wierd resonance, anybody else have this? Options

[blabla914]

OK. I'm going to try to make a long story short.

Last winter I did a six conversion to a 2.0L six with a rich johnson front mount with the club sport mounts. I am also using club sport mounts at the back. I originally fitted the car with a sport muffler. I finally replaced the sport muffler with a Bursch a few weeks ago. While the sport muffler sounded great I tired of my ears ringing for about 20 minutes after getting to/from work. I'm surprised my neighbors weren't throwing rocks at me, especially on those early sunday mornings on my way to an autocross.

Now that I've got a muffler that actually muffles I've noticed the car has a bit of a resonance at precisely 1900 and 2300 rpm. You kind of hear a waaaaaaaaaaa type noise and you can feel a little vibration.

Is this just the hard mounts transmitting some vibration to the car and resonating at these specific rpms or do you think I really have a problem?

Thanks for your help.

Kelly

[914GT]

You might check tension on your belts. It probably won't be this easy, but I did have a resonance-type vibration at a certain speed on my Chevelle at one time, and it was an A/C belt that was not tight enough.

[Cap'n Krusty]

A Bursch that's quieter than a sport muffler? Wait a few weeks .................... The resonance could easily be inside the muffler; anything from design, to a weld flaw, to a broken baffle. I've always found Bursch mufflers to have a particularly irritating tone, which is why I liked them when I was in my early 20s, and why I haven't liked them since. Of the cobbed up style mufflers, they're the most well made, and probably the longest lasting, but they don't hold a candle to the OE mufflers. The Cap'n

[blabla914]

Dude, the bursch is WAY quieter than my sport. I mean completely different universe. I think there's a big difference in sport mufflers depending on where you get them. I got the sport because I've heard other cars that didn't seem too loud at all. I got mine very lightly used so I don't know who made it. It's a liestritz can and it is LOUD. Even at idle it's like, DAMN that's loud. I suppose the resonance could be in the muffler, but I can feel it in the whole car a bit more than I'd expect for just a loose baffle.

I'm going to check alternator belt tension tonight, but I just checked it about a month ago and it was OK.

I'm just worried I have a problem that would affect longevity and I'm looking for people who are using 4 sport mounts to see what kind of vibration they get in the car.

Thanks for your responses.

Kelly

[914GT]

Kelly, was the engine balanced? Also, rev the engine up to 1900 in neutral, with and w/o clutch engaged to see if it's the same.

[blabla914]

Good question. Thanks for the response.

Yes, the same noise is made in neutral with clutch in or out. This leads me to believe it's somewhere in the motor. I don't like that.

The crank itself was not rebalanced or cut from stock. It is a forged CW crank from a '69 E motor.

The rods were all weighed and assembled with the lightest on 1 and 3 next on 2 and 5 and heaviest on 2 and 6 or however you are supposed to do it according to Bruce Anderson's book. I also weighed all the pistons to make sure they were in spec.

The clutch and pp are directly off my 4 cyl which did not resonate like this.

The flywheel was cut and rebalanced by the same house that used to balance all my engine speed sensor test targets. I used to work for Siemens VDO automotive making engine speed sensors and we used to make some really out of balance test targets. These guys would balance them so I could put my coffee on the test rig running 8k, so I'm pretty confident in their work, but it is possible they messed up.

I'm also DAMN sure the flywheel is tight with new bolts and red loctite. The pp bolts are also torqued to spec with blue loctite.

Is there anything I've overlooked??

Kelly

[914GT]

The list is getting smaller. I'm not familiar with a 6 - how tough is it to slip the belts off the fan and alternator (or anything else) then rev the engine up? (not for long of course) Just to rule-out any belts and accessories. If you have vibration down at the base motor then sure points to the rotating assembly. Double-check those mounts, try to lift the motor or move it around to see if there's any excessive movement. You've probably already done that though. Good luck Kelly I hope you find it and hope it's something easy to fix.

[blabla914]

Hmmmm. The mount is a good suggestion. You've reminded me of somthing I had to do on my bug. I had to just loosen all the mounts and then tighten back up. I replaced the rear mounts with sport mounts after the motor is in, so that might do it.

Slipping the belt off is a PITA, but it would definately be worth a try.

Thanks for the suggestions.

Kelly