Well this project hasn’t had an update for a long time, over two years which is pretty sad but an indication of the difficulty and my time constraints also. However, it is not abandoned, none of my projects are abandoned and I am looking to increase my tooling and ability to do these jobs all the time. I don’t have the specific tools for this job as it’s extremely technical.

‘’To recap I am not using the Corvette differential, I am not trying turn my car into a Corvette but use key parts that someone like myself can organise to modify and strengthen the Porsche drivetrain. So many parts of the drivetrain actually become mine in terms of design and manufacture. Those parts are; the flywheel, the upper bell housing the lower bell housing which is being designed to accept the LS starter motor. More on that later. The tail piece of the Tremec transmission and the ring and pinion.

The goals are are to have the most modern hand shifted street transmission that’s possible. Despite numerous lightweight parts, the weight will be close to identical.

Porsche 928 throw out bearing


Corvette C7 Hydraulic Bearing


There is is lots of lightweight parts in the Vette tube, however there is lots of heavy parts too like the couplings.




Now some of those bolts can get a trim, however if I replaced the alloy shaft with a carbon one the weight savings are not dramatic but would be noticeable in terms of rotational weight. So I am reserving my opinion on it’s worth till the car is driven. (Long way off). The carbon shaft often deletes the rubber couplers which is ok for the track but rather crappy for the street. The Corvette has a dual mass flywheel which I won’t have so deleting ‘the couplers would be a big mistake.

I was highly concerned about the overall length of the drivetrain and would it fit? I did take a gamble there. I have had to scrimp and save every mm. The length I got when I measured a manual car but with a auto crossmember (they sit back about another 15 mm) was from the tip of the input shaft to the centreline of the rear drive axles was 2186 mm.

Then we we put the Corvette’s tube into the 928 tunnel and did some measurements, it’s very tight with such a big tube! The parts need to be CADed but where I need to save a lot of length is where the Tremec box will mate with the Porsche auto differential housing. I must delete the sandwich plate to save 35 mm. Fortunately and I do mean really fortunately there is some space to steal from the Tremec box. It will integrate really well. The other area that will be chopped to save length is the front input shaft. 30 mm to be cut from there.



The pilot bearing ID is actually the same but this will be chopped as the pilot bearing is further into the crankshaft on the Vette. So the saving of a total of 65 mm gives me a total length of 2180 mm. If my measurements are out a bit, there is some wiggle room with the front shaft I believe.

Two billet bellhousings need to be created at the front of the tube. I will also take that opportunity to fit a LS starter. They weigh only 3 kgs and have better ground clearance. You’ll note that the throw out bearings have the same weight, however the Vette doesn’t need the cast iron arm or ball. The starter is lighter than even a lightweight 928 starter and the torque tube weights are 5 kgs in favour of the Vette. However the clutch is much heavier on the Vette. Overall it’s a wash in terms of weight. However the Vette box is much stronger and faster shifting especially with its billet shifter.

What’s the most difficult part, pretty much all of it, I have to design a very compact pinion and it’s a unique design due to the unusual nature of this conversion. It’s an extremely tight fit so I hope I don’t need any more space.

What needs Ed’s to be done? Well I now have a solid plan but I will not rely on anybody else to draw the components, generally I have been let down here by others. Currently the plan is to buy a 3D laser scanner and software to scan it into CAD. The issue here is two fold. I don’t have that training as yet. I have been starting to learn it though. SolidWorks which is my preference is very big bucks at around $7,500 USD. The same sort of price for the scanner and software. I’m treating like a potential business, not the 928 business but learning CAD and 3D scanning.

Solidworks did did years ago offer me a student edition to use because what I was doing was not commercial, this is what’s really needed I think.

Here’s the scanner...http://www.nextengine.com/company/news/rimac

‘All I can say is the reviews I have read have said don’t eliminate the couplers, our standard drivetrain is damped by the wind up in the thin steel shaft. I would imagine the NVH is going to be worse too. I tend to err on the side of caution on these issues, not that much to gain.

To move all my projects along but particularly this one, I bought a 3D scanner and some pretty impressive software. The scanner can operate hand held of desktop. Accuracy will hopefully be o.k, it’s 0.05 mm or just under 2 thou. There is lots of scanners more accurate but have you seen the price! Anyway the scanner can also be used to scan the entire car and convert it to a CAD file. The software includes a CFD parkage which I have no idea how to use but who knows later down the track. The aero mods I am doing on one car would be interesting to analyse.

So the plan is now to learn how to use this equipment and tools and create the CAD files needed for machining. Also the accuracy I need to ensure things like the spigot shaft lines up perfectly with the back of the crank when I make the new bellhousing. Also we can assemble the drawn parts on the computer to look at clearances and positional relationships. I plan to use the LS starter of previously mentioned, by having everything drawn we should be able to get the starter correctly positioned in relation to the ring gear. The bellhousing (2 pieces) is about $500 USD in material so I don’t want to have two goes at it.

Well im glad we’re on the same page, this is exactly what I’m expecting even though I have zero experience with CAD but have been doing various tutorials. I have Siemens Solid Edge ST10 which is the current version. It doesn’t have the cam tools but I think I have enough to learn..... How do you think I’m going to go at 0.05 mm tolerance? My plan is to go with this accuracy to develop the CAD model and then on parts like the housing for the 7 speed transmission get my model checked on a CMM or maybe with a touch probe on a mill. On other less critical parts I’m going with that standard.

O.K, we have some progress, albeit a little minor but important in the scheme of things, I am in a position where I had to learn CAD and 3D scanning. My CAD system is Solid Edge, it's quite good but my problem is that I don't know CAD at all and I have had to teach myself from YouTube videos, as you can imagine this is very slow and fraught with mistakes and not knowing the quickest way to do jobs. However trying to explain complex jobs to tradesman is also a problem and the cost might be astronomical.

So with the mini stroker engine, this engine will have a solid 500 HP and because I don't want to break a gearbox or clutch, I have decided to build the same clutch setup for the factory GT transmission as I will build for the 7 speed version. This means custom flywheel, upper bellhousing and lower bellhousing. The lower bellhousing will hold the LS starter. The LS starters are extremely good and small, they only weigh 3 kgs and that is less than the lightweight starters that don't turn over as quick. So that is a no brainer really. So those three parts are common to both systems. The other improvement is that I'm using the aftermarket billet shifter made for the Corvette. It's extremely precise, like a rifle bolt in fact. A one off piece that is required will be the rear housing for the torque tube on the Porsche. This is not needed when I do the 7 speed as that torque tube is complete.

With this project I was really in the dark, I had no idea whether it could be made to work. So far it looks like it's feasible. The big issue is length, the Corvette is a very long car compared to the 928 when you look at the wheelbase. So it's very lucky it it looks like fitting. I've had to steal every mm I could find. When I started I had no idea what the flywheel would look like, it was somewhat surprising, the new flywheel will still use the factory ring gear and timing ring. This is one of the issues, in this photo you can see the physical size of the Corvette clutch, it's quite massive.




So the current C7 Corvette uses a stepped flywheel, so this is the reason the factory/aftermarket flywheels look like the one below, those posts have to hold a lot of torque, so will mine. I can still tweak my design in fact it looks like the constraint for the ring gear needs adjusting which won't hopefully be too hard.




Then we have mine, I haven't drawn the ring gear or timing ring, it's not worth the time when you are an amateur like me. However it will get drawn when we measure the placement of the starter motor so that I can accurately match up the pinion to the ring gear. My flywheel will incorporate the Katech flywheel replaceable friction plate. I will also be able to change that insert so that I could use my Tilton carbon clutch with it different bolt pattern. I may but don't whether there may be an issue with the depth of the throw out bearing with the Tilton but I could adjust that a bit with a thicker insert plate.




Ring gear area to be widened

I will have to cut the rear of the crankshaft off where the pilot bearing is housed. It will then need to be rebored to hold the bearing. When I get the stroker crank made this change will be incorporated from the start. It appears I have just enough room to have a 4" thick (long) bellhousing. The Corvette torque tube bellhousing is quite big having 8 large bolts. Roughly it is a 200 mm diameter. I will do some more calculations and drawings in the near future.

That's a really good point about the hydraulic TOB, I didn't know of the problem but I did see a bleeding kit made for these. I wondered why initially, well I am going to try to use the Porsche master cylinder, so that's half the system... Will there be a problem? No idea on that. On my Porsche transmissioned car, I could in theory rotate the bearing so that the cylinder still faces upwards but on the side for easier access or maybe after investigation have a hose or pipe with a tap?

I did a measure today with a tape so somewhat inprecise, it appears that the 7 speed is exactly the same length as the automatic version of the 928 drivetrain. So it will be tight, that measurement depends on myself designing a the rear housing of the TR 6070 to fit the Porsche automatic differential and as such not adding to the length with a sandwich plate type adaption. This does appear possible at this point, it's a risky project as nobody has guaranteed this will work, just like the clutch arrangement nobody knew whether it would fit, it now appears to be not a problem. It is probably only a 1/3 of the job though.



C7 throw out bearing and bellhousing.