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Posted by: Britain Smith Jul 15 2004, 12:09 PM

Can anyone provide me with some random bits of information about the 912E transmission?

As you all know, am building a fairly powerful type-4 engine and I was headed down the path of mating it to a 915 transmission. I could either (A) use a conversion flywheel from Kennedy and machine it to match my dowel-pinned crank, or ( use a 923 transmission from a 912E which has a type-4 mainshaft and would allow me to keep the dowel-pined, lightened flywheel that I already have.

I know that the 923 tranny is a mag case with a 7:31 R&P which has it's own set of issues, but I need some additional information to sway me either way.

I am also interested in some further explaination of the gearing. The 912E gearset looks like the following:

1976 (5-Spd) 11:35 11:33 23:29 26:25 29:21 7:31

Is there any reason why the first and second gears are so close in ratio to each other? I plugged this into my calculator and found that the 1-2 shift is ridiculously short...this can't be right. The 72-73 & 74 915's gearing looks like this:

1972-73 915 11:35 18:33 23:29 26:25 29:22 7:31
(5-Speed)

1974 915/06 11:35 18:33 23:29 27:25 29:21 7:31
(5-Speed)

Do you think that a type-4 motor would be better off with the 912E gearing or the normal 915? Anyone? Jake...I know you have done a fair bit or research into this, any thoughts?

-Britain

Posted by: Britain Smith Jul 15 2004, 12:20 PM

This is what I am talking about in terms of the difference in shift points...

923 Transmission:


Attached image(s)

Posted by: Britain Smith Jul 15 2004, 12:20 PM

And here is the 74 915 Shift Points:


Attached image(s)

Posted by: bondo Jul 15 2004, 12:25 PM

Hmm, I don't see how it's possible to have an 11:33 and an 11:35 in the same transmission. Are you sure your data is accurate? An18:33 tooth count like the others seems more likely.

Posted by: Britain Smith Jul 15 2004, 12:27 PM

My thoughts exactly, does anyone know if those ratio's are correct?

-Britain

Posted by: bondo Jul 15 2004, 12:36 PM

I don't just mean in terms of the shift points being too close, but also physical meshing of the teeth. The centerlines of the 11:33 gears would have to be closer together than the 11:35 gears.. but they all have to fit on the same two shafts. If you scaled them up, I don't think they would mesh properly. I'm not a transmission designer, but I have rebuilt a few and the 11:33 just doesn't make sense. It's gotta be incorrect data. Someone here must have a 912E they can push in 2nd gear and count distributor and wheel turns

Posted by: ArtechnikA Jul 15 2004, 12:58 PM

near as i can tell, for 923 (912E)

1st is 11:35 (AZ)
2nd is 18:33 (HX)
3rd is 23:29 (NT)
4th is 26:26 (QQ)
5th is 29:21 (TL) -- very tall gear (fewer top-gear revs) 923. part number


911:
4th is 27:25 (RP) or 26:25 (QP)
5th 28:23 (SN)

Posted by: Eric_Shea Jul 15 2004, 02:41 PM

I want that TL

Posted by: Britain Smith Jul 15 2004, 02:47 PM

Ah, that makes much more sense. I will plug that in and see what it gets.

Posted by: ArtechnikA Jul 15 2004, 02:49 PM

923.302.905.53

however - it shows NLA at Stoddard ...

Posted by: Britain Smith Jul 15 2004, 02:51 PM

Here are the 923 shift points corrected...


Attached image(s)

Posted by: bondo Jul 15 2004, 03:00 PM

What data do you use to determine the shift RPM? I like your graphs and want to do something similar for my 901 re-ratio-ing.

Posted by: Eric_Shea Jul 15 2004, 03:07 PM

Yeah kinda figured. Now all I have to do is convince Britain that it's a useless piece of shit gear and he should send it to me for a nostalgic paperweight.

It would be cool with the 3.0 MFI in my 915.

Posted by: Britain Smith Jul 15 2004, 03:25 PM

I have a file for both the 901 and 915 tranny with all the gears available that I know off...try these. Let me know if you need some help using it...I added to comments to the areas that need to be filled in by the used.

-Britain

http://www.britainsmith.com/Porsche/901Calc.xls

http://www.britainsmith.com/Porsche/915Calc.xls

Posted by: bondo Jul 15 2004, 03:58 PM

wow, awesome... looks like I need engine dyno specs to get max benefit. I'll have to find those.

Posted by: ArtechnikA Jul 15 2004, 04:11 PM

basically - you shift when the HP available in the next higher gear would be not less than the HP available in the lower gear you are leaving -- this maximises area under the HP curve.

you want to be somewhere peak HP at the end of the longest straight - this determines your top gear ratio. you want to be somewhere near peak torque exiting the slowest corner on the course - this determines your lowest 'useful' (2nd...) ratio. then you plug in values for 3rd and 4th that sorta split the gap, keeping the engine between peak torque and peak HP ...