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The table below shows cruising speeds in 5th gear across all 5-speed 928s, taking into account all the gear sets, layshaft ratios, final drive ratios, and normalized for the stock tire diameters (between 24.2 and 25", depending on MY). It reinforces Sterling's point that the only meaningful change you can make to a later car is switch in a G28.13 from an '87-88.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
I have the 928MS C5 Z06 kit with SSK in my supercharged 79 Spyder and it is the best upgrade that car has other than someone chopping off the roof in the 80s. I had it installed by Carl. I had finished building the rest of the car and had planned to keep the stock transmission, but once the rest was completed, the trans became out of place in the way it shifted and the way the gear ratios were matched to the now higher performance engine - didn't need 1st gear at all, the synchros in 2nd & 3rd were very worn resulting in a quite a bit of grinding without double-clutching when shifting both up and down into those gears - just not that much fun. I drove the car from SC to WI for the swap and the drive home was night & day vs the drive there !
The 6-speed now let's my 40 year old 928 shift like a modern car - no double clutching, any gear to any gear, no pause in the middle between gears, short throw, ergonomic shift ****, and all of these things allow the car to be faster since I can go through the gears faster. At 85 MPH, I am running 2,000 RPM in 6th gear on the highway, which means better fuel economy and quieter ride (not that my car is quiet since it's decatted with a stainless Y pipe and Borla exhaust). Need to pass, no problem, drop to 5th or directly to 4th and nail it. This trans and its synchros are beefy and won't have any trouble handling the 350 ft lbs and 350 HP of my car.
I've had it installed for 3 years without any issues. I have it filled with Joe Gibbs Driven Fluid of the proper spec for the Corvette trans with LSD. If there is one negative it's that the mounts are semi solid and transfer some noise to the body, but also hold it firmly in place for proper shifting and alignment all the time.
The big question that's always in the back of my mind with this conversion, what are the negative effects from replacing or modifying the rear cross member? This isn't a trivial change or modification and most seem to overlook it.
Unless someone pays a ton of money to have the various ways to do this conversion fully stress tested compared to an original one, I'm not sure I'd ever be comfortable with this swap, as least with the various way's I've seen it done so far.
The rear cross member is one of the key elements of what makes the overall 928 chassis what it is. Cutting this up or replacing it shouldn't be taken lightly.
We agree, and that's why we make our rear cross-member from OEM rear cross-members. The rear cross-members locate all the suspension points and is responsible for the rear alignment. We segment the rear cross-member in such a way that the locating points are not disturbed. So the integrity of the rear suspension is maintained as the two outer (in the inner rear fenders) and one inner (near the transaxle mount) mounting points are just the same as they were. No changes.
We have a steel fixture here that was made around a known-good, unmodified stock rear cross-member. It accurately locates six different critical points on the rear cross-member.
This manufacturing fixture allows us to do two important things:
1) before we start making up a new 6-speed cross-member, we take the rear cross-member core and fit it into the fixture to check it for straightness. If it is twisted or bent in any way, it is discarded.
2) as we make our steel center section for the kit, it is assembled and welded in this fixture to be absolutely certain that the finished rear cross-member is an exact fit to the car, and maintains all the suspension and alignment points in their correct locations.
As to the original question posted by the thread-starter, I can tell you we are selling more of these 6-speed kits than supercharger kits at this time. I'm not surprised.
Going back to the Beach Boys songs, dropping 4.11 gears in your favorite car was a great way to get off the line and go fast. Sure, you can't go 160 MPH any more (how often have you done that?) but you sure can have fun jumping from 0 to 60 (and how many times do you do that?)
It feels like you have supercharged your 928, but without supercharging your 928.
No worries about detonation and tuning. No worries about overheating. Plus getting rid of the heavy and slow stock synchro rings and now you have these slick and fast carbon-fiber synchros that make all your shifts like butter.
We manufacture both - the supercharger kit and the 6-speed kit - and I don't want to sell against my own supercharger product, but honestly, the 6-speed install has such a fantastic end-result that a person should give it a serious look when seeking "more HP".
Sincros sincros sincros! Having owned 2 GT’s and driven many corvettes, i’d say the biggest advantage are the sincros. If you can get reliable fast shifting like a modern corvette on a 928, what’s not to love? I’ve always baby shifted my 928’s, with lots of care and love. But if I had a modern corvette tranny back there, it would drive it like I stole it.
I guess you could look up C5 Corvette 0-60 times and compare to 928 0-60 times. The differences in HP between the two cars will throw you off.
We do not have any before-and-after 0-60 times in a 928. I test-drive every install before I give the car to the customer, but I do not do hole-shots with customer cars, so I don't know. Maybe some our customers can report their findings.
I forgot to mention another item for the plus column: if you happen to own a slush-box equipped 928 and always wished you had a manual trans, about a third of our 6-speed installs are just that. Apparently the 6-speed was the tipping point and they have us convert their 928 from an automatic directly to 6-speed manual for them.
Sincros sincros sincros! Having owned 2 GT’s and driven many corvettes, i’d say the biggest advantage are the sincros. If you can get reliable fast shifting like a modern corvette on a 928, what’s not to love? I’ve always baby shifted my 928’s, with lots of care and love. But if I had a modern corvette tranny back there, it would drive it like I stole it.
.....I've got carbon fiber synchros for .the '85 and later transmissions to improve the shifting on the stock transmissions.
I might be in the minority, but I love the feel of my trans. I think it has a vintage feel.....back in the early 80's, I had a 1966 and 1971 911,
so the dogleg gives me some nostalgia
I forgot to mention another item for the plus column: if you happen to own a slush-box equipped 928 and always wished you had a manual trans, about a third of our 6-speed installs are just that. Apparently the 6-speed was the tipping point and they have us convert their 928 from an automatic directly to 6-speed manual for them.
What's the ball-park price for such a conversion? (It's already on my wish list.)
06-18-2018, 09:44 PM
Maybe some our customers can report their findings.
