915 Tranny Conversion Options Options

[Coloradocurt]

I've already done a good bit of searching and have yet to find what I'm looking for, so here goes-
Is there a good, comprehensive thread which explores/discusses the tradeoffs (functional performance, prices, availability) of the various options for converting a 915 tranny for adaptation to a 914-6 conversion project.
Thanks.

[0396]

.

[Steve]

It's been pretty hashed out in this thread.
http://www.914world.com/bbs2/index.php?showtopic=102988

I'm running Martin Bott's kit. I would recommend it for a 3.2 or 3.6 motor. Anything smaller I would stay with the 901/914 gear box. If I had a 3.6, I would look into the 6 speed boxster S trans and cable shifter.

[GeorgeRud]

Not many options out there. Martin Bott's Kit, Wevo, or an old Vellios Kit if you'd find one. Setting up a 915 with a cable shifter setup using the original 915 tail shifter position is another option.

They're all $$$, but are appropriate if you have a large engine conversion.

[Coloradocurt]

For those Bott, WEVO and Vellios options,
Shifting precision pros and cons;
Installation challenges pros and cons;
Pricing pros and cons.
The car I'm getting has a 901 side-shifter w/ 3.2. I'm told that at the very least a Rennshifter is a must if I stick with the 901. I've also gotten the impression that the 901's 1st gear is a problem. However, since I don't dag race (i.e., drop the clutch), is that a problem with 1st gear being able to take the 3.2's torque, or the ratio being such that it's MPH limited (i.e., too short)? And the 8x16 rears will wear street tires.
Although it's been 20-30 years since I've overhauled 915 trannies, I realize if I decide to go down that road a $5K budget figure is realistic.

[Steve]

The biggest issue for me with the 901 with a 3.2 is that 1st gear is too low to be of any value. So you end up starting off in second instead. Which means you more or less have a 4 speed instead of a five speed. My 3.2 is a euro 3.2, which means it has 231 hp and that's in a stock 911 with a catalytic converter. So it probably has more. So for me 1st was worthless and 2nd was too tall so I had to slip the clutch a bit to start off in second. If yours is a stock US 3.2, this might not be a problem, since they have less hp. Personally I would baby first and go nuts 2nd-5th. If your happy with it leave it alone. If only 5th bothers you because it revs too high at fwy speeds, then get a flipped H for 5th.

[Steve]

No matter what, the 901 is easier to shift, cheaper and weighs less.
10lbs less than a magnesium 915 and 20lbs less than an aluminum 915.
The Bott kit is a true 916 kit, so your buying a piece of history. It also supports a stock 914-6 muffler. It also has a reverse light setup. Wevo does not and requires a special muffler. The positive side for wevo is that it's been out longer, which means more shops in the US are familiar with it. CFR has installed both. If the 901 is too annoying for you then I would have CFR build you a Bott 916 gear box. Then you have to decide what box to use. The last year magnesium 915 box has an 8:31 ring and pinion, horse shoe spring and electric speedometer. This trans is highly sought after, because it's lighter than the alumninum 915. I'm running an 86 915 gear box, because it's the last year made and has 108mm cv flanges. The earlier 915's only have 100mm flanges, which is no big deal. The 108's are overkill.

[brant]

[quote name='Coloradocurt' date='Dec 26 2016, 12:15 PM' post='2437654'
Although it's been 20-30 years since I've overhauled 915 trannies, I realize if I decide to go down that road a $5K budget figure is realistic.
[/quote]

Real world pricing
Add another 5k to your number
10k if you intend to race/gearing/upgrades etc

[Steve]

[quote name='brant' date='Dec 26 2016, 04:19 PM' post='2437731']
[quote name='Coloradocurt' date='Dec 26 2016, 12:15 PM' post='2437654'
Although it's been 20-30 years since I've overhauled 915 trannies, I realize if I decide to go down that road a $5K budget figure is realistic.
[/quote]

Real world pricing
Add another 5k to your number
10k if you intend to race/gearing/upgrades etc
[/quote]
I have 10k in my trans. I only paid $5k for the car and $5k for the motor. However I bought the car in 86 and the motor in 2000.

[sixaddict]

Unless you really plan on this being a track car changing to a 915 is no t necessary in my opinion and experience. First is usable but only to get it moving. Flipping a couple of gears to get the ratios closer in 3 to 5 is very beneficial.
You can have many spare boxes for the money you spend going 915.
Really comes back to how You intend to use the car.
Street use means it would be hard to rationalize unless you just want it and have big budget.

[ClayPerrine]

Porsche went to the 915 with the release of the 2.4L engine for a reason. I have a 901 attached to a 2.4S motor, and I have broken it three times now. Big tires, torque biasing diff, and 944 turbo CVs means the weak spot is now the trans. Unless you baby it, a 901 wont last behind a high horsepower motor. And what is the fun of having a 914 you have to baby????

I don't like the 915. But I was considering it before I decided to upgrade to the 4.0 Monster motor. Lots of friends in PCA tell me the 915 won't handle a really hopped up 3.2 used for racing. I have seen lots of 915s eaten up at the track. Should not be as bad in a 914 because it is lighter.

[0396]

.

[Steve]

That's why I paid the bucks for a 915.
1. Gearing that matches the motor
2. Piece of mind, not having to worry as much about breaking it.

[Coloradocurt]

Lots of good input - keep it coming.
I've grown beyond the stage of high HP and brutal acceleration. My Rotrex supercharged 2003 MR2 Spyder (w/ 305 WHP) is now NA as I more appreciate the much-upgraded handling. My daily driver, also Rotrex'd, is a 2008 Honda Civic Si sedan w/ Mugen suspension. Although I now live at 6400 foot elevation, at sea-level that would be around 370 at the wheels.
So getting the 914-6 conversion takes me back 20 years when I built one with a 3.0, S-cam'd, Weber-carb'd and 915/Quaife tranny. And this one will be for weekend trips up into the mountains. "Maybe" some AX or DE.

[naro914]

We have both:
In our 3.2 'street' engine car (Huey) we've run a totally stock 901 for many years with no problems. It gets used for spirited mountain/street driving, autocross, track time trials, and the occasional club/vintage race. We beefed up the CV's to 930 turbo (I think) but that's it. We use first gear all the time from stop lights/signs, around the paddock, starts on autocross, etc... It's no less useless than the 1st gear in the GT3...

In the race car with 360-ish hp (Papa Smurf), we have a 915 with modified Vellios side shift. Modified meaning it was 're-engineered' to work right, and it works great. We also have a recirculating pump and cooler on it. But I can't tell you about longevity since we have only driven on the track with it once now.

As Sixaddict said, the thing about 901's is you can have a garage full of spare 901's for the cost of a 915 side shift. I used to keep 2 spare transmissions for the track when Papa Smurf had a 901. I don't have a spare 915 because I have about $8k into this one and can't justify another $8k sitting in the trailer.

If/when someone (Clay??) comes up with a reliable and cost effective set up to use a Boxster/Boxster S trans with an air cooled engine, I may go that route with Papa Smurf. Until then, we'll keep everything as is.

Curt, my advice: keep the 901 in it. It should be fine unless you get into hard core racing...

[PanelBilly]

I've had my 901 modified with different gears. Second was changed out so that the ratios make more seance with my 3.0. I had a new second gear welded onto the shaft so it can't be changed out. More $ than the stock 901, but a lot less than changing out to a 915.

[slivel]

My equipment: race prepared 901 with AFNSY gears and billet intermediate plate. 335 HP 3.4 l twin plug six. Track only 914.

My experience: after 16 years racing with this configuration, I have had two failures - 3rd gear and ring/pinion. Both failures happened I believe because I was trying to eeke out little faster lap time by riding the curbs and gators at the track. I think that the severe loading and unloading of the drive train when driving like I did prematurely caused the failures. I don't drive the curbs much anymore and consequently have not had a transmission failure in more than 10 years. I do change gear oil regularly and look for evidence of problems and I tear-down and freshen (bearings/synchros) about every three seasons. I use first gear to get moving and never use full throttle in first. The billet intermediate plate is important in high HP cars, in my opinion.

Your experience may vary.

Best,
Steve

[slivel]

I should add to my previous post that I do use a cooler that directs cooled gear oil to the gearset and the R&P.

[larryM]

what everyone says above is on target -

> be less concerned about gross HP, than about torque when you try to peel rubber in 1st or launch at the AX; - i don't use ever 901 1st gear except to get up the hill & out of the driveway

- the 901 has a known weak input shaft when it comes to smoking the tires - which can be "fixed" with a $1200 shaft from a 904 (when yu can find one)
http://www.ebay.com/itm/Porsche-Genuine-90...R-/121057562374

when i upgraded to the torquey 3.2 from a 2.7RS, i bought the Bott kit since i already had a rebuilt 915 left over from an aborted 911 track-car project;
- it is all still on the shelf (maybe for someday, but highly unlikely, since i will never again race the car) - i figure it is just money in the bank at no interest

during the 3,2 upgrade project, i happened to meet a SNR fellow who'd raced a 914 with a V8 chevy in scca - with a 901 - he never had a failure over several seasons; said the 901 would easy handle 250hp & the Chevy torque; so given that my chipped 3.2 makes about 235, i decided to leave the 901 in there & see how it went - was a lot cheaper & easier to get it all installed & on the road

during my deliberations on installing the 915, a friend was doing a 914-3.6 conversion with a wevo kit 915 - a lot of custom work was involved including the shifter, muffler fit, hydraulic clutch, etc -

i have had a 901 sideshift conversion (which shifts tightly & beautifully) with a simple Weltmeister short-shift kit, a custom GPR shift rod & bronze bushings in my car since i started racing it the early '90s, and raced it for 10 yrs behind a 210hp 2.8 - had only one failure where it chewed up intermediate plate & ring gear at Thunderhill - and that was a previously used 901 from a donor junker 914 of unknown history (fwiw: the 901 i'm currently running is from a similar donor with only the 1st-Rev fix done to it)

for that reason - like some others above - i have 2 more known real good 901 sideshifters also in storage; so if i do go track-ing again (PCA DE planned for 2017) & break the 901, the fix will only cost me 2 days time

(that pair of 901s is only worth about 20% of the value of the 915/Bott parts on the shelf) - cheap insurance

some good tips on the 901 here: http://forums.pelicanparts.com/porsche-911...x-how-do-i.html

. and here is a hot-rod thread with a lot of good links & Rennsport Systems transaxles info: http://www.fiero.nl/forum/Archives/Archive...9-2-086306.html

.

[mepstein]

One of my cars is a 3.2 with a 914 trans, one will have a 3.2 and a 915 trans with cable shifter and one will have a 3.3 suby 6 with a cable shifter suby trans.
I like options (IMG:style_emoticons/default/smile.gif)