915 vs 901 vs boxsterS - Opinions needed., pros/cons, difficulty moving to, etc... Options

[naro914]

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

[SirAndy]

pros/cons

Yes.

(IMG:style_emoticons/default/biggrin.gif)

[pcar916]

initial cost for a 915 conversion is simply more $ (IMG:style_emoticons/default/beerchug.gif)

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.

[naro914]

Thanks Ron, that is very helpful.

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

[pcar916]

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

[Richard Casto]

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

Richard

[mepstein]

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

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

[rohar]

Thanks Ron, that is very helpful.

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

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.

[PeeGreen 914]

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 (IMG:style_emoticons/default/beerchug.gif)

[naro914]

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

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...

[PeeGreen 914]

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.

[naro914]

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.

[PeeGreen 914]

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.

[pcar916]

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.

[Racing916]

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

[naro914]

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.... (IMG:style_emoticons/default/smile.gif)

[brilliantrot]

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.

[SirAndy]

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.

(IMG:style_emoticons/default/popcorn[1].gif)

[mepstein]

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

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...

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 (IMG:style_emoticons/default/biggrin.gif)

[naro914]

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... (IMG:style_emoticons/default/smile.gif)

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... (IMG:style_emoticons/default/smile.gif)