915 short gearset suggestions for 2.7RS

[obrut]

hi all

i'm planning on rebuilding the 915 in my 2.7RS replica shortly with some shorter gears. before i lose 10 hours in gearbox spreadsheeting i thought i would ask what people have tried (and liked) here.

depending on the event, the car weighs around 2200lbs, has 230/240 flywheel hp, revs to 7500 and uses 225/50/15 rear tyres.

i'm working back from a top speed of about 130mph.

any suggestions?

ps. is there any factory short gearsets (eg. equivalent to 901 airport gears etc) available for 915's?

[pu911rsr]

Ryan,
Go to carquip's web site they have a nice gear calculator. Also a good gear chart in back of Anderson's book. You can occasionally find some pretty good deals on short boxes, the gears are pricey so they are expensive to build.
Phil

[obrut]

thanks phil

checking out BA's book is a good idea. i've got a few calculator/spreadsheets already, i was just trying to shortcut the selection process.

[Chuck Miller]

Ryan,

This probably doesn't help but it does include the factory part numbers...

Unlike the 'early' Sport Purpose Manual, I found no application recommendations for the 915 gear sets...


This out of the 'later' Sport Purpose Manual...

Cheers mate,

[Alex Webster]

Ryan, the spreadsheet I gave you gives you all the available gears, and Albins in Ballarat have all the gears that you want for about $500 a set.

[Eric911S]

Hello: What 915 gearbox do you have? If it is a later 915 I would change the ring and pinion to a 7:31. That will shorten the box alot without changing gears. If you have a early 7:31 box use the gears from the later 915 box. You will need 4th and 5th only. That should give you a fairly good setup for the least amount of money. Then you can buy a limited slip. I like torque bias types for street/track cars since they don't wear out. Hope this helps. Thanks Eric

[Flunder]

Here is a set up for a short track 915 built on a '74 915/06 with limited slip. This was built for a track motor using 8000 as a redline:

1st 11:35
2nd 15:30
3rd 20:32
4th 22:29
5th 24:27

Final drive; 7/31 or 4.43; this would be too short for your needs.

In Michael Rossi's book Porsche Performance he describes the following short gearset for his 915 box. His engine is a 2.7 mfi RS spec but with 9.8 hi compression pistons. As his car was designed to spend alot of time on the track, he changed to the beefier 8:31 or 3.875 ring and pinion. Using the following gearset he calculates the mph numbers shown at 7100 rpm. This looks like a nice set of gears with an 8:31:

1st 11:35 49mph
2nd 15:30 69mph
3rd 21:30 96mph
4th 24:28 118mph
5th 26:26 137mph

[mjshira]

I am in a similiar situation as I try to decide what gearbox set up to run. I have a 2.7RS twin plug MFI engine. She's got head work and a little higher compression than an RS. Probably 250ish HP. I will never go faster than 140 in the car. But I want to have fun in between. Whats the best of the best 915 with mechanical speedo?

[gled49]

I have in a similar car: 11:35 15:30 21:29 25:26 28:23. Good feel, much better 1-2 split, even gear spread and a decent road 5th gear. 142 mph with 24.6 dia tire, 7:31 r&p, 7300 rpm and 845.52 revs per mile. I also have and 8:31 trans with the same gears but a 15:29 second. The ratio splits are the same but a little faster all around. Gordon

[YTNUKLR]

I think:

-the 7/31 is too weak for rallying with a 2.7RS-powered car on sticky tires

-that the 11:35 1st is all but useless on a 7/31. Have you ever driven a circuit car with a looooongg 1st?

-The 'Flunder' box is extremely short. I am sure that is fun on a hillclimb but it runs out of revs at 107mph! Flunder, is that what you have in the ST? (A common 901 equivalent hillclimb box is A-E-I-O-U or A-F-J-O-S; this looks like A-E-I-M-Q! [which is a rather uncommon thing I would think...])

-Here is my planned box, 8:31 final. You keep a stock 18:33 2nd, replace everything else. It's actually quite close to the 'Michael Rossi' box, but a little taller 1st and 2nd, very similar 3rd, a little shorter 4th and 5th (tighter spacing in general)

Tell me how you like it.