roysatikas

iTrader: (1)

built a 400hp/tq 4 bolt 350 60 over balanced rotating assembly, march pulley system. Problem: vibration noticable at all rpms, continuously cycles between vibration and then smooth every few seconds at all rpms when holding the rpm at 2000/3000/4000/5000/6000 and the rpms do not cycle or vary . 1 5/8 jet hot coated shorty headers with 3'' exit to 2.5 Y pipe and then the 2.5 single splitting to dual out of the muffler out back. Dynomax system, no CAT, this worked great with my warmed up 305 w/o vibration issues. I am wondering if this is an exhaust pulse vibration due to the limited 2.5 pipe. I have tried to chase this issue down from measuring crank runout to removing belts from the pulleys and everything cks out fine, the engine stayed together at 6500 on the dyno. I am preparing to install the Dynomax 2.25 dual exhaust part # 89009 and am cautiously optimistic this will solve the problem. thanks to all who reply, I will post when it is finished with pics etc.
Roy

Guest

1. Motor mounts. Your stock rubber ones could be crapped out or if you installed poly vibration can be a side effect.
2. Transmission mount. Could be loose or again if it is poly, same business.
3. The exhaust could be contacting the floor pan. Less likely as you would hear little gremlins banging on your car from below, but still worth a look. This happened to me.
Now, why the vibration is intermittent I have no idea.

roysatikas

iTrader: (1)

Thanks g92, all you mentioned are good including new Spon crossmember with TQ arm mount on the crossmember. Hope to have this done next week.

Z28/ZR1

Is this a fresh rebuild?
Has this been going on, or did it just start?

Other things to check:
Tuning
Spark Plug Wires
Ignition system in general
Torque converter (automatic)
Clutch pressure plate assembly (manual)

roysatikas

iTrader: (1)

Been occuring since I put the engine in last fall, everything has been checked, even have a new B & M 2400-2800 stall TQ converter. If the new exhaust doen't work/solve the problem I will need to start retracing possible sources. The engine did fine on the dyno breakin 6500rpm's held together just fine, only difference between the previous warmed up 305 and this engine is I'm running the same small diameter exhaust. Thanks

Z28/ZR1

An engine dyno won't necessarily expose a vibration. If you want to eliminate the exhaust as the culprit, just temporarily remove it.