911S3.6

My 915 trans needs a 3rd gear synchro repaired (at 166,000 miles) and I am debating over cost vs. performance options of A) change gears 2,3,4 to closer ratios at $530.00/ea;B) Add limited slip differential at $1440.00;C)leave it alone stock, as Dr Porsche's engineers intended. By the way, this 2380 lb 911S has a 3.6L varioram engine with Patrick motorsport lghtwt flywheel-centerforce clutch, and other...also I'm leaning toward Pete Zimmermann's advice on short shifts and gate shift, any opinions, advice Thanks.

Edward

Option "D" which is both A and B, but not C.

But if you can only do one, I'd say it depends on whether you're tracking the car avidly. If so, and I had to choose just one, I'd go with the LSD. If predominantly street with occasional track excursions, I think I'd opt for the shorter gears. Either way, I assume you're still repairing the synchro and checking the others while you're there. BTW, 3.6, eh...I'm jealous! Have fun!

Edward

Peter Zimmermann

I'll assume that you're using a magnesium case 915 (is it a 7:31 or 8:31?). Is so, your proposed project is going to be extremely expensive. The best way to maximize your engine performance is to get a gear ratio chart for the car the engine came from. I think you'll find that the ratios were pretty close to those in your 915. If your car is only a street machine you don't want 75 mph to equal 4500 rpm! Your trans reliability will really be stretched by the power/torque of your 3.6, but through careful use it should last a while. LSD is always a great idea, and if your trans is a virgin you will probably find that lots of parts will need replacement (other synchros, bearings, etc.). Hopefully the ring & pinion will still be OK. If you find an expensive mess when you open it up I think shopping for a late 915 (SC or '84-86 Carrera) with as few miles as you can find is your best way to go. You might get lucky and find one with LSD, then all you have to do is have your shop open it up to make sure everything inside is OK. Let me know more about what you've got, there's more to tell!
Pete

GrantG

I would've recommended a gearing change, but with the Varioram 3.6 and the lightweight chassis, I think you have enough torque to make pretty good use of the gearing in the 915. So, I vote for LSD, although if you can afford both, you might find that it's well worth it. While the box is apart, you might want to add tranny oil cooler and pump to handle that 285hp too...

911S3.6

It is a magnesium case (8:31). I've seen the costs and took my heart medicine! The 915 was only designed for up to 3.2L so stress is a factor. The car is street driven (total miles this year 17,000-hey it cost a lot to build, I'm enjoying it) and does 12 or so DE days per year. Note: here in the foothills of the Ozark mountains, this car gets seriously exercised everytime it goes out to play. Documents have shown gear sets help 3.0 and 2.7s more than the super torque of varioram engines. What are the benefits of LSD, is it like positraction?

GrantG

Yes, LSD and Posi-traction are the same thing AFAIK. It allows you to apply power to both rear wheels so that you don't just spin the inside one upon exiting tight corners. An LSD also provides more stability under braking and corner entry. With the large amount of torque you have, I imagine it would be quite helpful to avoid excessive wheelspin. BTW, the magnesium 915 was only used with motors up to 2.7L, I believe (for US models anyways). My 73RS replica (with 2.7L and 240hp) uses an Aluminum version for additional strength and improved shifting. At least you have the stronger 8:31 Ring/Pinion - that's a big help...

Peter Zimmermann

The 8:31 diff allows you a little room to play with gear sets, but the torque going through the magnesium box has me worried. The later aluminum transmissions (THEY HAVE THEIR OWN WEAKNESSES!) used through '86 were designed to cope with Euro 3.2 Carreras, about 230 hp, but they were fitted with a cooler. To get one of those and stay cost effective you have to find a totalled gray market car and get the trans from that. With your engine, for street driving, I think that the stock gears in a Carrera trans would be just the ticket. LSD gives you two rear drive wheels instead of one, which provides a nice assist to capable drivers. Regarding your shifter, I prefer the stock '85/86 Carrera setup. As we know, in '86 Porsche offered a "super-short" shifter option, which is good but you have to be prepared for the heavier shift change action.
Pete

DGaunt

IME, the syncros rarely wear out in a 915; it is the operating sleeves that are sacrificed.

Sounds like a great car. 17,000 miles/year...must be fun!

911S3.6

First, thanks Grant and Pete, your thoughts validate a lot of what I was thinking. Pete, your advise has been followed by me since we met at PDE 2001 (check your personal messages on Rennlist through control panel). It is a H model 911S 2.7L trans # 154148 (915/40) so I think it is mag case definitely no cooler. Rather than even spend for opening this trans, should I try and locate a 1985/86 Carrera 915 preferably with LSD and swap out? Are there folks who swap used 915's for what I want i.e partsheaven,...? Nick

JackOlsen

I have a 2390-pound 911T with a 3.6 that I drive on the street and on the track, so my experience might be useful for you. If you're only doing DE's, then I would say the cost and trade-offs of three more gear sets might not be worthwhile for what you're doing with the car. In my case, I time trial pretty extensively, so we decided to gear it for track use. I have a taller-than-stock 2nd, stock 3rd, and shorter 4th and 5th. What this give you on a track is shorter jumps between gears (about 5-600 rpm difference from one gear to the next, for the same speed, on my car), which allows you to keep the motor in its torque sweet spot for longer. My 5th allows me to hit 136 mph at redline, so all my gearing is distributed over a pretty short range of speed.

The trade-offs, for a street car, are pretty dramatic. My top speed is 136 mph. That's it. At that point, I hit redline, and fire shoots out of my taipipes... but I've hit the ceiling, in terms of speed. When I'm out with the 911 guys on some Sunday morning run, most everybody is able (assuming they're willing) to go faster than me. And cruising at 80 mph, my motor is spinning at just over 4000 rpms. Atermarket gears are cut differently than the factory ones, and they're louder. What's more, at 4000 rpms, gas mileage isn't so great.

Also, with the taller 2nd, my 0-60 show-off ability is compromised. The jump from 1st to 2nd doesn't matter on a race track, so it's not shortened in a custom gearing set.

The problem with gearing is that it doesn't give you anything for free -- it just allows you to move things around. It's not like adding a sixth gear. If you value a significant top speed, you can't do it. If you gear the car for coming off the line fast, you can't also get close ratios for your 60-120 range.

On the other hand, I'd vote yes on the limited slip. And for track use I'd recommend the ZF type over the torque-sensing type. The only thing it's going to really do for you on the street is to allow you to leave two black stripes on launch instead of one. Driving in the rain will also be scarier. On the track, though, it will lower your lap times, as it reduces wheelspin out of corners, and allows you to use the throttle to straighten the car out when things start to get scary.

However, the limited slip will also exacerbate a problem that you should be addressing with your setup: heat. A 915 with a limited slip, when tracked, should absolutely have a cooler. This is especially true with a 3.6, but it's a little-known fact about 915's with stock engines as well. The limited slip increases the amount of heat in the transaxle significantly in track use. If we all had temp gauges on our transaxles with limited slips, we'd all be adding coolers. Before I added my cooler, I added a temp gauge, and saw 260+ temps on the transaxle, when the motor was running at 220. Gear oil breaks down at lower temps than engine oil. Most transaxle experts recommend never allowing the transaxle to get above 250 degrees. I've talked to some who say you're speeding up part wear significantly every time it gets above 200 degrees.

In my experience, in street driving -- even super aggressive street driving -- the tranny stays cool. The track is a different story.

Your 3.6 is too much motor for a 915, generally speaking. After blowing out my 7:31 box, I built a 915 specifically for the task. I now have a 1977 magnesium box (for weight), but with 930 sychros, a billet reinforcement plate for the bearings, an electric-pump cooler with an internal spraybar shooting right at the gearsets, the Wevo internal gateshift, 80% limited slip, and custom GT gear sets for 2nd, 4th, and 5th.

The transaxle is important. To put things in perspective, mine cost me more than my 3.6 did.

Peter Zimmermann

NICK: Yes, try to find an '85/86 Carrera trans - preferably a Euro model w/cooler. Your trans is probably worth only core value, you might want to advertise it on E-Bay, or maybe a wrecker will take it as partial payment. In any case your money will be well spent by switching to an aluminum housing. You must weigh the pros/cons of the gear ratios, they are certainly a compromise.
Pete

AJAKE7796

I have a modified '77' Carrera 3.0 (euro), according to the 3.0 Carrera owners manual I should have a 915/44 transmission 8:31 with the M220 option (LSD) at the shop I noticed it did not have a transmission cooler, but appeared to be a magnesium case. A lot more fun to drive now. My I asked my P-mechanic if he used Swepco trans fluid and he said no. As Swepco 201 has a Moly additive, would this affect the LSD ?
Good luck on your rebuild,
AJAKE7796