Hmm. Interesting points, worthy of further discussion. As you said, I understand we aren't agruing, I just wanted to try out some counterpoints and see how they fly.
Not trying to argue, just stating my opinion for the sake of discusstion. For a uncompromising sportscar, I'm suprised that passenger comfort is held in such high regard.
If it were mine, the entire tunnel on the passengerside would be moved over as required. All the tubes in the tetrahedrons that terminated at the old vertices would need to be completely cut out and new ones fitted to the new vertices.
The ppf is outstanding. It is light, has no welds to fail, prevents angular changes in the driveline from engine loads, and best of all, it's free, sitting there in front of you.
True perhaps, but the PPF isn't plug and play for issues other than the width of the tranny tunnel. It would have to be shortened over a foot (no easy way to do that) and on top of that you would need to cut through or omit much of the bracing (if not all) on the bottom of the tranny tunnel, sacraficing some chassis rigidity.
Without the ppf, the pinion angle needs to be dropped to compensate for loads compressing bushings as the pinion tries to climb the ring gear under load. If you go solid without bushings, noise and vibration will be transmitted into the chassis.
What you said up top regarding passenger comfort applies to this too IMO. In an uncompromising sports car who cares about some vibration. (If there's that much vibration something is probably wrong anyway right?) Lock the Miata diff carrier down solid and you don't need a PPF, which also allows you to mount the engine/tranny and diff carfier in whatever spatial releationship that you need. Using the PPF locks you into the same geometry used in the Miata, which IMO is less than ideal since you are not installing them in a Miata, but rather a 13" tall sled. You have to move the engine tranny up a bit to get sump clearance unless you have a 6" ride height.
The long pinion rx7 will see less force at its mounts due to the longer arm but the driveshaft will be very short. The shorter the shaft is, the tougher it can be to get the ujoint angles you want. Ideally a driveshaft with ujoints should have a 1/2 degree angle at most to aid needle bearing lubrication and joint life. The steeper the angle, the lower the reliable rpm of the shaft. The trans output shaft should be parallel to the pinion except for bushing compression compensation. The trans output shaft should be higher than the pinion. Under load the pinion lift and reduce the ujoint angles.
Engine torque if the trans mount is bushed also affects the angles. Solid mounting a transmission while keeping rubber engine mounts can break the trans case. I have seen it happen on older domestics.
All this stuff is surmountable but it doesn't make any sense to do it. I can totally relate to the possiblity that a few of you guys don't feeling comfortable cutting up a chassis you bought or are concerned about modifying. I am not a total newbie. I have not spoken to McSorley in about 5 years while I have been caught up in other stuff. Just my humble opinion about something that is not being built for me.
Well as you've probably guessed I have the engine and tranny mounts bushed and the diff carrier mounted solid. I didn't do it that way to save passenger space, but that turns out to be an added bonus. IMHO trying to adapt the PPF would require much more work and for minimal gain and perhaps at the cost of some chassis rigidity. Sure I will feel more vibration, but it's a purpose built autocrosser and track day car that just happens to be street legal, so who cares.