this is from PMO's ad in the october '03 panorama.
I was thinking about solid motormounts for my racecar but this article makes you think. Maybe the 911 clubsport mounts are better.
dave
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i'm with him. fix your shifting problems where they occur and leave the old mounts in (make sure they're not broken) or get the sport mounts. they are there for a reason. if you car shifts bad, it's NOT because you have rubber tranny mounts ...
Andy
i've got stock mounts front, solid rear (trans).
It's unclear to me that Parr has actual data to support the contention that solid mounts harm horsepower, put a significant strain on the sheetmetal and engine, destabilize the carbs, or caused the Romans to eat lead. He just has some musings and a a single anecdote about this guy who gets a lot of power out of his car and airplaine engines. There's no evidence that the motor mounts had anything to do with this horsepower gain.
Conversely neither do I have any actual data that solid motor mounts don't degrade horsepower, or damage sheet metal, or cause "harmonic" disturbances, or fuck with your circadian rhythms. But my guess is: probably not. I don't think the solid mounts do any significant damage to the structure of the car and I really doubt it has any measurable effect on horsepower.
That said, I don't like solid mounts for street driving. They connect the engine to the body directly so all the engine vibration gets translated directly to the body. The body turns into a big speaker and helps to amplify engine noise (that should have been suppressed through isolating the engine vibrations at the motor mount level). This is partially why new cars are so quiet. On top of having counterballance rods that rotate in the opposite direction from the cam and crankshaft, canncelling out vibration, they also have liquid filled motor mounts that isolate engine vibration, and thus sound, from the passenger compartment.
The second major problem with solid mounts is that I've found that if you have a lot of power under the hood, say around 200 hp or so, is that the engine's own twisting force (under rapid acceleration or rapid deceleration) will press the suspesion down on one side or another of the car (depending on whether the engine is suddenly winding up or suddenly winding down) and this will destabilize the rear of the car. Essentially the car gets squirly at the rear end because the engine rocks hard to the right or hard to the left. Normally this sort of engine twist is cushioned by the soft motor mounts but with solid mounts all the force gets directly translated to the chasis, and thus directly translated to the rear suspension. This can be dangerous.
So for me, the sport mounts are the better choice, and not for the reasons Parr has given but for the reasons I discussed above.
Douglas
I agree with Andy.
Take this comment for the secondhand gossip it is, but I have "heard" of top-end performance and drivability issues with the PMO's in race applications. I betcha those full-blown race cars were running solid mounts too, and that was used as the explanation for the problems.
Now, I don't see many race cars with PMO's and would venture a guess that Richard's main market is Zenith/Weber/MFI/CIS replacement for street cars.
Right now I have the club-sport engine mounts and Weltmeister (semi)solid trans mounts on my 914 (w/ Webers), and have been thinking about solid mounts on both ends. I know of no legit reason right now not to do so. It's a track car - I agree that solid mounts are craziness on pure street cars.
I've read that ad many times. Normally I wind up laughing...
Seems to me that the avenue of experimentation would be with higher durometer rubber compounds, or maybe delrin. Stiffer, but still allowing some compliance.
Patrick Pickerell
I think one reason he condemns solid mounts is that they play havoc with the PMO carbs. A friend of mine replaced his perfectly good Webers with the allegedly superior PMO carb. He has had nothing but problems with the PMO's, and PMO claims his solids mounts are the culprits.
The Webers have been reinstalled, the PMO's are for sale, and the solid mounts are still in place.
The float assemblies in the PMO's look suspect, there is a teeny lil spring thats supposed to control things, and we think they just cant stand up to the vibrations from the solid mounts.
BTW I have solid mounts in my 911 w/Webers, have had them for about 5 years now
I never thought of it till I read the article but that may be the reason my carbs are consistently out of tune.
I only run the solid trans mounts
my solid mounts have been in for a week now and here is my impression (rears only).
There is a 'bit' more noise than the stock mounts, but in our cars its only noticeable during deceleration and it's not annoying. I kind of like the sound myself.
Front mounts? I dunno about.
Tim...interesting...that exactly echoes the story I was alluding to here in SoCal. 914-6 3.2 race car that had PMO's. They couldn't get it to run right, even with significant pro dyno tuning, so it was converted back to Webers. Makes you go...hmmmm....
Andy - I think on the 914 the trans mounts make the biggest difference in keeping things "in place" when shifting and under accel/decel forces; I am not really sure how much benefit the front mounts add to that. The main place the added stiffness can be felt IMHO is when downshifting under decel and especially also under g-loads; like those turns where you are going uphill, downshifting, and turning sharply all at the same time. With squishy mounts the gear you are looking for tends to "move" from its normal position on the shift linkage. I guess the trans is actually moving a slight amount on its mounts, which is exaggerated by the looong 914 shift linkage.
I'm in a bit of a quandry. My 914/6 trans mounts work just fine, thanks, but I've got a set of solid mounts that I need to try out. Come spring, they'll be in there for a test.
Reading all this stuff reminds me to keep an open mind.....even if sumthin' falls out.
I've got them old Weebers and not them fancy-schmancy PMO thingys so's I ought to be OK.
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