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Carrera Transmission Rebuild Suggestions Please

After the summer season has ended I want to rebuild my 85 Carrera 915. I want to plan it out and do it right. I don't plan to race it just street driving. What suggestions do you tranny guy's have as to the gear ratios, improvements, and the best place to acquire these parts? Any advice is welcomed. Thanks Much!

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1974 sahara beige 911 targa
1982 chiffon 911sc
1985 prussian blue metallic carrera

Last edited by kodioneill; 04-14-2010 at 12:55 PM..
Old 04-14-2010, 12:51 PM
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Quote:
Originally Posted by kodioneill View Post
After the summer season has ended I want to rebuild my 85 Carrera 915. I want to plan it out and do it right. I don't plan to race it just street driving. What suggestions do you tranny guy's have as to the gear ratios, improvements, and the best place to acquire these parts? Any advice is welcomed. Thanks Much!
Mike, here's what your transmission looks like now, you have a 915/73


It's actually pretty good all around street transmission, the only reson to change anything would be for some specific situation that you are currently not satisfied w/
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Old 04-14-2010, 01:25 PM
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gearhead
 
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As Bill alluded to, what about your gearing don't you like? What are you trying to achieve in changing it?
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Old 04-14-2010, 01:35 PM
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Hi Bill, The shifting is notchy and difficult at times, hard to downshift and upshift and, it seems tired. Looking to procure parts during the summer so the financial hit ain't so bad. I was going to give you a call and see how things are going .
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1974 sahara beige 911 targa
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Old 04-14-2010, 01:39 PM
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Quote:
Originally Posted by Matt Monson View Post
As Bill alluded to, what about your gearing don't you like? What are you trying to achieve in changing it?
What did I say about the gearing that makes you think I don't like it? I am surfing for ideas from others experience and thoughts.
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1974 sahara beige 911 targa
1982 chiffon 911sc
1985 prussian blue metallic carrera

Last edited by kodioneill; 04-14-2010 at 01:56 PM..
Old 04-14-2010, 01:53 PM
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Quote:
Originally Posted by kodioneill View Post
What did I say about the gearing that makes you think I don't like it? I am surfing for ideas from others experience and thoughts.
You didn't say anything about not liking it. You said you wanted input on ratios. So I asked a question about what you want out of your ratios. Pardon me if I insulted you with my probing question.
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Old 04-14-2010, 02:05 PM
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Quote:
Originally Posted by kodioneill View Post
Hi Bill, The shifting is notchy and difficult at times, hard to downshift and upshift and, it seems tired. Looking to procure parts during the summer so the financial hit ain't so bad. I was going to give you a call and see how things are going .
Take it easy Mike, Matt is the resident transmission expert, you did ask about gearing, which is what I was addressing too.

You can drive mine if you want to feel what a fresh late 915 feels like.
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Old 04-14-2010, 02:13 PM
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Quote:
Originally Posted by Bill Verburg View Post
Take it easy Mike, Matt is the resident transmission expert, you did ask about gearing, which is what I was addressing too.
Bill,
I appreciate the compliment, but I think you are being a bit modest yourself. Not to mention this place is populated with the likes of Pete Zimmerman, Grady Clay, Steve Weiner, John Walker and many many others with way more years with their heads inside of a gearbox than myself. I'm a relative noob in the company of this site's regulars.
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Old 04-14-2010, 02:22 PM
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Quote:
Originally Posted by Matt Monson View Post
Bill,
I appreciate the compliment, but I think you are being a bit modest yourself. Not to mention this place is populated with the likes of Pete Zimmerman, Grady Clay, Steve Weiner, John Walker and many many others with way more years with their heads inside of a gearbox than myself. I'm a relative noob in the company of this site's regulars.
Ok one of the resident transmission experts.

Mike is a good guy too and no slouch w/ a wrench
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Old 04-14-2010, 02:24 PM
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Sorry, Matt / Bill I didn't mean to seem short, I think something was lost in the true feeling i was trying to convey. Being a wise ass was not my intention. My apologies if anyone was offended.
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1974 sahara beige 911 targa
1982 chiffon 911sc
1985 prussian blue metallic carrera

Last edited by kodioneill; 04-14-2010 at 02:44 PM..
Old 04-14-2010, 02:32 PM
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No worries, Mike. You asked a question about changing gears and I didn't want to make any assumptions about what you wanted to achieve. That's where my question came from.

I literally can offer you a dozen alternative 2nd gear ratios, and an even greater number for 3rd and 4th. You see maybe half a dozen "stacks" thrown around here as popular but there are many more options than are usually discussed. I recently spent several weeks working with a guy who has a low boost 3.4l in his Carrera. He doesn't track it at all, but wants the car to be a rocket ship to 100mph. Then he wants to cruise down the highway at 85 mph in 5th gear with his engine at 2500rpm. We went back and forth on the merits of a new mainshaft, what he would compromise with a 100mph 4th versus a 110mph 4th, and many different ways to get to that goal.

That's a very unique stack and not something you see in the usual popular "race" or "3.6l conversion" stacks that people talk about. What I am getting at is I am here to help, even if thats to tell you that what you've got right now is pretty good for you. I'll give you advice that's specific to you and not just something generic. I just need you to let me get to know you first so my input means something to you.
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Last edited by Matt Monson; 04-14-2010 at 02:51 PM..
Old 04-14-2010, 02:49 PM
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Thanks Matt

So let's say keep the same ratios. Where would you aquire the parts? Are there any used sources or would you buy new?
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1982 chiffon 911sc
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Old 04-14-2010, 02:57 PM
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gearhead
 
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Well,
I would buy the service parts like bearings and synchros and sliders from our host. I don't deal that stuff and even if I did, it would be poor form to attempt to move in on Wayne's business in his house. We really only do the special gears, differentials and things like billet bearing plates and shift forks.

I personally don't favour rebuilding a 915 using used parts. If you've got a worn gear or need a set of dog teeth, then used is fine. But I like fresh stuff for synchros and sliders. Bearings are a toss up and case by case. You've really got to get it apart and see what you are working with as a starting point before you start buying things unless you just commit to the full monty from the beginning.

Regards,

Matt Monson
Guard Transmission LLC
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Old 04-14-2010, 03:09 PM
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Just to give you something to think about

here's a 915/44 that was used in my '76 Carrera 3.0, it's a good example of a trasmission designed by engineers as opposed to the later trans used in your Carrea that was strongly influenced by politicians. They have the same input parameters so they can be compared

915/44


915/73


the big difference is the smaller more progressive drops as you go up through the gears w/ a slightly lower top speed in the older boxes

the newer boxes were influenced by fuel economy imperatives

the /44 was a great transmission, the /73 is also good just slightly more biased to freeway cruise and fuel economy
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Old 04-14-2010, 03:24 PM
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Quote:
Originally Posted by kodioneill View Post
Thanks Matt

So let's say keep the same ratios. Where would you aquire the parts? Are there any used sources or would you buy new?
Buying trans parts is like any shopping trip, you explore resources and find out who has factory parts for the best prices. That said, I think that you'll appreciate my 915 repair Tutorial at... http://porsche.wikidot.com/forum:start

Click on "915." There are nine parts that will take you through the entire repair process. Enjoy!
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Last edited by Peter Zimmermann; 04-14-2010 at 03:37 PM..
Old 04-14-2010, 03:33 PM
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Bill:

Interesting comparison, especially since my '86 carrera is likely due for a transmission refresh soon. Looks to me like the only differences are that the '76 has slightly shorter 2nd and 5th gears.

I presume that the shorter second gear would make the car a bit more responsive acceleration-wise (as one would be higher up on the powerband when second is engaged). However, since I use 5th as a cruising gear in my daily driver 911 and don't see maximizing acceleration above 128mph as being a priority, I'm wondering whether a good improvement for my 135+k mile 915 (which is likely close to needing a rebulid) would include substituting this slightly shorter second gear for its stock second gear.

The only issue I'm having with my 915 at this juncture is a BIT of grinding going into second gear. Would a shorter second gear in any way be more robust?
Old 04-14-2010, 03:38 PM
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Thanks guys I've purchased thousands of $ worth of parts from our host and will continue to do so. Thanks for you're input. I'll always buy from Pelican first, but they may not have everything I need.
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Old 04-14-2010, 03:40 PM
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Quote:
Originally Posted by darrin View Post
Bill:

Interesting comparison, especially since my '86 carrera is likely due for a transmission refresh soon. Looks to me like the only differences are that the '76 has slightly shorter 2nd and 5th gears.

I presume that the shorter second gear would make the car a bit more responsive acceleration-wise (as one would be higher up on the powerband when second is engaged). However, since I use 5th as a cruising gear in my daily driver 911 and don't see maximizing acceleration above 128mph as being a priority, I'm wondering whether a good improvement for my 135+k mile 915 (which is likely close to needing a rebulid) would include substituting this slightly shorter second gear for its stock second gear.

The only issue I'm having with my 915 at this juncture is a BIT of grinding going into second gear. Would a shorter second gear in any way be more robust?
grinding is the result of worn syncros, even when new the 915 doesn't like to be shifted quickly and you can beat the syncros(even new ones) especially w/ something like M1 in there.

yes, the big difference is gear ratios over the years was a tendency to go to higher(lower numerical) gears, especially in the upper gear pairs. Besides the over all ratio you need to look at the drops between gears, smaller is better and you want progressively smaller drops as you go up through the gears. each gear set needs to support and complement it's neighbors

generally it is not advisable to make a change in only 1 gears set, the larger the change the less advisable.

I have a pure race trans in my 993 w/ a 2.0 2nd, yes it's great but it is surrounded by 1st & 3rd that compliment it
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Old 04-14-2010, 04:02 PM
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Slight hijack..
I have my '77 mag 915 apart right now and am trying to put my shopping list together. The obvious syncro's, sliders, and dog teeth will be replaced. The big unknown is the bearings - big $$ to replace them all. I've looked at them all very closely and they all look good and spin freely. I have no info on the history of the trans. but the general condition is good.
What is the conventional wisdom on bearing replacement?
($1500 + to replace) That is the money I was planning for the Wevo internals...
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Old 04-14-2010, 04:23 PM
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Slight hijack..
I have my '77 mag 915 apart right now and am trying to put my shopping list together. The obvious syncro's, sliders, and dog teeth will be replaced. The big unknown is the bearings - big $$ to replace them all. I've looked at them all very closely and they all look good and spin freely. I have no info on the history of the trans. but the general condition is good.
What is the conventional wisdom on bearing replacement?
($1500 + to replace) That is the money I was planning for the Wevo internals...
Post some Macro pics of your bearings and races. If the rollers arn't scored or pitted and the race is clean, you're good to go.
I believe you can source bearings from, FAG, Timkin etc...

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Last edited by kodioneill; 04-14-2010 at 04:44 PM..
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