I'd like some opinions from the general peanut gallery. Is it worth the cost go to a 915 transmission over a 901? What is the benefit, if any? And, what's involved?

How about a Boxster S transmission?

I've heard stories that a 915 is a pain to install, and doesn't every really shift well. I can't comment since I've never driven a 915 in a 914. But, with the bigger engine we now have, is it something I should be considering in the future? Being that I can get a complete 901 for cheap, and already own 3....

I know nothing about the Boxster S trans, other than a 6 speed with cable shifting would be very nice.

thoughts, comments discussion?

thanks

Yes.

initial cost for a 915 conversion is simply more $

I have yet to install a 915 because the 914 seems very hard to kill. But I just broke a pinion gear on the 914 trans recently so I'm building both.

915's can be good shifters too. But they both have the Porsche style synchronizers so neither will be super fast to shift. Built and maintained properly the 915 will shift almost as well and will handle more torque. A few thoughts.

1. The 915 ring and pinion gears are much larger and the ring has a bit longer lever arm from the mesh to the axis. Easier on both R&P. But to get the same ratio as our 914 boxes (7:31) you have to find an early (or custom) 915 R&P. Later ones are 8:31 so the gearing is much different.

2. Shifting them with either the 916 side-shifter style or the Wevo conversion is expensive. Fab'ing a cable shifter is less expensive, but more effort. There are some ready-made cable shifting systems out there.

3. Up above 250hp you should consider cooling/filtering the oil in either box unless you drive real slow, or live in super cold weather.

4. A few thoughts on early vs late 915s.
- The early mag case version is lighter (mag) has a 7:31 R&P ratio, and uses a mechanical speedo drive. It also has steel inserts in the mag case for the front bearings. The aluminum cases just mount the bearings into the aluminum. Diff carrier bearings are the same for either box.
- The mag case 915's use the same differential form factor as the 914 as well as course splines on the output flanges. Later diffs have fine splines and different dimensions on the spigots (reluctor ring for the electronic speedo) and ring flange. Also the late 915's use carrier spacers and shims that are notched for the reluctor ring. Why is all of this important?

If you already have an LSD or TBD diff in your 914 it will be useable in an early 915 mag case.

5. The clutches are very different so your flywheel adapters will differ.

I'm sure others will chime in on this one as well.

Thanks Ron, that is very helpful.

I've also added Boxster S tranny as an option. That was brought up to me today....

I thought about that as well but heard the starter at 12 o'clock high is a deal breaker.
Don't know since I haven't seen one yet. Did I hear that wrong?


There are two other recent threads that are pertinent if you haven't seen 'em

http://www.914world.com/bbs2/index.php?showtopic=158144 and

http://www.914world.com/bbs2/index.php?showtopic=161520

Should prove to be an interesting thread. Two other quick (probably obvious) differences... 915 is heavier and larger (particularly longer) than the 901.

Richard

what are you trying to accomplish? Will it be worth the 5-10K?

The starter is on the wrong side and the bellhousing doesn't match. With a jigsaw, a grinder and a tig welder it's an option. Short of that, it's out.

With everything done right the 901 can handle a great deal of HP and is light. The idea of a 915 sounds nice but I'd rather stick with the 901 with a bulit intermediate and proper gearing for what you want to do. Throw in a TB diff or LSD diff and you're good to go

I'm concerned that the 901 will not be able to handle the 290 RWHP engine that is now in the car. Or at least, not handle it for a long time.... That's what got me to thinking 915. In addition, the stock gearing is way too short on a 901 for this engine. 1st is useless, and top speed in 5th is only 140mph +/-. Works fine on tracks like CMP and Barber, but will not be good on the Glen, VIRand Road Atlanta. The class I'm in for PCA racing - GT3R - has cars like 993 RSR style that can easily hit 170+.

yes, I can re-gear it, but I'll end up needing multiple trannys for different tracks.

915's always seem to have taller gears and I was thinking they might be a bit stronger.

Boxster S is a 6 speed so the ratio's are better with a higher top end.

I'm just looking at options right now. Unfortunately, a huge crack in my suspension point has sucked away all my budget for this year (already...and it's only January!!) I have no idea how well the 901 will do...I guess we'll see!

Oh, and I do have a Guards LSD in the 901 right now...

The 914 that has won at parade the past few years has a 901. It's not even anything special. Granted, he is putting a 915 in it since he broke it last season.

I've been told by a few different engineers that the 300hp point is about the max for the 901 to run without issue. So maybe the 915 would be best.

AX and track racing put very different strains on a transmission. Most autocrosses use 1st, 2nd and MAYBE 3rd gears. and very little overall shifting. yes, the torque strain is strong, but for a very limited time, and very little heat is generated. In a 90 minute enduro with 10-15 shifts per lap and maybe 50 laps, thats nearly 750 shifts and a lot of heat.

I am thinking I definitely need a trans cooler regardless of the trans, and Ron I did read through your threads.

If you look at many of the Patrick Motosports 914s that they have prepped for the track most do have coolers on them if it is a 901. A few look like they have 915s as well.

These transaxle threads are really useful to me because I need 'em to get up to speed on the options.

AX and the 915 are not in my experience except a good (fast) friend in an early 911 with a 3.2, an aluminum case, and a 7:31 R&P would never go back to the 8:31. My assumption in this case is that, unless the 7:31 was significantly weaker when used with a reinforced sideplate, I'd want one on the AX circuit.

All of the AX and track 914's I know personally run a 914 transaxles (or Hewland) and just rebuild the the former every year or so as SOP. I have noticed one thing though not related to the p-boxes.

Their fluids are as varied as their personalities, from ATF to synthetics. I don't know how tough the EP packages are in ATF, but those guys are in dedicated AX cars and that's maybe OT for this thread.

My issue is that, ignoring the initial $/mile issue, whether or not the hit to my power-to-weight ratio with a 915 is offset by greater reliability than the 914 box. I haven't compared their weights in a long time but will over the next week since I'll have them all in the same place soon.

Also: If you use CRC's SP-400 to coat your rebuilt transaxle, it will smell like crayons... just thought we should all know that.

I have driven a crap load of track miles on my 901 with one issue, and have driven my dads 86 911 with a 915. There is not much difference in feel from the 901 to 915 in the "easy to shift" category. while the 915 can take more horsepower my 901 has been great for my car with a 200hp motor. My car is also faster than a boxster s in any trim. I would spend my money on a motor rather then a tranny. I would also first spend my money on suspension upgrades and weight reduction and beat up on some boxsters. I am not sure if this answers your question at all but on the track (not ax) this is my experience.

Steve

Note in my post above: my car has 290 rwhp 3.4 liter engine. Thats why I'm considering a different transmission...

And just for information, I have full coil overs around, 1800 lbs, 12x16 and 11x16 wheels with full slicks. I stopped beating up on Boxsters many years ago...Now....I chase down GT3's and even a few Cup cars....

IMO the 915 isn't enough stronger then a 901 treated right (little use of 1st gear) to justify the upgrade. I as well as many other have broken them in 200whp 911s. The 915 shifts slower then the 901 due to heavier parts so there is no gain there either. The Boxster S 6-speed seems to be a fairly strong transmission from what I have seen of the internals and it uses a G50 diff which is encouraging. The g50 however is still the gold standard of the factory transmission options.

My only beef with the 901 is that when you approach the 300HP threshold, 1st gear becomes a weak point.
You effectively have to treat it as a 4-speed and not use 1st gear anymore.

Surprisingly, this has been much less of a hassle than i thought it would be. But my car is driven on the street on a regular basis.
If it was a dedicated AX or track car, i would have switched to a 915 a long time ago.

In fact, i have a 915 sitting in my shed that already has the RP flipped and was prepped by Bill Pickering to go into "The Beast ™".
I'm going to add a LSD to it and fab a cable shifter (already talked to McMark about some options for that).

My (stock) 901 has held up just fine for the past several years, but like i said, it's mostly street driven these days with the occasional AX and track day.
So the stress put through it does not compare to what you guys are doing.

Sounds like from some of the above posts that the 915 would just be a stepping stone to the G50. At least a re-geared 930 trans would hold the horsepower but lack the 6 speeds. Since you run a rather unique car to this board, you might have to track down what some of the other big horsepower 914's are doing. Maybe Jim at PMS would have some good input?

I really like the way you are taking the 914 to the limit ...and beyond

I keep being told by guys I should ditch the old stuff and move to a more modern platform. Then I pass them on the track and they quickly shut up...

If you like driving a 914 (which I assume everyone on here does), you would LOVE driving papa Smurf. He's such a point-and-shoot car and so easy to drive. Nadine loves driving him - it's so funny seeing this her jump into this beast of a car...though she hasn't driven the with he new engine yet. Next day on the track is Barber the beginning of March. With the 2.2, I could keep up with the GT3's, we'll see what happens with the 3.4...

I'll side with the factory on this one. In 1970 they re-designed
the 911 trans with a larger, ribbed case, heftier ring gear, larger
clutch and cv joints. Then in 1972 they put out the 915 style and
carried over all their upgrades from the '70, and later added on the
Euro tranny oil pump, if memory serves that is. IMO the 915 is an
upgrade, but in the world of racing, things break.

Cryogenics is another "cooling" option.
I'm piecing together a cable-shifted 915 (923) as my budget allows...

We have a track car at the shop right now with a built 3.5.
I think its making at least 325whp.
Trans is 914 with billet intermediate plate and billet side cover.
It has a trans cooler.
20 minute DE sessions isn't the same as a 90 minute enduro though.

So keeping it cool is the trick, hey Chris?

The only place 222 runs that might put some heat into the transmission is Medford and that event is less than 4 minutes, but very aggressive gear changing. In fact I think most people would call it abusive. Got to install new synchros for second and third gears.

Now where is Dr. Evil when you need him?

I think, with that much horsepower, one needs to use every available trick.
I don't think an autocross or hillclimb car needs a gear oil cooler, but any high horsepower track car should have one.

I think the Boxster S is your clear choice. For no better reason that to be a pioneer and pave the way for the rest of us.

I also think that positive cable shifting would be sublime without giving up much driver to drive train sensitivity.

Nice, I'll be the guinea pig ...

I think for the time being, I'm going to stick with the 901, maybe put a cooler on it. Hopefully I don't destroy too many of them. This suspension crack thing put a serious crimp in my 2012 budget, but I'll see how this year goes and adjust accordingly.

No guinea pig risk...Renegade has developed (just finishing) a conversion to use the Boxster S in the 914. No having to try and figure starter location/mounting, hydraulics, flywheel, etc., etc.

The upsides naturally is it is already set-up for mid engine use and it shifts very nicely. The down side is the spider gears are known to have galling and for performance use/activities really should be replaced - youtube it. Depending upon your power aspirations, the Boxster might come up a little short and the gearing less than ideal (to short).

If they've got a conversion for the Boxster S tranny, it'll work with the Audi 01x tranny as well. Geared quite a bit taller and designed to take twice as much torque.

Only problem is finding them stateside. I've seen sources that will ship them CONUS though.

The Beast or the V-8 car?

The 915 he'd built for the V-8 car had all the great stuff built in. If he build another one then it probably has all the Wevo stuff as well. In any event you got a great transmission Andy.

I still think it would be worth the nose bleed to figure out how to use a boxster transmission. And my logic stems from the most basic element, i.e., shifting. The cable shifter on a boxster is magic. Night and day as compared to a 901 or 915.

But the deal killer for track guys would be availability of different ratios. The Spec. Boxster crowd don't change them (rules), so no one is doing much sourcing there. Maybe some other format does and has discovered someplace that is making different gear sets.

The other element (unknown) is how the boxter transmission would do with more power pushing it. The Spec. Boxster guys are all less than 200rwhp.

Longevity with the boxster transmissions is pretty good. Bill Pickering had over 6000 race miles on his before it broke.

The boxster tranny is externally identical to the Audi FWD transmission so if you can bolt one up, you can bolt up the other. The Audi units are available in a huge array of factory gearing in order to handle everything from the high reving 1.8t (and even smaller engines) to the v6 diesel in europe. Not sure what the interchangeability of the internals between the porsche and auid boxes is though.

Interesting.

I would go Boxster S...6 speed and stronger

I was just thinking the other day how robust the trans has been in my Audi S4 - probably a different unit than the FWD cars but it has withstood 165K miles with a chipped 2.7T with torque in the 360lb-ft range. It can be shifted quite quickly as well but the 'feel' is crappy. 2nd gear synchro is getting a tad crunchy but that's it.

For my 914 I'm sticking with the 914 trans prepped for track. It has the classic mods and I'm installing a cooler right now along with engine work. I have had no trans trouble thus far, knock on wood, with a high-HP 2.7 (trying to become a 2.8 now). I do plan to get another box with taller gearing though as a spare and for faster tracks. Spares are never a bad idea...

The FWD and AWD trannies from Audi/VW aren't that different. They stand up and shift about the same for the most part. The AWD versions take a little more pressure as they've got one more shaft to spread the torque around.

The problem I see is how the heck do you get a starter on one if it's bolted to an air cooled porsche engine? Stock, it's on the wrong side. I don't think there's enough room anywhere on the tranny side to blast a hole in the bellhousing and mount a starter there. Even if you could, some metal would have to be removed from the rear trunk as I think it'll only fit on the top.

I'll take some pics of the mounted fit on mine. I'm pulling the engine back out today anyway.

There is a member here that has installed a 6speed boxster trans and made some type of provision for the starter that looked like it would need to be done better for longevity. The car was red but I can not remember the name. I looked at the 6 speeds and from what I understood they are the 996 trans flipped.

Is it possible for someone with an engine out to measure from the center of the crank to the top of the lip on the case at the bellhousing on the engine? I've got a tape measure and some bad ideas...

It came out of the same stash as my 3.6L motor and if i recall correctly that was supposed to go into the Beast as well.

Ok, so I just pulled the motor tranny out from under the car AGAIN. It had to come out, time for some work.

I pulled out a pair of calipers and both a 914 and an A6 starter, turns out they have the exact same mounting.

There are literally 3 positions on the top half of the transmission where a starter could be fit on the tranny side. It would require some hacking and welding on the bell housing, but it'll fit. Bolt positions line right up. 2 would require relieving part of the rear trunk, the third interferes with the stock hydraulic clutch hardware, but it could be modified to allow the starter to fit there too.

I'm starting to think this could have promise, but I don't have a type IV nor a 914 flywheel to measure against.

I'll tear the tranny off the engine shortly and measure the radius of those mounting bolts. If they're the same as the audi starter mount bolts, it's a definite possibility.

I believe that a Boxter/ Cayman 6 speed transmission would work well using 17 inch wheels and an engine that revs similarly to a modern mid engine car. I am in the process to widen my fenders via Sheridan fender kits (I don't like the Porsche GT flares) and use 11 and 12 inch wide rims........ What should be the Wheel offsets be, to clear the front struts (non coilovers) and rear coilovers?
Since the Boxter trany does not have a starter, I would think that one would have to be designed in a new location, this, I think is the most difficult part of the transplant.

That and the output flanges are about an inch off center. There's gonna be axle issues.

Crap, I almost forgot, you need to throw a hydro clutch conversion in there too somehow.

oh that's a piece of cake

I'd like to see that. The starter on the 986/996 is on the engine-side. I think that problem alone makes the mating of that transmission to an air-cooled engine somewhat less than trivial.

Looks like an Audi 016 automatic, but after a bunch of measuring this weekend, pretty similar:

Link

You have a modern motor so a more modern gearbox makes sense. I would think you would be much faster with a modern lsd. would you ever add that to your boxster transmission, and also would you ever change gearing for a big track like watkins glen or vir?

Can you run a audi tranny in PCA club racing?

Nope, but the boxster tranny is identical to the Audi 012 which is very similar to the bellhousing on the 016. There's a lot of transitional assumptions going here, but if it can be done with an 016 and the measurements from an 012 match (I checked this afternoon) then a boxster tranny should work the same way.

I need a type IV in the shop again so I can bird dog this dumb idea.

dangit, doublepostage.

The Audi box I'm running is already very tall. It was geared for the 12v V6 with a redline at about 6k. With the solid lifter conversion on the V8 I'm using, I should see 8k easily. I'll be shaking the car down at Spokane County Raceway. I swear it's got the longest straight of any track in America. If I get bored half way down the straight, I'm ordering an 01x from Europe. A tranny built for a V6 TDI should be more than tall enough.

What kind of MPH will you see @ 8K RPM with the gearing in the 012? If you don't have the ratio's, give me the three letter code; I may have it in my files (somewhere). Are you building a track car, or street car?

For my street car with an LS1/01E, I get 202MPH @ 6K RPM calculated in 6th....not that it would pull it nor would I ever attempt it in a 914!

Andys