DIY Semi Solid Engine Mounts - modded OEM mounts

My OEM engine mounts were in good condition, but I felt my 2002 996 exhibited excessive play when accelerating, shifting, and transitioning left-right. I was not ready to spend $400 on aftermarket engine mounts, so I made my own. I gutted the OEM hydraulic mounts, drained the hydraulic fluid, and drilled out the rubber mesh membrane - leaving only bare metal and rubber. I then filled all the voids (top and bottom) with an industrial solid polyurethane of 80A hardness rating. This polyurethane replaces the hydraulic oil in the top of the mount, and fills the bottom void that surrounded the mount stud. This cures for 48hrs in room temp, and is then finished in the oven @ 150degF for 8hrs. The result is a much stiffer and durable solid polyurethane/rubber mount. Shifting is much crisper, accelerating and downshifting is smoother, and the car turns immediately upon steering input. The chassis sloppiness is gone.

Whats stopping that polyurethane from being worked out?

Interesting. Keep us informed as to the reliability of this modification. Im using the RS mounts which really transformed the feel of the car. Also, off topic, but your coolant tank looks like it could use replacing. Ive never seen one that discolored. Thanks for posting.

The polyurethane is completely encapsulated by the OEM mount: on top by the metal screen, and on bottom sandwiched between the OEM rubber and aluminum housing. The polyurethane is a 2-part liquid that is poured into all voids, forms itself to the mount, and then solidifes upon curing.

This does look very interesting and I too am curious as to what he long term reliability of it would be. Keep us posted!

It has been about 2k miles since I've installed, and I'll be doing autocrosses the next 2 weekends, so I'll take a look at how they're holding up and post some pics following that.

Interesting. Might have to give this a shot.

The pics and description are awesome... well done, great post! Question - What does engine mounts have to do with steering sloppiness or turn-in... unless the engine is a stressed member of the frame (e.g. Motorycles)... or maybe you say now it is after your mod? thx

The engine and transmission weigh about 700 to 800 lbs, and it is dead weight suspended by 3 mounting points. The problem lies in the fact that anytime the car changes vectors (ie. turning, accelerating, braking), the engine/tranny lag behind the chassis due to inertia. By stiffening the mounting points, the engine becomes increasingly one with the car - the change in direction is more immediate and the car is more settled (stable). From a performance standpoint, the ultimate solution is solid mounts that create a solid connection between the drivetrain and the chassis (these are available aftermarket, and racecars use them), but this too negatively impacts NVH comfort for a street car. Hence, the happy medium of stiffened mounts. But this idea is nothing new, as semi-solid mounts have been produced for years. This is just a less costly alternative.

The 996 turbo mounts are solid rubber and relatively inexpensive compared to aftermarket mounts also.

Laz - great job! Not something I mess with but was looking out for your post. I am impressed - keep us posted!!!!

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That's good to know turbo mounts are solid rubber - I was not aware of that. I would be interested to know the hardness rating of the OEM rubber. It is definitely softer than the 80A poly I poured, so mine should be stiffer than the OEM turbo mounts.

I made 2 sets of these and I'm interested to get feedback from a fellow Rennlister that has been using aftermarket semi-solid mounts. If anyone is interested to help me out, pls PM me. I'm located in NYC/NJ, but I am also certainly willing to ship. I will also be at the NNJR PCA autocross this Sunday (Jul 29) at Giants Stadium. Thanks.

The stock turbo mounts are not solid rubber. The X73 mounts are much stiffer, but I'm still not sure if they are solid or fluid filled.

993 375 049 07 should be solid (stock part # for GT2/turbos), I'll admit I've never opened one however. X73, and Carreras appear to have the same part number according to my supplier. The 964 RS motor mounts are solid also, but they're nearly 3x the price.

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I've always wondered if this was true. I've read conflicting info. I have the turbo mounts and can't feel any difference compared to my non turbo mounts.

My guess is the Turbo mounts have more rubber than the Carrera mounts, but they're still hydraulic. What about the GT3 - does it use Turbo or Carrera mounts?

The stock 996 Turbo mounts are definitely hydraulic. I cut one of mine open after I replaced them with the X73 mounts. The X73 mounts are visually identical to the stock mounts, but are a lot stiffer. I had assumed they are also hydraulic, but I'm not cutting one open to find out.

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Thanks for the verification. Bummer as I thought I had "upgraded" by getting the turbo ones. Did you feel any difference between your original stock mounts and the x73 mounts?

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Huge positive difference in handling, smoother shifts, and quicker off the line. Just by holding the motor in place better, less energy is lost from the motor torquing on it's mounts. No noticeable difference in vibration, ride, or noise. I'm not sure how much of that was worn stock mounts and how much is stiffer X73 mounts. However, I chould hold the old mount in my hand and easily flex the threaded shaft around in the mount. I couldn't budge the shaft on the X73 mount. BTW. I installed the mounts 2 weeks before the shocks and springs, so I got a very good read on the difference just the motor mounts made. It was significant.

Bump. Any updates on if it's still holding up good? One thing I'm still not 100% clear on is if there is a potential for the rubber cup portion to decouple from the metal housing and shoot up since the metal cap is no longer there. Also important to note is to get 80 A rated Polyurethane as their are several designations of which the two most used are A and D ratings. Different stiffness even though it's the same number depending on if it has the A or D designation. Another source to get varying durometer levels of Polyurethane is McMaster. http://www.mcmaster.com/ Thinking of going this route soon.

Yes, holding up very well. I've done a couple thousand miles and several autocrosses since installing. I'd like to go stiffer, so I'm working on another set using slightly harder 90A urethane. The metal cup is not structural - it's only purpose is to protect the pre-existing rubber enclosure for the hydraulic fluid, which I remove. It's also important to note that older mounts sag, and so it's necessary to make a jig to relocate the stud relative to the housing before filling. I took measurements from a brand new OEM mount and will construct and utilize this jig for my next set of mounts, ensuring perfect fitment.

The original mounts allow the motor to sag. When I installed RS mounts I realized the engine was lifted higher in the chassis. The RS mounts are not great at keeping a low center of gravity as a result. (lowering the car helps obvuiously) So, your mod actually keeps the height of the engine as low as stock but stiffens the movement of the motor. Pretty cool.

Hardback, thanks for confirming that.

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Great Info. Do you know roughly what the height difference is?

I am about to pull trigger on RSS mounts for my 40th AE riding on x74 suspension

I have the RSS mounts in my C2 and really like them. That being said if the Function First mount would have been available when I did mine I would have bought those. Based on the quality of their trans mount I think their engine mounts would be a good option.