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Thread: rebuild 901 vs. 915 conversion

  1. #1
    Junior Member
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    Apr 2004
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    rebuild 901 vs. 915 conversion

    My 1970 911E has a broken tooth on its pinion gear. It runs, but makes a "p" sound under load, and this noise is getting louder. I need to rebuild the 901 transmission and replace the broken gear with new or used ring & pinion. I received quotations from $900 (used ring & pinion) to $3,500 (new ring & pinion), plus $300 to $600 for R&R.

    I use my car as a daily driver, and a higher 5th gear will make those long highway cruises much more enjoyable. (Don't get me wrong, I like the sound of 911 air cool engines as much as anyone else, but my old ears would no longer tolerate this music for more than an hour at a time.)

    An alternative would be to upgrade to a 915 trans. I received one quote at $2,500 for conversion with used parts and a "refreshed" 915 trans.

    What are your opinions on 915 conversion vs. 901 rebuild?

    What engine rpm can I expect with an early (72 or 73) 915 trans at 60 mph? How about 915 trans from 74 thru 77 cars? How about 915 trans from SCs?

    Is there any "how to" article or book on 901 to 915 transmission conversion with list of parts required?

    Does anyone have a kit for this conversion?

    Thanks,

    Rob Hou

  2. #2
    Stick with the 901. I believe you can get a different 5th gear for it if you are rebuilding it anyway. You can't give up the goofy, I mean proper racing, shift pattern. I'd go with the rebuilt ring and pinnion and look for a new gear.
    '71 911S Targa

  3. #3
    admin_old
    Guest
    Not mentioned is the cost to rebuild your 901 (besides the R&P). Of the two options available to you, the 915 conversion sounds like the better deal, especially if the gearbox is truly "refreshed" (whatever that means - you should find out).

    The early 915s use the 7:31 R&P which should provide increased acceleration over the later 915s. However, the earlier ones also have an input shaft seal that must be replaced from the inside. The seal can be replaced externally on later 915s ('77- On?). You can use any 915 up to '86. If you need assistance with the conversion, come back here for tips.

    However, if you're after period correctness, stay with the 901.

    Sherwood
    http://members.rennlist.org/911pcars

  4. #4
    Moderator Chuck Miller's Avatar
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    FYI,

    I believe the external input seals started in March '73...

    I have an April build car... I lucked out

    Cheers,
    Chuck Miller
    Creative Advisor/Message Board Moderator - Early 911S Registry #109
    R Gruppe #88

    TYP901 #62
    '73S cpe #1099 - Matched # 2.7/9.5 RS spec rebuild
    '67 Malibu 327 spt cpe - Period 350 Rebuild

    98 Chevy S-10 Utility
    15 GTI Commuter

  5. #5

    Talking 915 trans

    Chuck,

    Your right on the date for the change to the external main shaft seal.
    I recommend the change to a 915 gear box. Much better clutch accuation arrangement and stronger internal parts. I think a late 73 to 77 915 with a 7:31 ring and pinion would be best for your 2.2 motor.
    I converted my 70 to a 915 years ago and was very happy with it.
    The conversion is very straight forward and I would be happy to talk you through it. There are small variations to the procedure depending on what year gear box you use.
    Rob, email me or give me a call. Hope this helps.

    All the best,

    Roger Grago
    R Gruppe #27

  6. #6
    Rob,

    Some extra 915 info for if you go that route.

    1972,3,4,5 will give you 7:31 CWP ratio

    1976,77 will have 8:31 CWP ratio

    1972 has the input shaft seal installation flaw.

    1973 (april on) has an improved input shaft seal installation - although effectively still a fixed guide tube.

    1974 has the removable guide tube - for bench top input shaft seal replacement.

    1972,3,4,5 will give you a cable drive speedo

    1976,77 has electric speedo - can not be retro fitted with cable speedo without significant parts.

    1972,3,4,5,6,7, are all magnesium, 1977 (915/61) are also available in aluminum, with the case material being the only variation.

    Sometimes overlooked when installing a 915 in a 901 chassis, is the main shift tube. At a glance, they are similar, but the 901 component is 15mm longer than the tube used for 915. It's trapped in the car once the transmission is installed, so needs to be exchanged prior to engine install.

    Regards

    Hayden
    Technical Director
    Windrush Evolutions Inc.

  7. #7
    On a related note, next week I should get my 1970 911E running with the new 3.0 with Webers and Bursch headers. (I'm replacing a worn out 2.7, and will boost the HP by ~50-60 with a lot more torque.) I have the original 901 which worked very well before I pulled the old engine.

    What are the drawbacks to the the higher HPand a 901? Possible breakage? And will I have to get out of 1st gear before I leave the start line at the next autocross? I'll give it a try with the 901, but if it doesn't work to my liking, might I prefer a 915? Of course, I prefer the cool shift pattern of the 901.
    Charlie
    '66 912
    '50 VW Bug
    '89 VW syncro Tristar Doka
    '83 VW Westfalia

  8. #8
    Moderator Chuck Miller's Avatar
    Join Date
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    Great info Hayden...

    Thanks
    Chuck Miller
    Creative Advisor/Message Board Moderator - Early 911S Registry #109
    R Gruppe #88

    TYP901 #62
    '73S cpe #1099 - Matched # 2.7/9.5 RS spec rebuild
    '67 Malibu 327 spt cpe - Period 350 Rebuild

    98 Chevy S-10 Utility
    15 GTI Commuter

  9. #9
    I own 2 1970 911 S coupes. One has the 901 with the 2.2 MFI motor. The other was converted to a 915 with a LSD when the motor was changed to a 2.7.
    I have to say the 915 trans is the way to go, UNLESS you are going for originality. It is easier to drive.
    While you are in there, you may ask about putting in a LSD/Quaiffe. Down the line you may want more performance from you car, i.e. bigger motor, suspension mods. etc. Then you will not have to back and redo the trans.
    [SIGPIC][/SIGPIC]
    Ray Crawford
    Early S Registry #271
    R Gruppe #255
    '70 911 S Coupe 2.9 w/MFI Twin Plug "Flairs n Chairs"
    '72 911 S Targa 2.4 w/MFI

  10. #10
    Junior Member
    Join Date
    Apr 2004
    Posts
    12

    Thanks

    Thank you all for your comments. I decided store the car and decide what am I going to do at a later time. For now, I will enjoy driving my wife's 89 Targa.

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