901 rebuild and upgrades!

[Tspringer]

Its toast, trashed, gots to be rebuilt.

The alloy cased 901 in my 911S will be getting rebuilt over the next couple of weeks. I am not going to do it myself because I have no idea how and I have no time. Next track event is 3 weeks away.

If I am going to have it rebuilt, I want it right and better. Currently around Road Atlanta, this box is basically a 3 speed. I use first gear just to start in the pits and I never use 2nd gear. The ratios are spaced pretty well for the 3 gears I am using.... but of course only having 3 gears is not an optimum situation.

I would really like to have a much higher 2nd gear (perhaps just a tiny bit lower than the current 3d) and coorespondingly higher 3d and 4th. The current 5th gear is good.

I would also like to have the tranny last longer next time.... what should I have done to make it more robust? Billet intermediate plate? What exactly IS that? What does it do? Where do I get one?

Limited Slip. Dont have one..... do want one. It would be REAL nice to be able to put the power down better coming out of turn 7 and 10a. What options do I have for aftermarket LS for a 901? Who is a good source?


Thanks for the suggestions and help!

[JWest]

From what I remember (it's been a long time) the aluminum intermediate plate is thinner at the mating surface than the mag plate. The aluminum case extends further forward than the mag case to make up the difference.

Because of this, the billet intermediate plate won't work (it's made for the mag case trans).

[Tspringer]

hmmmmmmm

Would the mag case with the billet intermediate plate be stronger than the alloy case? I do have a mag case spare I could use. Would it thus be lighter AND stronger????

[RPMClassic]

Terry -

I have run mag case trannys in my last few race cars and have learned a few things. Nothing you haven't heard before, it all comes down to " check book" and the back log of work at your selected repair shop.

I opted for different gear ratios and LSD a few years ago and waived off the intermediate plate and billet side plates. Guess what.....I am back in this year having those 2 items done.

Use the gear charts that are available and pay close attention to the RP too. The limited slip is awesome. A few guys are using the transmission external coolers and plumbing - where do you draw the line and end !!! Surely there are updates that you can fit in along with the normal rebuild process, no matter what your budget.

You can end up with some pretty funky gear choices if you're picking out of a catalog - probably best to stick with the proven gear charts that are in the sports purpose books or pick a track that you consider your " home track" and make a custom box for that. Airport ratios worked well for me.

Pm if you need any help.

Good luck and keep us posted.

-Jim

[Tspringer]

Thanks Jim.

I just spoke with Tom at Carquip. Very helpful. Before he can make suggestions on gear he of course needs to know what I have now so step one is getting it all apart.

He has torque biasing LS for about $1300. What kind do you have? I only have about 220hp and my car runs 225/45 hoosier race tires front and back. Its really only at turn 7 at Road Atlanta that I feel I am really limited by traction in putting power down. I am really debating whether any potential gain and fun is worth the cost of a LS. What gains did you get? How is it different in feel and such?

Thanks!


Terry

[Grady Clay]

Terry,

Jim has some good ideas. A ’70-’71 mag case will bolt in with only one mod. To maintain your 215 mm push-type clutch you will need to reposition the clutch release arm pivot. You can change to a 225 mm pull-type clutch. There are ’69 mag case transmissions also.

I agree use the aluminum intermediate plate and differential side cover.

What gears do you use now – AFMSX? Is a GA still too low for second? How high in X 5th do you get? What revs in the slow turns in M 3rd? Draw yourself a track map and shift points/RPM in each gear and post it. Going from three to four gears will probably make you faster and should be accounted for in the gear choices. No matter what you do you will want to change it. Stick to classic ratio progression; as you go up in gears, the RPM drop between gears decreases progressively.

On another thread I mapped out how to weld third gears in place of the fixed 2nd gear.

A LSD will change the handling and your driving style. The large ZF LSD from a 915 or 930 will fit in a 911 but not an aluminum case 901. If you ever drive on the street, don’t use a spool (solid non-differential).

Stick with a standard 7:31 R&P because they are easy to come by.

An electric pump, cooler, filter screen, and spray bar are very worthwhile for extending the life of a 901/911/914. I like to also put some large Alnico magnets inside the transmission to catch any errant chards of steel.

At a minimum you are going to want to learn to re-gear the transmission yourself. It is easy. With practice, you and a helper can do it between sessions at the track.

Best,
Grady

[Tspringer]

Grady, thanks for all the good info!

I am really debating the LSD. Guard has a 80% plate style LSD in stock but its like $1600 and it would not be good for a car that will do street duty.

I am planning on driving this car more on the street and eventually less on the track. My RSR racecar is still a ways off from beign ready to go and I will continue to track this car in the meantime but dropping lots of coin on a setup that would be short term at best just doesnt make tons of sense.

So, I may leave the LSD out. Still, I definitely do want different gears so I can have 4 active on the track. I will post what gears I have when I know.

The mag case 901 I have IS from a '69. I pulled it out of a '69 911S I parted out several years back. Joe at PAP "rebuilt" it. He put in "the gears you will need" whatever that means. During the interaction with Joe I learned what many know: Joe may be a really great racecar driver but NEVER trust the guy! I will not install that tranny without having it looked over again and the bearings/seals all replaced. I also know it does not have an alloy intermediate plate.

So, I may upgrade the mag case I have and use it after I figure out what gears are actually in it.

Re-gear it between session at the track?!?!? YOu can swap the gears without having to pull the tranny?

[Zeke]

I'm nowhere near the experience level of Grady, but I do have a fellow that I can talk to when it gets technical and experience counts. He was a racer like Grady, but of much less notoriety. Nevertheless, he has his share of the trophies to establish himself as experienced. He also happens to have a Porsche shop that he has owned for 20 years and worked there before that.

OK, so much for his qualifications. His take on the limited slip concurs with Grady on the handling issue. But, he also offers that a limited slip is more of an asset when you have the power to spin the tires. At 220 Hp, you can probably dial out any tire spin with the chassis setup. His statement, not mine. I just happened to pose the same question a few weeks ago.

If you go with the LS, you will have some new noises to listen to. Once again, his info.

What IS your 5th gear? I have a ZA, most likely and I can't use all of it at Willow Springs. How fast is Road Atlanta? I would think you would want to shorten the gears a little and close them up. WS is also a 3 gear track with the stock gearing, and that's not optimum AFIC. But to use 2nd at all out there even in one turn, it would have to be very tall.

I will run shorter, tighter tracks I'm sure and need some 2nd other than having it at, or near, the present 3rd ratio. That may be the case for you too. The question is, how much do you dog the motor in 3rd anywhere out at RA? At our "home track," the answer is not much.

Like Grady says, the answer to all these questions will determine the range between 2nd and 5th for your particular track itinerary. AFA changing gears between sessions, that is a LOT of work, practice or no. You have to be a serious racer to be at that level.

Some will counter, is there any other kind of racing? My answer is yes. I hope I'm not in all you serious guys' way out there, but I'm beyond changing gear stacks at the track. I've slaved thru my share of lunch breaks and I'm looking for some fun now.

Sorry, Grady, but someone had to say it.

"Re-gear it between session at the track?!?!? YOu can swap the gears without having to pull the tranny?"

Yes, you can drop the tranny a little under the car (if you can make the shift rod clear) and pull off the drive axle from the flange side of the trans. Pull the nose and the side cover, lift out the diff and pull out the stacks. They can be re-geared on the bench and reinstalled, re-filling the trans somehow quickly before shoving it back up on its mounts and re-attaching the shift linkage. Not for the weekend mechanic, which of course, Grady is NOT. (I think I remember that the diff has to come out, I could be mistaken on that. Grady?)

Edited for clarity and the answer to the question

[Plavan]

All good info here. If you use the car for the street too... get the 40% lockup instead of the 80%... You tires and wallet will thank me
I just put a Guard 80% LSD in the race car with a mag case with billet intermediate plate. My mod is similar to what Grady explains to make the LSD fit from Guard (They do not fit in Alum 901 cases.

GA is the tallest gear you can use without getting the pricey 904 mainshaft.

My gears in my race car are B,GA,M,S,X

I also have a ton of other gear ratios so send me a PM if you need a mainshafts or different gears.
FYI- Carquip will "bend you over" when it come to gears. Check with me before you buy anything from them.

[Grady Clay]

Milt,

No, the differential can stay in for a quick change of gears. What you are remembering is the input shaft must come out on a 914 in order to get the differential out. The important thing is to not damage the input shaft seal when reinstalling the gear cluster. I made a part similar to the clutch release bearing guide tube only smaller diameter and on the inside. This was to insure the input shaft would go in the right place and not snag the seal. It had the added benefit of protecting the seal from the direct splash coming off the ring gear at very high speed. Many of the race cars had a tube around the input shaft all the way through the differential area. Under normal circumstances the input shaft seal should be installed after the input shaft is in place.

I have done my share of thrashes. I usually have these kinds of things well planned out. All the tools laid out, all the hardware, gaskets, etc. laid out, all the gears ready to go with syncros installed, and every conceivable part that we might find damaged or dropped. That is why you have two breather hoses. You fill a pre-measured amount of warm oil in one while the air escapes in the other.

You will usually find me in a lounge chair under the tent sipping on watered down Gatoraid. Racers that thrash a lot don’t tend to win many races. Today, someone has to help me get down to a creeper.

I agree with your friend. The point I was making was a LSD or spool changes the handling, usually for the better.

Terry, I am only familiar with the friction clutch ZF type LSDs and a solid spool. Some swear by the gear type. I can’t offer an opinion one way or the other although I’m sure I have driven them and not known it. With a spool, at the end of an event the first chore is to jack up the car and undo one inboard CV and let it ride on a strap. The car will maneuver just fine driving one tire. A pita with both attached.
I’ll post some more about trailer tires on the thread about tieing down a race car on a trailer.


Best,
Grady

[Tspringer]

I am going to be taking the car to Franz Blam racing next week. They are going to swap the transmission in the car for the mag case spare I have. They are going to replace all the oil seals and bearings plus check everything else out before installing it.

I am probably going to order the billet intermediate plate and I will use the alloy diff cover off the trans. now in the car.

I am debating on whether to purchase the good used 904 mainshaft I have located. Its $475. I guess the decision will depend on what gears are in the car now, and what it will take to get a usable 2nd gear.

I am also debating still the limited slip. After speaking with Paul Guard, and searching the boards and reviewing all the info here.... I am still questioning it.

It seems that a 80% locked Guard diff would really help late braking and stability on corner entrance plus it would allow the power to be put down earlier once the car has turned in. I do have SRP adjustable anti-roll bars front and rear so I can use those to help tune out initial understeer that I may find and to balance the car. Currently, it is pretty well balanced.

BUT.... I do plan on driving this car on the street as well. Not as a daily driver or anything, but for fun from time to time. Maybe 3K miles a year.

What is the downside of driving such an LSD on the street? What will it hurt? I am honestly not sweating tire wear all that much, but I dont want a horrid handling car on the street and I dont want constant diff/trans. maintenance.

I am also really putting thought into what to do with this neat little car. ahhhhh another post on that topic!

[Plavan]

I just think that the 80% LSD on the street would be too much. We leave tire marks in the pits when we make 90 degree turns to park the car in our pit slot. Feels like the emergency brake is on in tight slow turns. I think the 40% would be a better fit for you since you are going to drive it on the steet.
That is a great price on a 904 mainshaft, however I think it is overkill if you are going to be using it on the street. I think if you find a good GA mainshaft you will be fine.