Is this engine knock ?

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mbaird

mbaird

mbaird
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I am working on getting the tune right on my 408 and every so often I get a whacking sound . Its as if someone rapped my aluminum intake with a small hammer. Only happens every 30 seconds or so unless I rev the engine ...then it happens every 10 seconds or so and is more frequent as engine get warmer. Could this be detonation ?
 
4 speed.
I will try to duplicate and video

This occured while setting my curve . I figured I might have had too much advance in under 2000 rpms . I now have my digital curve set .
If its detonation I can tune it out... but if its mechanical I have an issue.
You would think a guy that has been working on engines for 40 years would be familiar with knock/detonation ..lol
 
Possible I messed up and filled it with regular instead of premium .

408 with W2s , 9.8 comp 246/250 on a 108 cam. Shouldn't have an issue at lower rpms.
 
mbaird said:
Under load it does but what about freewheeling ? Like when I bring rpms up to check timing advance rate .
Click to expand...

I agree, should be under load unless your initial is whacked.
I'm thinking a valve snapping shut ?
without sound it's tuff
 
Dana said:
Pull the plugs, make sure nothing found it’s way into a cylinder.
Click to expand...

I had a baseplate screw on a Holley back out during breakin n an engine .
This sound is louder and less frequent .
Kind of a loud snap .
That baseplate screw would go in one cylinder then back up into the intake plenum in succession .
 
I had a screw stick in the top of a piston, then on cool down it would come out, and rattle on initial start up. Then get stuck during hard RPM. One in a while unloging itself. It will usually smash the electrode.
 
I doubt that it is detonation. I had a similar noise once on an engine that was running a 440 Mopar AVS carb. This model carb had the acc pump nozzle with brass inserts sticking out; one insert had gone into the engine & was getting hammered on the quench part of the piston. Just an example of 'odd' happenings....
 
I ran pump gas in my 12.2 to 1 motor on dragweek. Cast iron heads. Quench? None of that here! Thought I would need to adjust timing for the on road portion. Nope, I babied, I mean really drove it like it had an egg under the throttle. I would very gradually get to traffic speed. THE MOST BORING RIDE EVER!! No issues though. I think it would be hard to get it to knock without a load.
 
What’s the skirt clearance? Piston slap? Does heat effect the noise at all?
 
TT5.9mag said:
What’s the skirt clearance? Piston slap? Does heat effect the noise at all?
Click to expand...
It seems to happen after it warms up .
I got the advance map programmed in yesterday and did not notice it after so it may have been random detonation due to over advance and isolated hot spots . Although my chambers are polished so I shouldnt have hot spots.
 
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