911 4-speed. Some thoughts and questions.

[RallyJon]

Right now I have a 911/01 5-speed in front of the 3.0L SC engine. Love the light and accurate shifting in 2nd thru 5th H, but have some issues with it:

1) the 1st -> 2nd shift still drives me nuts, even with new bushings, Swepco, Rennshift, perfect adjustment, etc.

2) with the SC engine, first is too short and really just a get rolling gear. But 2nd is still a bit too tall to use from a standstill.

3) 1st is supposedly fragile with higher torque engines, but there are occasions where a hard launch would be nice (I know, get a Camaro…). If I rev 1st out, the shift to 2nd is even slower and more awkward, so I lose what I gained by revving it higher.

4) 5th gear rarely gets used since I primarily use the car for back roads fun. The SC engine is smooth enough that driving along at 3500-4000 RPM is no problem.

5) So what I’ve really got here is a 3-speed transmission with a low gear to get rolling only and an overdrive that I barely use.

What are my options for a decently geared box that will hold up to standing starts and general jackassery?

1) 915 conversion. Heavier. First would still be low. From what I’ve read 1-4 in a 915 is nowhere near as nice feeling as 2-5 in a 901. I’ve driven a stock SC once and my recollection was that it shifted poorly. Custom geared version with gateshift like Jack’s would solve those drawback, but at an extreme cost.

2) G50 conversion. Expensive. Very Heavy. Overkill. Not the best gearing. Hydraulic clutch. Lots of fabrication.

3) 930 conversion. Expensive. Very Heavy. Overkill. Not the best gearing. Lots of fabrication.

Which brings me to…

I’ve been reading the old posts--mostly from Grady--about how a regeared 4-speed is a good choice for autocross and other low speed fun in an early car. I just saw a recently rebuilt 911/00 sell on Ebay for $225. Hard to get much cheaper than that for he basic ‘box. It would drop right in to my car with zero changes. The first gear is much stronger. The shift pattern for 1st-4th is the same as 2nd-5th in my 911/01.

The only trick is regearing it to match the 3.0. Using Carquip’s cool spreadsheet, here’s what I came up with for four useable gears (in red) vs my current 5-speed. I could adjust the gearsets up or down slightly based on what’s available.

Comments? Suggestions?

[pwd72s]

I think you could be onto something. If auto-x is your bag, especially. Anybody who has indulged in that game realizes that time is lost while changing gears. Toss in that 901 and 915 boxes aren't the quickest shifting ones out there. Yep, you could be onto something.

[RallyJon]

One key question: I assume that 1st gear in a 901 4-speed is like 2nd gear in a 901 5-speed: the gear is part of the mainshaft. The only gearbox that used a "B" mainshaft is a Sportomatic. Can one use a Sportomatic mainshaft in a manual 4-speed case? Otherwise it's going to be a 904 mainshaft = $$$ ouch.

Alternatively, using a Sportomatic 7:27 R&P would raise the standard "A" 1st up to a usable ratio. All the other gearsets would need to be different, though.

[Grady Clay]

I’m impressed; you have done your homework. Good for you.

I agree with your basic philosophy. With a torquey SC 3.0 your proposed gearing should work great. You are correct, your 911/01 first gear is for carefully getting the 911 moving, not burn-outs. First is fragile with a two liter.

Your 911/01 has a GA (18:32) second gear which is pretty tall. The shortest standard 2nd is an F (18:34) cut on the input shaft (’69 and earlier). I think the shortest 2nd gear is the 904 E gear. It is easy to machine off an A first gear cut on a 4-speed main shaft and weld on a 904 E gear. I don’t know of any lower gears that would work. The first gears A, AA, B, C are totally different parts and can’t be fitted as a 2nd gear.

If you buy an F 2nd main shaft, make sure you get the mid ’67 and later reinforced version. Since you are building a 4-speed the early version might be OK but it should be free.

A 4-speed has lots of different parts from a 5-speed. You need to start with a donor 4-speed.

From a donor 905 Sportomatic, gears 2, 3, 4 are desirable. The 1st gear input shaft won’t fit a 901 or 911 transmission – it is a 905 only part. I think the B Sportomatic gear could be cut off that input shaft and welded on an A 4-speed shaft – lots of work.

The X (28:24) 5th is a good top gear; about 3000 RPM at 60 MPH with normal tires. For your use you might consider something slightly shorter.



The wonderful thing about these transmissions (901, 911 & 914) is how easy they are to work on and change ratios. We would do it between practice sessions. Only a Hewland on a formula car is easier.

I encourage you and anyone else fussing with gears to collect and share. This is one of the more remarkable parts of Porsche compared to any other automobile.

Best,
Grady

[RallyJon]

Assuming I'm starting with a 911/00 4speed:

If the Sportomatic 1st gear is not an option, what about using the Sportomatic 7:27 ring and pinion? That would make the stock "A" first gear almost as tall as the Sportomatic 1st gear would have been with a 7:31.

Will the tall Sportomatic 7:27 ring and pinion work in a 911/00?

If so, then the rest of the ratios have to be short:

2nd gear should be F (18:34) or G (18:33).
3rd gear should be M (22:29) or N (23:29).
4th gear should be T (26:26) or U (26:25).

Can the above gearsets be used in those positions?

Thanks for your knowledge!

[RallyJon]

Here's what it would look like with A, G, N, U and the Sportomatic ring and pinion:

[Grady Clay]

No the Sporto R&P won't fit - another 905 only part.

I'll reply more later - about to meet with pelicans at the track.

Best,
Grady

[Grady Clay]

About the only parts that you can swap between a 905 and a 901 are the 2-3-4 gears and the syncro parts. The differential housing and the ring & pinion are smaller to accommodate the torque converter. The input shaft is shorter for the same reason.

Best,
Grady