I have asked about this before, but still looking for some suggestions here as I am not quite sure what is going on with my new engine...

To make story short: I recently replaced my old engine with a neutral-balanced 383 stroker built using forged Eagle crank / SRP pistons etc... When I made the purchase, I paid extra to get the rotating assembly balanced; as such, I assumed they did a good job so we didn't double-check the balance prior to installing these items. The flywheel and the clutch have also been neutrally balanced, and I am using a Fluidampr for good measure. We went with poly engine mounts for this build, and for what it is worth, the car is a six-speed.

After all this, I have a noticable vibration between around 2000 - 2500 rpm that can be felt through the shifter and also rattles the dash around a little bit. The vibrations only occur when the engine is under load and not when I take my foot off the gas. In addition, the vibrations seem to get worse once the engine warms up and the car goes into closed-loop. Since everything has been neutrally balanced, I am kind of at a loss for what might be causing the vibrations. Could the poly engine mounts be causing this? Any suggestions are welcome. Thanks.

 

Could be anything....loose tranny mounts etc. Did they give you a balance card with the engine? I had an issue with mine where I asked them to neutral balance, but they balanced it to a weighted flexplate instead. So if you have documentation that says for sure that the assembly was balanced to the neutral (no weight on the flywheel/flexplate) then your problem lies with mounting or tuning.

NOTE* does it vibrate while you're moving or while parked? If it only really does it while moving, could be the driveshaft/wheels/etc that need balancing aswell. Also did you have the assembly balanced to the flywheel at the same time? If not they should be rebalanced to eachother and not separately. Hope that helps, let us know if any of these things turn out to be the BAD VIBE issue.

 

Actually, the more I drive the car, the more I am starting to think that what I am calling vibrations are more like rapid pulsations that travel through the car. It is almost as if the car is missing radidly; it actually shakes the dashboard around and you can feel it very strongly through the shifter but not through the steering wheel. Since it only happens when the engine is under load and when the car is in closed-loop, I am starting to think it may have to do with the tune? Do you guys think this is possible?

 

Does the engine visibly shake at idle?

Was the crank balanced with the hub, flywheel, clutch and pressure plate attached or accounted for?

Only thing I was told when I had this problem was to disconnect everything from the trans back, including the clutch and PP (unless they were balanced with the motor). Then run the engine. If it runs smooth, then start adding things back on to see when it starts vibrating again. If the engine doesn't run smooth, then you have a problem in or near the motor.

For me, it ended up that the pressure plate was way out of balance.

 

No extraordinary shaking at idle; it actually idles very well at around 900 RPM with a slight lope you would expect from a GM 847 equipped 383 ci engine.

The fact that it only occurs when the engine is loaded (i.e., although it only seems to do it at certain RPMs, it appears to be not related to the revolution rate since the symptoms disapper when I take my foot of the gas at the same RPMs) and when it is warm is leading me to believe that it does not have anything to do with any mechanical imbalance...

Could a lean condition be causing this?

 

One cylinder not firing?

Seems to be the same situation I had when I burned a wire.