'overdrive' fifth for A 915?

[haycait911]

does anyone make an overdrive or much taller fifth gear for the 915? I get tired of turning 3K + rpm at highway cruising speed. thanks, Don.

[GrantG]

3k? Puhleeeese. I run 5,000 rpm when I'm making good time on the highway (not triple digits - my gearing and 15" tires gives 19.3mph per 1,000rpm). Your car is perfectly happy running 3,000rpm indefinitely and I would highly recommend against gearing it any taller than stock (where 5th is already a tall overdrive gear). 911's are sluggish and unhappy much below 3k.

Are you asking on behalf of your 67 with 250hp 3.0? Where is the power peak and redline for this motor? Does your 915 have an 8:31 rear end or 7:31 (what year is the gearbox)?

I have changed my 3rd, 4th, and 5th gears (much closer and lower) with Guard gears. They make a wide variety of 915 gears and probably have something taller than stock (for high power engines that can make the car go 180mph), but for a 911 with less than 300hp or so, I'd certainly not gear it any taller.

Didn't mean to sound insensitive to your question, but you will be giving up quite a bit of high-speed acceleration by making 5th taller (especially if you have an 8:31 R&P and stock diameter tires).

[Steve@Rennsport]

One of the big concerns is adequate cooling with the higher loads imposed by air resistance.

Porsche was pretty smart in their road car gearing to ensure sufficient fan output at higher loads.

JMHO, but unless one made significant reductions in drag, I would not install a higher 5th gear than the factory offerings (for all around usage), especially if I lived near mountains.

[haycait911]

yeah Grant. it's for my '67. it's an 8:31 915. at 2000 lbs I don't think I'm in danger of lugging the engine at 2500-2700 rpm. as for peak hp and torque, see below. where can I get a look at Guard Gears? a website? Don.

[Jim Richards]

haycait, what size (diameter) tires are you running? Taller tires will lower your engine's cruising RPM.

[GrantG]

Don - Here are ratios available from Guard:



Their new website (Matt Monson is owner): http://www.guardtransmission.com/

Guard Transmisson LLC
4699 S Nautilus Ct Suite 207
Boulder,CO 80301
(303) 530-1094

[john walker's workshop]

76 912E 0.72:1. 2200 @ 60 with a 8:31 915, 225/50-16.

[Matt Monson]

Do you know the year of your gearbox? There's been a number of stock 5th gears over the years. You could have any one of them.
Compiling 915 transmission gear ratio documentation

I'm out of the 29:21 TL 0.724 ratio right now. It's really popular with the UK guys who are always building RS replica gearboxes and are often building cars to FIA homologation specs. But I believe I've got pretty much everything else on that list that Grant posted. The .679 would be an option. You don't want the .595. That's for the v8 guys. It also requires modifying the case to fit.

[haycait911]

well, it seems like maybe a little more trouble and $$$ than it's worth right now. I might be better to put in a later 915 with the taller R&P. thanks for the input guys. Don.