Race car transmission, LSD and shifter design

[JohnJL]

My 915 has given up the ghost, limping home on the last run on Wednesday. It had 70,000KM on it before I got it and judging from the engine it was mated to, had a hard life.

Around 10 days at the track in front of 240 hp of track abuse at my hamfists did it in.

This is the instigation I was waiting for to do an engine swap, so the race engine is going into the weekend road toy (and Targa entrant) with a supercharged 375HP going into the track car. Its the race car we are discussing here.

I'd appreciate some suggestions for the following objectives and available parts;

The car is a race-only open class 1983 SC tub and suspension and transmission. It weighs in at 960kgs without me (80 kgs) with heavy seats and tail. The latter go for lighweight units soon so I wil be down to 920 kgs with a full cage. Doors may go to get to 900kgs eventually.

Suspension is torsion bar but I am considering coilovers and accompanying reinforcements with cage connections.

Engine is in progress now so the old one will be in temporarily. The new one will be around the following (I hope):

375hp at 6000rpm
300 ft-lbs from 3000 to 5500

Eastern creek is my home track, so I expect to do 50% of time here and there are only a few longer straights in the country (which is surprising given how bignfreakin flat it is.) Top speed on the main straight is around 260 kph. I think I want to leave 10% left on the top of top gear to allow for different tracks.

Tires will be 16" 9" fuchs in back with 2010mm circumference. These will get wider over time but assume that circumference.

I am interested in opinions, priceranges and availability of the following three choices:


1) Keep my 915 and modify/reinforce/spraylubricate with an lsd and sort gears.

2) 87-89 G50 which I understand will fit right in with the existing suspension. Cases seem to be at an extreme premium, with none to be had in Australia. I would need to purchase a donor and gearset from the US and ship here for assembly. Again, with LSD.

3. An investment in something like a heyward sequential shift. Although less exotic I also put the G50/50 in the same category as it would also require an entire new rear suspension (which I was considering anyway) so if I go that far I guess I might as weld in suspension points anywhere, which opens up all kinds of expensive options. And sequential is cool.Since I'm "all in" in Porsche anyway maybe the G50/50 investment is worth it anyway.

I dont think the 915 will handle the power anyway so #1 might be out but my experience with 915s is limited (to destroying them.)

So other points of debate could also be Hargett vs Wevo shifters and LSD choices. I am all but settled on an GT based on my experience with Paul. Options there are whether a welded spider, 40/80 40/65 or otherwise are likely to best fit the job.

My Excel seems to be corrupted so no good on running a gear chart but here's the program I've been playing with http://www.carquip.com/parts/gchart/index.asp

Thanks all for your help.

[Bill Verburg]

A 915 in full RSR trim, w/ all the coolers, spray bars, reinforced plate w/ scheduled rebuild weekly is rated @ 275ft-lbs.

I wouldn't even think about a 915 for 2sec w/ 300ft-lb

[Steve@Rennsport]

Bill is (as usual) spot on.

You are a prime candidate for a G50 conversion and IMHO, it will be a lot cheaper in the long run with the kind of power you will have.

The G50/50 is VERY expensive and the most rugged, durable 5-speed they ever made but its not for the faint of heart (or wallet) now.

The very first question I always ask my customers is, "What is your budget" as that answer really frames what one does,...

[JohnJL]

Thanks guys. I am thinking within $4-5000 total installed for all the gearbox & LSD bits exclusive of any necessary suspension bits should the torsion bar need to go. I'd very much like to avoid cutting the torsion bar though.

[Bill Verburg]

Originally Posted by JohnJL

I'd very much like to avoid cutting the torsion bar though

The alternative of shortening a g50 isn't appealing either.

I'd just bite the bullet and reinforce the new suspension pickup points, cut the tube and install an effective coil oversetup.

[Walko]

John,

How have you arrived at the top speed for Eastern Creek.

I know that the A Class Porsche Cup cars (993RSCS) pre Carrera Cup were only acheiving 225km at the end of the main straight. That was on Data logger.

I have only raised this isssue cause i would hate to see you get a box that is to long in the gearing for the track.

Michael

[TRE Cup]

Like the others above have said: G50 is the way to go and expensive. Now it is rare as well; many kit /replica car builders are searching them out for their projects. Finding a G50/50 for less than $5k is a huge deal
best of luck

[kojo911]

It may sound like a primitive solution but what about a 4 speed 930 tranny. Given the weight of the car and assuming a fairly flat power band it might be good compromise, and it will save a lot of work.

[TRE Cup]

a 4 speed could work well. But unless the box is a 76 or 77 turbo unit, the torsion tube will need to be cut , or bellhousing/mainshaft shortened. Gearing is less than desired, so changing ratios will need to be done too

[petevb]

I had a similar problem, and ended up biting the bullet and going for a 6 speed G50. Lower shifting effort and the extra gear are nice to have. Of course you need to do some significant surgery to get it into the car due to the extra length, likely blowing your budget in the process, but there may be some good options coming along in that regard- pm if you're interested.

One thing I'd suggest is that your requirement for a 10% taller top gear is actually more than you'd want, as 286 kph (177 mph) will be at or very close to the absolute drag limited top speed of the car, requiring miles of straight. 5% headroom will likely be plenty as long as you're not open road racing.

[kojo911]

Peter what are some other options? I'd like to put a g50 in my car but would like to do it without using the brake booster set up. Is there a way to get around the hydraulic clutch fork?

[petevb]

I'm going to have to fill you in with what I can off-line, as I don't think the info is ready for public consumption at this time.

[JohnJL]

Thanks guys, all good input. If I am willing to cut the tube and go to coilovers (wow, how's THAT for a "while you are in there?!?!") then any G50 donor would do since I'd be regearing and LSDing anyway?

I arrived at that top speed estimate through the following process....ahem....I am getting an indicated 210kms at the end of the straight currently. That speedo is calibrated for 15" wheels with street tires which are actually about the same diameter of my 16" fuchs with RA1s.


I'm surprised the 993CS didnt get faster than that...997RS and Turbos are clearly getting at least 30kms faster than me on the straight...I agree though overgearing is a waste.

[antaircooled]

There is a guy here in NZ who runs a 3.8RSR engine with 390HP and 337ft/lbs torque through a modified 915 gearbox. He has won the Porsche championship in 05/06 and runner up 06/07 so it is possible. He is an engineer and machined new gears etc himself and installed them in a late 915/67 with the OEM cooler and LSD. He doesn't seem to have broken anything over those two seasons of racing.

[Peter Bull]

John,

does it have to be a Porsche box? Both Hewland, Sadev and Quaife have sequential six speed boxes that would more than cope with the torque and horsepower numbers you are quoting. Some of them are silly money new, but it should be possible to get a used one. There are also a lot of ratios available to them so it should be possible to tailor such a box to your needs.

/Peter

[911-32]

Originally Posted by antaircooled

There is a guy here in NZ who runs a 3.8RSR engine with 390HP and 337ft/lbs torque through a modified 915 gearbox. He has won the Porsche championship in 05/06 and runner up 06/07 so it is possible. He is an engineer and machined new gears etc himself and installed them in a late 915/67 with the OEM cooler and LSD. He doesn't seem to have broken anything over those two seasons of racing.

Thats Jon Waring - he used to work on my first 911 many years ago trading as Rennsport in Auckland. As the engine from his racecar was recently for sale, the gearbox may also be for sale. If not, Jon could possibly build one for you and that is by far the cheapest option of the lot. He is in Christchurch now and is semi-retired.

I would note that Jon has been racing 911s for a lot longer than I have been driving them - and thats 15 years - and he is anything but hamfisted. He drives with a mechanical sympathy and is a very skillful competitor but perhaps not ultimately the very fastest driver. That said, he has been at the sharp end of NZ Porsche racing for a long time, so he is no slouch.

Richard

[JohnJL]

Thanks all.

I am getting quotes of 8-13000 for what I need, I am shocked to say the least. It seems 915s are out of the question (notwithsanding the fellow across the Tasman, I am not as mechanically delicate a driver) so I kind of view this as an investment.

While I see prices of 3-5K on rebuilt, shortened g50 but by the time LSD, shifter, axles, pedal clusters, hydraulic bits and everything are quoted it gets way up there.

[JohnJL]

Could someone please confirm for me if a standard G50 out of a 1988 911 will fit right into a 1983 body with the torsion tube intact and mate to a standard SC motor?

[Bill Verburg]

Originally Posted by JohnJL

Could someone please confirm for me if a standard G50 out of a 1988 911 will fit right into a 1983 body with the torsion tube intact and mate to a standard SC motor?

It fits when the '87-89 torsionbar tube is used, but not w/ an '86 or earlier tube
late Carrera torsion bar tube

[TRE Cup]

and you change the torsion bars as well, spring plates too
btw- we have a g 50 tube and spring plates if you need one. pm me for details