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The curious case of petrol and sound

Off Topic Discussion

If it's not really about tuning or wiring. Then it belongs in here.

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Hi, I recently changed fuel fro ma 95 octane to a 98 octane to help control knock, and to be sure it was not a false knock. And that is all good.

There is a side effect which is actually quite welcome, the car makes less exhaust noise, especially when not under much load but even when accelerating it is noticeably quieter.

I am hoping someone can explain why the fuel change has had this effect as I would like to understand a bit of what is going on


Slightly slower flame speed and possibly a touch more total mass if fueling wasn't tweaked with the change? Might be enough to take the edge off?

has the logged spark timing changed between fuels?

Slides, more mass of air? or fuel? The tune wasn't tweaked. So if it is more fuel, slightly richer = not all fuel burning = quiter engine? If that is the case then I could make a neighbor friendly morning map :)

Raymond , I will have a look tonight after work.

Higher octane hydrocarbon fuel is usually a touch denser as it has cyclics and stuff added in.

Raymond, yes the ignition timing has changed, firstly there was a lot more knock, so when accelerating the previous timing was less advanced. But while cruising or mild accelerating the timing was pretty much the same as now, so I don't think thats the secret to noise control.

have the variable valve timing angles changed?

No, nothing was changed in the tuning, just the petrol being used.

So you didn't change the maps. Did you log before and after to see if the logged variable valve timing angle has changed as an effect of the changed octane? Engines can tolerate more intake cam advance with higher octane.

Yes, I didn't change the maps, just went to a different pump and the logged again to see if the knock sensors detected more or less etc, adn they did so that was a confirmation to me that it is genuine knock they are picking up. Yes, the timing is now about 4 or 5 degrees more advanced under load at high RPM.

But the thing I am curious about is the car is now a lot quieter during a cold start, and at idle when cold or warm, and at low loads and low RPM (less than 2500rpm)

I am also suspecting my reaping of the MAF sensor is not upto scratch yet, and that maybe it was running a bit lean hence the knock. But I tweaked that, then when still knocking swapped the fuel.

If the logged variable valve timing changed (expected direction: more advance), it affects the overlap and the exhaust gas flows in the manifold, which can affect sound. If the spark timing changed (expected direction: more advance), the peak cylinder pressure and possibly the pressure rise rate can increase which can affect sound.

Has the ignition timing changed at the rev/load points youve noticed a change ? If so - To me this points to combustion being used more efficiently . Less energy/sound going out of the exhaust . Possible ??

Hi Ken, you might be onto something. I didn't really look before. but checking the logs now shows that the 95 octane idle timing as 13 degrees, and on the 98 it is 15 degrees.

I had assumed the ecu would only change the timing when it thinks it is detecting knock (at higher reves)