slate blue

Guys I have been working on this conversion since the beginning of the year, I went off it for a while due to the shop selling the transmissions saying they went suitable for the high RPM use I intended. Given there is a distinct lack of options for the 928 in terms of transaxles. The Corvette trans is really the only game in town. There is conversion kits around and this method I plan to implement is just another method of achieving the same but with a few of key differences.

This transmission based on the TR6070 is modified in terms of its ratios to give hopefully an excellent selection of cogs. More on that later. The other issue this addresses for me is that my exhaust is of large diameter that being 3 1/2" and with that diameter you cannot run the Corvette differential. The other issue and I am not sure of the absolute importance is that of the hypoid offset. The 928 has zero offset and the corvette has 1 1/2" of offset, there is more friction with that offset but I am thinking there could and I stress "could" be a transmission height alignment issue. Again for myself who will run a flat floor and diffuser this could be a problem. The goal is to keep all alignments as factory.

Given my solution which is very similar to Mike S's solution that is the TR6070 bolted to the Porsche differential, the output shafts are the same and LSD and PSD operation can remain stock.

The ratios I have selected and I am just waiting on a confirmation email given the custom cogs are below and the proposed final drive gearing is 3.67 which is 34 teeth on the crown wheel and 9 on the pinion. The comparison is to the GT gearbox ratios with 2.73 rear gears. The driveline is the combined gearing;

...........................….Corvette…driveline……..Porsche….driveline
First gear...............…2.97…….10.90………….3.764….…10.26
Second gear.............2.07……..7.60…………..2.515…….6.85
Third gear.........….....1.43………5.25…………..1.79………4.88
Fourth gear............…1.00………3.67…………..1.35………3.69
Fifth gear.................0.82………3.01…………..1.00………2.73
Sixth gear................0.68………2.50……………..8% less rpm
Seventh gear......…….0.48……...1.76……………35% less rpm
Reverse...............….2.90

So to address the gear change quality, btw this is a super slick box to 6,500 rpm and maybe with lightweight components in the driveline like alloy flywheel, carbon clutch and carbon driveshaft the inertia is greatly reduced. The other big advantage this transmission has is a special sensor on the gear change mechanism which tells the ECU which gear is being selected. This allows when employed with a fly by wire actuator to "Rev Match" and this may take away any concerns about shift quality above 6,500 as the engine speed is being matched to transmission speed. My max revs have been reduced to 7,700 RPM to accommodate a syncro type trans as really above these RPMs you should have a dog box which are horrible on the street.

The transmission is reworked with carbon blocking rings on all gears, shift pads are brass instead of nylon and all the gears are super finished (REM) and cryogenically treated.

The only issue I have is the cost of the custom ring and pinion, that cost is greatly reduced when a small quantity is ordered, it is the first set that is very expensive. So if there is any interest in this conversion please let me know. I would anticipate a ring and pinion costing around $4k AUD. I was advised it would be a little more expensive in the States for a similar quality and that was from a top gear manufacturer and he only wanted to do a run of 5 sets. So I am lucky to get someone who is top of their game to produce a small run. A custom or customised output shaft is also required although the cost is this is relatively low along with the custom sandwich plate which replaces the Porsche to Mercedes sandwich plate.

One of the key reasons to retain the Porsche differential is that the packaging is perfect and I will be able to implement my custom software to drive the diff operations. You here 928 owners complaining that the PSD can't handle track conditions or at least when slicks are fitted. We haven't been able to work out where the actual problem is but given the system uses an accumulator the system should be very fast. One of my concerns about the PSD for racing is the fluid getting too hot. I am not sure how you might cool this. Maybe a larger reservoir would serve some purpose? I am also looking at some ducting to the pump area.

Also regarding fitment of this 7 speed transmission, it is 42 mm longer than the TR6060 or 6 speed version. If I run into trouble regarding this I will deal with it but going forward from there it is possible to use the 6 speed but you will miss out on the "economy" gear. (Something that is important if you have a large capacity engine) Still that will be better than the current 5 speeds. These 6 and 7 speed transmissions are very strong and not subject to the same issues as the factory boxes.

IcemanG17

Mike Simard did something similar before he dropped the project

He took a racing 4 speed manual and attached it to an automatic 928S4 rear end....the goal was a racing strength 4 speed....not sure if he ever finished it

slate blue

Quote:

Originally Posted by IcemanG17

Mike Simard did something similar before he dropped the project

He took a racing 4 speed manual and attached it to an automatic 928S4 rear end....the goal was a racing strength 4 speed....not sure if he ever finished it

Hi Iceman,

I believe he did finish it and just after that sold the car as some other project took his fancy. From the accounts I have read it was a great success. I think we have to apply that success to the area of endeavour it was aimed at, that being racing. I think if you drove it in traffic on the street it would not be the most enjoyable transmission you have ever used. Horses for courses.

My car will be mostly car show and track events based, we also do interstate driving so the need for an economy gear with a 7.5 litre motor comes into play. While the carbon clutch is a consideration it is not a sole determining factor in the choice but it is for having a fairly similar first gear ratio so that I don't have to slip that expensive clutch too much. However I believe there is an even closer ratio with a taller first gear available if that suited.

As far as racing goes the new C7 Corvette manual seems to change gear pretty well, it comes with an in built oil pump for use with a heat exchanger so that is also bonus.

FBIII

Do you really need 7 gears in a 3000 lb car with a 7.5 liter engine? Seems like you would be stirring the gearbox a lot more than necessary.

slate blue

Quote:

Originally Posted by FBIII

Do you really need 7 gears in a 3000 lb car with a 7.5 liter engine? Seems like you would be stirring the gearbox a lot more than necessary.

With regards to everyday drivability for myself, where I live there is a lot of highway driving, average speed limit is around 50 to 60 mph, around most suburban streets it is 30 to 35 mph, I don't think for the suburban driving anything will really change, look at the end ratios comparison, you don't need to use all 7 speeds, maybe just the first 4 gears is enough. At present I have a very steep driveway and need that first gear for circumstances like that and I don't know how good the Tilton carbon clutch will be for street use. So that slightly lower first gear is insurance.

For the highway I think you can just skip gears when you have 7.5 litres of torque. Porsche of course also reduced the final drive for US Models to 2.20 and I am just going a bit further than that and I believe for good reasons. The other matter is that using 6 or 7 speeds the cost of the transmission probably doesn't cost a dollar more except for my custom gear selection. So it is not a money thing.

Top speed will be set in 6th gear and I have tailored that the best I can without having completed the engine. We are working right now on the 944 2.7L cylinder heads for adaption and fitment of 2.20" intake valves! The cylinder head expert believes from what he has seen from my work and what he has also achieved with my 104 mm bore engine that he can get around 350 cfm at peak cam lift. He said it then should safely make 750 hp at 7500. The only issue I see is the exhaust as that is a bit small in the headers but I hope that this will just add to torque and drivability.

Just to add a little bit regarding the head flow, with my test port I managed to get 305 cfm @0.500" with a 2.10" valve in a 4.060"/103 mm bore, extrapolate that out a bit and we may get around 330 with the bigger bore and valve and peak lift will be in the 0.625 to 0.650 range and you can then see that 350 cfm with the larger bore 4.285"/108.9 mm is a realistic goal. It is an interesting and challenging area.

Rob Edwards

Anytime I'm driving in normal traffic around here ( 0 to 50, to 0, to 45, to 0, to 50...), I shift 1-3-5. Lots of opportunities to skip-shift with enough torque.

FBIII

That's my point exactly. A lot of unnecessary gears. One of the aspects of the early 930's I like is their 4 speed gearbox. The cars are light with significant torque and each shift resulted in a longer pull to redline.

hacker-pschorr

The new Vette has plenty of torque, silly them for not keeping it simple and just installing a 2-speed power-glide.

Chalkboss

Or just do away with shifts all together and go with a CVT. Those things are atrocious... I look forward to seeing this car when it's done.

dr bob

"Street" CVT's are atrocious because they are optimized for fuel economy not performance. Many opportunities for software 'fixes' to make them more sporty. Need more than just the "ECO" button as driving options. ASSuming a decent CVT that would handle the torque, building software to hold the engine at peak horsepower point is hardly a challenge. Reduced shock on driveline components from shifting, eliminating upsets to suspension from shifting, lots of other possible benefits. Downsides include less predictable engine braking and the friction load inherent to current street CVT designs.

Current street CVT's certainly take some 'getting used to', particularly if you are rote with manual gear selection, or even like to 'feel' an autobox upshift. Soggy and disconnected seems the norm so far.

RedPanda

What is the smoothest of the bunch?

GregBBRD

Buy a Corvette, save a whole bunch of time, lost money, and not end up with an abortion, when you are done.

sstrickstein

Or just use the Porsche Box and get a few extras for back up. Hey I have a automatic 4 speed you can have..... Its even been rebuilt..... and its dead now.

123quattro

The new Vette has a 7 speed because the 911 has a seven speed. It's purely for marketing.

FBIII

I drove an 84 US 928 5 speed with the 2.20ish final drive as a daily driver and found myself doing the 1 to 3 to 5 almost constantly and that poor thing only had 230 some horsepower. If I was starting out on a descent I'd start in 2nd as I always felt 1st was too short.