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Posted by: r_towle Mar 15 2003, 05:24 PM

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

What gears would you all recommend???

Rich

Posted by: ChrisReale Mar 15 2003, 06:15 PM

Not a professional answer (ill leave that to the "others"), but a friend of mine has a taller 3rd gear. While I spend about 95% at high rpm's in second, he used 3rd almost the whole way through. I think it also depends on the course layout. Just bring like 3 tranny's and bolt them on as neeeded Somebody else will have a better idea, but thats a start

Posted by: Zeke Mar 15 2003, 06:42 PM

In that case, Chris, I think you're friend has a shorter 3rd gear (closer to 2nd than yours), otherwise he'd be traveling a lot faster thatn you on the course. I hear a shorter 3rd gear is the hot ticket. Maybe 4th as well leaving 5th for the freeway. On my race car, I've shortened 3rd, 4th and 5th. We'll see how it works.

Posted by: J P Stein Mar 15 2003, 07:57 PM

I would like an A or B first. We are pretty much SOL with 2nd. My only solution is to either shift to 3rd or put in the 7300 rpm chip.....that's good for about 30 feet more. I don't have any problem with my 3rd (stock 4 banger gearbox). It doesn't drop the motor off the cam, so... what the hell.

You may be able to tell that I seldom get into 3rd at out local AXs. I agree, tho, it does complicate things......but not enuff for a 904 mainshaft.

Maybe larger diameter rear tires?

Posted by: ChrisReale Mar 15 2003, 08:50 PM

Zeke, he IS going faster than me, by about ten f'in seconds! He runs GTU. I could have it backwards though!

Posted by: J P Stein Mar 15 2003, 09:13 PM

Well.......comparing your stockish set up against (as I understand your class structure) a GTU car is a bit absurd......then there is the driver (no offense ). What's the seat time comparo?

Posted by: ChrisReale Mar 15 2003, 09:22 PM

we dont run together, we just shoot the shit before, during and after. I have a total of five runs under my belt, he has a few years under his.... :cursor:

Posted by: Dave Cawdrey Mar 15 2003, 09:26 PM

So, 5 or 10 minutes, huh chris

Posted by: Racer Chris Mar 15 2003, 09:27 PM

Try building an engine that has a higher redline and a wider torque band instead.
If the rules allow alternate gears, they must allow engine mods too, no?

OTOH, a taller first gear, as J.P. mentioned will get you at least through the first turn before shifting.

Posted by: ChrisReale Mar 15 2003, 09:35 PM

He has a 1.7L with big bore, 22mm torsion bars, different tranny, 22mm sway bar, 225 rear springs, and hoosiers.

I have a stock 2.0L with stock bars, 22mm swaybar 180# rear springs and F'in all season yokohamas!
Im going to steo up to 21 or 22mm torsions, stick with my tire size. That way I can stay in P1.

It may take me a few years and a feew rebuilt butts (sitting on seat without cushion so my fat head and helmet can fit) but I am young and determined and greedy, so one of these days....

Im curoius what Brad has to say about racing trannys.

Posted by: J P Stein Mar 15 2003, 10:11 PM

He,he.....thas what I figured.
3-4 seconds from the car.....the rest is operator error

NOBODY is competive the first time out (write, if you're an exception)....or the first year or 2. Track guys come out with a big Johnson and....oops....the excuses flow like beer....."I got lost" is a good one "My car ain't set up for this" is another. I've used them both (tho I'm not a track guy). Everybody nods and says "Sure"... which really means "This guy won't be back".
My latest one is "I'm workin' on it".....which really means "I'm learning to do this". I'll hang with that for a while.

The only way to measure your progress is to pick a consistently fast guy and start working at improving your times vs his. Get the car set up and leave it alone for several events(or the whole season). Then it's just driver.

Chris:
You're right.
1st is good for about 60-80 feet....tain't enuff, sometimes. When that is the case, I've been short shifting as soon as the rear tires hook up.....just to get it done with. I haven't come up with a good launch sequence other than to abuse the trans. One of these days there's gonna big this big BANG :toilet: and I'll be needing a new 901.

Posted by: ChrisReale Mar 16 2003, 03:03 AM

err...umm...cant comprehend this statement... system overload....malfunction...... :toilet:

Posted by: Dave Cawdrey Mar 16 2003, 03:38 AM

Its like tellin' a crack head to not smoke all summer

Posted by: Dave_Darling Mar 16 2003, 12:43 PM

Locally, Scott Winders showed up to the Zone 7 AutoX school (same one I attended) and by the beginning of the next season, was an instructor and a consistent top-10 finisher.

One or two of the people B has built cars for have wound up very competitive in their first year.

The common denominator, I think, is motorcycle racing. They didn't road race or autoX, but the fine art of balancing a motorcycle on the ragged edge of control teaches you some very good reflexes and skills. And a good mindset.


...Hmm, so in a way those weren't 1st-year drivers, were they??


As for the original question--
The easiest way to change your gearing is to change your tire sizes. Taller tires will make your gearing taller, shorter tires will make your gearing shorter. Taller gears will lose you some torque but you won't have to shift as often; shorter gears will gain you some torque but you will have to shift more frequently.

If I were you, I'd just drive the car.

--DD

Posted by: Brad Roberts Mar 16 2003, 01:39 PM

I'll answer in more detail later:

Take off in second and bog the motor. Keep both hands on the wheel for the first turn.


B

Posted by: r_towle Mar 16 2003, 04:13 PM

Ok,
I'll be more descriptive.

I short shift to second ASAP. first in the 914 is only good till 13 mph, so basically useless.

the problem I have is that the courses are set up to keep the 911 drivers coming back, so they are fast courses.

This means that I must redline second, or shift to third. I would like a second gear that goes a little faster, or a second/ third gear combo, where I can short shift to third and keep it there through a 180, and back down a long straightaway.

I have listened to another 914 in our club, and he can stay in third an alot slower than I can...

I was just wondering if there is a standard swap to accomplish this.

So to recap,
Option A is a second gear that is for higher speed (not sure if that is taller or shorter)
Option B a second/ third set that are closer to each other to enable staying in third at slower speed.

BTW, I am really asking if this can be done, and what do I need to buy???
Thanx
Rich

Posted by: TimT Mar 16 2003, 04:27 PM

Option A is a toughy,

second gear is part of the input shaft. so to change second you have to get a 904 input shaft, and a taller second gear. Porsche did make a few different ratio input shafts so getting another is an option, but they are expensive too.

Option B is the way to go. find out what gears you have in the gearbox. then "build" your ideal gear box. Often you can shift gears around use 4th as 5th,3rd as 4th, and buy a shorter 3rd.
The 4th and 5ths in hat come with the cars are comprimises, and usuallu much to tall.

If you have a Z 5th gear, that is good for about 145 at 700rpm, but you will never see that in a ax, and you wont use it on most racetracks either.

Another option is to keep the stock fifth ( so you can still drive on the freeway and keep the rpms cool), throw away the existing fourth, move 3rd to 4th, and get a shorter 3rd..

you have lots of options just depends if you want to make a dedicated ax car, or a car that will still see some street use..

<<<

Posted by: mskala Mar 16 2003, 04:35 PM

Rich,
Some people were saying it in a roundabout way in their
messages; You can't change second gear in a 901 trans.
It is part of the mainshaft. To change it, you have to buy
the '904 mainshaft', which allows changing 2nd gear ratios.

And I assume you mean 31mph in first, I can do about that.

I can only get to 52mph in second with 205/50 tires and a
6300 redline.

Mark S.
'70 914-6

Posted by: J P Stein Mar 16 2003, 05:56 PM

I watched a 914 w/alloy Buick at the last AX.
He did 2nd gear launches. I figure he has about (maybe more ) the same torque than ulf...he claims about the same hp. I was not impressed.

In a heads up drag race, I would put a bus length on him using 1st with my car. When the trans is nice and warm, I can bang off 2nd pretty quick. I short shift (when necessary) to get both hands on the wheel before the first turn.

Mark:
52 mph @6300?
Humm, what's the link to CDs gear chart? I wanna do some figuring on my car.

Posted by: Racer Chris Mar 16 2003, 06:10 PM

Mark's numbers closely match what my speed graph tells me for 23 inch tall tires.

Posted by: J P Stein Mar 16 2003, 06:52 PM

I wasn't doubting him, just a bit surprised.

I was doing some calcumulating and came up with about the same thing. I was going faster than I thought.

7300 rpms with 23.5 inch dia tires in second comes out to a bit over 60 mph.....38 in 1st. I've been known to.....ahem... bounce off the limiter while trying out a new course. I need one of them shift light thingys.....a big RED mofo.

Posted by: Bleyseng Mar 16 2003, 06:53 PM

Chris, you must be talking about Jason or Chris...can't remember who drives the white 914 or the blue 914.
For Bremerton the shorter tires are great! Drop down to 205x50's and you will see what I mean. I have put switching 3rd gear on the list when I go thru my extra tranny but that isn't happening soon.

I agree with DD to just bring and run what you have instead of changing the car all the time. Seat time will let you learn how to make the car faster. Later on you can improve the car to how you want it to drive.
If you watch Chris and Jason they both drive very smooth and effortless and don't seem to be going fast at all until the time is posted! They beat alot of 911 with twice the hp all the time.
I'll be there this Saturday even if it rains.....probably will too!

Geoff

Posted by: ChrisReale Mar 16 2003, 07:02 PM

Yup, its Chris. He is actually my girlfriends parents neighbor. Small world...well at least in West Seattle

Posted by: Bleyseng Mar 16 2003, 08:08 PM

Ahh the blue car is nice, very fast too. The story he tells is his grandfather setup/built it.
What size tires are you running?
Geoff

Posted by: ChrisReale Mar 16 2003, 08:28 PM

This season Im just gonna beat the shit out of the Yoko's I have on. 195-60-15. Id like to find a set of Mahle's so I could mount dome 205'50's on. Prolly Kuhmo Victo's. Yea, his grandfather was the one who had it, and I believe he and his dad rebuilt the engine

Posted by: Brad Roberts Mar 16 2003, 08:36 PM

I have a wicked Excel gearchart that shows shift points and lists all the 901 available gearsets. If anybody would like me to put it on the server..I would be more than happy too. You can plug in any RPM and any ratio to "achieve" what you want.

B

Posted by: TimT Mar 16 2003, 09:11 PM

Brad do you have the excel sheet from Stomski? or another one? I have 2 different excell apps for 901 gears, and was going to offer them up to the site.

( I developed neither, just would have to credit the authors)

Posted by: Brad Roberts Mar 16 2003, 09:17 PM

The one I have is from a friend of mine who raced a GT2 914 in SCCA. He is a computer engineer (David Ferguson) I was getting tired of doing all the math on my own and voiced this to him.. he told me he had a sheet and that I could share it since he sold the car and all the parts.

B

Posted by: TimT Mar 16 2003, 09:23 PM

Yea ferguson wrote the excel app I got from stomskis site. a link to a gear calculator would be cool.... it doesnt take up much server space either..

Brad get me a icon or gif I can use from my site.. Ill post a link to here

Posted by: campbellcj Mar 16 2003, 10:01 PM

Count me in for the spreadsheet

If my budget ever recovers from all the brutality in the last year, I will dive into the trans (LSD, 904 mainshaft, gears, cooling mods)

Posted by: Brad Roberts Mar 16 2003, 10:07 PM

Chris,

I' have a 901 I can sell you. I'm going to a WEVO 915 before summer. Its the one I built last year for the tube car using a new Guard LSD (80%) It has 4 races on it with a mild 3.0 in front of it.

Tim,

You can use the logo on the front page of the website:

Posted by: Brad Roberts Mar 16 2003, 10:11 PM

Oh.. email me the Ferguson gearcal... I have it on a computer at the shop that is turned off right now.

B

Posted by: campbellcj Mar 16 2003, 10:15 PM

Let me know when it becomes available. I still haven't gotten the six in the car and the trans is not gonna happen until after that. Too many friggen bills in my life lately. Damn job and finances always get in the way of the important stuff...

Posted by: Brad Roberts Mar 16 2003, 10:21 PM

I didnt figure you where in a hurry. I'll even gear it for your engine. It should be close right now.

B

Posted by: campbellcj Mar 17 2003, 12:28 AM

Yeah but the big question is, will I keep driving the car mainly at the "short" tracks like Streets of Willow or partial Buttonwillow, or the longer/faster ones like Willow? Seems like gearing would be pretty different. I don't even know about the NorCal tracks yet; haven't ventured up there (soon though.)

Posted by: Mike T Mar 17 2003, 08:53 PM

Here's my setup in my green car.
weight =1776 lbs
engine = Chevy 2.8 V6 approx 190 hp, 180 torque
tires = 23" Goodyear slicks
transaxle is internally stock except for 3rd gear, it's a close ratio(shorter) 1.41:1 I got by using a stock .071 5th gear set and switching their positions on the shafts where the drive becomes the driven gear. I've been using it this way for 3 seasons and it works great. Speeds in 2nd and 3rd at 7k 58mph and 77mph Good for most local autocrosses. It's actually like having a gearbox with a choice of two 2nd gears.

I can launch in 2nd and if the course warrants it, shift up to 3rd and just leave it. You don't want to shift too much on course, wastes time.


I learned about this trick on the old dgi.net Renegade forum. I remember some of you from there. I think. Did one of you tell me about this 5th/3rd gear switch?

Mike T

Posted by: Brad Roberts Mar 17 2003, 10:27 PM

I didnt tell you, but we call this gear flip "KA" in the industry since they never actually made that ratio. This is THE cheapest way to get a shorter third.

I'll start a new thread.... for the downloadable gears stuff I have.


B

Posted by: Lawrence914-6 Mar 18 2003, 09:17 AM

Yeah.. post it, please. I'd certainly be interested.

-Rusty

Posted by: porsche735 Mar 18 2003, 07:19 PM

The trick is thus:

Stock trans is:
1st 3.09=A
2nd 1.89=F 39% delta from 1st
3rd 1.26=N 33% delta from 2nd
4th 0.93=V 27% delta from 3rd
5th 0.71=ZD 23% delta from 4th

Do this:
1st 3.09=A
2nd 1.89=F 39% delta from 1st
3rd 1.41=KA 25% delta from 2nd (swapped 5th)
4th 1.08=R 23% delta from 3rd (swap 4th around)
5th 0.79=Z 26% delta from 4th (swapped 3rd)

This gives you a beautifully spaced gearbox (once you get out of 1st)

Do it, it's great....

Chris Kolstad
1973 914 2.0L (bummer stock ratios in my class, BP)

Posted by: Brad Roberts Mar 18 2003, 07:46 PM

Chris,

how did the time trial go ??

B

Posted by: mskala Mar 18 2003, 10:09 PM

Except if you have a -6 trans, then

Stock 6:
1st 3.09=A
2nd 1.78=GA 42% delta from 1st
3rd 1.22=O 31% delta from 2nd
4th 0.93=V 24% delta from 3rd
5th 0.76=ZA 18% delta from 4th

same final drive 4.43
If I calculated correctly
Mark S.
'70 914-6

Posted by: porsche735 Mar 19 2003, 02:06 PM

Brad,
The Time Trial and HSDS went great. The weather actually cooperated with us. Friday was 3 sessions dry and 1 session wet (funny watching the newbies spin like crazy after getting fast after 3 sessions).
Saturday was basically dry. Sunday was really wet in the morning. I had fun, and I am not kidding, passing EVERY car in the red group. And that's on bald Kumhos (we corded them prior to the time trial). It's nice to have an equalizer for our low horsepower cars. The weatehr dried up after the first session and it was great. We started to get sprinkles around time trial time, so they caled them off and went to practice instead. We went once more until we really had no tread left and exposed about a 6 inch patch of cord. You know, running on corded tires is a lot like running in the rain. We were still putting down 2:19's on corded tires.
So, no times, but a great weekend....

chris

Posted by: TimT Mar 19 2003, 03:46 PM

FWIW I have AFKQV in my 911's box. My 914 has shorter gears dunno which though, someday Ill open the box up and find out.

Posted by: campbellcj Mar 19 2003, 04:07 PM

There are too many Chris's on this board...

I guess I will just have to start going by my middle name:

JOHN

Posted by: r_towle Mar 19 2003, 05:56 PM

I like the sounds of this KA route.

So, what do I buy for this.....and where!!!
I am not clear on swappping fifth??

Please explain...

Thanx
Rich

Posted by: Bleyseng Mar 19 2003, 07:17 PM

We know who you are Campbell whether it's Chris or John or somethingelse....
Geoff

Posted by: Mike T Mar 19 2003, 08:33 PM

I used the 5th gear right out of the same trans. A friend did it for me. Another local 914er. We used 5th to replace 3rd and stuck the old 3rd, reversed, in the 5th gear spot. 3rd was now 1.409:1 and 5th was 0.793:1. Since I never see 5th gear in this car it was just expedient to do it that way. Works good, is cheap to do. I like it.

Mike T.

Posted by: porsche735 Mar 19 2003, 09:24 PM

Rich,
As I said, no gears to buy. All you do is move 3rd to 5th and swap the pinion and drive gears. Flip 4th pinion and drive gears and use the old 5th in the 3rd location swapping the pinion and drive gears. It is really very easy to do. I have a brand new transmission that I have done this to if there are any takers...

Chris

Posted by: Brad Roberts Mar 19 2003, 09:35 PM

I normally use the terms (Drive and Driven gears) this normally makes it a little clearer for people to understand. I say Drive gear and they can figure out that is the input shaft gear. Driven being the pinion shaft gear.

B

Posted by: Brad Roberts Mar 19 2003, 10:52 PM

Richard,

We flip 5th gear to make a new shorter 3rd gear. Think of fifth as a overdrive gear. Now flip it (make the drive gear of the two a driven gear) This now makes a underdrive for 3rd. You physically take the driven gear and put it on the input shaft making it the new 3rd drive gear.


Got it ??

B

Posted by: Mike T Mar 20 2003, 01:46 AM

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.

Posted by: James Adams Mar 20 2003, 08:19 AM

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.

Posted by: drew365 Mar 20 2003, 09:46 AM

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.

Posted by: Joseph Mills Mar 20 2003, 10:18 AM

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

Posted by: Brad Roberts Mar 20 2003, 11:08 AM

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

Posted by: Brad Roberts Mar 20 2003, 11:10 AM

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

Posted by: James Adams Mar 20 2003, 11:19 AM

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

Posted by: Zeke Mar 20 2003, 11:50 AM

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

Also, what did you mean here:

Posted by: Joseph Mills Mar 20 2003, 12:42 PM

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

Posted by: James Adams Mar 20 2003, 12:51 PM

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

James

Posted by: Travis Neff Mar 20 2003, 01:19 PM

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

Posted by: TimT Mar 20 2003, 03:58 PM

Brad

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

Posted by: TimT Mar 20 2003, 04:03 PM

oops helps to attach the pic

Posted by: Zeke Mar 20 2003, 04:31 PM

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

Posted by: bpick84 Mar 21 2003, 09:35 AM

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.

Posted by: Zeke Mar 21 2003, 10:46 AM

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?

Posted by: Mueller Mar 21 2003, 11:13 AM

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

Posted by: Demick Mar 21 2003, 12:04 PM

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

Posted by: Brad Roberts Mar 21 2003, 12:17 PM

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

Posted by: r_towle Mar 21 2003, 05:33 PM

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

Posted by: TimT Mar 21 2003, 05:38 PM

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

Posted by: Zeke Mar 21 2003, 05:53 PM

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.

Posted by: Demick Mar 21 2003, 06:05 PM

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

Posted by: Brad Roberts Mar 21 2003, 06:22 PM

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

Posted by: TimT Mar 21 2003, 06:38 PM

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...

Posted by: porsche735 Mar 21 2003, 07:08 PM

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

Posted by: TimT Mar 21 2003, 07:24 PM

Milt did you get that spreadsheet I sent your way?

Posted by: Racer Chris Mar 21 2003, 11:17 PM

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.

Posted by: Brad Roberts Mar 21 2003, 11:49 PM

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

Posted by: TimT Mar 22 2003, 08:31 AM

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.

Posted by: Racer Chris Mar 22 2003, 05:29 PM

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.

Posted by: TimT Mar 22 2003, 06:28 PM

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.

Posted by: mskala Mar 22 2003, 06:57 PM

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