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Old 03-28-2010, 12:00 AM
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Loud knocking at WOT

I'm completely stumped. I had a SB 350 in a 41 Buick. I had airbagged the car at which time we replaced the exhaust pipes and installed a new balanced driveshaft. After the install, the car had a bad knocking that I couldn't reall place it's source. There was a place where the exhaust pipe was hitting the body over the rear axle that we fixed but the pipes are less than a 1/2 in from the frame.
The knocking occurs when I stomp on the throttle and gets progressively worse at high RPM until it shifts and then diminishes. I had the car looked at and was told that the cam went flat and the knocking was a bad detonation occuring because of it. It made sense so I pulled the motor rather than digging into a motor I didn't know the history of and replaced it with a new turnkey crate ZZ4 350.
The knocking is still there under the same conditions. I am thinking that it's either the exhaust hitting the frame ( it doesn't do it in park and I can't see how far the motor torques over the see if there is a clearance problem. The motor mounts looked ok and I used the old ones when I replaced the motor. I don't see any marks on the new driveshaft. PLEASE HELP. I have already dropped a huge bundle on a motor I didn't need and everyone I ask scratches their head.

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Old 03-28-2010, 12:16 AM
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Knocking

You sure its not the Flywheel cracked buddy? If it has a crack it will crank and run but as you rev engine it sounds like a hammer hitting soming really hard.
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Old 03-28-2010, 06:57 AM
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knocking

I think that when he replaced the engine he either got a flywheel with the new motor or he would have seen a crack on the original when he changed it.

Did you check the tranny mount? Being you changed the engine, and still have the knock, it would have to be something from the flex plate on back.

Ordinarily, I would suggest checking the torque converter bolts being it's rpm related but you just changed the engine so their tight. Might want to get a friend to ride in the back seat and see if you can isolate where the knocking is coming from.
Another trick is to set the parking brake and hold the brake peddle while you change from 1st to reverse to torque the engine over both ways to see if you can hear a knock while sitting still. Hope this helps.
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Old 03-28-2010, 01:50 PM
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Well I replaced the trans mount and the knocking is still there...it's slightly less loud but still prominent at WOT. How much will a motor torque to one side on hard acceleration and which way will it towards pass or driver side. It still sounds like its coming from the pass side but I thought the motor would torque with the top of the motor rotating towards pass side and that would make the pipe hit the frame on the drivers side. There is only a 1/4 to 1/2" clearance on each pipe on the frame rail. I only have about that much before the pipe would hit the ground before the frame rails because of the air bagging. Could I flatten out the exhaust pipe to get more clearance on top and bottom without messing up the back pressure too much? It's a cruiser with a 350 HP 350 CI.
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Old 03-28-2010, 05:07 PM
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1/2-1/4" from the frame is not enough clearance for the exhaust pipes on a normal ride hight let alone a low rider with a 350 hp engine. Only thing I could suggest is a torque strap between the left front of the engine to the frame to reduce the amount of travel of the engine under high torque conditions.
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Old 03-28-2010, 06:39 PM
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Quote:
Originally Posted by ron2424
Well I replaced the trans mount and the knocking is still there...it's slightly less loud but still prominent at WOT. How much will a motor torque to one side on hard acceleration and which way will it towards pass or driver side. It still sounds like its coming from the pass side but I thought the motor would torque with the top of the motor rotating towards pass side and that would make the pipe hit the frame on the drivers side. There is only a 1/4 to 1/2" clearance on each pipe on the frame rail. I only have about that much before the pipe would hit the ground before the frame rails because of the air bagging. Could I flatten out the exhaust pipe to get more clearance on top and bottom without messing up the back pressure too much? It's a cruiser with a 350 HP 350 CI.
The drivers side lifts up on acceleration. I've used a turnbuckle arrangement bolted to the front motor mount bolt hole (the OLD, front-of-the-engine motor mount hole, that is) to keep the engine from lifting- you might try the same thing. Kits are sold, or fab one up.

AFA exhaust system hitting, you would think you'd see 'witness' marks from the contact, but if in doubt, use wooden wedges/spacer blocks to isolate the exhaust system from any close by parts, frame, etc. to see what changes.

You mentioned a new d-shaft- is there sufficient room for the yoke to telescope in and out of the tailshaft?

Good luck.
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Old 03-28-2010, 07:02 PM
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All good input. Thanks..so what about ovaling the pipes around the frame rail to get more clearance?
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Old 03-28-2010, 07:09 PM
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Quote:
Originally Posted by ron2424
All good input. Thanks..so what about ovaling the pipes around the frame rail to get more clearance?
Sure. That will give additional clearance, if you're not able to use more hangars strategically located to hold the system away from the frame- they're ALL different! lol

Let us know how it turns out.
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Old 03-29-2010, 05:30 AM
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You also ask how can the pass. side can hit if the torque drops that side, well it works on the same basis as a teeter totter. Depending where the exhaust hangers are placed, when the engine rotates downward on the right, the pipes towards the rear will go up. It's a geometry thing! Perhaps better placement of the hangers would help some.
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Old 04-06-2010, 11:20 PM
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I reworked the exhaust for better clearance and I still have the problem. I realized something tonight. I have been running the V advance off the plate and not the ported. Is it possible what Im hearing is severe engine knock at WOT because the timing is getting major VA from the wrong port. I think that would explain why when it shifts and the engine drops RPM's it goes away because of the less VA. The response was a bit better when I retarded it a few days ago but it was still connected to the wrong port. I found it ran best at 4 BTDC when GM recommends around 10. I will retime tommorow at 10 and run the ported vacuum and see. I am also going to replace the motor mounts as I am seeing a pretty hefty motor shake when I shut it down with a corresponding clunk down under the pass side. I could be a have a 2 headed monster. Any thoughts on the timing issue possibility???
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Old 04-06-2010, 11:53 PM
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I have a 79 350 stock and run 10* initial and ported vacuum for the advance
and run reg 87 octane with no problem. If knock or ping was caused by the timing it would happen at a lower rpm under load.
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