Building a 915 - Need Advice

[Fishcop]

Hi All

It's just about time to rebuild my car's 915. Here are my choices: I have two 915 boxes, a '74 4 speed mag case (user serviceable guide tube) with mechanical speedo drive and 7:31 R&P; and an aluminum '83 915 5 speed with electronic speedo and 8:31 R&P. I presume the gear-sets in both are stock.

The box will be in my '69 and bolted up to my 2.7RS spec engine and I am happy to have a hybrid box built from the best parts of both... I will keep the '83 (SC) drive flanges and the mechanical speedo drive.

My driving is weekends and spirited through country roads and the occasional track day, fuel economy isn't a big deal in this car.

Presume both boxes are shagged and need all the usual bits replaced... There is also probably budget enough to stretch for a Wevo Gateshift and bearing retainer if necessary.

What do you guys reckon would be a nice box?

Thoughts and opinions appreciated.

[Flat6pac]

The early R&P in the late model box gives a responsive box in all gears but its a little too short for long trips because the higher RPM at road speed.
Bruce

[Joe Bob]

The early R&P is nice for low end but runs outta gas up top. You can always look into a higher final drive gear.

The later side cover is a better idea for a "banger" trans...less flex. Ask Tyson or Dave from TRE on that.....

I'd recommend adding a bearing retainer plate on whatever you choose.

[RWebb]

I would put a 901 or 911 not a 915 in there! I would of course, strengthen the early trans. as much as I could with various afternarket parts - int. plate; mainshaft - maybe there are others.

Why go with a trans. that [1] lacks the light shifting feel of the earlier boxes, and [2] lacks the outstanding road-racing shift pattern?
- the only reason I can find is if you have gobs of torque to deal with

there is a thread on the best 915 - which turns out to be a hybrid of several years....

you will want to look at your motor's torque characteristics, on what roads or tracks it is driven, and then do some thrust calculations

[Fishcop]

Originally Posted by RWebb

Why go with a trans. that [1] lacks the light shifting feel of the earlier boxes, and [2] lacks the outstanding road-racing shift pattern?

Hi Randy, 'cause that's what's in there now and that's all I got

Seriously, over here in oz we just don't have the luxury of picking and choosing. When I converted the from a sporto some time ago, a pelican here gave me smoking deal on all the bits I needed - it just so happened that the 915 box was the only one available.

I picked up the second mag 4spd off evibay - all the numbers were missing and its condition is unknown - but it doesn't have an LSD

I just figure that rebuilding what I've got is the best idea... cost wise; my dilemma is really which R&P suits a) the car, and b) me, the best...

Hey if anyone wants to swap me both boxes for a known good 901 and clutch pack then I'm real keen! Postage to/from the USA is a killer though

[RSTarga]

the 915 is a superior box to the 901. If you drive in traffic at all the offset 1st is a pain in the A**

[Bill Douglas]

Excuse my ignorance, but can you get a REALLY high geared 5th gear for the 915 and use the low ratio ring and pinon.

[MBEngineering]

HI John
I would strip both box's, and keep both on 2 benches for now, check the 7:31 CWP for damage and if OK use the 7:31 CWP, and the mag case's (if not damaged/worn) and all the bearings from that case's, now remove the input shaft from the SC case and 1st gear, slider and spider, as a base, swap the sincro parts from the SC 2nd gear onto the 2nd in the early box, fit the 3rd (NT) from the SC box with the slider/spider, now flip the old 3rd from the 4speed into 4th, discard the 4th gear set from the 4speed box, now remove the spacers and fit the 5th from the SC box, check the 5th/Rev slider in the 4speed box has teeth for the 5th dog teeth, a general check round for the obvious worn parts and you now have a close ratio box with a cruising top/5th , fit the 1 piece clamp plate and the shifter gate if the $$ will permit, should give you a box that is usable for now until something turns up better??

regards mike

[RWebb]

Originally Posted by Fishcop

Hi Randy, 'cause that's what's in there now and that's all I got
* * *

Hi John, As usual, your reasoning is unassailable!

I'd stick with the original R&P ratios. To some extent it depends on what type of driving & on how easy it is to find different gears and put them in. Remind us what your motor is like...

Here is a starter for some background reading:

http://forums.pelicanparts.com/porsche-911-technical-forum/332597-best-year-915-transmission-post4242866.html#post4242866

BTW - can you repost that old pic of you in your engine compartment with the big "toothbrush"? It was an all time great.

Best wishes for a fun - and inexpensive - rebuild.

[Fishcop]

Hi Mike

That's exactly the info I was after. I just need to work out if I do want to go with the lower overall ratios from the 7:31 diff. Also, I know the mag case seems to be the preferred base to build upon, but will I notice the loss of the omega spring?

Hey Randy



This space is filled with a 2.7RS spec engine - built as per factory except for the webers, so I suspect the 7:31 would be a lot of fun!

PS You're a bird man? I've been involved investigating the big ship oil spill over here - seen a lot of dead birds

[MBEngineering]

HI John
there is nothing to stop you modifying the bottom of the mag case to fit the late clutch arrangement, if you have the later clutch cable, if you need to go back to the old type cable, a bolt on bracket is available form Wevo and myself to sort the prob' of no cable support.
check the 7:31 CWP before you do/commit to stripping any thing and have a pile of bits and no box.

regards mike

[RWebb]

Thanks John - I saved it!

Yes, I am more ornithologist than mammalogist. If you need info on oil spills and birds we have a lot of it in the US.

My former MS adviser did the workup on effects of the Exxon Valdez spill - his name is John Wiens. There should be a ton of info for NEPA documentation here in the US. It is a law that requires all federal agencies to explain the likely env'l effects of any major actions they take.

Send me an Email and can contact some people for more info.

[RWebb]

oh yeh - re the omega spring - the early cars never had one and I have no problem using the clutches in various 911s I've owned w/o the spring... '73 - '75 and some others I've driven.

[Bill Douglas]

Hi John. What's happening with the cleanup and the wildlife? I couldn't believe it, all that oil on what is probably the most beautiful beaches in the world