Transmission: rebuild or buy?

[gliding_serpent]

I am planning on reinforcing and upgrading my 915 drivetrain and transmission. It is fine but 1st and second are showing wear signs. My main goal is to reinforce and futureproof. stock gears, wevo shifter, racing coupler, freshen as needed, bearing retainer plate, new side transmission plates, maybe a wevo streetlite clutch/flywheel, wevo internal gateshift, new mounts, guard 930 pro Lsd.

Tranny will be open this weekend for a look. The question is... With my wants, is buying a rebuilt tranny a more feasable idea? I am thinking not, unless i sell my original.

Thoughts from those who have been down this road before would be appreceated.

[Josh D]

Rebuild yourself. You will find that these transmissions are not difficult to work on. Read Peter Zimmermanns tutorial. There are a few special tools, some of which you can fabricate yourself.

How-To: Porsche 915 Transmission Repair Tutorial Part 1 - Porsche Wiki

[Matt Monson]

With the recent spike in values, I would argue that any car with a 915 is worth keeping the original gearbox with the car.

And used is used, even if it has all the upgrades you want.

[BER]

I just did my 915 last fall. I just replaced the 1st and 2nd syncros (i.e dog teeth), the rests of the syncros were fine. I also replaced the drain and fill plugs because the 17mm hexes were starting to "round-off".

I also replaced the clutch and my flywheel. The flywheel had been resurfaced sometime in the distant past and was showing it's age and needed to be replaced.

My recommendation is to fix the tranny you have.

BER

[gliding_serpent]

I like to keep a car as original as possible... So will likely rebuild.

Would like to do the work myself, but young kids and work do not give me the time. Time is the most valuable commodity in my life right now.

[911 Rod]

Maybe do both?
Pick up a used one so you can take your time upgrading your old one?
When you are done sell the used one.
Like renting.

[sundevil64]

Originally Posted by gliding_serpent

I like to keep a car as original as possible... So will likely rebuild.

Would like to do the work myself, but young kids and work do not give me the time. Time is the most valuable commodity in my life right now.

I hear ya there! I am in the same spot.

[gliding_serpent]

Maybe do both?

Pick up a used one so you can take your time upgrading your old one?

When you are done sell the used one.

Like renting.

Interesting strategy but Canadian dollar just took a dive today with lower interest rates. Local option non-existant and shipping/currency conversion from states would kill. Plus the engine is out this winter anyway, so the time is now.

[KTL]

"Rebuild" is a relative term. Lots of DIY rebuilds consist of new synchro bands and dog teeth and slap it back together. But that is ignoring other important components like:

>Operating sleeve (fork moves this back & forth to engage the free gears' dog teeth) & op hub (sleeve is guided by hub)
>Brake bands are usually OK but can get distorted from hard use
>Synchro retaining circlips are cheap to replace ($10 ea.) but most reuse these, which have been known to pop off and eject the synchro, destroying other parts in the process. Usually only happens with hard use like in racing
>How do bearings sound & look? Gears with "shadows" in the inner race are having needle bearing & lubrication issues. Many people ignore the expensive bearings because, aside from being expensive, they look OK to the untrained eye. Plus they don't want to reestablish the pinion depth because they don't have the expensive setup tool.
>How are the bearing races in the diff housing? it's becoming quite common for the pinion bearing race to be loose in the housing. I believe that's the fault of the two piece bearing retainer .

Once you start itemizing everything and adding things up, just the basics to refresh the trans gets quite expensive.

I agree 100% that the one-piece bearing retainer is a must. The two individual bearing retainers do a poor job of supporting those ball bearings and thereby poorly support the main & pinion shafts.

Pete Z's tutorial is a must for people new to the inside of the 915. It does a fantastic job of explaining the inner workings of the 915 and what to look for when repairing it. He should get an award for putting together that tutorial.

Ask yourself what you're really doing or going to do with the car. If its just enjoyable street-based driving, the Wevo streetlite clutch, gateshift and LSD aren't really things you'll reap the benefits of IMO. Plus they're quite expensive. But if you avidly track it, you'll get a much better benefit from those nice parts. For street use the OEM style Sachs clutch is very durable and the aluminum one is a nice upgrade to the original iron clutch.

What you'd really get a kick out of is replacing some gears with quicker ratios. A few sets of new gears also will test your wallet! But you feel that benefit every time you roll on the throttle & accelerate. It's "free" horsepower.

[gliding_serpent]

Originally Posted by KTL

"Rebuild" is a relative term. Lots of DIY rebuilds consist of new synchro bands and dog teeth and slap it back together. But that is ignoring other important components like:

>Operating sleeve (fork moves this back & forth to engage the free gears' dog teeth) & op hub (sleeve is guided by hub)
>Brake bands are usually OK but can get distorted from hard use
>Synchro retaining circlips are cheap to replace ($10 ea.) but most reuse these, which have been known to pop off and eject the synchro, destroying other parts in the process. Usually only happens with hard use like in racing
>How do bearings sound & look? Gears with "shadows" in the inner race are having needle bearing & lubrication issues. Many people ignore the expensive bearings because, aside from being expensive, they look OK to the untrained eye. Plus they don't want to reestablish the pinion depth because they don't have the expensive setup tool.
>How are the bearing races in the diff housing? it's becoming quite common for the pinion bearing race to be loose in the housing. I believe that's the fault of the two piece bearing retainer .

Once you start itemizing everything and adding things up, just the basics to refresh the trans gets quite expensive.

I agree 100% that the one-piece bearing retainer is a must. The two individual bearing retainers do a poor job of supporting those ball bearings and thereby poorly support the main & pinion shafts.

Pete Z's tutorial is a must for people new to the inside of the 915. It does a fantastic job of explaining the inner workings of the 915 and what to look for when repairing it. He should get an award for putting together that tutorial.

Ask yourself what you're really doing or going to do with the car. If its just enjoyable street-based driving, the Wevo streetlite clutch, gateshift and LSD aren't really things you'll reap the benefits of IMO. Plus they're quite expensive. But if you avidly track it, you'll get a much better benefit from those nice parts. For street use the OEM style Sachs clutch is very durable and the aluminum one is a nice upgrade to the original iron clutch.

What you'd really get a kick out of is replacing some gears with quicker ratios. A few sets of new gears also will test your wallet! But you feel that benefit every time you roll on the throttle & accelerate. It's "free" horsepower.

Thanks for the info. I will do search for the Pete Z tutorial.

I will not be doing the work, but we are having a look this weekend to decide what needs doing and what does not (engine included). I will then estimate my budget and try to figure out what the cost/wife anger ratio will be.

Agreed on the gateshift, clutch, an LSD not being needed. If my clutch still has lots of life, I will probably delay that for now. The gateshift can be added later after I gauge the benefits of the Wevo shifter. The LSD is a must.

The short ratio gears are not on my radar as I quite like the current ratio, even if shorter would be of benefit. Our track is tight, so I was near maxing 4th on the back straight on my first day out. I will likely need 5th as I push the car and get more comfortable. With my 2002Tii I just swapped in a 30% LSD with a 3.91 ratio in place of the original open 2.64. It was perfect for our track and reasonable for highway cruising.

[gregwils]

"I agree 100% that the one-piece bearing retainer is a must."

I buy all my new parts here, I sometimes pick up used stuff from fleaBay. I could not help noticing Carquip is selling a new billet plate for $195. Does anyone have any experience with them?

[Jdub]

Unless I'd rebuilt it myself or had a trusted mechanic do it for me I'd forever wonder how the internals were.

[Matt Monson]

serpent, you might want to explore the cost of a new ring and pinion versus a few gears and the install costs. Changing the final drive on a Porsche costs a lot more than the rear end on a BMW.

[gliding_serpent]

Originally Posted by Matt Monson

serpent, you might want to explore the cost of a new ring and pinion versus a few gears and the install costs. Changing the final drive on a Porsche costs a lot more than the rear end on a BMW.

A quick search brought up 2.3K+ (3k once I got it to me) for ring and pinion.
5th cost 500$ in 2009. probably nearing 750$ once at my door.

My bmw lsd was a rebuilt unit, so I got it with the case for just under 1K.

As I said, I like my ratios as is.

[Matt Monson]

You are pulling results on the weaker 7:31 set. Plug in the 8:35 and the price doubles, for the B grade Albins units. Does your local mechanic have a 385 bar? Most mechanics won't put in a ring and pinion for under $600. That's not including the price of labor to rebuild your gearbox.

You gear numbers are also a touch low(over 50% in price, dude, it is 6 years!) but changing the ratios doesn't change the labor cost of your rebuild. They are direct replacements.

You say you like your ratios. The 2-3 shift rpm drops are pretty bad on track. 3rd is what the factory used for 4th and 4th is what the factory used for 5th on the SC-RS. That was after they fixed 2nd and 3rd.

[gliding_serpent]

You are pulling results on the weaker 7:31 set. Plug in the 8:35 and the price doubles, for the B grade Albins units. Does your local mechanic have a 385 bar? Most mechanics won't put in a ring and pinion for under $600. That's not including the price of labor to rebuild your gearbox.



You gear numbers are also a touch low(over 50% in price, dude, it is 6 years!) but changing the ratios doesn't change the labor cost of your rebuild. They are direct replacements.



You say you like your ratios. The 2-3 shift rpm drops are pretty bad on track. 3rd is what the factory used for 4th and 4th is what the factory used for 5th on the SC-RS. That was after they fixed 2nd and 3rd.

How fast do you want to go? How fast do you want to spend? :-)

Thanks for prompting me to think of these things. Great discussion. I get a 2k rpm drop from 2nd to third. I do this shift once on our track. I shift to 4th for the back straight once. Mind you i was just feeling out my car and i only tracked it once (a week after i got it). I suspect to be needing 5th and using a lot more of fourth, but our track uses a lot of third in most of my cars.

In truith your advice makes me think i should be more sequential in my upgrades. I need a few more hpde's to feel out my car, and really determine the gearing i need, otherwise i could just be adding a lot of shifts in inconveniant places, that might even hurt my times.

After my second hpde in the speed3, i was a full gear higher in almost every location compared with the hpde before. Mind you, that car is a (relative) torque monster.

With my ssi's my mid range should be ample around 4k, so a shift at hp peak at or just above 6k should keep me near torque and hp peaks. I think peak torque for 3.2 ssi setups tends to be around 4.5k if memory serves.

Mind you i don't have larger diameter headers. Compramise is part of my street/hpde build.

But yes, i would love to be maxing 5th on our back straight.

Not sure about the 385 bar. Need to search that one. Learning on the fly here (half the fun). I know he has done a number of 915's, including reinforcing and upgrading gear sets for some sc 915 3.6 guys. I think one, maybe even two of those guys chewed up second in our second gear turn mentioned above, so he replaced/upgraded that and did the retainer plate, side cover, etc. I know (at least) one used a guard second (from a turbo?) that is longer and more robust. I think he also replaced 1st with one of your options but i am weak on details.

Without looking at the math i am guessing that would help with the second to third problem by making second longer. A longer second Might be a good upgrade because i was already thinking i will soon need more second to prevent a 2-3 shift on turn exit on our second gear turn.

My 5th is a touch loud, which is not uncommon i guess. Who knows, if it needs replacing, i might start thinking alternate gears. A shorted 5th, and longer second might work. Will digest and research. Your thoughts would be appreceated. I should end up with a 30-35hp bump from stock which will give you an idea of the power my car will have.

Again, i should know more when we look this weekend.

[KTL]

Here's the Pete Z tutorial. Great info on how the 915 comes apart and goes together, along with great tips on what to look for.

List All Pages - Porsche Wiki

Agreed the LSD is a big win when you are tracking the car. The difference you feel, most notably the increase in stability under heavy braking, is huge. Drive the car on the track w/the open diff for a while and then feel the difference w/the LSD. You may notice that the car previously felt "floaty" under heavy braking, whereas w/the LSD the rear end feels much more planted.

Agreed w/Matt that the gear ratio thing is a collective look at your whole set of gears. Well, all gears except 1st since most people aren't inclined to replace that gear, plus a new mainshaft, on account of the shaft having 1st gear permanently attached to it. A gear chart spreadsheet is a must to see what you want to do in finding a compromise arrangement of gears for the track(s) you frequent.

If your 4th is great, great. But 2nd-3rd is quite a large drop and you can adjust that by using a slighty "longer" 2nd. I have a 1.684 2nd (stock is 1.833) in my 915 and it's great on the track because most of my tracks are such that we want a 2nd that has more legs. Basically for me stock 2nd is a bit too short (or the engine redline is a bit too low- Carrera & SC are most affected by this) since stock 3rd (1.261) can be rather lame in slow-ish corners.

I have in my '84 trans:

2nd = 1.684 (stock = 1.833)
3rd = 1.318 (stock = 1.261)
4th = 1.125 (stock = 1.000)
5th = 1.000 (stock = 0.789)

Here's what it looks like on the move. https://www.youtube.com/watch?v=AqMSh93drWo I use the long 2nd gear a lot at that track (Blackhawk Farms Raceway)

Do a search on the forum and you'll see a number of threads on gear ratio discussion. There's many gear charts posted in those threads to give you an idea of the rpm drops & speeds in each gear.

[gliding_serpent]

Matt sent me a great spreadsheet. I will play with it tonight.

I also thought the longer 2nd would help in our only second gear turn. A shorter 5th might help, but i am near maxing 4th on the back straight so i would not go as agressive as you. Plus add a lsd, r comps, increased experience driving the car, 10 more hp, and 200-250 less lbs.

Will digest. I have not given this topic much research to date, but now i am thinking.

Thanks for the link. I read it through last night. Good stuff to know. I will read it again before i review the open transmission with the mechanic this weekend.

[Bob Kontak]

Originally Posted by Matt Monson

With the recent spike in values, I would argue that any car with a 915 is worth keeping the original gearbox with the car.

+1

The things that caught my eye when Ed Petrushka at Mayo Performance rebuilt my box were:

1. Parts prices were the same as Pelican

2. The price of the "work" was $750 (I shipped unit - 7 years ago)

3. I don't have them things in the pic hanging on the wall in my garage.

[gliding_serpent]

3. I don't have them things in the pic hanging on the wall in my garage.

Haha. I am smart enough to know i am not smart enough to do this work.