Spark Knock

[dean_acheson]

I run 91 / non-ethanol in my car, which is as good as I can find in my area. I seem to have a great deal of what sounds like spark knock under light throttle, which goes away at WOT. Anyone else notice this, have it? It's starting to bother me.

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[firestarter2]

Do you mean detonation? Might be piston slap. Do you have it all the time or only when cold

 

[stanglife]

I run 91 / non-ethanol in my car, which is as good as I can find in my area. I seem to have a great deal of what sounds like spark knock under light throttle, which goes away at WOT. Anyone else notice this, have it? It's starting to bother me.

You're describing piston slap, to a T. No one likes it but it's pretty common and Ford says it's normal.

 

[dean_acheson]

Under real light throttle? It's pretty noticeable and goes away?

I guess I should see if it goes away after it gets to operating temp? It's not like there are a lot of 350s around to compare it with!

 

[NipSC4328]

91 is the best we can get here and I've never heard a hint of detonation in mine. I'm nearing my first 1K on mileage and couldn't be happier, no worrisome noises and no oil usage. If I had to complain about something it would be panel fitment, but mine's probably better than most.

 

[Hack]

Spark knock is generally most exaggerated at mid to full throttle and lower rpms and higher temperatures with load. Such as going up a steep hill.

When the thottle is not very far open, air entering the combustion chamber is limited, so the effective compression ratio is reduced. That's why spark knock generally doesn't happen under light throttle openings.

I'm not sure what the sound is you're hearing. I know some people swear they have piston slap, I think it might be associated with the cam chains or valve train.. I do know that the sound I hear from the Voodoo with light throttle and lower RPMs is similar to the sound the Coyote makes as well as Ford's 3.5L TiVCT V6. I also believe that the sound I heard when I first bought the car is almost identical to the sound it made this morning with almost 12,000 miles on it. In other words, I don't think it is associated with a problem that will cause premature failure.

 

[stanglife]

Well more than 50% of owners who say that they DO know what piston slap sounds like stated that they DO have it. Ford released a notice explaining that it's pistonslap and that it's normal. I'd say the evidence is overwhelming at this point.

If the sound diminishes greatly or goes away once the oil temp is 200+, then that's how piston slap usually presents itself.

Hack - if I'm ever in your neck of the woods - you'll have to let me hear your car. Beers on me if it doesn't have it. Tequila is on you if it does, lol.

 

[dean_acheson]

After reading the other posting on piston slap- I'd say that's what I have. It just sounds different than what I had in my Boss.

 

[stanglife]

I'd be curious to see the internals of a voodoo after 5-10k miles but for now, just living with it.

 

[Hack]

If the sound diminishes greatly or goes away once the oil temp is 200+, then that's how piston slap usually presents itself.

However, just because the sound decreases doesn't 100% mean it has to be piston slap.

Hack - if I'm ever in your neck of the woods - you'll have to let me hear your car. Beers on me if it doesn't have it. Tequila is on you if it does, lol.

You can definitely hear my car, and I don't mind buying you a drink. I think the likelihood is high that you will say mine has piston slap. I would probably say that yours doesn't. :cheers:

 

[Agent_S550]

Definitely piston slap. Light throttle application usually brings it out. Since the speed of piston is slower at the lower rpm range, it has more hang time to wobble around and make noise. Plus, with the introduction of a lower rpm throttle application you will ultimately increase load. Load is directly related to piston slap. In the upper RPM range you're more apt to get spark knock due to heat, compression, and the speed of the combustion cycle.

 

[stanglife]

However, just because the sound decreases doesn't 100% mean it has to be piston slap.


You can definitely hear my car, and I don't mind buying you a drink. I think the likelihood is high that you will say mine has piston slap. I would probably say that yours doesn't. :cheers:

Lol fair enough!

 

[Hack]

In the upper RPM range you're more apt to get spark knock due to heat, compression, and the speed of the combustion cycle.

Spark knock is less likely to occur as RPM increases. The reason why is that the spark plug will fire earlier the higher RPM the engine is running. This is because the speed of the flame front is the limiting factor for pulling work from the F/A mix.

Spark knock is when the mixture ignites prior to the spark plug firing. This is more likely to occur when the spark plug fires later (at low RPM). At low RPM the F/A mixture also resides in the hot combustion chamber for a longer time.

 

[crs2879]

Plus, your car has knock sensors so in order for you to hear actual "knock", I would imagine the fuel would have to be so bad, or some other abnormal condition present within the motor, that the ECU couldn't retard the timing enough to stop it...but I'm no combustion scientist....

 

[NipSC4328]

Back in the days before ECUs existed when we built or modified an engine we couldn't just set the timing with a strobe light on the harmonic balancer. We would advance the timing (by rotating the distributor) until the engine started to knock and then retard the timing until the knock disappeared. Then we would road test the engine since it was more prone to knock when being lugged, and adjust accordingly. Hell, I've seen a guy ride sitting on the air cleaner adjusting the timing during the test drive, lol. Damn I'm getting old! Anyone else here ever roll in a set of main bearings without pulling the engine?

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