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Old 06-27-2005, 01:18 PM
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383 stroker: odd noise at cold start

Hey guys. First off, I have to say that this message board is the best. I was on thirdgen.org for a long time but this place seems to be where it’s at! I have done many searches on this but have not been able to rest easy yet…so here it goes…

I have this noise, it sounds like a slight “knock”, almost like a diesel engine, off idle during slight acceleration. It is not a rod knock, I have had those and I remember vividly the sound they made upon deceleration. It only seems to happen when the motor is cold. I first noticed it the day after it ran its best. I got 25mpg on the freeway and it ran like a total champ; I was so happy. That was with 15* initial + full vacuum (35* at idle) and the mechanical is set with the tightest spring for now. So the next day is when I noticed this noise. I backed the timing to 10* and kept my vacuum, thenI didn’t notice it. The next day it came back. I put timing back to 15 and stopped using the vac. It went away but came back the next day. It goes away after driving for about 10 mins. I can shut the car off and it will not come back if I start it again while still warm. It only seems to happen when cold. What scares me is that I do not know what detonation sounds like!! I know that I am knocking on its door (no pun ) with my 10:1 and iron heads, but with my quench and all I think I should be good.

Does NOT happen at idle. ONLY when in gear at tip-in under light acceleration.
Seems to go away after engine is warm.
Does not seem to care about timing
Running Sunoco 94.
Plugs are R42TS, no sign of detonation.

Engine specs: http://wan.kicks-***.org (replace *** with a.s.s., without the '.'s)

Any ideas?

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Last edited by IROC_385z; 06-27-2005 at 01:32 PM.
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Old 06-27-2005, 01:35 PM
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Forged pistons do this. They swell more than a standard piston and need some extra cylinder wall clearance...so they rock in the bores when cold. common
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Old 06-27-2005, 01:46 PM
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Thanks for you quick reply.

How does this affect cylider/piston/ring wear?

If I were to hear detonation vs this piston slap noise, would it be a safe assumption to say that 'i would KNOW it'?
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Old 06-28-2005, 05:19 PM
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I think that it is still making the sound when warm, just not as loud. It is no-where near as loud as the GM at pistonslappers.com, but it does sound like it.

I should just forget about the sound and accept it as a noisey engine?
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Old 06-28-2005, 08:18 PM
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Normally I would concur with NXS, forged pistons do make a lot more racket when they're cold. That's where my nickname came from.

The reason I'm concerned in your case is because of the other thread where you have inconsistent, and low, cylinder pressure readings. I think you may have installed, or your builder did, one or two out of spec pistons. That or you have one or two out of spec cylinders.

I could be very wrong about it but based on the info from both posts that's what it sounds like to me.

It's not a great big deal, it's just going to be noisy and it won't run a million miles but does any hotrod last forever?

Larry
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Old 06-28-2005, 10:24 PM
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35 degrees of timing at idle?
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Old 06-29-2005, 07:46 AM
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As a followup to my other thread, after adjusting all of the valves all cylinders are now within 3psi of each other. I am confident that everything is square as far as that is concerned.

My timing is only 15 at idle. I had the vacuum can connected to full vac and that was giving me about 35 at idle, then when I noticed the noise I pulled the vac off.

The other day I noticed that the sound lingered a bit past warming the motor. What other mechanical sounds would do such a thing? It is not a very pronounced sound, it is very light and you almost have to be listening for it to hear it. My girlfriend was able to hear it, but only after I pointed it out. The flexeplate and balancer/pully are all fine.
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Old 06-29-2005, 08:42 AM
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Put about two gallons of race gas in the tank and see if it goes away.
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Old 06-29-2005, 01:41 PM
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my brother many yrs ago has a 400 Pontiac that had a rattle at start up, he would punch it to the floor and let up and punch it to the floor till it quit. i though it would blow apart but never did. (it sounded like something internal was loose) he abused this 400 for 3yrs. then sold the car, the guy that bought it and tore it all down, it turned out it was the flex plate.




Mustangsaly
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Old 06-30-2005, 10:30 AM
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I am wondering if it may be the flexplate also. I have had busted ones before that sounded like a rod knock, and I had a loose one before that, if I remember correctly, did sound similar: but it also made the noise at idle. I am ganna tighten up the flex-to-converter bolts and see what happens.
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Old 06-30-2005, 05:07 PM
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This may sound dumb but sometimes the simplest things are overlooked. Very slight header or flange leak??
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Old 06-30-2005, 05:22 PM
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al37ford, that is definatly not a dumb question! Yes, I DO have header/flange leaks but unfortunatly that is not the sound. This is more of a dull sound, whereas the headers make that raspy 'tick tick' sound.

Thanks for you idea though!
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Old 06-30-2005, 06:11 PM
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What temp theromstat...? 180...? If so move it up to a 195 and see if it goes away completely at full operating temp...? This would most likely point to the forged pistons... If it for some reason increases it may be a spark knock...? More temp means you need less advance or more octane to controll the burning process...

One other thought... wrist pin...?
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Old 01-07-2007, 06:51 AM
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when I put my harmonic balancer on I noticed it hit the timing stuff on the cover,and it had a knocking noise.Found it with a stethiscope,I took the end off and you can pinpoint any noise.
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