When mine was done I wanted to flip the 4th gear set too, make a 1.08:1 out of it. Leave it right there in 4th position, just flipped. The guy doing the work said it wouldn't work. Was he mistaken?

Mike T.

No, you can't just flip a gear set in its same position (4th to 4th). One gear is splined and the other has a bearing and the syncro parts, these end up in the wrong place if you were to try to just flip drive for driven.

It does work for the 3rd to 5th swap that is being discussed, though.

This sounds like a good mod for a short road course also. I know on the Streets of Willow there are two corners I'd like to take in 3rd but my gear is too tall. But if I also run longer tracks like Buttonwillow I'd need to have a second trans setup. This hobby keeps getting more serious. So what would be a good mod to a stock -6 trans for the longer road courses.

I've been given two spare trannys from an ex-racer. He can't remember their condition but thinks one may be limited slip. Presently, I have NO low gear syncro and have future plans to rebuild one of these trannys so I'm interested in this gear KA flip for auto-x. But how driveable is this setup for highway speeds?

I'm a little confused (again). When 3rd & 5th is swapped, what is the end result for the new 5th gear combo? Is it taller or lower?

Anyone have any idea what the estimated rpm difference (from stock to KA flip), for a given speed (say 70mph), in 5th gear would be?

Joseph
75 2.0L AX bound

Joseph.

Flip a 5th from the tranny you want to use. Put it in place of your current third. Now use the stock 5th from the spare tranny and it put it back in the stock location. Now you have a close ratio box 2-3-4 with a stock 5th. There will be a stock split from 4th to 5th. but the car will pull like a freight train.

B

Oh,

we chat about this like it is nothing. NONE of this is easy. Plenty of physical work to get 5th off the pinion shaft. We are leaving out about 150 steps in the process.


B

Gear chart for about a stock diameter tire (calculated a 205/55/16)
Red is 914/4
Blue are flipped 3rd and 5th

I think i understand what is being shown. Could you post the formula to calculate some other combinations?

Also, what did you mean here:

Wow! Thanks James, for that excellent chart. So on a typical flip, at 70mph looks like you'll run about 400rpm higher in your new fifth. Not a good idea for my car. And it looks like there would be about a 10 to 15mph loss in top end speed (I'd have to leave for work earlier).

Brad, borrowing an additional fifth from the extra tranny is a perfect solution for me. I can have the best of both! Thanks.

"Oh, we chat about this like it is nothing. NONE of this is easy. Plenty of physical work to get 5th off the pinion shaft. We are leaving out about 150 steps in the process."

Uh oh! Looks like I need another chart. One that calculates the inverse ratio of rationalizations vs. geometrical expenditures and the estimated time & distance of budget repair expressed by the thiness of my wallet in mm's please.

Joseph
75 2.0L AX bound

I'm not touching that one, mostly because I don't want to know the results as it applies to me!

James

So, for a hot street setup - you would want to obtain a stock 5th gearset and use that in place of the original 3rd, use an "S" gearset for 4th with a stock 5th gear - here's how it looks on paper

1st 3.09 (stock1st gearset)
2nd 1.89 39% delta from 1st (stock 2nd gearset)
3rd 1.41 25% delta from 2nd (stock 5th gearset, flipped)
4th 1.04 26% delta from 3rd (S Gearset)
5th 0.71 32% delta from 4th (stock 5th gearset)

or go ahead with the swap of 5th gear with 3rd

1st 3.09 (stock1st gearset)
2nd 1.89 39% delta from 1st (stock 2nd gearset)
3rd 1.41 25% delta from 2nd (stock 5th gearset, flipped)
4th 1.04 26% delta from 3rd (S Gearset)
5th 0.79 24% delta from 4th (stock 3rd gearset, flipped)

At least it looks good on paper, never drove a custom geared 914

Brad

I took the liberty to play with the pic you posted to show what a gear looks like that has been flipped.

oops helps to attach the pic

I just did this and that's exactly what it looks like. I wondered about it, but I guess it's OK.

It will run okay in reverse cut, but it may whine some. I did it on my track only car. I flipped 5th to 3rd. Put a Q gearset in 4th position and moved my 4th back to the 5th position. A little steep but close and useable.

No one is gonna explain to me what "delta" is (I think I know) and the formula for calculating? That looks useful to determine the amount of rpm drop off between gears.



Yeah, I wondered about the thrust. Is that backwards too?

3.09-1.89=1.2
1.2/3.09=39% differance (delta)

1.89-1.41=.48
.48/1.89=25%

and so forth.............

I think the picture in Tim's post above shows 5th and 4th switched, which cannot be done. This pic shows 5th and 3rd swapped and shows the new orientation of the gear teeth on 3rd gear.

Demick

Bill Pickering ?? If that is you... welcome. Thanks for cleaning that up Demick, I was scrolling through the posts and thought "hum" cant flip 5th to 4th and was going to remove the pic "fix it" and put it back in his post.

B

I am close to understanding this...please forgive my ignorance here, I have only done motors, never doen a tranny.

So I have three side shifters, and a tail shifter.

I am going to do this to a "spare" side shifter...

When I get going, I will take a pic of two and ask my dumb questions.

Brad, I am flipping the fifth Drive gear with the Third Drive gear???
Or the fifth drive with the third driven...
Or aaaarrrrggggg...

Rich

Oops, I know you cant flip 5th for 4th, I was just showing the cut of the gear teeth goes in a different direction. was just trying to help

Rich, I just did this, so it's fresh on my mind. What you are doing is swapping both complete sets for each other. I don't know about the 3rd to 5th swap, that might be too low for the street. But the 5th to 3rd drops 3rd down closer to 2nd. With that, you would be better off dropping 4th down a liitle too, IMHO.

The 5th is on one cluster and 3rd is on another cluster. Except for the gear sizes, these clusters look/work the same. When you install the gear from 5th into the 2nd-3rd cluster, it's in the same position realtive to the cluster as it was before. Except, it's upside down, hence the reverse diagonal cut.

Hope that's clear as trans fluid. None of this makes much sense unless you are racing the car.

Rich

Drive gears are always on the input shaft, driven gears are always on the output shaft. For the 5th / 3rd swap, it goes like this:

5th drive becomes 3rd driven
3rd driven becomes 5th drive
3rd drive becomes 5th driven
5th driven becomes 3rd drive

Don't associate the drive vs driven gears by which one has dog teeth which engage the sliders. For 4th and 5th, these gears are the drive gears. For 2nd and 3rd, these gears are the driven gears. When you do the swap, be sure and move the bearings and bearing races along with the gears (in other words, always keep the gear, bearing, and bearing race together).



Tim. You are correct with the opposite orientation of the gear teeth. Notice that with the stock setup, all gears on the input shaft have the same tooth orientation, and all the gears on the output shaft have the same orientation. Using helical cut gears is stronger and much quieter than straight cut gears, but creates an axial thrust on the shafts. By reversing the orientation, you are reversing the thrust direction on each shaft. So when you shift from 2nd to 3rd and then from 3rd to 4th, you reverse the axial thrust on both input shaft and output shaft each time. I have no experience with doing this, but think that this *could* be a very bad thing if the transmission wasn't designed to do that (and I doubt it was).

Demick
'74 2.0

It wasnt designed for it, but I personally have been building the boxes this way since 1989 or so and was taught how to do it by a old guy who built 901's for 911's way back. I can think of 3-4 boxes that I have done in the last 9 months behind some serious HP that are'nt having any problems.

I used to worry about flipping the ring gear in 930 boxes way back also. When you flip it...the drive side ends up with a smalling bearing than original drive side. The pro tranny builders told me not to worry about it. Porsche over engineered a lot of components.

Lets just say I havent seen one fail because of these mods.

B

I wondered about the thrust issue. Then being an engineer I reasoned in a roundabout way that the change in direction of thrust couldnt be bad. Since your spending minimal time in the inverted gear (3rd)

At Lime Rock I use 3rd and 4th. in a 1:15 lap, in 6 turns you would only be in 3rd for a small portion of the time.

I also know a guy who has a 750+ hp 911 that is running with some flipped gears...

Back to this stock 5th thing.... Why would you want to do that? 5th is completely useless on a road course and even more useless on an autocross. Once you make that shift from 4th to 5th on a road course you gain no rpm unless you are going downhill;-) Do not use a stock 5th and you will be very happy. I have a transmission that I did the 3rd to 5th reverse-swap on AND replaced the 4th gear with an "S"= 1.04. So I have:

1st=A=3.09
2nd=F=1.89
3rd=K=1.43 (actually swapped 5th=1/ZD)
4th=S=1.04 (new gear)
5th=Z=.79 (actually swapped 3rd=1/N)

This gives spacings of..39%, 25%, 26%, 24% It's a beautiful thing...

I have a transmission opened up right now that this is done to. Anyone want to help rebuild it?

chris

Milt did you get that spreadsheet I sent your way?

Tim, your signature says you have a GT3 PCA 914. I haven't done a 1:15 lap since my car was a bone stock street car. When I ran ITB SCCA (stock 1.7 and 195/50 tires) I did 1:08s easy with stock gears and a 6:29 R&P (4.83 final reduction) in 4th and 5th only. Never needed 3rd, and only in 4th from Big Bend to the entry of no-name straight. You have to push a little harder.

Chris,

The thread started out as a AUTOX thread... no real need for a AutoX guy to change to a lower 5th...LOL


I agree 100% that your S gear is the hot ticket for road racing.


B

Chris, I just bought the GT4 car, havent turned a lap in it. The 1:15 at LRP is a number pulled from thin air. :finger2:

In my 2.2l 911 I turn in the 1:05-1:10 area (that is with a stock gearbox). My 911 at LRP is uses 3rd and 4th

I just rebuilt the 911 box with short (KQV) gears, and a quaife. So now I have 2 cars to sort out, my new to me 914, and my recently rebuilt 911.

Is 6:29 the stock 914 final drive? the 901 box in my 911 has a 7:31 final drive

I have a number of suspension questions about a track only 914, so Ill start a thread on that subject instead of diluting the ax subject on this thread.

914s only came with the 7:31 AFAIK. The 6:29 is the only alternate R&P that I am familiar with for the 901 box. It provides almost 10% increase in rpm at the same speed. That plus shorter tires helps make the stock gears more usable on a racetrack with a stock powerplant. The spread is still just as wide, but the rpms are higher. For SCCA Improved Touring it's the only legal way to get more usable torque to the ground.
Tim, are you planning to run at the PCA race at LRP this spring? BTW, I was just teasing in the last post.

Sorry for hijacking the AX thread. Back to the topic. In an AX car the 6:29 might help, not sure. It would mean you have to shift into second as soon as the car is rollling, and shift up to third for any straight section. Second might become just right for the real tight stuff, too.

Chris I know your were teasing just wanted to use that emoticon at least once LOL

I think my first race this year will be Watkins Glen. Even though the my 914 has a current PCA log book, I want to go over the car and make myself confident that everything is as I would like it.

I also am completing my 911 project there arent enough hours in a day it seems.

This may not apply to many, but for PCA
AX in our area, NONE of the things mentioned
are legal in any stock class. If you do them,
you will be in modified, and will be up against
people who've done a lot more to their cars.

To remain in stock, I've heard of people
using the 14x6 (5.5?) Fuchs, and nice low profile
tires to get the different 'ratios'.
Mark S.
'70 914-6