AX transmission Options

[r_towle]

So,
I want to stop shifting so much at the Autocross.

What gears would you all recommend???

Rich

[TravisNeff]

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

[TimT]

Brad

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

(IMG:style_emoticons/default/driving.gif)

[TimT]

oops helps to attach the pic

[Zeke]

I just did this and that's exactly what it looks like. I wondered about it, but I guess it's OK. (IMG:style_emoticons/default/driving.gif)

[bpick84]

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.

[Zeke]

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.

It will run okay in reverse cut, but it may whine some.

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

[Mueller]

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.............

[Demick]

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

[Brad Roberts]

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

[r_towle]

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 (IMG:style_emoticons/default/pray.gif)

[TimT]

to show what a gear looks like that has been flipped.

Oops, (IMG:style_emoticons/default/smash.gif) 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 (IMG:style_emoticons/default/beerchug.gif)

[Zeke]

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 (IMG:style_emoticons/default/pray.gif)

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. (IMG:style_emoticons/default/driving.gif)

[Demick]

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

[Brad Roberts]

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

[TimT]

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...

[porsche735]

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

[TimT]

Milt did you get that spreadsheet I sent your way?

[ChrisFoley]

in a 1:15 lap, in 6 turns you would only be in 3rd for a small portion of the time.

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. (IMG:style_emoticons/default/laugh.gif)

[Brad Roberts]

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

[TimT]

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.