Solid versus Semi-Solid engine and transmission mounts
 

So it turns out my 73 needs a new clutch assembly and flywheel (I had posted here a while back about clutch disengagement issues), and I thought while the engine is out would be a good time to at least replace the transmission mounts (rear ones don't look hard to get to with the engine in.. confirm?). I've got the Hargett shifter, and noticed some issues with alignment of the body mounted shifter since the transmission seems to move around a lot (no surprise with 36 year old rubber mounts).

I was thinking I want to get solid mounts to keep everything located as well as possible. This is an occasional driver, I don't mind vibration or noise (I don't even have carpet in it), and autox. Eventually want to do track days and long-term is vintage racing once I get the engine built up. I'm also planning to go with a lightweight clutch/flywheel combo (have Sach's aluminum pressure plate, and probably Fidanza flywheel)
However, I have concerns with solid engine mounts having negative effects on the engine. Vibrations causing bolts to rattle loose, etc.

I found the Wevo SS mounts
http://www.wevo.com/Products/ChassisProducts/WevoChassisProducts-911SSEngineMounts.htm

which look promising. One question is... would there be any negative or positive effect to using solid mounts for the transmission, and SS for the engine? Seems like that would help keep the transmission tied down but allow a bit of damping for the engine. However this could start trying to turn the bellhousing into a structural member?

You are right, it could cause the bellhousing to take more stress. Vibrations can also be an issue with more than bolts, the carrerra tensioners had to have the pipes tied down because vibration lead to leaks.

For the street I would just get the "Sport" mounts from our host and spend the extra money on something else. I actually run them in my race car..

The bellhousing is a structural member, there are many many out there pumping I would assume much more HP thru the units than the 73 using solid mounts on track cars and street.

+1 on the Wevo set up very nice stuff no worries on negatives, I would go with the Blue mounts on both the trans beam and motor mounts, bought them for my 71.

Regards

THe PMO carb people recommend against for just the reasons you mention...vibration shakes loose vital parts.

So sounds like the best bet is just go with WEVO for all 4?

I'd go WEVO for all four mounts. According to WEVO the SS mount development was encouraged by PMO.

I follow Porsche’s admonition that a 911 engine and transmission are not ‘stressed members’.

Even in a race car, I use semi-rigid mounts at the transmission and very flexible mounts at the engine. With the high stresses from cornering, etc., the four points of the mounts can twist out of a plane. If the engine and transmission are rigidly mounted, that twist will be seen by the engine and transmission castings.

We all go to great effort to have the main bearings properly support the crankshaft. Imagine bending the casting while at high revs.

Porsche had the ideal opportunity to use a 911 engine as a stressed member when they designed the 956/962. They pointed did not rigidly mount the engine.

If Porsche wouldn’t use an aluminum engine and transmission as a stressed member, think about our more fragile 7R and earlier magnesium castings.

If I were to improve on the situation, I would make a single transmission mount using a spherical bearing. There is a reason for a 3-leg stool. It can’t twist.

Best,
Grady

Thanks for the replies.. I think I'll go for the Wevo. Not much price difference, and sounds like probably the best compromise of rigid vs damping.