914-6 Gearbox Ratio Change

[Neven911]

Hi

I'm currently restoring my 914/6 and as part of that will take the engine out to 2.45 (70.4 mm x 85mm) it will probably produce 180HP but am tuning for mid range torque over outright power as it will be a street car.

Question: Should I change the 5th gear ration to a taller gear, Porsche picked the shorter gear I think to extract maximum top speed from 110HP @ 5700 RPM (max power point) so with greater torque and probably max power @ 6K I feel it would be a better street car with a taller 5th

Speed limit in NZ is 100k (62Mph) so this would give a cruising rpm of 2600 down from 2800

Opinions, Thanks Neven

[Dave at Pelican Parts]

I have read that Porsche used the shorter top gear (compared to the 4-cylinder cars) to keep the fan speed up to make sure the engine cooled properly while in top gear.

Make of that what you will.

--DD

[djpateman]

The 914/4 had a ridiculous gear set, as it had the widest range of any 911, 912 or 914. Essentially you were lugging the engine in top gear in the name of fuel economy, but the engine was under-cooled.
The 914/6 was a little stretched as well. Fifth gear was essentially the same as the 911S and RS with almost double the power. With your proposed engine, the transmission will finally be balanced with the engine.
For an awesome amount of fun, convert the trans to A-F-M-S-X

[Neven911]

Thanks David
I have a friend whose 2.2 911 he converted to A-F-N-S-X so I'll try it, Comparing to the ratios in my 2.4 911E, I see your point, the 914-6 box is actually tall but with more torque and power it could work well, strangely they end up with the same ratio in 5th with 1st to 4th still slightly taller

[John Rogers]

Several things to do with the 901 gear box in a 914/6 to make driving/racing much better.
- Install a limited slip if not already in the box.
- Change the input shaft so 2nd gear can be adjusted to allow 2 through 5 to be used well.
- Change 3-4-5 to M-S-X for track racing, makes the box much better on a long track like Willow Springs or just for running around on the street.
- Change 3-4-5 to K-Q-V which is lower than above, much better for a tight track like PIR outside of Phoenix.
- Add a lube/cooler system and temp gauge where speedo plugs in the case to make parts last longer and keep you informed of the temp.
- Buy a second 901 transmission and have the M-S-X and limited slip in one and the K-Q-V and limited slip in the other. The gear box can be swapped in less than an hour.
- Get a shifter like one of the racing ones, better firewall bushing, aircraft type coupler and better bushings at the transmission. This will allow you to use all 5 gears when rolling or racing.

Note: NONE of these items are low cost!
John

[Cornerlot]

My 2.6 was fitted with AA, F, M, S, Y years ago, seem well spaced. Turning 3200 on the freeway at 70 MPH is comfortable but I still would like a 6th gear to further lower it. Rear tires are 225/50/R16.

[Dave at Pelican Parts]

Wow, the 904 mainshaft is a lot less expensive now than it used to be! Still over a grand, mind you, but that's about half of what I remember! Still need to get the gears for it, of course.

But I feel that most of what John is talking about is not very relevant for a street car. Race cars are different animals, and those mods make a real difference for a race car.

--DD

[Emo993]

with my 2.2 E/S, I did the 3rd- 5th swop with a Q for 4th. So, A, F, KA, Q, Z. Mark

[tx93086]

I have tried several versions of gearing and motors in 914s, and here is my experience. I had a 2.4t with A,F,M,S,X and it was too low geared to enjoy on the street. I went to the stock 4 Cyl gearing A,F,N,V,Zd and it was much better. I would have preferred to have a Ga second gear but didn’t have one at the time. If you have a factory 914-6 I think they came with the Ga second, so if you get the 345 gears out of a 4cyl box it should be fairly cheap and I think you will like it. I did build one with a 3.0 Ltr that I built with C,H,n, v,flipped Hb, but the flipped Hb was a little too tall so I went back to the Zd and liked it better. In my 4 cyl, I have an AFMSZd and it works well. The Zd is like an overdrive on the freeway, but close spaced gears in town.

[Charles Freeborn]

Originally Posted by Neven911

Thanks David
I have a friend whose 2.2 911 he converted to A-F-N-S-X so I'll try it, Comparing to the ratios in my 2.4 911E, I see your point, the 914-6 box is actually tall but with more torque and power it could work well, strangely they end up with the same ratio in 5th with 1st to 4th still slightly taller

I have that gear set in my 814/4 track car. Great close ratio box- would be great on street if top speed isn’t a goal. I’m up against the rev limiter on long straights at 120 or so mph