Transmissions, Porsche Transmissions Options

[Randal]

I keep hearing about all the different transmission formats that can be put into a 914 (4 or 6).

One of the guys at Sundays event is building a 914 with a 993 engine with the transmission turned upside down. Now how can that work, i.e., shift patters, etc.

So what are all these different transmission numbers (914/915/?)and what are the differences?

And why are 915 transmissions so expensive?

[ArtechnikA]

One of the guys at Sundays event is building a 914 with a 993 engine with the transmission turned upside down. Now how can that work, i.e., shift patters, etc.

So what are all these different transmission numbers (914/915/?)and what are the differences?

And why are 915 transmissions so expensive?

i'm not the expert here but i can rough out some of it ...

one way an inverted transmission can work is that you can do anything you want with mechanical linkages, including change the direction of twist. another possibility is that the gears have no idea what position they're in on the stacks - on a transmission without a fixed 2nd (like the 901/914) there's nothing to keep you from assembling the clusters in any order you want - all the rotational portion of the linkage does is decide which pair of gears is going to be selected by the fore-aft motion.

i'm not going to write the book on the various transmission types - that's been done, the information is out there. check out Barth's "The Porsche Book" or Frere's 911 Story, or "Excellence Was Expected" for some of the development history.

nowadays 915's aren't -that- much more expensive - but they're bigger, stronger, and able to transmit a lot more torque. they have provisions for a built-in gear oil pump for external cooling.

the cost comes in because they're not set up for mid-engine use - modifications are required. they're enough longer that you have to choose between some kind of custom exhaust or sideshift - you can't tail-shift a 915 in a 914/6 with the original muffler - no room. the linkage bits for side-shift 915's are very low volume parts - basically lots of custom machining.

but they do support lots more ratios than the 901, and for racing purposes, 5th is overhung in the nose cone so it's pretty easy to change - and top is a gear you'd like to be able to change in a hurry...

[Brad Roberts]

One of the guys at Sundays event is building a 914 with a 993 engine with the transmission turned upside down. Now how can that work, i.e., shift patters, etc.

Did he tell you what tranny ?? Did he tell you that the tranny will be hanging out the back of the car by 3 inches...LOL Unless he uses a early G50 upside down... NONE of the 993 boxes will even fit under our cars. Typically they would cable shift these cars.. the pattern is always in the box... you just have to twist the cables to match the upside down box.

So what are all these different transmission numbers (914/915/?)and what are the differences?

901/914/915/930/G50/G50-50/ << the ones you need to worry about. As the numbers go up.. the tranny gets bigger/stronger and more expensive. Every 911 from 1973-1987 had a 915 box in it. Everything after 87 had a G50 in it until 92 or so. The G50 was Porsches best shifting box. ALL the Porsche tranny's after the G50 (except the new 996 non-turbo) are based on the G50 platform including the 996 TT box and the GT3RS race cars.. they just added gears and made them cable shifted.

And why are 915 transmissions so expensive?

1200$ for a 915 is not expensive... we will have 7000$ in the 915 for the 9146 that I am building. This is WEVO everything from the shift knob back. New limited slip... blah blah..

[nine14cats]

Hey B,

I was at that lunch time conversation about the flipped 993 gearbox. I think the guy is going to use the tub and have custom carbon fiber body panels all around...a hand built car from what I can tell...

On the topic of tranny's....my 901 is an open diff. Would I can a bunch of time on the auto-x/TT circuit by going to a limited slip? I see 40%, 80%, locked....which one do you recommend? More $$$ for the winter..... (IMG:style_emoticons/default/barf.gif)

Thanks,

Bill

[Brad Roberts]

ZF style from Paul Guard. 80%

I see you slowly moving away from AutoX.. so 80% will be fine. You'll have to learn to drive it again in AutoX (it will want to understear) but within a couple of runs you will have the driving part nailed.

Dont get me wrong.. the whole tranny upside down thing is cool.. but unless he is Lawrence (english speaking guy) or 1-2 other people in our area (Like Hayden from Wevo)... this guy has NO idea what kind of hassle it is to flip a gear box. The four bolts that hold the tranny to the engine are not symetrical. The tranny case has to be cut and welded (while in a jig). Should be cool.


B

[ArtechnikA]

....my 901 is an open diff. Would I can a bunch of time on the auto-x/TT circuit by going to a limited slip?

i'm not B, but :-) ...

are you spinning the inside rear wheel ?
LSD lets you apply traction where an open diff will spin a wheel.
it can help a lot if you want (for whatever reason) to run a rerar antiroll bar - which will tend to raise the inside wheel.

it's very useful for big-track events with elevation changes -- AX is typically flat.
it's nice in limited-traction events if you race in the rain. or drive in the snow.

you also get to decide between a Torsen 'torque biasing' type (Quaiffe, Guard) or a ZF-style clutch-pack locker. each has its advantages and i won't pretend to be able to quote them all. either way you go Paul Guard is the go-to guy, i think.

[nine14cats]

Geez...you mean more decisions! (IMG:style_emoticons/default/laugh.gif)

Yep...I'm spinning the inside wheel of my car in auto-x's or in tight track corners like #2 at Buttonwillow....I've decent stick with the car since I'm running cantilever slicks on it, and I have plenty of HP with my 250HP+ 6 cylinder....I have to really take care in putting the power down when accelerating out of a tight turn...

I'm hoping the limited slip will help my times and my car control....and I like running the rear roll bar hooked up...I've tried it hooked up and un hooked and I prefer it attached.

How much for the 80% LSD?

Bill

[nine14cats]

Randal,

I didn't want to hijack your post....are you thinking of swapping your 901 out? Going to a 6?....I have to say...having a 6 keeps me grinning ear to ear...and with your car running slicks now...you looked like you were gripping pretty good out there!

Bill

[Brad Roberts]

Guard LSD is 1450 $ and another 300$ or so to have it setup in your box.

I dont want to burst any bubbles... but Randal was 15 seconds behind us at the last time trial we attended with him. Unless he is running 2:15's consistantly now at THILL... he needs more seat time before he needs more power.


B

[Brad Roberts]

Where you will notice the LSD working for you:

Top of 5 at Thill. Top of the corkscrew at Laguna. Top of 9 at Buttonwillow.

Any place the car becomes unweighted when you should be on the gas.. the LSD will end the tire spin.


B

[EdwardBlume]

At the AX we notice Tom Provasi's Orange Crate with a 901 in it. I'm fairly sure he runs a prepped 3.6.

[campbellcj]

The LSD also stabilizes the rear end of the car under heavy braking. I have less than an hour on mine so far, but Me Likes.

[Randal]

Hey Brad, the reason for more power is to move up in my primary focus, i.e., AutoX.

On the other hand I really love time trials and hope to be spending more time there, but it is tough on equipment and costs lots of dollars.

But in the end your right as seat time is what it’s all about. Rich told me that the first day I met him.

By the way all us little 914’s did pretty well Sunday at the Marina. Check out the results on GGR website. I don’t think anyone was holding back.