Hey fellas, i'm having a hard time gettin hold of high octain fuel (95 and above) where i am.. i was using octain additives but finished the supply i had and they are being ordered at the moment.

I had no choice but to fill up with the available low octain fuel we have here (in yemen) and by the coastal towns, there's a knockin weird noise when i rev the engine.. the noise isn't there when the car is in motion. Any idea what the noise is and how it affects the car?

I read in the manual somethin about a 'knock sensor' when usin low octain fuel but i'm not sure if this is it nor not...

p.s. the noise goes away when i use octain additives

 

Hey, I don't think low octane is the reason for those sounds since we are using 87 - 93 octane fuel here. Octane Knock Control will adjust the amount of fuel needed for proper combustion in the engine.

 

how about if you use even lower octain levels? i'm not sure what it is here but i know it's pretty low

 

First of all, it's written as "octane", not octain, for your information

I don't think a car would agree to run on octane levels less than 87, so, you better go to your dealer for an inspection, maybe bad coils?

 

This is a fact that if you put low grade fuel in porsche or BMW mercedes etc. especialy in newer motors you will get a knocking. Switch back to the right **** it will go away.

I know from experience

 

yeah but what causes the knocking sound? like the mechanics behind it.. anyone know?

 

I'm no mechanic here but I'm sure it has to do with lower grade fuel being not as consistant? Or the obviouse that higher octane helps the engine run more efficent.. Like puttin water in the tank if its not used to running that way

 

Agreed - even my 96 daily driver's engine computer is smart enough to adjust it's compression and timing when I'm being cheap and putting low octane fuel in the car.

I think you may have other issues than just having a low octane fuel. We don't get anything more than 91 out here in California and my 911 runs fine.

 

If the octane is really low then it could be pre-detonation also known as "knocking" or "pinging".


In a typical modern high performance engine the compression ratio is high.

In such an engine low-octane gasoline will have a tendency to ignite on its own as it is being compressed in the compression stroke way before the spark plug actually fires. Thus it explodes out of synch with the engine cycle and produces that sound. If left to continue can potentially destroy the engine because it produces extremely high pressures inside the cylinders.

High octane gasoline has a lesser tendency to ignite on its own and thus can complete the compression stroke and wait for the spark plug to ignite it at the appropriate moment.

This is the same principle used in diesel engines which is why they do not need spark plugs.

 

right - but wouldn't modern day car engines be smart enough to also detect that and adjust accordingly?

 

Yes there is an anti-knock sensor on most engines nowadays that would tell the engine computer to adjust accordingly but I don't really know how effective those really work or if there is a limit to what the ECU can do to mitigate the situation.

We can only guess using the limited info from the OP. He is in a different country so I don't know how low the octane is or how different the gasoline blend is.

 

Hehe... maybe they "cut" their gasoline with mouthwash to save money

Then again, considering oil comes from where he lives, hard for me to believe they'd cheap out.

 

update: higher octane fuel has been around for a while now and even though it costs too much, its worth it because it sorted out 95% of the knocking noise problem also its increased throttle response and I can feel more power now so no complaints.

 

My wife's '07 BMW did the same thing if I didn't put 93 in it. But as soon as I refilled it with higher octane, the noise would go away.

 

This is a topic of much debate, research, and opinion. If you really want to learn more, search for pre-ignition, and detonation. Entire books have been written about the subject!

In a nutshell, the metalic clacks you hear are the more harmless variety of "improper" ignition (providing you have a computer that back back off timing each second as required). This is detonation and the sound is caused by multiple flame fronts. The flame fronts take place at "roughly" the correct time and sorta compliment each other.

Those typically not heard are detonation events and if bad enough or often enough, bend connecting rods, spins bearings, breaks rings, melts holes in pistons, etc. This is pre-ignition and the pre-ignition front is far earlier than is needed and this flame front forces the piston downward before it is ready. Think of a hammer hitting the piston when it is still on its way up in the compression stroke. Bad Bad Bad.

Maximum heat/pressure occurs at peak torque. Theoretically, after around 3800 rpm, the pistons are moving so fast and spark events taking place so quickly, that detonation pre-ignition is no longer a big concern.

Enjoy!