jmartpr

Last time we all talked about this issue the problem was the cost of doing this mod and how many were really committed to dropping the $$$$...But I still wonder if just starting with a base Cayman transmission and changing 3-5/6 would be more economical and get you similar results....with the benefit that you have the spare OEM transmission in case something goes wrong.

RDCR

Matt: I'm assuming updated 3-4-5 eliminates the potential 3rd gear issue with the stock tranny?

With regards to your LSD I saw a comment you made about these cars starting to eat rear brakes as they accumulate more track time. I'm running PCCBs and had them density tested this week. After 1100 track miles this season I'm only down 2mm on the front pads and lost 5% on the front rotors. However I'm down 4mm on the rear pads and lost 40% density on the driver's side rear rotor and 50% on the passenger side rotor. I run with traction control on. I realize my driving style probably has something to do with this along with the track (Laguna) I run the most. I'm wondering how much the stock diff may be contributing.

GrantG

I think the PTV is at least partly responsible for the rear brake wear. The torque vectoring is achieved by braking the inside rear wheel for better turn-in.

mark kibort

Gents, you really need to step back a little and understand that the function of the gear box is the HP optimizer. what works for a GT3 will not necessarily be good for the GT4Cayman. why, the HP curves are different shapes.

to think that you ge more torque by changing gear ratios is a misnomer. all you do is shift the optimization of the HP up or down the scale.. this means, all you are doing is changing vehicle speed range and HP utilization. in other words, you change a gear ratio, you shift the optimization point up or down in speed. its a trade off..... example: a change of gear ratio might now make your cars sweet spot from 55mph to 75mph instead of 60mph to 80mph. if you race the car, you want to optimize for every single turn, and track, but that usually is impossible, so you look at the speeds you usually run, and where and select the best gear ratios possible...... if you dont have the luxury of changing the gears, JUST make sure you are in the RIGHT gear. This means you want to be shifting at redline to maximize accelerative force during all gears and resultant gear shifts.

Below Yargk puts it all on the table... its almost all you need to know. whats missing is the HP curve.

what does the comparison say? it says that you really have a 4 speed for the track, and possibly really only a 3 speed for most tracks. 2nd for most speeds over 40mph and a top track speed in 4th gear at 147mph.. (road america mani straights, unless you have boosted the HP like the POC racing caymans pushing 350rwhp) so, with that known.. there is NO issues.

i added some color to the gear ratio RPM changes post shift by adding the %. this means 1st is to get out of the paddock.. redline to 46mph. perfect for auto cross tight turns, and usless on a big track, for good reason.

2nd your lowest gear on the track, from 40 to 80mph .. most GT4 drivers dont use the gear properly.. are in 3rd where they should be in second on critical turns. and some dont use 3rd when they can and stay in 4th.

what this means is that you are NOT using the engines available HP.. in the above example, if you are ever in the 4-5000rpm range on the track, you are giving away near 50hp vs being at 6Krpm. this 50hp, is near 50ft-lbs of torque loss too. and the same percentage loss of torque at the rear wheels. this is the main point here. keeping the engine RPM in the sweetspot. the cayman has perfect ratios for most all applications. even on the hyway, with a redline of near 7800rpm, you cruise around at 80mph at near 3000rpm, it sounds like the engine could lug down a bit more .. but no worry, its designed to be operating at slightly higher RPM... there is no proof that the MPG you would get with a taller 6th gear would give more efficiency. if you want hear lower RPM, get a taller tire in the rear. But generally, dont worry about it.

in summary: the GT4 gear box allows for all shifts, after 1-2nd gear, to remain above 5700rpm which is the goal and sweetspot of HP for the engine. no changes can improve the perfection already achieved by porsche designers. if you think otherwise, i would love to hear any idea of a better spread or ratio for the GT4

just a few thoughts to put your minds at ease... you have an optimized gear box for the engines HP and HP curve of the GT4

this is what i was talking about above. ... the 5% change is not required because of the HP curve. as long as you are optimized for the entire HP curve for each gear, there is NO reason to change. GT3 needs to rev higher as the HP peaks and shape happens in a higher range of the RPM. thats the ONLY reason. you don't change the torque or anything by 5% by changing. sure in each gear you would have a change, but you also have a trade off of the exact same amount.. so, dont be too quick to judge. a common mistake in gearing decisions are made in using that logic. in other words, if you change your gear ratios 5%, you migh gain torque ini 2nd, but lose torque in 3rd in a more dramatic way... especially if you use 3rd more often in the lower RPM ranges of that gear.

so, enjoy the car.. its already optimized for your purposes of track and street. the close ratios are great for keeping the RPM close to the max HP region of your HP curve.

Plus, just for discussion what would the optimal gear ratio be for you? make it real simple.. give me speeds at redline for the Gt4 and we can discuss the trade offs... this entire discussion can be wrapped up by investigating this desired changed, the perceived value and performance gain or loss, by how it compares to the already near perfect GT4 gear set for most all uses , track and street.

I hope this has helped answer any concerns of the past 5 pages.

below... GT3 hp curve vs the GT4 HP curve... the answers to what works and what doesnt are found here

remember, its all about usable HP. why??
acceleration = power/(mass x velocity) newton said so!!!!

points

Welcome back. The above is one of your better posts.

mark kibort

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


i think there is an argument for the stock gears here. 78mph in 2nd seems to be a sweet spot for most race tracks , even autocross because your post shift RPM out of 1st is still in the max HP range. i have a lot of experience in varied tracks and this speed range is what my race car has had .. in fact, its almost identical, but slightly taller and i wouldnt change a thing.
55 1st gear MPH
80
118
155mph

all the ratio have good points about them, but the amount gain would be very small if there is an advantage for any use. there are some drawbacks... the 4.75 would be awkward for Laguna turn 11 downshifts, as it would just be too low, and harder on synchros, vs stock. because the cayman GT4 keeps you in the sweet spot of the HP curve on those turn exits, there is no gain . any same hp any same speed will yield the same rear wheel forces. so, with a flat style HP curve of the GT4, the gains for such changes might not be worth the effort or worth anything in terms of gains at all.

stout

Hey Mark! This is/was understood from the start, and why the 996 GT3 comment was interesting but kind of a non-sequitur. Point of original post was to start an intelligent discussion on gearing from two premises:

1. The GT4 is an amazing car, and an amazing value. This thread has made me think more about that, and I am glad the GT team spent the money it had where it did. They put a 3.8-liter engine into a Cayman, added GT3 front suspension, developed new rear suspension pieces, added a full GT3 brake system, did real aero work that required unibody changes, and added the best shift linkage any Porsche has ever had. The result is a new GT car that's fast, handles beautifully, brakes hard, and is amazingly practical. For $85k, with a warranty? Wow.

2. Many have found the gearing of the GT4 to be unsatisfactory, and a recent road trip really drove it home for me. Some members within the GT team itself made it clear with facial expressions that they would have loved to do a gearbox for the GT4. In the grand scheme of things, gearing is something that the aftermarket CAN do, and do well.

To your post above, yes, this is exactly what we're talking about—just not about GT3s. And the GT4 gear set is a less than optimum hp/torque multiplier because it is bolted directly to an engine it wasn't designed to work with. The GT4 uses a 991-1S engine delivered with a gear set PAG optimized for the 981-1S engine according to its emissions and fuel efficiency needs with only six cogs to work with. The 991-1S uses a 991-1S gear set PAG optimized for 991-1S engines according to its emissions and fuel efficiency needs with seven cogs to work with. I suspect 1-5 plus 7 from the 991-1S gearbox—or something close—would make the GT4 a better car on all fronts except maybe cases where the 991-1S sixth gear is useful. I'm sure that scenario exists somewhere in North America (Bonneville? oval racing?), but I haven't experienced it—and it sounds like there are already two or three 6th GT4 gear ratios to choose from for the few who need that.

GTgears

I'm just going to bold within the quotes...
Lastly, I will not manufacture a batch of taller than stock 6th gears. Stock 6th is already too tall for Daytona.

stout

All good to know and get answers from someone who is knowledgeable. Thanks.

Understand the gear spinning faster will wear faster; probably pretty negligible however, right? Much in the way guys running small tires on their GT4s are wearing everything out sooner but will probably not feel the effects unless they keep the car until 200,000 miles, etc.? Increased rpm didn't stop me from running 205/55R15 tires on my 914 for 15+ years, and while I am back on 185/70 Avons, I am not sure I have seen any adverse effect from running the short tires.

On the point of not manufacturing a taller sixth because of Daytona. Does that mean you won't make them under ANY circumstances? Such as if you had more street customers requesting a taller sixth gear than race customers?

Yargk

Hi Mark, I've been reading your posts for a while. I saw your car parked at Thunderhill the end of September when I was leaving the track and said to my passenger "there's the guy who talks about hp and torque on rennlist!"

GTgears

Hello Pete,

Since day 1 we have made 5 "kits".

We make three 3-6 four gear kits for the racers. I call them 160mph, 165mph and Daytona.

The 3 gear 3-5 gear sets I call 145mph and 155mph. They were intended for the DE drivers who are going to drive to and from the track and wish to maintain the quiet 6th gear for the interstate. The NorCal guys all run the 145. You are never going faster than that at Sears,Thill, LS. The 155mph is for everyone elsewhere in the country who might drive somewhere like Road America or VIR or Atlanta and need a little more legs.

GTgears

Sorry, I should have been more clear. I will not manufacture them as part of a kit. They won't be integrated into a package. I will always do custom manufacturing. If we released a 1-5 kit, for example and you wanted a special tall 6th done for yourself we would make one. We just require a prepayment on custom work and then you wait for it to get done, usually 3-4 months.

GTgears

That math is wrong. $25,000 is the production cost. That's how much it will cost me to develop and make the very first one. $2500 ea is below my actual cost AFTER development costs.

There's a thread asking people to speak up if they want this. It's the old final drive thread here. Nobody ever came forward. One member in Hong Kong has a standing commitment to buy 4 if it ever happens, but that's nowhere near enough units. He's the only one who has ever expressed real interest and checks back with me every 4-6 months. I am sure he's reading this...

GTgears

I'm going to skip the actual gearing conversation that has started. Like many here, I don't play well with Kibort. I'll just say you guys need to look at RPM drops if you go down this rabbithole. At MPH GT3 996 and GT4 look similar but GT3 runs a 3.44 FD and GT4 runs a 3.89. There's huge gains to be made by fixing the actual ratios on the GT4, whereas the only difference between a GT3 Street and GT3 Cup in a 966 is a 3.44 versus 4.00 FD. Stock GT3 ratios are actually what one of the 3 gear 3-5 kits I have done are based on.

mark kibort

I promise to play nice.... the point is , not many want to hear the truth about gear ratio changes .
why would the 3.44 vs a 3.89 be any better? you shorten the gears for what purpose? the torque at the rear wheels , and you can work backward from there, is related to the input HP. wear is related to input torque, and that doesnt change. so, im trying to see the problem that you are trying to solve

in other words. give me (us) a set of ratios that are better than stock, be it the rear end final drive ratio or individual gears, and show the advantage and reasoning.
curently, i see absolutely no benefit in changing for all purposes. (meaning varied use, multiple tracks, mutliple speed ranges, mutlple performance packages that achieve differnet speeds over segments at the track, etc etc)

The "rat hole" is not the RPM drops that i and others have laid out. the rat hole is not knowing or arguing that the HP curve attched to the gear box looks like..... if you have a flat HP curve, and it covers the RPM drop , then there is no need or advantage to change the ratio, agreed?? in the case of the GT4 vs GT3, the GT3 needs shorter ratios to get the same speed and hit its higher RPM and needs close ratios due to its more peaky HP curve.

this is not a "rat hole" this is basic race car mechanics and physics. so, basing a gear set, on another car with another shaped HP curve, doesn't make much sense unless it is trying to solve a specific deficiency. lets stay on factual gains, no perceived gains here.