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> 914/6 Transmission Question, why a shorter 5th gear? + more trans gear talk
McMark
post Sep 28 2007, 02:51 AM
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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. (IMG:style_emoticons/default/wink.gif) 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.
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ClayPerrine
post Sep 28 2007, 07:57 AM
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QUOTE(McMark @ Sep 28 2007, 03:51 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. (IMG:style_emoticons/default/wink.gif) 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.
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Eric_Shea
post Sep 28 2007, 10:51 AM
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(IMG:style_emoticons/default/agree.gif) 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?
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DanT
post Sep 28 2007, 10:55 AM
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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. (IMG:style_emoticons/default/smile.gif)
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Eric_Shea
post Sep 28 2007, 10:58 AM
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I have an 8,000rpm redline and no torque. Now what?
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ClayPerrine
post Sep 28 2007, 12:09 PM
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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.
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McMark
post Sep 28 2007, 12:12 PM
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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. (IMG:style_emoticons/default/wink.gif)
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TravisNeff
post Sep 28 2007, 12:13 PM
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QUOTE(Eric_Shea @ Sep 28 2007, 09:58 AM) *

I have an 8,000rpm redline and no torque. Now what?

Shorter gears
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McMark
post Sep 28 2007, 12:21 PM
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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. (IMG:style_emoticons/default/wink.gif)
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McMark
post Sep 28 2007, 12:32 PM
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What about a D first gear for a torquey 2270?
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Eric_Shea
post Sep 28 2007, 06:00 PM
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QUOTE
But if you want my ZA gear Eric, we can probably work something out.


OK. (IMG:style_emoticons/default/biggrin.gif)

QUOTE
The 912 5 speed is you MSX gear source.


I'm a gear dummy. What does that mean?
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sixnotfour
post Sep 28 2007, 07:27 PM
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Six Gear Chart


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sixnotfour
post Sep 28 2007, 07:39 PM
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Four geras are ; A-F-N-V-ZD


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Brando
post Sep 28 2007, 07:44 PM
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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?
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McMark
post Sep 28 2007, 07:53 PM
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QUOTE(Eric_Shea @ Sep 28 2007, 04:00 PM) *
QUOTE
The 912 5 speed is you MSX gear source.
I'm a gear dummy. What does that mean?
The MSX trans is the 'hot' setup for AXing, and most likely what's in your close ratio transmission. The MSX transmission has:

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. (IMG:style_emoticons/default/wink.gif)
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Aaron Cox
post Sep 28 2007, 09:39 PM
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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)


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brant
post Sep 28 2007, 09:43 PM
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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
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Eric_Shea
post Sep 28 2007, 11:23 PM
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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.
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Johny Blackstain
post Sep 29 2007, 08:42 AM
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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 (IMG:style_emoticons/default/blink.gif) .
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