901 Z, ZA, or "flipped N" 5th gear?

EarlyPorsche · 05-01-2008, 08:15 PM

I need to replace my 5th gear in my 66 911. Its a 912 transmission and I guess the X. Too many RPMs on the highway. So what gear do I go with: Z, ZA, or "flipped N." Could anyone tell me what a flipped N is? Which one will give me good 65-75 cruising as the X gear is doing too many RPMs at that speed.

TimT · 05-01-2008, 10:57 PM

I have a ZD gear that is the uber tall fifth if you need it, should be about 75mph@ 3000 rpm. I am almost positive I have a Z in another box-o-parts.. Ill check when I get into my trailer tomorrow.

Let me know if interested

304065 · 05-02-2008, 04:52 AM

Thou shalt not flip gears because the thrust axis of the gear is reversed.

Get yourself a 914 ZD.

EarlyPorsche · 05-02-2008, 05:19 AM

Thanks for the help guys. What is the difference between the Z v. ZD compared to what is in there right now? I just don't want my engine bogging down on the highway. I fear that 3000RPM at 75 is too few RPM's for my 2.0(a little built and bored to 2.2)?

john walker's workshop · 05-02-2008, 05:32 AM

ZD is a 914 5th. very common and the highest ratio available stock.

EarlyPorsche · 05-02-2008, 05:46 AM

John, will cruising at like 60-65 be too hard on my little engine with the ZD?

cgarr · cgarr's Garage

Does this have the 6:29 R&P? I run about 3850 @ 70 with a ZD in my 912 as I remember, with the stock 5th it was around 4500

EarlyPorsche · 05-02-2008, 06:11 AM

R&P is whatever is stock on a 66-67 aluminum transmission from a 912. I think 3850 on the highway would be enough but I am not sure what 55MPH will feel like with a ZD. Any input on the engine situation, I don't want to burn up an early 2.0!

EarlyPorsche · 05-02-2008, 11:40 AM

TTT for the night afternoon crew.

Matt Monson · 05-02-2008, 11:53 AM

I personally find the ZD in my 914 to be too tall. When I rebuild it I will be going to a Z for better acceleration with a minimum sacrifice for highway cruising. If I drove on the highway a lot, I would probably go with the ZA on it, but my "highway" driving is a state highway through farm country at 65mph. I almost never have my car on the interstate.

304065 · 12:01 PM

OK, stock 912 is AFQX with a 7:31. A Vredestein 185/70R15 has a diameter of 25.24", if you are using smaller tires you're on your own.

At an RPM of 3500:

I-19
II-31
III-53
IV-69 MPH

Switching for a ZD fourth gives you 84 MPH at 3500 in IV. At a speed of 65 MPH, with the ZD you will be turning about 2700 RPM, which is too low (too far from the torque peak).

If you want a nice comfortable cruising gear, something that will give you about 60 MPH at 3000 RPM and 65 MPH at 3300 RPM. . . may I suggest. . . .

. . . the X gear that's already in the transmission!

btw a "flipped N" is a Z, this was the fifth ratio in the 232 911s but was changed to X because the engine wasn't revving up high enough.

Matt Monson · 05-02-2008, 12:06 PM

john,
He's using smaller tires...

john walker's workshop · 05-02-2008, 12:19 PM

putting along on a level freeway at 2200RPM should not be a problem for a properly tuned 6 cylinder. you don't need to be in the torque band for that. if you need fast acceleration, or hill climbing power from 60MPH, downshift.

TimT · 05-02-2008, 12:48 PM

OK, in addition to the ZD, I also have a Z gear looks to be in pretty good shape.

Ive run lots of 901 boxes with gears "flipped" with no problems, and I know many people that have all sorts of odd gearstacks in 901's with gears flipped every which way.

kenikh · 05-02-2008, 12:50 PM

Quote:

Originally Posted by john_cramer

Thou shalt not flip gears because the thrust axis of the gear is reversed.

Get yourself a 914 ZD.

I keep hearing it, it makes perfect intuitive sense, but I just don't hear of these failing, either first hand or from the mechanics I know that do it.

EarlyPorsche · 05-02-2008, 01:17 PM

My transmission is a 5 speed and not a 4 speed.

I won't be keeping the X because there is really no need for that in there, I don't know what terrain the Porsche engineers in the early 60's planned these vehicles to run on but my guess is that they were thinking about people driving at 60MPH over the Rubicon trail, I do not

.

My tires at 195/60/15 so pretty much stock sized. I think that being the case, the 2700 RPM at 65 would be ok but I thinkn 55 (the speed limit in many places) would be too low an RPM? Perhaps the Z is better? Someone sell me one way or the other please!

kenikh · 05-02-2008, 01:19 PM

Odd; what R&P are you running? I use the standard 7:31 (pretty sure) and my A-F-K-Q-X gears are perfectly suited to driving from Seattle to San Francisco at 75 MPH.

Matt Monson · 05-02-2008, 01:44 PM

Quote:

Originally Posted by kenikh

I keep hearing it, it makes perfect intuitive sense, but I just don't hear of these failing, either first hand or from the mechanics I know that do it.

Yep,
My shop and many others use them flipped on a regular basis. We flip ZD's and run them as Ka in 3rd. We also flip H 3rd for tall 5ths for the v8 crowd. It's been done by racers and street rodders for several decades now. There have always been those who feel uncomfortable about it, but I have never seen one fail because it was flipped...

fbarrett · fbarrett's Garage

You wrote:

"My tires at 195/60/15 so pretty much stock sized. I think that being the case, the 2700 RPM at 65 would be ok but I think 55 (the speed limit in many places) would be too low an RPM?"

That's 24 mph per 1,000 rpm, not a bad range to be in for a 911. The 2-liter isn't exactly a lugging engine like some of the later torquier big motors. I think you're in the right rpm range. If you don't like it, swapping tires would be a lot easier (and reversible) than swapping gear sets; try 195/65x15 or something; after all, many of these 911s came with 185/70s.

Frank ('65 912, AFMSX, even with 50-series tires)