901 Up to the task?

[joetiii]

OK, so I've had issues with my 901. 2 years ago the clutch pivot fork broke from being poorly adjusted and I had my Porsche garage repair that. A new clutch and flywheel were done at that time. Last year, the magnesium wore out right by where the Push cable attaches to trans housing. Same shop modified the mount. This year I started AutoXIng the car. Broke the clutch pivot pin the pedal cluster. I now have the car being repaired and my wrench thinks I should consider installing a 915 when we upgrade the engine. He feels the 911 trans push type isn't a great design and I will continue withe other issues like the Bowden tube.
Currently, I have a 3.0 SC engine installed and have plans to open the small ports up a bit and install Mod S cams. I figure around 215 ft lbs of torque. When the engine comes out, I was planning to get the 1rst gear synchros done as I can not shift down unless I'm crawling to a stop. My mechanic is thinking about a bigger engine upgrade and feels that the 901 will not hold up. I know that the 911 was only a 2 year build and I'm wondering if it is both a weak link and a poor design. I will not be racing this and like to do maybe 5 autox per year.

I really like the 901 shift pattern in 2 thru 5.

Any sage advice would be appreciated

[tomkirkcis]

Joe,

3 things KILL 901/911 boxes. torque , 1st gear launches and heat. Engines larger than 3.0 can shred a 901/911. This is particually true if you are lauching in 1st. You can strengthen the box with Al billet plate and side cover, but even with the billet parts, it still is no where near as strong as a 915. And for the cost, you can pick up a core 915 I know you have discovered autox and it is hard to give up that second by easeing off the line. You are bound to lauch it. If you install a LSD, you just added heat to the equation. You know I have this issue with my 914-6. I have stayed with the 901 because of the complication of changing shift mechanisms and fliping the ring and pinion. with a 915. the 914 boxes are also a dime a dozen so shredding one is not the end of the world. In a 911, I would not hesitate to slap in a 915. Bigger clutch, more robust and no fitment issues. And build the 3.2ss. You wont regret it.
tom

[joetiii]

Joe,

3 things KILL 901/911 boxes. torque , 1st gear launches and heat. Engines larger than 3.0 can shred a 901/911. This is particually true if you are lauching in 1st. You can strengthen the box with Al billet plate and side cover, but even with the billet parts, it still is no where near as strong as a 915. And for the cost, you can pick up a core 915

Tom, If I understand you correctly, the Billet parts new would run the cost of a 915 core. correct?

I know you have discovered autox and it is hard to give up that second by easeing off the line. You are bound to launch it. If you install a LSD, you just added heat to the equation.

Funny you mention about LSD because I did pick up a Gaurd TB diff that is going in this winter. Does the diff build heat during all driving , or only when its being pushed in corners?

You know I have this issue with my 914-6. I have stayed with the 901 because of the complication of changing shift mechanisms and fliping the ring and pinion. with a 915. the 914 boxes are also a dime a dozen so shredding one is not the end of the world. In a 911, I would not hesitate to slap in a 915. Bigger clutch, more robust and no fitment issues.

So go over for me what I would need to change in a 915?
new clutch? Cable? pedal cluster? what else?

And build the 3.2ss. You wont regret it.

ummmmm I have a line on some Max Moritx 3.2 P&Cs.
tom[/QUOTE]

[tomkirkcis]

Tom, If I understand you correctly, the Billet parts new would run the cost of a 915 core. correct

About $700. If you buy a 915, avoid the 1972 unit, seal problems in the first year. A decent core may run more, but not a ton.

Funny you mention about LSD because I did pick up a Gaurd TB diff that is going in this winter. Does the diff build heat during all driving , or only when its being pushed in corners?

tb should note do much to add to the heat situation nearly as much as LSD-slipping clutches generate the heat


So go over for me what I would need to change in a 915?
new clutch? Cable? pedal cluster? what else?

Someone here knows this much better than I but I believe you are looking at a different flywheel, and clutch package (much stronger) for sure. Pedal cluster is the same. I suspect cable and shift mechanism will be swapped out. I do not think the TD you bought will work in the 915. no idea on the speedo cable or calibration

[Flieger]

Torque biasing differentials generate more heat when they are driving straight. They want to turn.

Limeted-Slip Differentials (clutch/friction-disc type) generated heat when turning. They want to go straight, both wheels at the same speed.

[Bert]

I am planning to build a 2.8 twin plug engine with approximately 250 HP. I would like to keep my current 911 transmission with the new hot rod engine. Of course, I would avoid drag strip starts in 1st gear. But I wonder whether the 911 transmission can cope with the extra torque?

Has anyone installed a similar set up? Any reliability issues?

[Flieger]

What kind of torque are you planning on? 906, 911R, plenty of other customer 911s raced with 250PS and 901/911 gearboxes. They only had around 180 lb*ft of torque oon the 2.0 liters, though.

250 horsepower should not be too bad. An aftermarket cooler would help at the track. The factory qould likely have done this and they also rebuilt the boxes more frequently.

I think 200 lb*ft torque force would be about the limit wihtout significatnt reinforcement and some re-engineering.

[Bert]

According to Bruce Anderson:

The 2.7 RS had 188.4 ft/lbs.

The 2.8 RSR had 217 ft/lbs.

I figure my 2.8 twin plug engine will have close to 200 ft/lbs. of torque.

My 911 transmission already has the billet aluminum intermediate plate but not the billet aluminum side cover. I guess I will have to upgrade the side cover when I install the LSD.

Cooling would also be a good suggestion for track events.

Thanks,

[chris_seven]

I would think that a 901 would only be good for about 150/160 lbs/ft torque on a regular basis. A 906 motor would have produced about 210 BHP at 8000rpm which is about 140 lbs/ft.

I think that the expansions in a 901 can unload the pinion if they get too hot and this would be a real problem.

I agree that ATBs generate heat in all conditions as thery have gears running in mesh and an reasonable estimate for a helical gear pair would be about 1% per mesh engaged.

Assuming that during spirited driving mean power being applied is 125 bhp which is equivalent to around 100 kW the box would need to reject an extra kilowatt, most of which would end up in the oil.

Just as an aside has anyone ever tried fitting a Hewland Mk9, which shares transmission case design with a VW Transporter and in 5 speed form deals with 180 lbs/ft had dog change, cheap an plentiful ratios etc.

[joetiii]

OK... this is all good info. My 3 liter puts out 184 lbs at the rear tires and this is just over the limit for a 901/911. Adding a trans cooler would be required if I went with a TB diff even when I'm just driving on the street. So my options at this point are to keep the open diff and go easy launching the car in first gear. (which I normally do since the engine spins up so fast).

If/when I go the 3.2SS route, a 915 would be the preferred box for my autoX activities.

I checked out the ratios in the 915 and 1,2,3 are fairly similar to my 901, but 4,5 are a bit different.