mark kibort

ahhhhh, you should have stopped and said hello!! did you race that day?

yeah, i dont think spinnning faster is an argument for the change at all. remeber, the rate of rotation for the internals is based on the input shaft (engne RPM) and you want to maximze the engien RPM regardless of your final drive. nothing will ever spin any faster or slower if you are in the sweet spot of the HP curve and you never stay in one spot anyway. usually, 5500 to 7800 is where you want too be. the wear and forces will be the near same regardless of the gear ratios. i dont know where the reduction of speed is coming from.... do you know?? curious ..........am i missing something?

this is why i posted what i did.. you see, the engine is even more powereful than the original , but it has a 7 speed gear box normally on the 991. however, if the HP curve, is as broad as the GT4, then there is no reason to have the 7 gears.. most of that is for road noise on the hyway, so you can chug along at 2000rpm rather than 3000rpm at 80mph in 7th. however, if you look at the gear spacing and the HP curve of the 991. 3.8.... you can see that the broadness of the HP curve alows for the spread of gear ratios just fine. there is no advantages i can see.... do you see any? if so, what gear would you want to change and why?

I have a problem understanding how anything spins faster with a change of rear end. if your target speed is to be in the sweet spot of the HP RPM, then how does the rear end change matter in some cases, you would be spinning more RPM for example, if you made the rear end taller. why, because in those cases of speed, you would want to be in a lower gear with even higher, much higher input RPM. the entire premise that going shorter or taller RPM changes anything over all , is not logical and a little misleading. (unless I'm missing something and ive i am.. please explain why)

mark kibort

I found this from a guy posting dynos of the following cars
check out the pink line.. is that the 991 engine you are refering too? what is teh redline... isnt it over 7500rpm stock?

edit: added a 3.8 L cayman dyno and 991 specs with gear ratios.

The chart are 911s dyno'd on a Dynojet 248. As follows:
dark green: 2.4 MFI
teal blue: 3.2 Carrera, chipped
red: 964 3.6
bright green: 993 3.6
brown: 2003 996 3.6
pink: 997S 3.8
blue: GT3

Alan C.

Brian Copan built several gear boxes for me. On the first 993 I picked the gears against Brian's suggestions. In 4 weeks I was back having the box done again. This time per Brian's suggestions and his rpm splits chart. Huge difference and an expensive lesson.

Yargk

I wish I was racing (only racing I've done is Lemons); I was at the DE on the West side. It was a NASA day on the East, right? I'll definitely say hi next time!

mark kibort

Lemons is racing!! thats right.. i came over warming the car up to check out what was going on. cool! next time stop by

check out the HP curves and plot the ratios or RPM drops .. you can clearly see, if you have a wide HP band, the gear ratios clearly dont matter much , especially if you are visiting lots of different tracks, and or spending time on the street at different speeds. always trade offs.
by the way.. the redline for the 3.8 looks to be 7500, so you need to take that into account too.

just think about it.. take the 997 3.8 dyno graph above with the pink line... if you can use the HP curve from 6000rpm to 7500rpm there is never a need to change out the gears. there is a dip in that particular graph... it would be far cheaper to fix the dip in the HP graph with a tuner like Vision motorsports then replace the gears in the gear box. once you have the HP curve working for you and optimized, there is not a gear change that can improve the effectiveness of its ability to put accelerative forces at the wheels at any vehicle speed , anywhere on the track.. IF the driver knows the operating RPM range to stay in.

stout

Hey Mark, simple question, asked honestly and with zero desire to start a flame war: How much seat time do you have in GT4s? 0 miles? 100 mile? 10,000 miles? I ask because a lot of professional road and track testers have noted their dissatisfaction with the GT4's gear ratios, as have a lot of owners. This, after gleaning real-world impressions.

There are charts and theory, and then there is rubber meets the road. I've tested a lot of Porsches over a lot of years, and probably have 8,000-10,000 miles in GT4s on a variety of roads and on track, as well. I feel the gearing in the GT4 offers an area for real improvement—and that the gear ratios from the 991S that the GT4 gets its engine from would make for a significant improvement.

d00d

My first VW had short gears and I was always shifting, hated that car, I thought it would be great if it had long gears.
Now forty years later the company has finally put the engine in the right place, and I'm thinking it would be great if it had short gears.

mark kibort

That's a valid question.. i have seat time of two days driving and instructing in one. and some street miles. Im sure as much as many of the road and track, or bloggers that talk about cars.. (by the way, many of these "professionals" change their tune when they start driving on the track. ) but in the end, im only speaking from a performance perspective, and this transcends models and types of cars.
so, my question back to you would be what is the deficiency in the ratios?
sure the 3.8 liter that came from the 911 , has a different gear box in that car. there are plenty of reasons to change the gear box.. porsche had the same model of car with the same HP with several different gear boxes based on where the destination was for the car, and options.
if we are talking racing, a change to a different set of ratios or a final drive change seems like a lot of expense for little gain, unless you need that little gain for a particular track. (this is not a one fits all discussion)
however, in respect to hyway use, sure depending on your neighborhood and driving style, and speed ranges, a different gear box might suite you better.

contrary to GTGears comments, putting a taller rear end vs lower rear end ratio shouldnt change much, other than cruising RPM. (which is a concern and preference of mine by the way), so for that reason, i would endorse the change, but he doesnt want any part of that, because " the car as is, can go Daytona speedway speeds in 6th". so, what are we left with? changing the gears for what reason. this is my question to you . what are the deficiencies of the gear box with its different ratios to the 991, where the base engine comes from and what do they mean to you? seems like a fair question to have summarized about now, no?

Mark

ps here are the speeds in each gear from this thread . so , all we need now is the 991 stock gears with its stock final drive. do you have that handy?
to keep the discusssion simple.. lets see the stock 991, with the stock GT4 speeds in each gear at redline.



Speeds at redline with a 4.75 final drive -
39.85
67.64
93.55
116.73
138.84
162.84

@stout, Curious if you thinks this will address the 2-5 being too long issue (obviously, it won't help your 6th not long enough complaint)? For comparison, 1-5 speeds with 911S ratios mated to stock GT4 3.89 final drive -
41.19
70.33
103.91
123.89
149.13
and compare to stock gearing what does it buy you ?

Gt4 gearing as was posted. (if this is at the 8200rpm, then slow down by 5%)
1 48 45.6mph at 7800
2 82 77.9
3 114 108.3
4 142 134
5 169
6 198

from specs:

Number of gears
6 Speed
Transmission type
Manual
Ratios
1st
3.31
2nd
1.95
3rd
1.41
4th
1.13
5th
0.95
6th
0.81
7th
-
Reverse
3.00
Front Axle Ratio
-
Rear Axle Ratio
3.89
Final Drive Ratio
3.89

Water-cooled horizontally-opposed six-cylinder
911 Carrera
911 Carrera S
Gearbox
Manual

Gear Ratios
1
3.91
2
2.32
3
1.56
4
1.28
5
1.08
6
0.88
7
0.62
Final Drive
3.44

you know the reason for that, right? the bug had about 70hp on a good day, and needed the gears to keep the hp up in the RPM range , so they needed to be very short... just like the old datsun pickups. i remember driving one for work in highschool and it woudl go through three gears before it got through the intercection after a stop light! tall gears would fry clutches in 1st, as you would need big slip to get rolling, AND, the next gears needed to be low to get the car accelerated to highway speeds. they couldnt go much over 80mph so, they were geared just about right back then. race car versions with more HP could go taller..............like the love bug.

stout

When you say two days, do you mean a few 20-minute sessions, split between driving and right seat? Even if they were open track days, the thing is, the problem isn't evident on track, or at least wasn't at Portimao.

It's evident in other situations—street, autocross, back roads, freeway cruising, gas mileage—and how.

Those aren't the ones I listen to—much, any way. Luckily, you've got "many" in there as a qualifier, twice. Thing about auto reviewers is that while only a tiny number can really drive, they tend to sample a lot of machinery and the best ones have their 10,000+ hours spent conveying what they learned. The good ones can drive a car well enough to glean useful information for the average reader, as well as for readers and racers who can drive better. The good ones can push a car as hard as the best owner/drivers might in certain safe situations, but can't turn a fast lap time because that is not needed for their toolbox, and presents a risk factor that may shorten their access. And career. The very best ones can turn lap times to match a pro. They are rare, rare, rare. I am not one of them.

Conversely, the best racers can outdrive all but a handful of writers—and I am not one of those—but a large percentage of them can do precisely bupkis to improve the car or tell you how it works. I'm told Milt Minter was the classic example: He was fast in anything (anything), and could get all it had to offer—but he could not tell you how it drove or how to improve it. He sure told great stories about Vasek, though. I've been told there are a lot of racers today who are in the same boat—they are fast, and can extract a lot from the car, but can't tell you much that is useful to you, or help develop the car. At all.

Disagree. This statement implies book knowledge beats all. In this particular case, you're flying in the face of a lot of knowledgeable folks with book knowledge and seat time.

Explained from the start of the thread forward.

This appears to me non-sensical, and implies that you haven't taken the time to read through this thread, or brush up on the story of the GT4's development. I may be reading it wrong.

Which would exacerbate the problem noted in the first post of this thread...

I don't understand what you are saying, but do have a mild concern that you may want to grind an ax with Matt, and I hope not—because this is a fun and enlightening discussion with Matt, who can offer specific knowledge as well as services of use to the subject at hand.

Explained in the original post, and then throughout the thread forward.

Fair question, but again this implies you skipped a lot of posts....

stout

That's what we need to do. I lost a lot of that type of info when I left Pano, as I left it with the club. I can do some digging for speeds in gear, or someone may have it handy. Per Porsche, the gear ratios are:

991S (PDK in parenthesis)
1st 3.91 (3.91)
2nd 2.29 (2.29)
3rd 1.55 (1.65)
4th 1.30 (1.30)
5th 1.08 (1.08)
6th 0.88 (0.88)
7th 0.71 (0.62)
F/D 3.44:1 (3.44:1)

981S (PDK in parenthesis)
1st 3.31 (3.91)
2nd 1.95 (2.29)
3rd 1.41 (1.65)
4th 1.13 (1.30)
5th 0.95 (1.08)
6th 0.81 (.88)
7th -- (0.62)
F/D 3.89 (3.89)

This info may have appeared earlier in the thread, but maybe it can be filled out with the maximum speeds in gear, and perhaps even a better way of comparing the effective gear ratios given the 3.44 vs 3.89 final drives. I suspect that means that, since custom gears would be required, those gears could be made to take into account the difference in final drive between the two while trying to create effective parity with the 991S in gears 1-5 plus 7. If, of course, that was the goal. It would be mine, and I suspect it would be a good setup for a lot of GT4 owners. Or maybe it could be optimized from there if a gear expert laid out a better plan to maximize acceleration and flexibility in gears 1-5 with a nice, tall cruising cog for 6th.

Manifold

We're talking about how the power delivery of the car feels on the road, in comparison with other manual Porsches. No theoretical argument can override that empirical experience. If it feels sluggish, it feels sluggish. The question is how to fix that, not how to rationalize the problem away.

stout

Mark brings something important, as others have, in that to fix the feeling, we need to look at some numbers. A simple, digestible comparison of the effective gear ratios in 1-5 and 6th between a manual 991S and a GT4 would be very interesting—as would the max speed in each gear. That may be a bit above my pay grade, or involve that pesky stuff called math. From there, it would be easier to chart a road to fixing the feeling so many of us don't like. And that's that "sluggish" you speak of. I feel like I can set the time on my wristwatch while waiting for the GT4 to wake up in second gear. I can set it and your wrist watch's while waiting for 5000 rpm in third.

d00d

Had the high performance 65HP fuel injected 71 Fastback, with a weird oil bath air cleaner, while the same year Bug had 60HP.
I thought of it as Hitler's revenge, when I got a face full of asbestos changing the rear drum brake pads.
;)

neanicu

The 991S has more power,less drivetrain loss and better traction. One thing I've noticed in the GT4 that I didn't notice in the 991 S is wheel hopping. If you throw the Powerkit in you're talking 45HP more than the GT4. You feel that! The talk we hear during GT cars presentation about " GT horsepower " is just marketing talk. The GT4 makes exactly what it is rated for. Same goes for the GT3. It's been proven on the dyno.
The only complaint I have is the 2nd gear,that's it. If you want to go to the redline and shift it takes a while... But I've learned to live with that. Most drivers don't have that fine tuned sensibility. Perhaps I'm most drivers... And that's ok...
I wouldn't risk losing the warranty for the " gains " I'm reading about here.
One thing that the GT4 is not it's " sluggish ". Pete,this thread combined with your other one and I'm starting to believe your GT4 is defective...

Manifold

I've driven the GT4 back to back against the Cayman R and 997.1 C2S, all 6-sp manual. On the road, the latter two feel significantly peppier than the GT4, despite having less power and peak torque, whereas the exact opposite should be the case. This can only be explained by the torque curves and how they work with the gearing in each car.