Thoughts on using a 911/01 vs 915 gearbox in front of a 2.8 twin, in a '68 SWB 911

[jmz]

OK, I'm getting close to assembling the '68 911 Rat. It will have a 2.8 twin plug built as a street motor so maybe 250 h.p. -ish I'm guessing.

I have both a 911/01 and a 915 box on the shelf. ...the 915 box is an 8:31 (maybe not ideal gearing) with electric speedo so that I know will be an issue to deal with.

Any of you SWB guys have a thought or two about using a 911/01 vs 915 box? Pros/Cons?

At some point in it's life someone was going to use or did use a 915 because the tunnel has already been massaged with a hammer for clearance.

How much stronger is the 915 really?

Thanks for any feedback any of you can share with me.

[Frank Beck]

Gearing is your biggest concern. 915 is absolutely more robust than 901 and I think you’ll find 1st gear a little too short on 901 but 5th too tall on the 915. People often forget that gearing should be matched to motor and driving preferences.

No easy answer here if you’re leaving both boxes stock. My recommendation is a shorter geared 915 since the body is already modified. If you drive on the F/W a lot you may want to even leave stock gearing.

If you’re concerned as to whether the 901 will take the beating... no worries. We thoroughly thrash a 901 in a 350hp race car and it handles it. But it’s FAR from stock.

[jmz]

Thanks Frank.

I totally agree on gearing but I'm trying to meet a short time line on this one as I'd like to have it ready for our Texas Hill Country Rallye in about 10 weeks.

I'm not sure what the gearing is on the 911/01 but the 915 is stock out of a '79 ROW.

Would you suggest a custom axle or can I just change the output shaft on the 915 to match the small SWB axles? If I use the 911/01 I have I will have the same issue as it has the larger output shaft that looks the same as the one on the 915 box I have. ...sorry if these are dumb questions, this is all new to me!

Is that 350 h.p. in that 914/6 of yours? ...I've got one of those too but previous owner put a 915 Wevo box on it and it is not making anywhere 350 h.p.

[Frank Beck]

If you have a fine spline diff and want to retain the 94mm CV joints and small axles the simplest option may be to buy flanges from this company. Great guy who is very easy to deal with. Parts are top notch.

https://www.bugat5speed.de/en/porsch...flange-911-930

[jmz]

If you have a fine spline diff and want to retain the 94mm CV joints and small axles the simplest option may be to buy flanges from this company. Great guy who is very easy to deal with. Parts are top notch.

https://www.bugat5speed.de/en/porsch...flange-911-930

Very much appreciated!

[Jon B]

If you have a fine spline diff and want to retain the 94mm CV joints and small axles the simplest option may be to buy flanges from this company...

https://www.bugat5speed.de/en/porsch...flange-911-930

Frank, those flanges are for CV joints with 94mm bolt circles, not 94mm diameters. They're basically aftermarket 108mm 85> flanges that use six M10 bolts.
Maybe I don't understand the fix you're suggesting?

Jon B.
Vista, CA

[Jim Breazeale]

Frank, those flanges are for CV joints with 94mm bolt circles, not 94mm diameters. They're basically aftermarket 108mm 85> flanges that use six M10 bolts.
Maybe I don't understand the fix you're suggesting?

Jon B.
Vista, CA

Jon

Correct me if I'm wrong but, cant you put 914 output flanges in the 911 trans and drill out the locating pin pockets to accept the 2 other Lobro bolts from the 68? Seems that would be a quick and inexpensive way to do it.

Regards

Jim

[jmz]

Jon

Correct me if I'm wrong but, cant you put 914 output flanges in the 911 trans and drill out the locating pin pockets to accept the 2 other Lobro bolts from the 68? Seems that would be a quick and inexpensive way to do it.

Regards

Jim

I was reading on Pelican and this is exactly what user Haycait911 did. His car was a '66 and he used a '72 915. Mine is a '68 and I have a later 915 from a '79 that I will be using. I wonder how much difference that makes?

I will also have to figure out the clutch cable and pedal assembly when switching to either the 915 or the 911/01 vs the original 901.

[Jon B]

Correct me if I'm wrong but, cant you put 914 output flanges in the 911 trans and drill out the locating pin pockets to accept the 2 other Lobro bolts from the 68? Seems that would be a quick and inexpensive way to do it.

Jim, the 4-cyl 914 flanges will fit in both 70s transmissions, if his 911/01 doesn't have a simplified diff, and if his 915 doesn't have a fine-spline diff. The 914 flange and SWB CV joint have the same M8 bolt circle, but the flange pin pockets are too large to drill for M8 bolts, although you could probably drill out the CV joint for locating pins. There would also be an issue in how far the different flanges extend from the transmission. I'm not familiar with all the non-stock conversions and mods necessary to update early cars, only what will work with the various transmissions. I'd be wary of any fixes that involve welding. With a "2.8 twin plug... 250 h.p.-ish" engine, I think he'd be better off to update the CV joints and axles, but I don't have enough first-hand experience in such conversions to suggest exactly what needs to be done.

Jon B.
Vista, CA

[gled49]

I have drilled out the roll pin pockets on drive flanges and installed timeserts with no issues. G