I need some help. I just got my Mav up and running and have a terrible harmonic vibration that resonates in the car at constant engine speeds, usually between 2500 rpm and 4000 rpm, it is more of a whrrr, whrrr, whrrr sound. I have a 351w, C4 and 9" with 3.89 gears, everything is new including the driveshaft w/1330 ujoints. I set the pinion angle and it is just about even with the trans. I have already replaced the rear ujoint thinking I picked up a bum one, but to no avail. Someone told me it might be tires, I had those recently balanced. I'm not even sure the noise is coming from the but thats what it feels like. Any suggestions would be helpful. Thanks.
First of all, is the noise vehicle speed related or engine speed related? When you go down the road, and put the car in nuetral, does the same noise continue? You need to isolate vehicle from engine to get a start at determining the source.
I have not tried putting in neutral while driving, I will try this next. The rearend is also new from Currie Enterprises, complete with new bearings (big, torino bearings). I will pull the driveshaft and have it checked for balance as well. Any other suggestions?
I set the pinion angle and it is just about even with the trans. Pinion angle should be down 2 - 4 degrees from what the trans is set at. The front of the rearend will rise as the pinion gear rides up the ring gear during forward movement of the car.
If all else fails put the car up on jackstands , take rear wheels off and put a couple nuts back on to hold the drums in place . Run car and note if you still have noise . If not then it is wheels . If vibration still there pull off drive shaft and run in gear again . If noise is persists then is engine or tranny related . If it is not there then it is somewhere in your driveshaft or rearend. Be careful when running doing this kind of thing on jackstands !
Well I took it on the highway today, brought it to a constant 3500 rpm and shifted into neutral, the noise went away and no vibration. I'm going to pull the driveshaft and have it rebalanced, hopefully this will reveal something. Oh, my pinion angle is appx 2 degrees negative. I'm running a 3:89 US Gear, ring and pinion with an Auburn Posi, would this contribute to the harmonic vibration?
If the vibration goes away when you shift into neutral then your driveshaft is probably not your problem . It has something to do with your engine balance . Check if your cooling fan has all of the blades , your accessories , vibration damper , and stuff like that . When you shift out of gear at highway speeds all the stuff from your output ( transmission ) shaft back are still spinning and if they were out of balance then they would still vibrate . I think more trouble shooting is in order before you spend money on a driveshaft balance job
Lightning, good point! However, I'm running an electric fan and had the bottom end of the engine balanced at the shop with all the matching components, 28oz. I was thinking, I have 1 5/8" header tubes, worked over to make them fit. Could it be that under load one of the tubes is rubbing against the body? I checked the tubes at rest, both with the car running and not and did not see anything hitting. I have brand new motor and transmission mounts and a "torque link" from Competition Engineering to keep the motor in place. So I don't think the engine is flopping around. I have not welded in my frame connectors yet, could it be the body is twisting just enough to make the header tubes touch the body? My 351w is putting out just over 400 hp and 400 ft lbs, I haven't brought it to the dyno yet to be sure, but these are conservative numbers. I wouldn't think this body would be that flimsy.
Lightning is exactly right and that is why I suggested that test. Sounds like you made the engine mounts so solid that a lot of the engine harmonics are finding their way into your cabin. many people I know who have tried solid mounts or firmer poly mounts have switched back to rubber.
