mark kibort

Hey, no problem. just a bunch of wacked out car guys here , right!!??

mk

]

GlenL

Here's an old email list post y'all might be interested in.


Hello all, I had lost 3rd gear in my 83 and was looking for a newer
tranny with limited slip and a better ratio than the current 2.20's with
open differential in the back of my shark. And of course I found nothing
affordable. But I want to go faster and I want Posi. Well I tear down the
tranny and I had worn the dog teeth down on third gear. Ouch. I have a 78
tranny all in pieces, no reverse but with limited slip. In looking for
tranny parts I had called 928 east and he said don't put the 78 tranny
together your car will be slower. I said how? it has 2.75's instead of
2.20's,it's got to be quicker. He then told me that the 78 had taller
gears in the tranny, and that really wiped out any advantage of the 2.75's
because the final drive ended up being worse than the 2.20's.Little did I
know that that would have been the key to this madness. So I call Jim
Baily to talk about options and what he had experimented with in the past,
liking messing with ratio's and such. First I can't buy just the dog teeth
I have to by the whole third gear. Over 700 bucks Ouch. So we start
talking about options and we come up with "I've never heard about it being
done but it should work in theory." 2nd 3rd and 4th gears in the 78 have
more teeth than in my 83. That is why even though the 78 has 2.75's in the
back it is slower than my 2.20's. because every tranny gear in the 83 has
less teeth thus faster acceleration.But then Porsche puts 2.20's in the
back to slow it down.(WHY) So I somehow get the dogteeth off the 78's
third gear and it fits on mine. Yeah. Put all my tranny gears on the 78
pinion gear and they fit, even though the manual and all part numbers say
they wont.(remember the cup is half full not half empty) and use the 78
ring gear and limited slip everything lines up except the last part. (of
course) the tail shaft, well a little modifying with the dremel on some
shims and it is back together. Pull out the manuals to adjust the backlash
and it is done. Put it in the car and take it out. Now this is a stock 83
4.7 i take it out easy breaking things in and I find myself in fourth
when I usually am in third. And things just feel wierd. So time to get on
it. Holy crap. Ist gear dump the clutch and go. and go and go posi rubber
all the way to redline shift rubber to almost 5,000 rpm's in second ,
hooks up, shift to third tires break free again. wow. doing like 15-20 in
first, floor it tires break free and car will go side ways. Driving in
second you step on it you get rubber. I'm sure I lost some top speed but I
shouldn't be going over 120 anyway. :-) This car is like a knew machine it
is so fun from stoplite to stoplite. Yes I am so excited I can barely
type. So Jim Baily- in theory and in real life it worked. And also no
howling from the ring gear and shifts great even with a pryed off dog
tooth gear and dremmeled shims. It is awesome, if only I could of got borg
warner sychros to work somehow it would be perfect. Now I may need a euro
intake and throttle body ANYBODY? Let the madness begin

Shad
83s black/black LSD, I wonder if I could add 6th gear?

Gregg K

I'm not affraid of math. But it has been so many years, I've totally lost my thought process. I think it's the torque with respect to the ground that I was talking about. Same engine, same tranni, different overall ratio. I've done this enough times from 3.73 to 4.54 rear end changes to know, without math, that the change is dramatic.
I do not drive my car over 100. And to get all five gears inside that speed would be really cool. But no, I have to wait until I'm doing 70 to shift into fifth gear. In the twisties, 70 translates to over the guard rail and cliff. But that car would be half way down the straight as the other cars were flying away, at Laguna Seca. Apples and orangutangs.

mark kibort

[Just keep in mind that the spacing never changes. do a radical ring and pinion change and you can end up with the same gear box, and loose 1st gear.
you dont gain torque to the wheels over the accleration curves. you have an equal number of trade offs. However, what a change will buy you is an optimization of the HP to the wheels over the application.

I dont care if you make your 5 speed fits into a 100mph max speed. the spacing doesnt change and at some point, the gears . (and in this case, the 3rd 4th and 5th would equal a non 100mph max car like an '84 2.2 of 1st 2nd, and 3rd gear. This means no gain, except having 2 more gears between 0-25mph.)

Look at the ratios, do the math and get back to me.

the internal ratios are differnt for all the 2.2, and GT/GTS/79 and even S4, so that the final ratios are not as dramatic as the 2.2/2.75 change would indicate

MK





QUOTE]Originally posted by Gregg K
I'm not affraid of math. But it has been so many years, I've totally lost my thought process. I think it's the torque with respect to the ground that I was talking about. Same engine, same tranni, different overall ratio. I've done this enough times from 3.73 to 4.54 rear end changes to know, without math, that the change is dramatic.
I do not drive my car over 100. And to get all five gears inside that speed would be really cool. But no, I have to wait until I'm doing 70 to shift into fifth gear. In the twisties, 70 translates to over the guard rail and cliff. But that car would be half way down the straight as the other cars were flying away, at Laguna Seca. Apples and orangutangs. [/QUOTE]

Gregg K

Wait a minute. Isn't there more torque to the rear wheels in first gear than in fifth? Isn't that why it's easier to lay rubber in first than in fifth?
I think we are talking about two different things. The thread started out as a tranny thread and split off into a differential gear ratio thread. I'm only talking about from the ring gear and back.

mark kibort

Yes, but we are talking about spacing not changing. if the gears get closer with a 6 speed, then it buys you something.
Yes, the threads overlap, as the concepts are the same. gearing doesnt make hp, it just changes efficiency of the applied torque

You cant just look at the ring gear, as it just gears down the entire gear set. do it far enough, and the spacing doesnt change, but you just loose the 1st few gears. If you look at the total ratio for all gears, you then can make the assesment on whether you will get advantages or not over the stock gear boxes.

as I mentioned in the other thread. Just concern yourself with the total resultant ratio. forget about the ring and pinion ratios. each box has different individual gear ratios, but the spacing seems to be the same, except for 4-5th in the 2.266 and S4 box


MK

DFWX

3.09 gears? Where? How? How much??
Thanks.
Mark
DFWX

DFWX

Rear gearing choice has a lot to do with what type racing/performance work, weight, horsepower, tire bite etc, so the argument seems somewhat apples and oranges.
As a comment, "only an 0.5 second difference" is not an "only" at all. It is the difference between winning and losing.
There is another factor little discussed in "gearing", and that is wheel/tire diameter size. Candidly, I hope to have at least two sets of wheels/tires and ideally 3. Call me wacko, but I'd like a set of 12 inch wide 15 inch wheels (though could live with 16 inch) for performance work - and 18 inchers for more ordinary driving.
As many in this forum lament of their 1/4th mile ets in their street 928s, the trend towards 18 inch wheels is a trend towards appearance at the cost of acceleration. Many model year 928s have too long a gearing for gas mileage reasons to being with - then worsened by larger diameter tires.
There is a notable difference in acceleration between my 81 with lower ratio and the longer geared 84.
As horsepower goes up (and/or weight down) at some point lower gear ratios does not help due to more power that tire traction can use and time lost shifting. However, for those most concerned with street acceleration in closer to stock horsepower and weight form, they would be much happier with lower gears and shorter tire/wheel combinations.
Lastly, there is the option of more than one transmission each geared differently.
Where and how much for the 3.09 gears? Already have OEM short and long geared 5 speeds.
10 grand for a 6 speed mod is absurd, unless it also comes with a real 1,000+ horsepower rating. I suspect, however, this is one of those send them the money and then they are going to try to figure out how to make it. Someone would have heard of this set up if it is real.

Mark
DFWX

Skunk Workz

The T56 is a Viper gearbox... also found in some Camaros and Mustangs... Small displacement engines? Nope. If it is a Viper 'box,the 4th is 1:1 .....5th is 0.75 overdrive....and 6th is 0.5 ( ! ) overdrive. The Mustang and Camaro versions have 0.72 and 0.64,if I remember correctly. No matter what box you use,you end up having tight first- to 4th gear ratios,and two serious overdrives to go cruising the highway.

This guy who advertized about the conversion for the 928,might be a hoaxer...but I'm actually working on putting a similar 'box into a 951 over here...

mark kibort

again, its not the number or gears or the final drive. ITs about closness of the gears and the ability of the gears to keep you in the max hp range. a 3.09 as someone tried on a GTS and thought was great, turns out to be the exact same gear box ratios as a S4 with a 2.2:1 rear end. ( just the gear designators have changed to protect the innocent)

we do a lot of testing with gear ratios at the track, with telemetry output as wel as basic lap times
depending on the track and HP, there can be little , none, or even worse results by any gear box changes. we often change from 25" diameter tires to 26.25" diameter tires with no changes in anything, which is equivilant to a 5% change in final drive ratio.

Look at the final ratios, cause they are what matters. doest matter if you have a 2.2 or a 4.11 or something in between. doesnt matter at all. what does matter is your final , resultant ratios. And, more importantly, the spacing. for example, the GTS with the 3.09, gave a 1st and 2nd gear set that had only a net 8% more torque for those two gears, (16% gain in 1st, 7% loss in 2nd) then from 3rd -4th -5th, they had almost the exact same ratios as a stock S4 's 2nd , 3rd and 4th.
People get caught up in the numerical rear end ratios and gear numerical designators . they should be looking at effective ratios and how you can keep the engine at max hp for the most amount of time. (if you are looking to be fastest, not the drivability factor)

as a note, a 3.09 on an S4 creates a mess of a gear box. it bascally brings the entire gear set down one ratio. this means you have" 1/2th", 1st, 2nd , 3rd and 4th. (instead of 1-5th gear) . oh and by the way, you now have a new 4th gear that is really 5th with that crazy autobaun 50% ratio drop. meaning you shift out of 3rd at 115mph, and then BOGGGGG in your new 4th (which is now your top gear and was 5th) so, basically you have 4x4 Low first (i called it " 1/2th"), and then 2-5 are the same as 1-4th, but the big final gear drop. get the idea??



Mk

m21sniper

No kidding.

m21sniper

Add another gear on the top end without changing the ratios of 1st to 5th and you'll gain 'something' as well....a lot of fuel cost savings at cruise speed, and depending on the application, perhaps quite a lot of top end.

"Look at the final ratios, cause they are what matters. doest matter if you have a 2.2 or a 4.11 or something in between. doesnt matter at all. what does matter is your final , resultant ratios."

That part is absolutely correct though.

mark kibort

if you are talking about a stock S4 box, or even a GTS 2.75, another gear on top of the one that gets the car to 185mph , wouldnt do much for top end, as the GTS would barely get to 185mph on its best day. however, Gas mileage may be effected as long as it didnt bog too much crusing at 1500rpm at 65mph may just cause cam wear and build up in the cylinders.

if you want the answer to the question of what gear box would be better for the 928, the answer is one that puts in a gear right between 2nd an 3rd and between 3rd and 4th. (ie that would be close to the ratio closeness of the porsche cup car that puts down about 8% more hp to the wheels over the operational range vs our 928 and other cars such as the vet even though it has a 6speed) (see PTG M3 race gear box too)

right now the GTS or S4 box is pretty good for most tracks, however, a single gear in the middle of the ratios added would be nice! both boxes have the same gear spacing (besides the useless 5th on the 2.2, but thats advantage to driving to the track with regards to gas mileage, but its never used racing)
the S4 with a 155mph 4th gear is pretty effective at tracks like road america and Laguna seca, vs a GTS box, with a top speed in 5th of 175mph with 500hp!
Lets face it, while racing, the S4 uses 2nd, 3rd, and a little of 4th. so , basically its a 3 speed. the GTS uses 3rd and 4th, and sometimes a little of 5th depending on the track. while the GTS can use 2nd on tight hairpins, the S4 can dip into 1st too. both are pretty darn close and can be used for different tracks effectively. to bad they are so hard to change out!

mk

SharkSkin

Note that usually there is no difference in rolling radius. My 17's are within an insignificant RCH of the rolling radius I had with 16's. IIRC, DR's 18" wheelsets also have the same rolling radius, and are lighter to boot -- which should improve acceleration.

mark kibort

yes, the rolling radius is pretty close in most wheel changes, however, any weigth savings is insignificant in HP savings. (ie acceleration difference potential)
i often change back and forth between 5 lb ligher tires (toyo vs S03 hoosiers) on the dyno , there is no differnce,and why would there be. acceleration time range is too long to effect HP . 10lbs for both 17-18" diameter masses, accelerating from 700rpm to 1800rpm (ie 4th gear 3000rpm to redline) over 7-8 seconds requires less than 1hp. however, this number goes up significantly in 2nd and 1st gears, to a level of around 5-10hp . remember the reflected inertia is reduced to the engine through the gear box . on a flywheel , that amount of weight , say 5lbs, becauses it is rolling is like having 7lbs in the car in 4th , but more like 50lbs in 1st gear as far as acceleration effects. on the wheel, a rule of thumb is that for every pound of wheel weight, double it for its effects of acceleration as if it was in the car. 5lbs wheel weight acts like 10lbs if it was in the car. 100lbs, in the car for example, acts like 10hp for most cars in our weight/hp range.

Mk