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914World.com _ 914World Garage _ 914/6 Transmission Question
Posted by: McMark Sep 28 2007, 02:51 AM
I'm looking at 914/6 transmission gear ratios because I have one in my shed. I'll be the first to say that transmission gearing is not my strong suit. 
According to Charlie Davis the 914/6 transmission has:
1st: Same
2nd: Taller
3rd: Taller
4th: Same
5th: Shorter
I'm not sure why the 5th would be shorter. My guess would be to keep the 6 cylinder revs up for cooling as well as to keep it in the high 6 power band. Am I wrong?
If I rebuild this tranny to mate with a 2270, I'm thinking that it would be smart to put the 4 cylinder ZD 5th on there since I'll have plenty of torque down low.
Posted by: ClayPerrine Sep 28 2007, 07:57 AM
I'm looking at 914/6 transmission gear ratios because I have one in my shed. I'll be the first to say that transmission gearing is not my strong suit.
According to Charlie Davis the 914/6 transmission has:1st: Same
2nd: Taller
3rd: Taller
4th: Same
5th: Shorter
I'm not sure why the 5th would be shorter. My guess would be to keep the 6 cylinder revs up for cooling as well as to keep it in the high 6 power band. Am I wrong?
If I rebuild this tranny to mate with a 2270, I'm thinking that it would be smart to put the 4 cylinder ZD 5th on there since I'll have plenty of torque down low.
I have the guts from a /6 tranny in my garage. The factory reason was to spin the fan faster to compensate for the 10 degrees hotter the engine ran in a 914 chassis. They also changed the redline on the temp gauge because it ran too close to the original line.
Posted by: Eric_Shea Sep 28 2007, 10:51 AM
Clay is spot on.
I'm putting that gearbox you tore apart back together via Evil and I'm thinking ZD but I have my reservations.
1. That 2.5 runs hot in my 911 with a jailbar front cooler.
2. Now put it in a teener...
I'll have the front cooler which is ducted and radiator style vs. the jailbar in the fender well so I think it will be OK.
Thoughts? ZD or...? I spoke with a friend who has a factory 2.0 twin plug teener and his tranny had the 904 shaft and 912 gears. How does the ZD compare to the 912 final?
Posted by: Dan (Almaden Valley) Sep 28 2007, 10:55 AM
and the fact that the 911 engine had about a 1,000rpm higher redline and cruising torque was not as good as the 2.0L 4s. 
Posted by: Eric_Shea Sep 28 2007, 10:58 AM
I have an 8,000rpm redline and no torque. Now what?
Posted by: ClayPerrine Sep 28 2007, 12:09 PM
I have a ZD fifth in my 914/6. It runs at 3k at 70MPH all day long with a 190 degree oil temp.
But I have a 911 Carrera cooler mounted in the right rear corner with the fan hooked up.
Posted by: McMark Sep 28 2007, 12:12 PM
Front cooler = freedom. I wouldn't worry about heat. Plus you can always upgrade to a larger cooler.
But if you want my ZA gear Eric, we can probably work something out.
Posted by: Travis Neff Sep 28 2007, 12:13 PM
I have an 8,000rpm redline and no torque. Now what?
Shorter gears
Posted by: McMark Sep 28 2007, 12:21 PM
912 4 or 5 speed final is X 28:24 (0.8571) versus the ZD 31:22 (0.7097) and the ZA 29:22 (0.7586), so it's even shorter than the ZA, by quite a bit. The 912 5 speed is you MSX gear source.
Posted by: McMark Sep 28 2007, 12:32 PM
What about a D first gear for a torquey 2270?
Posted by: Eric_Shea Sep 28 2007, 06:00 PM
OK.
I'm a gear dummy. What does that mean?
Posted by: sixnotfour Sep 28 2007, 07:27 PM
Six Gear Chart
Attached image(s)
Posted by: sixnotfour Sep 28 2007, 07:39 PM
Four geras are ; A-F-N-V-ZD
Attached image(s)
Posted by: Brando Sep 28 2007, 07:44 PM
Real reason was: To not be as fast as Porsche's 911 flagship.
You can take that as subjectively as you want.
Why is a boxster engine 'detuned' compared to a 996 engine of same displacement?
Posted by: McMark Sep 28 2007, 07:53 PM
1st: A - stock first gear
2nd: F - stock second, which is part of the mainshaft
3rd: M - shorter than stock
4th: S - shorter than stock
5th: X - shofter than stock
The stock 912 five speed has MSX gears for 3,4,5. So if you were building an AX box, then a 912 five speed would be your donor trans.
Posted by: Aaron Cox Sep 28 2007, 09:39 PM
gear chart time bitches....
(this is for my car... but the ratios are shown... the MPH will be off by a few % due to my tire sizes)
Attached thumbnail(s)
Posted by: brant Sep 28 2007, 09:43 PM
MSX can be fine
but they are kinda tall for a -6
they are really better suited to the -4's because the -4's often have a lower redline
MSX is great for 6K or even up to 7k
above 7K they are too tall
choosing gears is kinda like jakes description of a complete system
it depends completely upon your redline, track length, wheel sizes, torque curves, etc
when you start reving a motor over 7 and up to 8+ you really need shorter gears than MSX
I ran MSX (well actually Ka, S, X) on both a -4 and my current 911S motor at 7300. You wouldn't believe the difference when I went to the FJOSV. With the MSX I couldn't even use the X and S was even too long of legged for above 7K
brant
Posted by: Eric_Shea Sep 28 2007, 11:23 PM
I'm starting to catch on with Brant's description and Boy-Wonder's chart... especially given the engine I have. It doesn't wake up until after 4k and would top out over 8.
Posted by: Johny Blackstain Sep 29 2007, 08:42 AM
I too, run @ 3K RPM doing 70mph. No oil cooler up front, yet, so I've run around 205*/210* this summer. However if I drop down to 60mph the temp drops dramaticaly: 180*/190*.
Never goes over 200* in the winter no matter what my speed. No idea what my gear ratios really are- supposed to be the same as a 906 
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