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I have suspect a revised gear set won't be cheap, but I wonder if there is a way to simply bring over 1-5 and 7 from the 991-1S MT or 981-1 PDK gearbox? I know it's a different case, but the gears, at least, exist. It's a worthwhile project, as this is a rare Porsche where I'd put new gears in well before turning to engine mods. Of course, the right gears plus better breathing and a linear power band? Well, that would be heavenly...
I have NOT read anything but the first post...
This is impossible. The GT4 uses a Getrag 6spd manual gearbox. The 7MT is made by Aisin and PDK based. The PDK is made by ZF. There is zero compatibility or ability to cross over the parts. On a PDK the gears aren't even in the right order within the gearbox. This article should help people understand that there is no way no how these parts can be mixed and matched.
I've now read the whole thread. A couple comments. I in no way disagree with Pete's position on 2nd gear for a street car. Why on earth Porsche didn't use the 2.7 2nd gear ratio is beyond me. There is no strength difference between the two gearboxes. One can put in a 2.7 gearbox with no concerns, just switch the axle flanges for the GT4 larger CVs.
That said, I'm going to make a controversial suggestion... Sell your GT4s. Buy a PDK GTS and send the car to Sharkwerks or BGB for engine work. /thread. You want the PDK ratios of the 991? You can get them in a Cayman. Just start with a PDK Cayman. You just lose your 3rd pedal. In fact, I am going to go out on a limb here and say that given all the b1tching about the 6spd gearbox coupled with the excellent performance of the GT4 Clubsport racers, that the next iteration of the GT4 street car will come from the factory with a PDK.
Oops, almost forgot one other thing. The SW mule has a 3rd gear in it that is shorter than guys are getting in the 3-5 package. They've got a proper 3-6 race package in there. The 3-5 package people are buying will be 3-5mph higher in 3rd gear and a slightly improved street feel on the gap than what Pete experienced.
There is zero compatibility or ability to cross over the parts. [/url]
This is good info, and what I suspected/feared. Knew about both the different suppliers as well as the gear order, but was still curious to know if the gears to be made to work, whether by machining out their centers and using a billet adapter or?
Okay, so the questions become:
1. In your experience, do custom gears present the same longevity problems you've seen with aftermarket R&Ps?
2. With six new and custom gears, could one put the ratios of a 991-S 1-5 and 7 in a 981 gearbox? Or maybe even idealize them a bit, with a tight stack 1-5 and a tall 7th (or a stock 6th or your short 6th for those who prefer it)? The car as geared now essentially has three 5th gears and compromised lower gears.
3. OR, is there a way to supplement the 3-4-5 you already have available with a new 1-2 and tall 6th to put your company two steps ahead of other suppliers?
Even with six gears @ $1100~ each plus other parts (?) and labor hours (??), six new gears would be money very well spent on the GT4. Maybe even set it up as a "production line" service with a better diff installed at the same time at one price. I'd need to know I was keeping my GT4 for a long time before doing that conversion, but many here spend $6-12k on a wheel/tire set or headers/tune/etc without blinking—for what I suspect is nowhere near as meaningful a change as this would be. The GT4 would remain smog legal, overcome many of its performance issues, access what low- and mid-range torque it does have sooner in each gear, get better gas mileage, and be far more fuel efficient and comfortable on the freeway.
The only available options present serious compromises. Your comments on the R&P issues make me wary of that route, which also makes the too-short stock 6th even shorter than it already is (fail). The 3/4/5 re-gear is a race setup that works in that realm and seems to make some street drivers I respect happy, but to me it seemed extremely compromised on the street—yielding, essentially, two 2nd gears. In a race car, I wouldn't care so long as it gave me the 3-4-5-6 I needed to get around Daytona or Road Atlanta or wherever. While I prefer the new 3rd to the stock 3rd on the street, and it is a nice upgrade, the new 3rd gear is VERY close to the stock 2nd. That meant carrying 3rd on good roads with less shifting, but also no real gear down when you need it—unless you want to drop to 1st on two downshifts in a hurry. (Anyone who doesn't believe me should try downshifting from the revised 3rd to the stock 2nd. I pushed the clutch in before letting it all the way out the first time, as I thought I'd missed a shift. Nope. It's a tiny split, maybe 9~ mph or something IIRC.) The revised 3/4/5 option also leaves the GT4's too-tall 6th in place. Again, the available setup as used in the SW car may work for some, but I suspect that's a very limited group, and I worry that some customers who step up will be unhappy in short order. That's a big do-over for the customer, cost wise.
I suspect you had enough of a business case to do the new 3/4/5/6 for the race cars. I suspect in this case it would come down to how many customers you or another supplier would need to make it worth your while to do a new 1-6 for the street or a 1-2 + tall 6. But I have to think the market is potentially bigger on this than the race team market. Why? I cannot think of a single new Porsche so poorly geared from the factory in the last 20 years, and I cannot think of a single area of improvement from which the GT4 would benefit more than this one.
Last edited by stout; Dec 1, 2016 at 09:35 PM.
Reason: gear price correction per Mooty
Why on earth Porsche didn't use the 2.7 2nd gear ratio is beyond me. There is no strength difference between the two gearboxes.
Been wondering about exactly this—thank you. Interesting data.
Originally Posted by GTgears
One can put in a 2.7 gearbox with no concerns, just switch the axle flanges for the GT4 larger CVs.
So...with a GT diff and the ancillaries hooked up (will they hook up?) is the 2.7 gearbox a good fix?
Originally Posted by GTgears
That said, I'm going to make a controversial suggestion... Sell your GT4s. Buy a PDK GTS and send the car to Sharkwerks or BGB for engine work. /thread. You want the PDK ratios of the 991? You can get them in a Cayman. Just start with a PDK Cayman. You just lose your 3rd pedal.
Most of us got GT4s precisely because it has three pedals—and the 3.8 plus a lot of hardware that isn't available on an S or GTS or would cost $$,$$$ to add, and you still wouldn't have the unibody. So this is a non-starter.
Originally Posted by GTgears
In fact, I am going to go out on a limb here and say that given all the b1tching about the 6spd gearbox coupled with the excellent performance of the GT4 Clubsport racers, that the next iteration of the GT4 street car will come from the factory with a PDK.
Only the crystal ball knows, but I'll go out on a limb too and say I doubt that is correct re: PDK only in the next GT4, unless the GT3 offers both transmissions and the GT4 goes PDK only—but the GT4 model line is meant to appeal to the purists; Porsche learned a very interesting lesson with the GT4, mainly with regards to listening to the wackos, and sold 2.5-3.0x what it hoped to. Going PDK only next time would be an epic fail. Also, I don't think people are b1tching. I think they are looking for an upgrade solution much in the way they do when it comes to toe links or mufflers or headers etc. This is a taller order and a bigger project, but I think the first company to listen to this potential customer base will make a tidy sum on this project—and will wholly deserve to. Porsche sold a lot of GT4s here in NA, and even more of them abroad. I think the GT4 has a serious achilles heel, but what do I know?
Oops, almost forgot one other thing. The SW mule has a 3rd gear in it that is shorter than guys are getting in the 3-5 package. They've got a proper 3-6 race package in there. The 3-5 package people are buying will be 3-5mph higher in 3rd gear and a slightly improved street feel on the gap than what Pete experienced.
Didn't know this. Good to know. Surprised you are making two 3-5 packages. What prompted that?
Gearing should be shorter 2-5! Even if the stock gearing works ok at track, IMO it's fun to shift. And like said before, the gt4 is a road car that can be tracked every now and then.
For auto x and weekend fun running shorter gears is more fun.
Short gears Lee Keen
I cannot think of a single new Porsche so poorly geared from the factory in the last 20 years, and I cannot think of a single area of improvement from which the GT4 would benefit more than this one.
Same gearing relative to slightly different redlines, but the GT4 makes more power and torque at low rpm so it shouldn't be as bad as the 996 gt3.
The redline of the GT3 is more than 5% higher, so equal speeds in gear would mean 5% taller gears in the GT4 (mph per 1,000 rpm). Not trivial (5% gearing change is easily felt), and the 996 GT3 was not that well geared to begin with (agree with your point)...
Going to make this a drive-by post because I'm just getting off work and have some stuff going on this evening. I'll see if I can make time to answer Pete point by point later this evening or in the morning. In the short term, I think it warrants repeating why "just" doing a 2nd gear is problematic because it appears we've got some new readers here who don't know what's going on inside this gearbox.
1st & 2nd gear are integral to the mainshaft. What does this mean? You can't make one without the other. It's not as simple as the additive math Pete used above to add in 2nd gear, for example. Here's a picture of what the mainshaft assembly looks like (ignore that it's a different gearbox, same concept):
From left to right is 6-5-4-3-2-1. 1 and 2 have already been pulled off the pinion. On the mainshaft side, 1 and 2 are part of the mainshaft. So, instead of $1095 per gear, if one wants to change 2, one must change 1 AND the mainshaft. $4000 for 1-2 and mainshaft. Twice the price. $4000 even if all you care about is a different 2nd gear ratio. Can't do one without the other.
Twice the price. $4000 even if all you care about is a different 2nd gear ratio. Can't do one without the other.
Gotcha, and you've explained the mainshaft as the hurdle before, and it was understood then but no price guesstimate was offered IIRC. So lets say it's an $1800~ hurdle in addition to the gear costs, theoretically speaking (I don't think anyone should hold you to prices from this thread—we are shooting the breeze with someone who knows their stuff about the subject at hand, and should be thankful for your input).
Spending more time with the GT4 makes me wonder what an all-inclusive, all ratios optimized for street GT4 re-gear would cost—including 2nd rather than 2nd alone. To me, the ideal ratios are 991-1 first to fifth plus seventh, or something close to that. No reason to do 2nd on a new mainshaft without idealizing 1st while you're at it. Question is: What would such a set of gears (all six, including the integral mainshaft for 1st and 2nd) cost, roughly? That number becomes pretty interesting. Sounds like about $8400~ plus labor ($4000~ for 1-2 + $4400~ for 3-6?).
Again, I'll have to commit to my GT4 before I'll drop $10-15~k on its transmission (diff while we're in there), but that expenditure would be FAR more interesting to me than power mods for a number of reasons. Optimized gears would present strong value in terms of the performance improvements on several fronts, from speed and fun to efficiency and comfort. Of course, there are other RLers who don't think twice about dropping $10-15k or more on a GT4 if they think it will improve it—they are already doing it with wheel, track prep, headers, you name it.
The redline of the GT3 is more than 5% higher, so equal speeds in gear would mean 5% taller gears in the GT4 (mph per 1,000 rpm). Not trivial (5% gearing change is easily felt), and the 996 GT3 was not that well geared to begin with (agree with your point)...
This. 996 GT3 was hardly "magical gears," but it never felt hampered by its gearing the way that the GT4 does.
Also, the GT4 pace gains at any track have a lot more to do with its chassis, tires, aero, etc. than its powertrain. While its specs match up to the 996 GT3 nicely, consider its Ring time against the 997-1 GT3...
The redline of the GT3 is more than 5% higher, so equal speeds in gear would mean 5% taller gears in the GT4 (mph per 1,000 rpm). Not trivial (5% gearing change is easily felt), and the 996 GT3 was not that well geared to begin with (agree with your point)...
Point well taken, and I agree that a 5% gear ratio change, corresponding to different redlines having the same max in-gear speed, would be felt. However, if you increase the gear ratio by 5% and simultaneously increase the torque by 5% and reduce the redline by 5%, I think it would be unknown to the driver (it all cancels), outside of the pitch heard . I think the GT4 makes more than 5% more torque too.
Looking up some dyno plots, if we normalize to 360ish peak whp (just what cobb has for stock), then the GT4 hits 300 hp at about 5300 rpm and still makes 340 whp at 7400 rpm. The 996 GT3 doesn't make 300 whp until almost 6400 rpm, although it still makes 340 whp at 8200 rpm. 2100 rpm of good power vs. 1800 rpm of good power to me.
$4000 for 1-2 and mainshaft. Twice the price. $4000 even if all you care about is a different 2nd gear ratio. Can't do one without the other.
Would it make business sense for you to develop 1-2 set up for GT4 if you can get 10 or so to pony up (pay half for more up front so you have development $ in hand) for this set up and achieve group buy pricing for early adapters/capital providers? I know when this gearing discussion started back in 2015 at one time you indicated with $25,000 (so 10 at $2,500) you might be able to make a business case? Is that realistic today?
You put together a set 1-2 that works well for all uses - then let the customers pick their 3-6 gears to add for their needs (street or track or both). seems like as long as the 1-2 is developed to suit the car, you have plenty of gear options for 3-6 for customers to choose from.
Getting a new 1-2 is a much better solution than a R&P - even though it is more $ - you get what you pay for!
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. I think the GT4 makes more than 5% more torque too.