2 trans which should i use?
 

bought a wrecked 82 sc a while back and promptly stuck the 3.0 in my 75, with the 75 model 915 (lower gear ratio) My 82 915 is geared higher (thats what i read anyway) and is also a limited slip unit. The only other steet car Ive had with limitd slip was my old vette, and i loved the way the vette would squat and pull out of corners. so, im kinda stuck on which one, lower geared (pulls awsome, ive popped a few necks) or taller geared (more top end?, posi, and newer) getting ready to pull motor out to fix a rear main seal, and would like to get this sorted. any suggestion from anyone?

type of driving?

d
 

weekend hot rodding thru tennessee hills, ocasional run to red line in fourth. putting down the main drags in memphis on sat nites.

That's a really tough choice (handling or acceleration). How much trouble would it be to put the LSD in your 75 Box or put the lower Ring &Pinion in the 82 box?

t
 

no problem for me at all since i cant do it, I thought of this a while back, and i dont see why it cant be down, can it? the 75 915 needs new syncs anyway, the reasoning porsche went taller i read, was the taller gearing made the 3.0 feel more torqy for the american customer.

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

Issues to Consider:
- LSD can make it feel more understeery...
- Al cases are heavier than Mg
- Guide tube
- some say 7: R&P is inherently weaker; others say it is not the R&P itself but associated parts
-Gearing - which is better for your roads?
- Speedo drive
- Condition of each box...

You COULD rip them both apart swap R&P (I guess that can be done; or change the end housing too); install the 2 Wevo fixes; and pick thru all the gears for what you want...

Try to get Bill Verburg's and Grady's attention on this.

If you keep the '75 then definitely grab the side cover off the SC box.

tran
 

thanks rwebb, now im a little more educated on the 915. this would not normally be a problem for me as i would just swapp one out and see which i like best, (pulling my motor is no big deal for me) BUT, i have the early 75 with 4 bolt lobros, and the 82 as u know has 6 bolts, not a big deal as i have the complete 82 car with all suspension parts, but it would be a major pain swapping back and forth 4 bolt hub to 6 bolt , back to 4 bolt, Oh, and the axle flanges will not interchange, tried that, spines are differnt.

IIRC, the 4 bolt ones you describe have 2 roll pins (Spirol) in the same position as the "extra" 2 bolts on the 6 bolt CV.

Check it out as it may be a lot easier than you think...

hub
 

please tell me u are saying that four of the holes on the 6 bolt axle will line up with the 4 holes on the flange, that would make things alot easier on me.

OR, are u saying that 4 of the holes on the 75 axle will line up with 4 of the holes on the 6 bolt trans flange, did i say that rite? do u get what i mean?

yes, think they may line up - you are gonna have to play around/measure...

Don,

Good advice above.

Obviously the best choice would be to pick & choose all the best parts between the two transmissions. You could rebuild the one you intend to use so it has all the best parts and all the appropriate new parts. You could reassemble the other as a ‘spare’.

To expand on the above pros & cons: The ’82 has smaller CV joints than the ’75 – 6-bolt (M8) and a 100 mm CV joint compared to the ’75 4-bolt (M10) plus two 10 mm pins on the larger 108 mm CV joint. The 108 mm are far better. That said, you can configure either transmission with either size CV joint.

I’m in the camp preferring the 8-tooth pinion gear for strength and longevity.

You will need the mechanical speedometer nose piece on whatever transmission you use in the ’75 car unless you convert to the electronic speedometer. Very doable but requires an extra harness in the tunnel.

You will NOT be able to use the 8:31 ring & pinion with a mechanical speedometer as the pinion shaft does not allow for the speedometer drive gear.

Both transmissions allow for changing the input shaft seal without dissembling the transmission. This problem was only on ’72-’73.

Although slightly heavier, the ’82 aluminum case suffers from potential case damage where the bearings seat in the main (differential) casting. If currently OK, the life can be extended with a Wevo part. The ’75 mag casting doesn’t have that issue.

Yes, you should use the differential side cover from the ’82 on whichever transmission you use.

The ZF LSD in the ’82 is a good street part but has fewer clutches than the earlier version. It will require occasional service (replacing friction discs).

The LSD will tend to induce a little ‘push’ under power but tends to add a bit of trailing throttle oversteer. It may add to rear tire wear. Your ‘test’ for seeing if you need an LSD is if your driving conditions and style has inside wheel-spin.

There is a difference in the sliding sleeves and ‘spider’ hub. That was made wider/stronger with the ’77 model. You can fit those parts from the ’82 if good.


So, what to do?

First, I recommend you carefully dissemble the ’82 and see what is inside. The condition of the parts may render any choice moot.

When making decisions, keep in mind that it is a great deal of work to reset the ring & pinion specs (preload, position & backlash) when swapping parts around. It should be done properly with the correct tools or you will have premature R&P failure.

How are the syncro performance in the ’75? Even if reasonably OK, I would always replace some parts with new.


Assuming that everything is ‘normal good’, I think you would be best with the ’75 transmission. You could replace the differential side cover but should install new tapered roller bearings and the two main pinion bearings when you set the R&P. I would also use the stronger ‘spider’ hub. The LSD decision is yours. If you don’t need it, it can help fund the project. They seem to go for $1000 to $1500 if good.


Get out your tools and digital camera and let’s diagnose the ’82.

Best,
Grady

PS: I’m open to other’s opinions - not all this is cast in steel … and cost is always a consideration. :D
G.

hey Grady, man do i like it when you respond to my post. it seems the way to go is using the internals from the 82 915 and the case from the 75 915. in anticipation of using the 82, i sent the 82 elec gauge with the rest to hollywood speedo, and have over 600.00 invested? in that, and i like the idea of having lsd. Grady, are you the trans man? i have no problem shipping or delivering both trans to you for an expert overhaul, or if you could recomend someone. Money has turned into a no issue on this car, i quit keeping recpt. at aroung 25g and that was quite a while ago. I love col, how far are you from wolf creek pass ski resort? i havent been skiing in a while.