383 knock goes away on warmup

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Done Swimmin

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Hey All
I have a knock that is only heard on start-up when cooled off or not run for a few hours. it goes away at higher RPM or as soon as the engine is warmed up.

Oil pressure is good (near 80 lbs at idle and over 40 when driving).
I checked the plugs and they are black with soot and wet signaling rich mixture and possibly too cold a plug.
I have added octane boost and it helps but not 100%.

Here's where i am right now;

I ordered a set of plugs one heat range hotter
I added a can of Seafoam treatment to my fuel to clean system
I backed off the timing and it helped a bit.
I am trying to nail down the CR but i didn't build the engine. I have bore, stroke, gasket compressed thickness, head volume, flat-top pistons, so best guess it is not over 10:1.

I don't believe it is a rod bearing. I think it may be excessive carbon build up in the combustion chambers so we'll see if the fuel additives help.

It may be piston slap. I have been told it could be stuck lifters but sounds low in the engine bay. I will check proper rocker gaps when replacing the plugs.

I will post results when I have run the chemicals thru and installed the new plugs. Perhaps jet size changes to lean mixture?

Any other suggestions?
 
What is your basis tune up? More information would be helpful.
 
Can you post a video of the sound? Otherwise, we're just guessing.

Have you checked your header and exhaust flange gaskets to ensure you don't have a leak? If the sound goes away once it's warmed up, something must be expanding/contracting.
 
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Had a Chevy 327 that did that. It was a wrist pin slap. Only when cold.Dang thing ran for years till the car rusted away.
Just a thought.
 
If it sounds like all eight knocking like a diesel, it's piston slap. If it has forged pistons in it, that's all it is and it's normal. All modern forged pistons and some high end modern cast pistons are "cam ground". That means at room temperature they are not round, but oval. They have excessive clearance when cold, so they rattle. Once warmed up, they expand and get round and the clearances get right and the sound either diminishes greatly, or goes away. You're probably spending money for nothing.
 
Done Swimmin said:
Hey All
I have a knock that is only heard on start-up when cooled off or not run for a few hours. it goes away at higher RPM or as soon as the engine is warmed up.

Oil pressure is good (near 80 lbs at idle and over 40 when driving).
I checked the plugs and they are black with soot and wet signaling rich mixture and possibly too cold a plug.
I have added octane boost and it helps but not 100%.

Here's where i am right now;

I ordered a set of plugs one heat range hotter
I added a can of Seafoam treatment to my fuel to clean system
I backed off the timing and it helped a bit.
I am trying to nail down the CR but i didn't build the engine. I have bore, stroke, gasket compressed thickness, head volume, flat-top pistons, so best guess it is not over 10:1.

I don't believe it is a rod bearing. I think it may be excessive carbon build up in the combustion chambers so we'll see if the fuel additives help.

It may be piston slap. I have been told it could be stuck lifters but sounds low in the engine bay. I will check proper rocker gaps when replacing the plugs.

I will post results when I have run the chemicals thru and installed the new plugs. Perhaps jet size changes to lean mixture?

Any other suggestions?
Click to expand...

Loose converter bolts often sound just like a knock, and can also quiet down when warmed up.
The sound can also go away under load.
It would be worth checking.
 
Done Swimmin said:
Timing was 28-30 degrees and now is 20 or so
Click to expand...
So your initial timing is 20?

I know it can be tuned to run there but if you have stock ignition components that might be 10 to 15 too much.

My stock 67 273 is designed for 5 after (CAP) If I was to advance it to 20 before I would end up with over 70 with vacuume advance and around 55 without it.
 
Most likely piston slap.

Wrist pin noise [ worn pin &/or pin bore ] gives a double tap.

As long as the rest of the ign curve is altered to suit, running 20* initial is a good idea. Engine will run cooler, use less fuel & have better tip in response. The GM LS engines have a very good combustion chamber, only need about 27-28* @ WOT. They idle with 22*.....
 
Bewy said:
Most likely piston slap. Maybe--OP hasn't provided a vid or audio example.

Wrist pin noise [ worn pin &/or pin bore ] gives a double tap. <This noise won't go away when warmed up

As long as the rest of the ign curve is altered to suit, running 20* initial is a good idea. Engine will run cooler, use less fuel & have better tip in response. The GM LS engines have a very good combustion chamber, only need about 27-28* @ WOT. They idle with 22*.....
Click to expand...
J.Rob
 
Well I guess well never know unless the OP tears the entire drivetrain down and has it all laid out piece by piece on the bench... :lol:
 
OP needs to start with the basics to eliminate what he can from the outside before tearing into the engine.
 
RustyRatRod said:
If it sounds like all eight knocking like a diesel, it's piston slap. If it has forged pistons in it, that's all it is and it's normal. All modern forged pistons and some high end modern cast pistons are "cam ground". That means at room temperature they are not round, but oval. They have excessive clearance when cold, so they rattle. Once warmed up, they expand and get round and the clearances get right and the sound either diminishes greatly, or goes away. You're probably spending money for nothing.
Click to expand...
Very true assessment here. I would add that due to bore size/fitment differences, you can also have this sound on 1 or any number of cyls as well.

Been to too many "listening parties" for this type of concern as we assessed various piston profiles for our OEM engines. Pin knock takes longer to warm up/go away than skirt slap but they can sound similar.
 
Support the "Piston Slap" comments. I built a 383 with forged pistons and set-up "Loose" for street/race application. Had to walk away on initial start-up and drink a cup of coffee. After operating temp no knock. This engine had 40,000 miles befor a tear down inspection. No issues found with pistons or cylinders.
 
Racer359 said:
Support the "Piston Slap" comments. I built a 383 with forged pistons and set-up "Loose" for street/race application. Had to walk away on initial start-up and drink a cup of coffee. After operating temp no knock. This engine had 40,000 miles befor a tear down inspection. No issues found with pistons or cylinders.
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My brothers 69 Camaro 402 was the same way. He would say "Build them loose,they will take the revs"
 
Demonracer said:
OP needs to start with the basics to eliminate what he can from the outside before tearing into the engine.
Click to expand...
I talked myself into building a hotter 318 in my truck because of a knock just to find a cracked flex plate when swapping the two engines. It needed replacing anyway, is what I told my wife.
 
go-fish said:
I talked myself into building a hotter 318 in my truck because of a knock just to find a cracked flex plate when swapping the two engines. It needed replacing anyway, is what I told my wife.
Click to expand...
yep, in my early years i rebuilt the heads on the 318 in my challenger to cure a misfire. then i fitted a new distributor cap and rotor arm to actually 'fix' the problem :lol:
neil.
 
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