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MichiganMike's Avatar
 
Join Date: Aug 2007
Location: Corona Del Mar, CA
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Another 915 Trans question - 5th gear swapped with reversed 2nd?

Sorry - newbie question here.

I believe I have heard in the past that with a Porsche trans, you can replace the 5th gear for a reversed 2nd gear to gain a lower ratio. I am not sure if this is true or not, but curious if someone can clear this up for me. Maybe it was more true for a 901 or other trans, I cant recall. I have tried to search it on here, but all discuss aftermarket 5th gears, (which leads me to believe this swap isn't possible)

I currently ask because we are rebuilding my 915 trans and would like to upgrade my 5th gear to something lower and have a few gears at our discretion. Note - I do have a 8.31 R&P. With my Subaru motor powering the car, I am already pretty high in revs on the highway, and would def love to lower it to gain even better mpg, and perhaps even less muffler drone . Motor would be happy around 2500rmp I think, as the car weighs nothing and isn't even under and boost when cruising at 80.

Any help / insight would be appreciated. TIA!
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Last edited by MichiganMike; 10-18-2018 at 12:13 PM..
Old 10-18-2018, 11:07 AM
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Never heard of that gear swap thing, but the best solution to lower RPMs
while cruising is to find a TL 29/21 .72 5th gearset. They are hard to find but but show up occasionally. They were used in some early '70s 915s and also in the '76 912 type 923 trans. I have one for my '80SC with a 964 3.6 motor. Cruising at 80mph=3000RPM.
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Old 10-18-2018, 01:49 PM
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gearhead
 
Join Date: Nov 2007
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You're thinking of the 901 gearbox. NO such option with a 915.
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Old 10-18-2018, 02:40 PM
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AHHH OK - Thanks Matt! I knew you'd know. Appreciate sharing you expertise. Looks like I'll start looking for a lower 5th. (May be asking you soon on options!)
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67 Porsche 912R-STi - Betty White
06 Yamaha R6 50th Anny Ed. - Bumblebee
04 Ford F-150 FX4 - Boris

#912RSTi or follow me @912RSTi
Old 10-19-2018, 07:41 AM
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gearhead
 
Join Date: Nov 2007
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Quote:
Originally Posted by MichiganMike View Post
AHHH OK - Thanks Matt! I knew you'd know. Appreciate sharing you expertise. Looks like I'll start looking for a lower 5th. (May be asking you soon on options!)
You're welcome. I make a TL 5th gear new, but nothing taller than that anymore. Most of the V8 guys have gone over to the 3.10 final drive that Albins makes. They are noisy as hell, but what LS or LT swap isn't already noisy? My 5th makes a bit of noise too.
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Old 10-19-2018, 08:22 AM
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