracks involve.

(As a side note, I run slightly larger-diameter tires on the track, so those numbers in the chart are now on the slow side, for my car, by about 5 mph.)

I don't know at what point a 3.6 in a '75 911 would run into enough wind resistance that it wouldn't be able to reach redline. With my shorter gearing, I can hit redline in 5th without any problem. I've read that there's a kind of aerodynamic envelope at 180 mph that requires gobs more hp to push through. But the stock 964 listed a top speed of 161, so I would guess that you could do at least a little better than that.

(That's kind of scary fast, though.)

[k9handler]

that is scary fast...and I really would not expect to push that envelope, just kind of cool knowing the car could! I also have some suspension and bodywork to go as well, so the gearing may come somewhere down the road. Thanks, cant wait to see BB2 running through her gears!

[Eric Mckenna]

aahhhh! COme on Smitty! hahah The idea on running the ole gal up to
161 mph! HAHAHA what a hoot it would be scary due to the fact the isn't set up to travel at that speed! be careful BRO!

Eric

[arcsine]

As I understand it, aerodynamic drag increases with the square of speed. A 10 mph increase in speed means a 100 fold increase in aerodynamic drag. Lots more ponies are needed to go from 140 to 180 (1600 fold increase in drag).