What are the best engine mounts to use?

Looks like the left one I replaced a year ago has let go.... again
It was a rubber mount style with the metal tab which limits travel.
Just as well I put that style in.


Are poly mounts strong enough to tolerate some abuse?
or is there something better?

I've heard a few guys talk of putting a bolt through the mount to limit extension of the rubber under torque.... is this a suitable long term solution or is there something better?

Any pics of a bolted together mount would be appreciated so as to visualise the location of the bolt.

cheers

 

i would like to know about that bolt trick too ozzy tom. i put in GM brand ones in my other car which the power is so bad you would swear it runs on solar power but yet in one year the mount has broken. if that car has broken one then guarantee the one in my vet is broken. im pretty sure they are both interlocking ones .

 

Managed to get the offending mount out yesterday....



Just as well I used the interlocking type.... check out the wear on the contact locking tabs....

anyway,
I chased up some local prices on poly mounts.... $195 each ?
Salesman was telling me they're the best available, triple chrome plated etc etc etc ...

thanks but no thanks.


Well here's the two types of mounts I did find....


The top one is the #A1128 sold through parts suppliers here in Oz as direct replacement.... but there's NO interlock mechanism. OK for use on the right side, but definitely NOT for use on left engine mount!

The bottom one was hard to find, but I managed to source it from a specialist speed shop here in Adelaide.

I modified the new one by simply drilling 2 x 1/4" holes through the rubber and top side of mount... (note the bottom metal plate [block side] has two existing holes through which the rubber exits during the manufacturing process).


Countersunk the top side for maintaining the screw heads flush.


Used 2 x 1/4" x 2" stainless countersunk head bolts, and fitted a washer and nylock nut on the other side (engine block has raised mounts which provide clearance for nuts mounted on the plate) then cut the excess thread. I also used Loctite on the threads, just as an extra measure.
Nuts were torqued up reasonably tight... without compressing the rubber too much.



Got it all together yesterday,and took it for a spin. I can't notice any increase in vibration... so much for all those that told me it would vibrate badly.

I guess you could call it a "semi-solid" engine mount

Gave it a couple of full blooded clutch dumps... and all is good. I'll keep an eye on it and see if it can hold together like this.

 

For things like this I like to use Solid mounts and or use 1 Solid mount on the side that breaks and use the interlock and or a poly mount.

I hope this works out well for you.

 

I hope it works out too.....
Given the tensile strength properties of 2 1/4" steel bolts vs a 2" square rubber block, I figure this mount won't let go. (see calcs below)

Now that I know there isn't much vibration coming through with this modified mount, I have more confidence that a solid mount would probably be OK (regarding vibration transmission)

If this one ever does let go, I'll just put in a solid mount



PS.... assuming my references are correct...
tensile strength of rubber is 15 Mpa (2175 psi)
tensile strength of high density polyethylene is 37MPa (5365 psi)
tensile strength of stainless steel is 860 MPa (124700 psi)

Given that there is 4 sq inches of material cross section in rubber and poly mounts,
and there's 0.196 sq in cross section in a 1/4" bolt ( 2 bolts provides 0.392 sq in)

then the force (load) to break the rubber block is 8700 lbs (tensile strength x cross sectional area)
and the load to break the poly mount would be 21460 lbs
and the load to break 2 stainless bolts 1/4" diameter is 48970 lbs

If my calcs are correct, my modification should be over 5 x stronger than a rubber mount,
and at least twice as strong as a poly mount.

If I've got this wrong, please tell me. Some how I can't believe that the motor would provide over 8000 lbs of force at the engine mount to break the rubber ? what have I missed?

 

Well, this is just my opinion, but I've never been convinced that motor mount breakage is caused by "monster motor torque". I've broken a couple over the span of a bunch of years, and if it was engine caused, they'd be breaking all the time because the car gets driven with lots of throttle for a good percentage of its miles. (Lately I have as many track miles on it as street miles.) My thought is that the mounts are frequently broken due to frame flex. These frames are notoriously poor in torsional stiffness, and potholes and gas station entryways twist the crap out of these frames. If the occasional small block broke a motor mount due to its torque, then big blocks would be breaking them every time you pulled into the passing lane.

 

Good point Mike.
That's a logical possibility.

 

I recently completed a C4 suspension transplant onto my spare C3 frame. I made several modifications to the frame to add a bit of torsional stiffness to it. One modification, in an attempt to reduce twisting in the motor mount area, was an additional crossmember and torsion tube to connect the motor mount horns to the front crossmember. Here's an old picture while the frame mods were in progress.



I made several other changes, but they were to address frame shortcomings elsewhere.

 

Moroso solids. Issue gone.

 

Correction: Issue bandaided.

 

What issue?

 

I have a pair of poly mounts I'll sell you for $195. I'll start pulling them out as soon as I get home. I paid $70 for both of them. OK, they're used, how about $150?

 

Yes Tim, it's ridiculous the marked up prices on Corvette components down here in Australia....
but like I told the local speedshop salesman... at $195, NO thanks!

Talking with a mate today, regarding tensile strength of rubber mounts.... he brought up another valid point.
The break in the mount is NOT through the rubber itself, but rather at the bond between the rubber and the steel plate. I think they are vulcanised onto the steel plate using heat and pressure.

Also, the load is a sheer stress, rather than a straight extension load pulling the rubber apart. Sheer stress acts to peel the rubber off the plate, and the sheer rating is much lower than the tensile load rating.

Another interesting point he made....he also went on to say there should be a noticeable improvement in throttle response using solid mounts....
Energy is lost when throttle is applied and rubber mounts compress under torque load.
With solid mounts, the torque transfer is much more pronounced.

Anyone with solid mounts care to comment?

 

So I gather your thoughts are that using solid engine mounts, the block acts similar to a cross beam which ties the front chassis rails together to improve torsional rigidity.....
hence not necessitating an additional cross member as 69427 has suggested.

Are there any detrimental effects of solid mounts
(apart from the obvious increased transmission of vibration to chassis from motor)

 

That's how I see solid mounts. And keeping proper alignment of driveline angles could also be an issue.

One of your posts mentioned the rubber being vulcanized to the steel plates- exactly correct.

 

I thought the issue in question seemed to include concerns about motor mount failure, no? Having experienced OEM type 2-pc mount failure in a couple of high-perf applications, I can understand why an approved torque strap is required unless either solid mounts or motor plates are installed in many forms of racing; and it isn't for stiffening the front of the chassis. Believe me, one of the last things you want to see is your engine climbing out thru the hood when you're *****-to-the-wall.

However, as to stiffening, installing solid mounts and a spreader bar should prove adequate for the majority of C3 owners looking for some improvement without necessarily re-engineering their chassis. Beyond that, if strategic gusseting and fully boxing the frame aren't deemed sufficient, for anyone who's got legitimate concerns over the torques going thru the block, IMCO it's probably time for a full cage and perhaps motor plates, anyway, given the bigger picture which ought to be in view. Besides, I've raced a number of cars where the block is a stressed member of the chassis, so unless we're talking about tremendous amounts of torque IMCO block integrity (as it relates to carrying loads) doesn't often rise to being a matter of the first order.

Not to say anything against 69427's innovations, which I'm all for, but quantifying the return on the investment of effort and weight into additional supplemental sub-structure bracing is likely to be a somewhat subjective exercise without hard before/after rigidity numbers. Please, don't take that as flames, as my honest skepticism is simply borne out of having spent a good portion of my life in and around racing cars which incorporate the tried and true (by rule if not by election). I sincerely hope all of your work pays off.


Given that I don't place much priority on my land shark being comfortable, I've never paid any particular attention to whether there's additional noise and/or vibration. FWIW, don't ever run solid tranny mounts if you don't want to greatly increase the risk of cracking it's case.


TSW

 

TSW.... no remarks considerred flame at all...
I'm always keen to hear other people's viewpoints, and I take on board what makes sense and actually works, whilst discarding that which doesn't.

That is, after all how progress is made.

 

I went with Moroso solid mounts and love it. Stainless hardware is great but they are pretty soft so aplication has limits. You may need to bump it up with some grade 8's w/ nylocks. I'd keep an eye on that current set up just in case.

 

Just a follow up on this thread to provide feedback.

The bolt modification I used on the rubber engine mount failed after 18 months during a session at the track. (back in 2010)
Scared the bejeezus out of me when the throttle jammed open as I exited out of a corner and put me into a spin
I installed a solid Moroso mount on the left side and it has given no further problems.
Slight vibration only noticed during cold start idle, with no detrimental noise/vibration once warm.

Like quite a few other posters stated.... put a solid mount in and forget it. Problem solved.

 

I currently have the Energy suspension poly mounts in my car, however if the poly mounts ever break or bend then I will be replacing them with the Moroso solid mounts.

I don't track my car so I'm hoping that the poly motor mounts will be okay for the street.