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Engine sound vs combustion health #5456097 06/17/20 01:41 PM
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keantoken Offline OP
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I've changed spark plugs on a few different cars and I've noticed the idle engine noise changes.

For instance on an Acura 3.2L J32A1 engine, The exhaust was boomy and the valve ticking was clearly audible. I did nothing but change the spark plugs, and now the exhaust is no longer boomy and weirdly, the valve noise is much less. And there is no longer the rapid tapping/spitting noise of the valve heads themselves. I've always wondered what causes this tapping/spitting noise to come and go.

Mainly what I wanted to know is whether you want to go from boomy exhaust to quieter whooshing noise, or the opposite. I've heard a very bad spark plug choice will give a sort of jet engine sound, but that was for racing and perhaps not applicable here.

In one vehicle I noticed the change in exhaust sound, and checked the spark advance. There was no change in the spark advance.

My theory at the moment is that with bad combustion, the fuel burns slowly and for longer, so the cylinder pressure is higher when the exhaust valve opens. Hence the spitting noise and increase exhaust pulse noise. But maybe this is completely backward. Maybe slower burning fuel will always have a lower cylinder pressure when the exhaust valve opens. But then again maybe the booming exhaust comes from the unburnt fuel hitting the cat?

Re: Engine sound vs combustion health [Re: keantoken] #5456101 06/17/20 01:47 PM
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ragtoplvr Online Content
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One of the reasons Harley Davidson motorcycles have a the exhaust note they have is the lower flow of the 2 valve heads. The more modern 4 valve heads have a different, more staccato sound. IMHO the 2 valve heads sound better.

Rod

Last edited by ragtoplvr; 06/17/20 01:47 PM.
Re: Engine sound vs combustion health [Re: keantoken] #5456104 06/17/20 02:00 PM
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The_Nuke Offline
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I think it’s your mind playing tricks on you. If there is no mechanical change other the plugs, valve train noise shouldn’t suddenly change, nor should exhaust note.

Record some audio of the before and after for us to hear if you really think it’s happening.

Originally Posted by keantoken
I've changed spark plugs on a few different cars and I've noticed the idle engine noise changes.

For instance on an Acura 3.2L J32A1 engine, The exhaust was boomy and the valve ticking was clearly audible. I did nothing but change the spark plugs, and now the exhaust is no longer boomy and weirdly, the valve noise is much less. And there is no longer the rapid tapping/spitting noise of the valve heads themselves. I've always wondered what causes this tapping/spitting noise to come and go.

Mainly what I wanted to know is whether you want to go from boomy exhaust to quieter whooshing noise, or the opposite. I've heard a very bad spark plug choice will give a sort of jet engine sound, but that was for racing and perhaps not applicable here.

In one vehicle I noticed the change in exhaust sound, and checked the spark advance. There was no change in the spark advance.

My theory at the moment is that with bad combustion, the fuel burns slowly and for longer, so the cylinder pressure is higher when the exhaust valve opens. Hence the spitting noise and increase exhaust pulse noise. But maybe this is completely backward. Maybe slower burning fuel will always have a lower cylinder pressure when the exhaust valve opens. But then again maybe the booming exhaust comes from the unburnt fuel hitting the cat?


2012 Charger Pursuit aka "The Bacon Hauler"
- 5.7L + NAG1 (A5) + 3.07 (215mm)
Re: Engine sound vs combustion health [Re: keantoken] #5456197 06/17/20 03:48 PM
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Barkleymut Online Content
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I have definetly heard a difference in valve ticking before and after. On one car it had a very noticeable tick and after I installed plugs it was completely gone. No video needed.


09 Vue 2.4L 4spd AT/ 09 Flex 3.5L / 96 F150 5.0 5speed / 07 Mustang GT auto
Re: Engine sound vs combustion health [Re: keantoken] #5456478 06/17/20 11:12 PM
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keantoken Offline OP
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Yes, it wasn't subtle. Approaching the car with the hood open, the first thing you would hear would be the valvetrain noise. Now you have to listen for it. Human ears aren't good at discerning absolute levels but when one sound is louder relative to another you're much more likely to hear a difference. In this case the valves became quieter than the whoosh of the engine. Frankly I don't know why an engine makes a whooshing noise, I would think that would come from the air box and not from the engine itself. Another one of those mysteries. Coolant noise?

The engine was running inconsistently after all. Not total misfires but some cylinders were randomly louder, not following a rhythmic pattern.

As for the valve tapping/spitting noise, you can verify what this is by sticking the stethoscope as deep into the plenum as you can go, close to the valves. What you hear through the stethoscope will be the same as what you hear outside the engine, if it is audible. Some engines have it and others don't. The same engine has it sometimes and sometimes it doesn't. One could be forgiven for thinking it might be useful to know why.

Re: Engine sound vs combustion health [Re: keantoken] #5462963 06/26/20 01:53 AM
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keantoken Offline OP
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After thinking about this, I think the valve noise change was mainly from temperature differences. I still want to investigate the exhaust noise vs combustion efficiency angle.

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