Vibration at 3000 RPMs?

Hello. I have a 99 C2 with 3000 miles on a rebuilt motor. Eternally Fixed IMSB at rebuild. Recently installed Function First motor mounts and now notice a vibration that starts at about 3000 rpm's. Smooth up to that point and smooth after that point. It occurs in all gears. Thanks for your expertise. Cheers.

I can tell a bit more engine vibration beginning at just under 3000 RPM and it goes away again above 3K. I believe it is an inherent vibration characteristic of the engine. The Function First mounts are a bit stiffer than the stock mounts and so you will probably feel the vibes more with those. Edit - I changed out my motor mounts with OEM units and the vibration was greatly minimized But if I pay attention to it, I can still feel the engine vibrations as they come and go up the rev scale.

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I think you answered your own question. Somewhere in the installation process something went wrong or was not done right. Go back to whoever did the installation and have them recheck it. If you did it yourself, perhaps have a shop double check your work. Personally I think OEM would have been just fine.

My conclusion from this forum was that the 996 engine has a natural vibration at about 3K. I never noticed it with my original (soft) motor mounts but I'm now aware of it with my RSS semi-solid mounts. In my case it is only noticeable within a small RPM range and I don't consider it a real problem. I also wouldn't revert back to the OE mounts since I like the increased engine noise and feel :-)

now you know the reason why manufactures use mush mounts.what was your reason for going with aftermarket mounts ?

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I have RSS mount, and something does happen around 3000RPM, but it's gone as soon as it appears. But it's not really a vibration, it seems like something slips for a milisecond, or a misfire.

at 3 grand it may be the cam timing changing...just one of many reasons why mush mounts. why did you put aftermarket mounts in ? it doesnt look like a car that sees more track than street use

Any "performance" marketed motor mount is likely to increase NVH. I wouldn't worry too much about it. Maybe some buyer's remorse kicking in?

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The same reason why Porsche uses dynamic engine mounts in the GT3 and Turbo. Soft motor mounts reduce noise, vibration, and harseness but they have a negative impact on handling. The biggest difference is that with softer mounts the engine can take slightly longer adjust to direction changes which is noticeable on the track. In addition stiffer mounts can reduce wheel hop when accelerating and can improve the feel of the shifter. http://www.porsche.com/canada/en/mod...engine-mounts/ I haven't been able to compare new OE mounts against my RSS mounts since mine were worn when I replaced them, but I definitely noticed a big improvement in handling in corners with the new mounts. And I like the slight but noticeable increase in cabin noise (when accelerating hard) that was a result. The car sounds a little more like I think it should.

Thanks for your great responses. My car is strictly a daily driver, but I like a sporty feel. With the new FF motor mounts there is definitely more vibration, shifting is more direct, easier and it is noisier inside. Additionally, the minor interior rattles got more pronounced with these motor mounts. But I love the feel and sound. Next step is a roll bar. That should reduce the body resonance another .08% but it will definitely look cool....to me. Prior to my original post, I did some extensive searching for an answer on the 3K resonance and this topic seems to fit into the unresolved category. A few posts on this subject but no one gets past the opinion stage with a definitive or plausible answer. Cheers.

I have the F1 transmission mount. It definitely vibrates at 3K RPM and slowly goes away with higher revs. The first couple of times I thought there was something wrong with my car and brought it back to the shop. I rode in another vehicle there with the same mounts and it too made the same noticeable vibrations. I'd go into the camp as normal for the modification. Shifting is better and driving is more precise but the vibration on a daily driver is a bit much. Fortunately its a 60 minute swap out in my own garage.

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Better than stiffer motor mounts I think would be installing a torque strap to keep the engine from twisting so much. I installed one of these in my Datsun 510 while leaving the engine on stock mounts, but mounts that were in good condition. (I was forced to use a torque strap as the engine would twist so much the mechanical throttle linkage for the duel side draft Weber carburetors would be affected and the engine would *NOT* run right.) Since I do not auto-X my Porsches I do not feel a need to strap the engine down, but you might explore this rather than run stiffer motor mounts. There is it appears some kind of natural harmonic or resonance at 3K. I notice it in my Boxster -- on stock motor mounts. In the Boxster engine there is no change in valve timing at 3K. (The intake resonance flap is opened at 3K (or closed at around 2900 RPMs) but I do not think this really plays a role in the vibration.) The severity of the vibration is not bad at all, it is really only noticeable because the engine is so sewing machine smooth at all other times. The road surface plays a role in how bad the vibration is which suggests it is something to do more with the car's running gear and less with the engine.

It's the motor mounts. I installed FF mounts and I get the vibration between 3-4,000 rpm up to fifth gear. It goes away the higher I go in the revs. Not bad vibration but more than the stock mounts.