[67_Malibu_Danno]

Hi guys,

My first ignorant motor swap a year or so ago. I picked up a rebuilt '68 Hi-Perf 396 which i don't know much about, other than the fact it seems to have a radical cam (registers only a bouncy 7" of manifold vacuum, very hard to tune the holley), not even sure it is still a 396. When I got it, there was an old balancer but wasn't a flywheel. I used the flywheel off my 283 but didn't pay any attention to if/how it was weighted (honestly didn't know about internal/external balancing at the time, still not confident on the subject). It fit, and I wanted to hear that motor run so I re-used it.

To get to the point, I mainly drive it around the neighborhood, peeling tire, having fun, but when I'm out on the highway, I notice a periodic vibration with a shimmy cycle of about every 4 seconds (1/4Hz for you nerds) when holding a constant speed. I don't have a tach, but i'd guess it is about 3000RPM (although I can't say it is exclusively at one RPM). Seems like it has been getting worse over the past few times I drive it.

So I'm looking for some advice before I start taking the wrong path on this. Does anyone have any inputs if this issue seems like a driveshaft, balancer/flywheel mismatch, tuning/timing or just a characteristic of an aggressive big block motor.
Should I seek some professional help on this one or does this sound like something I can debug myself?

Questions about external balancing: Would the balancer have a number which might point to information about how it would need to be matched with a flywheel?

Thanks for your help,
dan

 

[65cayne]

That is odd...I have seen other's report the same problem (if I understand you correctly) and I experience it myself as well when running down the highway. Appears to feel like a tire slightly out of balance but it fades in and fades out in a 4 second envelope. For the nerds, this can best be described as amplitude modulation at 1/4 hz as stated.

I do not recall a solution being posted, but it has been attributed to motor out of balance, driveshaft out of balance, wheel out of balance, and flowmaster exhaust systems, or a combination of a few.

You could, theoretically, take all of the RPM values for each of the components listed, and do the math to see how a vibration in each one of them would attenuate or amplify each other over a continuous sine to produce the effect that you speak of. Or not. If there is an engineering major in the house, this might be a good research project for them (regardless of your solution).

 

[OLDED]

Check the drive shaft and u-joints too, maybe a harmonic vibration set off by an out of balance condition or a worn u-joint. The front driveshaft yoke might be worn and a bit loose where it goes into the trans. Water pump and fan tight? The flywheel should not be the issue - both the 283 and the BBC are internally balanced - if you are sure it is a 396 and not a 454.

 

[Mike]

Check for loose converter bolts.
Get some of these numbers and we can verify what engine it started life as

 

[Tom Mobley]

probably a u-joint or driveshaft out of balance. pull the shaft and take it to a shop that works on driveshafts, ask them to replace the u-joints and straighten/balance the shaft.

 

[BillK]

Dan,
If the engine is a 396 then your flywheel should be ok. Try this test .... depending on what transmission you have, leave it in a lower gear (second on an automatic, 3rd if its a 4 speed) and see if the vibration happens at the same rpm. If it does, it is something in the engine or clutch/transmission. If the vibration is dependent on vehicle speed as opposed to rpm, then it is probably something behind the trans .... driveshaft, u-joints, axles tires etc.

Also, some engine/camshaft combinations have a "bad" spot in the rpm range that can cause a similar feeling. Sometimes there is absolutely nothing you can do about it other than avoid that rpm range The engine in my Camaro race car would vibrate the entire sheet metal interior at about 2500 rpm on the return road but was smooth as silk up to 7800 going down the track. It was just an rpm that the cam did not like at all

 

[Splitter]

I would suspect a clutch or transmission problem. My dad has a field tractor that vibrates at road speed about every 3 seconds.

 

[67_Malibu_Danno]

Hey thanks for all the good suggestions. They are all worth following up on before I get into the motor.
I did recently add a high-flow water pump, and don't remember using lock washers so I'll check that. I am fortunate to have a mutual friend with a driveshaft shop. I've never changed the U-joints on this car but I had to on a '78 cutlass when it dropped on the ground in a parking lot and wasn't too bad. I'll take that pportunity to check the shaft for imbalance. Tires are a good one too. I've had vibration at 70MPH for years in a Tahoe that went away with new tires. Must have been a bad radial belt or something.
Thanks again, I'll get back on the weekend with my debug data
Dan