mark kibort

Exactly!! however, sometimes a taller 1st gear can allow a better launch, especially when traction is the most dominant variable. Also, depending on the target speed, as the spacing is the same, the idea will be to make you run to red line just as you cross the finish line in a drag based on hp and traction. now, increase the engine hp and the varibles change and you may actually need a taller rear end. (ie if you are running 13.3s and hit a final speed of 105mph in 3rd gear and you are redlining (at 320hp) now, if you put on 50hp, you may be able to run to 110mph, but that will be in 4th gear for the last 5mph. going against the same car, a taller ring and pinion may be the answer. looseing a little at the beginning, but gaining vs the original where you spend the most time in that last gear.
This example is the best way to explain the actual trade offs in racing or drag racing. its about trade offs.

Mk

mark kibort

The Gear ratios for a 3.09 GT vs a S4 with a stock 2.2 rear ring and pinion

GT/GTS/'79 with 3.09

1st 11.6 stock GT 10.2
2nd 7.7 stock GT 6.8
3rd 5.53 stock GT 4.8
4th 4.17 stock GT 3.6
5th 3.09 stock GT 2.72

Stock S4 with stock 2.2 rear end
1st 8.9
2nd 5.9
3rd 4.24
4th 3.2

stock 84

1st 9.67
2nd 6.45
3rd 4.6
4th 3.48

now, if you dont believe that the 3.09 will just give you an S4 transmission, here is how a race would more than likely pan out

GT/GTS/'79 with 3.09

1st 11.6 :1 (close to useless) (note: average ratio of 1st and 2nd= 8.6:1)

2nd 7.7 (15% taller ) S4 1st gear 8.9
3rd 5.53 (15% shorter) S4 2nd gear 4.8
4th 4.17 pretty close S4 3rd gear 4.24
5th 3.09 pretty close S4 4th gear 3.2

Anyone can see that in a drag race, with the exception of 1st gear, the 3.09 box on a GT would have a off the line advantage, assuming perfect hookup, then after a shift, (which takes a larger percentage of the time vs other shifts, and that could negate the gains quite possibly)
then, you have the S4 having a 15% advantage in 1st, but a 15% disadvantage in 2nd, but then a very close 3rd and 4th. (almost no diff).

Also note, that if you average the 3.09GT box 1st and 2nd, assuming absolutely NO loss for a shift, (impossible, but for argument sake) you wold have an average ratio of 9.5:1. this is a mir 1.08% shorter than the S4 1st gear. NOW, take in account the shift time, and you see, there is NO gains to be made here in launch or otherwize

so, meaing to a race to 145mph, these two gear boxes would propel a S4 and a GT with the 3.09 to pretty simular times and acclerations.

If the race was to 135, the 84 may have the best gear box, while if the race was to 125, the GT /GTS would have the best box. Interesting that the 84 seems to be one of the best ratioed boxes. Now, Ive run the same engine with an 84 gear box and a S4 gear box. didnt notice too much difference in my times at Laguna which I frequent and times for the same car usually dont different more than .5 seconds on any two days! So, the difference in most all gears is between 8-10%, so this is a big change too, and I saw no difference, and actually at laguna, the taller box worked better.

hope this helps

MK

SHRKBIT

I didn't say anything about horsepower Mark, and Mark Anderson doesn't drive the same courses I do, nor is my car anything like his. You'll just have to trust me when I say that I don't have a "correct" gear to use in some important places, because I don't care to kibort about it.

SHRKBIT

Jim, you are, of course, correct. I completely missed the layshaft ratio, which amounts to a whopping 8% difference. I need to go back and fix my spreadsheet....

mark kibort

But you now see that the 3.09 on a GTS or GT only creates a S4 gear box, with no 5th gear. cant argue the math here folks. it is what it is.
The key point, is that the 1st and 2nd gears of the 3.09 then would average to only 8% different than the current 1st gear on a stock S4 , and ive been through a 8% gear box change before (over all gears) and didnt see or feel much of a difference. Keep in mind, these kinds of changes are usually result in precieved value, not actual gains.

If this all makes sence, you have broken the code and can make gear changes with the right expected outcome
.
If it doesnt make sence, read the last post of mine again and get back to me with questions or comments.

bottom line, look at the final ratios. They tell ALL!!

dont believe me still, just ask Jim. pretty elementary stuff.

Mk

mark kibort

Now, we were talking about a 3.09 on a GTS box. now, if the question becomes putting a GT/GTS ring and pinion on a Stock S4 2.2:1 box, you will really have a mess of a gear box. 1st gear would be totally unusable, and 4-5spacing, and you would then get into 5th occasionally, would have a 50% spacing. not good. I could work the numbers, but that would not be a very efficient combination. (ie 4th gear wold get you to only 116mph, as the stock 3rd usually gets to 111mph. so you would get a 5th gear that would now be at a 50% drop and at an EXtreme torque disadvantage after 116mph. ) But, if your goal was to never get any faster than 116mph, it would be marginally better than the stock S4 box. you would have a 11:1 1st gear, that would be pretty uselss, but not that far off from the 10.2 of the stock GTS box's first gear.

Mk

kjcracer

Well since my name has come up in this thread, maybe you'd like to know the story about what we were up to with 928 gearing and modifications, as I see (saw back over 10 years ago) them? If so, please read on...oh, it's a bit long... it's a complicated subject.
There is a standard old trick: narrow up the gear "splits" in the car...so it can have a narrower power band, and then cam it wilder for that narrower rpm band. A lot more power in a narrower rpm band...and the (new) narrower gear splits make it work effectively. When I looked at the manual 928 gearbox, I saw factory gear splits that look something like this: 1st to 2nd and 2nd to 3rd, approximately 2000 rpm drops (when shifting near the engine redline) and 3rd to 4th and 4th to 5th about 1500 rpm drops. The exception is the USA 83-88 "EPA" gearboxes that also put in a 2000 rpm drop between 4th and 5th, along with the 2.20 r&p. (OUCH, if you ever need 5th gear at your racetrack with that gearbox!)
Why was Porsche up to? (When not trying to get around some arcane EPA rules.) Well, notice that they're "narrowing" the splits some, as the speed goes up. They know the engine had better be in the best part, think of it as the "sweet spot," of the power band as the speeds go up (I'm sure you've all noticed the available acceleration is less at higher speeds/gears.)
My idea? A real close-ratio gearbox for the 928...really enabling us to "cam up" the engine, too...just applying classic theory to our car. So, building on all the discussions so far on this thread, here was my three part plan:
1) 3.09 ring and pinion (deepest that fits the case, keeping-up the strength
of the parts)
2) close-up the layshaft ratio even more (makes the 4-5 shift "closer")
3) pull-up 1st and 2nd gear, too... (close them up, more, too)
This would give gear splits like this:
1-2 shift 2000 rpm (vehicle is least sensitive to power band at this speed)
2-4 shift 1,750 rpm (closer)
3-4 shift 1,500 rpm (stock)
4-5 shift 1,300 rpm (really close)
I think you get the idea. As the speeds go up, we'd operate in a narrower, just the best, part of the rpm band of the engine. We'd be able to highly cam the engine, and really make it really work...there is a synergistic effect. We'd have more ideal gearing for nearly any roadrace track, as we weren't working so close to the edges of the engine band.
So what happened to this idea? I had a couple PCA 928 S4 and a new GTS owner who wanted to try this idea. Finding a company to make parts for an o.e. transaxle wasn't easy (do this aftermarket limited slip, please!) but I did find Houseman Automotive in London, Ontario, Canada. I sent o.e. samples to Jamie, and he went to work.
The first part was the 3.09 ring and pinion. I drove Jim's car at Lime Rock in 1995 with it. Fast. The in-car camera videotape "agrees"...we made mincemeat out of the instructor group, after being first told they didn't feel 928s were fast enough for the (red) group. That notion got settled, then...
Next? It was time for the next steps. To change the layshaft and also make the custom 1st & 2nd gearset. While the 3.09 had been utterly reliable, these parts had subsequent failures. To this day, I can't even be sure what happened....I couldn't believe that some shop had machined the parts after they got them, installing them without consulting anybody. And then took no responsibility. I think we may have worked through the problems on these parts...but...the affected members were out of patience. Hey, development work sometimes is painful, so while I was disappointed, I understood the choice to continue/or not was theirs.
About the same time, PCA National nixed my proposed 4-valve 928 follow-up story to my 2-valve "Project 928" trilogy (from back in the summer of '91) so I now had little personal reason, or funds, to move forward alone. The whole project was stopped, "mid-stream." The 3.09 ring & pinion is the only product that I could call reliable, without any further work.
This concept would have fullfilled all the criteria to make a great close-ratio 928 gearbox...allowing us to do classic camshaft hotrod power boosts! Thought you'd like to know what the original idea was, and what happened to this little-known project...the close-ratio 928 gearbox.
If you look at details, you'll see that the stock 4-5 shift is much tighter than the 2-3 shift (on the "good" USA gearboxes, anyway ) I feel the 3.09 r&p, alone, is often a small improvement (again, not the "EPA" boxes, those need more changes.) Why? At least you're using the closest "gear splits" the factory offered...the 3-4 and 4-5, 1500 rpm splits...eliminating the 2000 rpm drop 2-3 shift at all but the slowest tracks. Yes, you will be limited to 150- 155 mph top speed depending on your actual engine rpm limit...but there aren't many tracks where you actually go faster than that.
If want to argue about this, please spare me. Take it up with the race guys at the factory! Yes, I was reading a copy of the new/revised "Excellence Was Expected" three-volume history book series on Porsche last week. Great reading, with plenty of 928 details in the appropriate chapters.... including? Guess what ring and pinion the factory came up for a race 928 Euro S, in 1983, for several races? A 3.09. Page 1070, Chapter 47 which is called: "Ultimate Eights" (in Volume Three.) You can check it out yourself...oh, and send congrats to Mark Anderson for getting mentioned on the same very page! I am not foolish enough to argue basics with the factory motorsport engineers, people like Helmet Flegl...those guys have probably "forgotten more" than any of us will ever know. Certainly if they thought enough of this idea to make a custom race part, there is merit. I hope I've explained (somewhat) the overall view of "why," and I'm only sorry to report that the close-ratio 928 gearbox project never got finished, only the 3.09 ring & pinion made it through the development process. With 20/20 hindsight, I am glad to see the factory agreed, valdidating the idea for motorsport application. Hope all this clarifies things some. Thanks for listening. Kim

mark kibort

Kim, Please go over the ratios of a 3.09 on a GTS or GT 2.72 gear box. As I made EXTREMELY clear before, it creates an identical gear box to a S4 .

It doesnt take a rocket scientist to show why. just do the simple math!
the gear spacing of a 928 gear box, whether it be GTS, or S4 or 84, are all around the .75 per shift spacing. the only tall shift is from 1-2nd or 4th to 5 ONLY on the 2.2 rear ends. 2-3 3-4 (and then 4-5 on the GTS/GT are all pretty close)

as I have provided, the 3.09 change only, provides you with near idential gears to the S4 thru its 4th gear. The ONLY difference is that you end up with a much lower 1st gear, but slightly taller 2nd gear compared to the 1st gear on an S4. drive the 3.09 to the top of 5th gear to 150ish mph which the same as the 4th gear on the S4.

Like I said, this doesnt need verification from some porsche engineers. its basic gear box math. as i will say at the end of this post, making the gears closer will be a big benefit.

Now, if you are running around instructors with a 3.09 GTS before and didnt before, id look at the nut behind the wheel first ( nothing personal)
remember, Speed Touring cars (ie bmw 328s, audis, civic, etc) are running 1.01s at lime rock on 280 tread wear street tires!!

Now, go punch in the ratios and tell me how the numbers are lying. Heck, dont shoot the messenger here. Im just trying to help.

by the way, a recent post of mine, also confirmed that the GTS ratios are the best porsche ever produced for any car. (besides in my mind , the GT3R) anyway, the Carrera GT gears are the same as the GT/GTS 928 with the exception of a 6th gear placed between your 4-5th gear. otherwise, the other gears are almost identical. for a car that almost made it to lemans, its a testimate to the ideal spacing for most all uses. (please note that the 1st gear on the Carrera GT is 10% lower, but in racing this car and the GTS 928 would be seeing 2nd through 5th on most tracks)

Again, put down the ratios as I have and put up an arguement. Also, forget about saying " such and such rpm drop" this is misleading, as it is a pecentage drop of RPM between gears. Most all of the 928s, and most other cars , have about a 75% drop in RPMs or 1/.75 (1.33%) gain in rpms between gears.

so, after looking at the numbers, a 3.09 on an GTS gear box , is usless.
you only get an ave 8% gain but have to shift once, while a stock GT/GTS would only keep his foot in it in 1st. (in a drag) all other applications, there is no point. Look at the numbers!!!!! please.

Now, as far as making using a 3.09, (which would bring down 5th gear) and making the intermeadiate gears closer, you have NO argument from me! that would be a great gear box, if it could be made. however, top speed with a 3.09 rear end would only be 150mph or so, and that would kind of put you in the back of the pack at Road america wouldnt it?



MK

mark kibort

Here are the actual ratios of the GTS, GTS with 3.09 and carrera GT

US gear box 1st to 5th

8.9, 5.9, 4.2, 3.2, 2.2:1
---\----\---\----\
GTS/309
------\---\---\----\
11.4, 7.7, 5.6, 4.2, 3.09:1 ( 1st and 2nd average to be 7% shorter than the S4 1st gear)

GTS

10.3, 6.8, 4.9, 3.7, 2.7:1
|----|-----|----|----\
Carerra GT------------\

11.2, 6.8, 4.9, 3.8, 3.3, 2.7:1 ( extra gear is placed between the GTS 4 and 5th to make the GT the 6 speed.)
(Note: ratios are adjusted to corresponding max HP rpm difference proportions of the GTS vs the carrera GT. 6800rpm vs 8300rpm)

this all goes back to the original post, of what would be the optimum rear end ratio or gear set for a 928. the GTS pretty much has it. However, on some tracks, like laguna and sears, it would be debateable that the S4 or S gear box , is the best gear set.

Kim mentiones that the S4 gear box takes you to 145-150 in 5th gear. cant think of a single track besides Road america where that would be an issue, with up to 400hp flywheel. with 500hp flywheel, there could be tracks where you could get slightly higher, but not much!

close ratios help not only cam'éd cars, but more especially, not cam'éd cars.

remember, we always want to be as close as possible to max HP, as much of the time as possible.

Mk

mark kibort

Kim,

now that i have shown the exact ratios for a 3.09 on a stock GTS/GT gear box, how can you say that it was "fast" when that resultant gear box will basically give you an S4 gear box, as it would have almost the exact same ratios as an S4 box in its 1-4 gears (up to 150mphish) , while the new 3.09 ratio'ed gear box would have 1-2n gear averaging 8% shorter than the S4 first, and then the next gears all the way to 5th matching the ratios of a stock S4 box?

The reason I ask, is that you drove a 3.09, and said it was fast.. puzzeling!
more puzzeling, as on a race track, LimeRock or anywhere else for that matter, you would never touch 1st gear in an S4, let alone a 1-2nd gear average only slighly lower than the stock S4 1st gear. Basically, 3rd through 5th on the 3.09 would be thesame as 2nd through 4th. make sense. so, any perceived gains, are just that.

In road racing, there are some tracks where the S4 box could be better than the GTS box anyway. so , maybe the reason you liked the S4 (equivilent , with 3.09) box, was that the straights at Limerock, only allow for a 111, mph top speed, (ie 3rd gear max in an S4, or 4th in a 3.09 GTS) so that if you had a true GT/GTS gear box, in 3rd, you would have ot shift in to 4th for a while while a competing S4 would be taching out in 3rd for an advantage . Just a thought. But the bottomline, is that the 3.09+ Gt/GTS = an S4 gear box from 2nd through 4th and 150mph

thoughts?

MK

=throughQUOTE]Originally posted by kjcracer
Well since my name has come up in this thread, maybe you'd like to know the story about what we were up to with 928 gearing and modifications, as I see (saw back over 10 years ago) them? I
The first part was the 3.09 ring and pinion. I drove Jim's car at Lime Rock in 1995 with it. Fast. The in-car camera videotape "agrees"...we made mincemeat out of the instructor group, after being first told they didn't feel 928s were fast enough for the (red) group. That notion got settled, then...
Next? It was time for the next steps. To change the layshaft and also make the custom 1st & 2nd gearset. While the 3.09 had been utterly and I'm only sorry to report that the close-ratio 928 gearbox project never got finished, only the 3.09 ring & pinion made it through the development process. With 20/20 hindsight, I am glad to see the factory agreed, valdidating the idea for motorsport application. Hope all this clarifies things some. Thanks for listening. Kim [/QUOTE]

worf928

MK, I will point out your use of the word 'basically' above as it relates to Kim's use of the phase 'fast.' Neither choice yields precision.

Below is a table of speeds in mph at 4000 and 6400 rpm for four gear boxes - a GT, S4, GT/3.09, and S4/3.09. Readers will note that there are very few gear choices that show the exact sames speeds for each box. The differences are small and subtle. (Note that these speeds are not calculated based upon a specific tires' loaded revolutions per mile. For simplicity I used 93% of the mathematically correct diameter of the stock GT 245/45-16 rear which yields speeds within a few percent of the 'spec-book' speed for each gear. I used 4k and 6.4k rpm values since they are in the peak torque area of the curve.)

Based upon the above I'd say that a GT w/3.09 is basically like an S4 box only if a GT box is also 'basically' like an S4 box.

mark kibort

kind of a silly quote, dont you think, when you are looking at the comparison of a GT with a 3.09 and see only 2%,4% and 6% differences in the equivilant 3rd through 5th gears on a 3.09 GT gear box, vs a stock S4 gear box's 2nd through 4th gear. your post states if these are basically the same ratios, (and I would have to say, when your gear ratio differences equal the range of tire diameter variances, its basically the same),that the S4 is " basically" the same as the GT!! The GT to S4 ratios even gear to gear are a DOUBLE 15% different.

so, if you look at my post again, you will see the coorelation of the gear comparisons that show that the 3,09 for racing purposes is practially identical.

GT with 3.09 vs S4 with 2nd 3rd and 4th (S4) compared to 3rd , 4th and 5th (GT 3.09)

74 vs 79mph 6%
105.6 vs 103.9 <2%
137 vs 143 mph 4%variances

while a GT to s4 comparison from 2nd to 4th gears is
137 vs 119
104 vs 90.5
64vs 74 ALL 15% variances.

anyone that has raced , knows that the benefit of gearing is resultant of the fact that you want to keep the rpms in the max hp range (not max torque range) of each gear. the 3.09 GT vs S4 comparison, actually shows an advantage for most tracks, as your top speed on most straights are never faster than 137mph. in racing, even that 6mph is a slight advantage going to the S4 gear box as a racing gear box.

Hopefully , you understand the point. " basically "means, for all practical purposes. basically, "equals" a 3-4% change of something, while 15% doesnt!! there are substantial differences in a GT vs S4 gear box, yet depending on the track, each has its advantages. the correct answer is" it depends"

my term basically, doesnt equate to Kims term " faster" , as we all know it is impossible to tell the difference between a 25.5" diameter tire and a 24.5 diameter rear tire( ie 3%) or 10hp for that matter. however, a 15% change could be detected, so it is not " basically" the same.

the FACT that ive shown you that the 3.09 GT gear box, vs a stock s4 gear box having gears that are average 3% difference, shows that they are too close to tell a differnce. But, a GT to S4 would and is an obvious change. I know, ive had both gear ratios on the track.

As far as your S4 3.09 numbers, pretty interesting, the s4 becomes closer to the GT 3.09, but has a unusable 5th gear due to spacing from 4th. However, its only 7% or so everywhere else. so, the 3.09 is such a big drop from the 2.2:1 stock ring and pinion, it basically just shifts all the gears down a ratio. (now this basically is in the 5-6% range)

Semantics folks!! GTS 3.09 becomes the S4. hard to argue, although there has been some attemps.


MK




QUOTE]Originally posted by worf928
MK, I will point out your use of the word 'basically' above as it relates to Kim's use of the phase 'fast.' Neither choice yields precision.

Below is a table of speeds in mph at 4000 and 6400 rpm for four gear boxes - a GT, S4, GT/3.09, and S4/3.09. Readers will note that there are very few gear choices that show the exact sames speeds for each box. The differences are small and subtle. (Note that these speeds are not calculated based upon a specific tires' loaded revolutions per mile. For simplicity I used 93% of the mathematically correct diameter of the stock GT 245/45-16 rear which yields speeds within a few percent of the 'spec-book' speed for each gear. I used 4k and 6.4k rpm values since they are in the peak torque area of the curve.)

Code:

RPM	4000	6400	4000	6400	4000	6400	4000	6400
Gear	GT 28.55	S4 28.13	GT (3.09)	S4 (3.09)
1st	26.9	43.0	30.9	49.4	23.7	38.0	22.0	35.2
2nd	40.3	64.5	46.2	74.0	35.6	56.9	32.9	52.7
3rd	56.6	90.5	64.9	103.9	49.9	79.9	46.2	73.9
4th	74.8	119.7	85.8	137.3	66.0	105.6	61.1	97.7
5th	101.3	162.0	125.5	200.8	89.4	143.0	89.4	143.0

Based upon the above I'd say that a GT w/3.09 is basically like an S4 box only if a GT box is also 'basically' like an S4 box. [/QUOTE]

mark kibort

David, why did you use 4000 to 6400???? no one should care about the max torque range of any gear, its the ave HP in any gear that should be looked at. That becomes the range of the ratio . As Kim said, its in the 1300 to 2000rpm range in any gear. not that it changes our discussion, but its just a point you should understand as well. correct ranges would be the appropriate rpm drops for each gear run to redline. (or as high as possible)

as another note, my 928 with the 2.2 rear end is running 111mph in3rd gear at 6400rpm.
The tire calcuator page someone posted is good to see what the speeds will be with any given tire size and gear ratio. vs RPM.

mk

worf928

Because they bracket the region of ...
... peak horsepower - more specifically the RPM range that you stay in on track. The use of 'torque' in my previous post was a brain fart at the end of a 17-hour work day. Pick any RPM range you like for the speeds. The speed numbers previously posted were an effort to cast the f/d differences into numbers with units that were familiar - m.p.h.

Ideally you would calculate based upon the RPM drop from gear-to-gear from redline (or close to redline) in order to compare the different boxes.

Last, if you go back to the near-start of this, you'll see that I wrote "folks that have the 2.20 rear end."

mark kibort

David, No problem with the torque vs RPM. That was just a side note.

Now do you see why a 3.09 GT box is basically a S4 gear box? basically, for our discussions will mean. "some change less than 3% or so"
certainly not precise, but certainly accurate for the point. anyone that says "yeah, i put on 24.5"diameter tires and man, i was faster than my 25.5"tires", is saying the same thing as someone that uses a 3.09 on a GT, right??? right! now, I just get all geared up (pun intended) when folks say they notice small changes like this discussion subject matter. Been doing this long enough and consistantly enough, to tell you what you can see (as far as times ) and feel on the track when you change them.
when its a sub 5% change, you wont notice it. when its a 10% change , you still may not notice it, but the times may show something!

my last 3 races at Laguna seca had times of 1:40.2, 1:40.5 and 1:40.8. Lots of 3% changes happened in all of these races. weather, tires, tire sizes, tire compound, and adrenaline levels. You got to look at the entire system and make your decision and decide whether a particular change did actually make a difference!!

MK