Recently in Egypt, gas prices went up by a lot. We have three octane ratings here: 95, 92, and 80 RON. (I am not sure how 80 is so low; maybe it has lead in it?)

Most of the cars here are supposed to use 92, but after the prices went up, a lot of people started to use 80 RON instead.

I have heard to prevent engine knocking when using the lower octane fuel, people retard the timing. They do this by either increasing the electrode gap in the spark plugs or by rotating the distributor in older cars.

Now, my questions are:

  • Could the increase in the electrode gap in the spark plug cause wear on the ignition coil/charge distributor?
  • Will retarding the timing cause any damage to the internals of the engine?
  • Doesn't the engine knock because the lower grade octane fuel ignite before the piston reaches TDC?
  • How does retarding the timing solve this issue? Shouldn't it be the opposite?
  • I have done that many times, since almost all the cars I have owned are carburatted and they work fine with lower octane gas. They all came to work with 92, but I ran then just fine with 80 by retarding the timing a bit. Curiously, then they would drink better gas much faster then :) No engine damage, maybe a bit of power is lost but almost undetectable with normal load. However if your car as a computer that controls the spark, it may be impossible to do since the computer will always adjust the spark timing at its own will :) Nov 15, 2016 at 3:24

2 Answers 2


Could the increase in the electrode gap in the spark plug cause wear on the ignition coil/charge distributor?

Increasing the gap on spark plugs does cause wear on the all the parts you mention. I wrote this answer some time ago which I believe answers this question.

Will retarding the timing cause any damage to the internals of the engine?

In and of itself, no, other than the possibility of carbon buildup due to reduced burn efficiency. This isn't really damage, though. It can be cleaned out by any of several methods.

Doesn't the engine knock because the lower grade octane fuel ignite before the piston reaches TDC how does retarding the timing solve this issue, shouldn't it be the opposite?

Knock is pre-ignition. This is the air/fuel mixture combusting before its time which causes the burning fuel wave front to hit the top of the piston as the piston is still traveling up the cylinder towards top dead center (TDC). Pre-ignition in most cases is caused by hot spots in the cylinder causing the dastardly deed. This is quite often carbon build-up which holds enough heat during combustion, carries it over to the next compression cycle.

With lower or higher octane fuel, as the octane number increases, it becomes harder and harder to burn. This helps prevent pre-ignition.

Retarding the timing is moving timing closer to TDC towards the end of the compression cycle. Advancing the timing is moving back from TDC, earlier in the compression cycle. Due to the length of time it takes the air/fuel to burn after it's ignited, it is ignited prior to TDC, giving it time to burn and gain peak combustion, which then forces the piston down. When pre-ignition occurs, there are two colliding wave fronts plus ignition happening too soon, causing all sorts of little nastiness. This also tries to stop the piston from traveling up in the cylinder the rest of the way, which causing the knocking sound you hear. This can cause damage to an engine. With today's extensive use of hyperuetectic pistons, shattering them is not out of the question if the pre-ignition is severe enough.

You may have heard the term of "pulling timing" in modern fuel injected cars. This is done when the knock sensor(s) hear the ping from pre-ignition. Pulling timing is in actuality retarding the timing. In doing so, the engine is firing the air fuel mixture later which can help prevent pre-ignition, but only to an extent. If, as I've stated above, the pre-ignition is caused from a hotspot, you can pull as much timing out as you want to and the engine will still have issues (this is where dieseling a gasoline engine can happen). Dieseling is where an engine will continue to run on, even after it is shut off. Dieseling only occurs with carbureted engines, as when a fuel injected engine is shut off, so are fuel injectors, which is the source of fuel. Carbs will continue to flow fuel as long as there is air being introduced through the venturis and there is gas to be given in the float bowls.

  • So if I understood your answer correctly, by having the mixture combust later it decreases the hot spots in the cylinder thus reducing/eliminating pre-ignition? does not that seem a little bit counter intuitive though, we want the lower rating octane fuel to compress less than the recommended grade, I guess hot spots are a much bigger contributor to pre-ignition then?
    – method
    Nov 15, 2016 at 2:32
  • 1
    Increasing spark gap has NO effect on ignition timing. It might create misfire. I wouldn't advise it unless you know you have an ignition system ready to cross that gap. Unless you feel that misfire represents a change in ignition timing... The "beauty" of lower octane fuel is that it actually has MORE power (BTU content). High octane (RON) fuel might resist predetonation, but the crazy fact is lower octane fuel has more BTU content. Retarding timing might be fine, but messing with spark gap is just silly.
    – SteveRacer
    Nov 15, 2016 at 3:32
  • @method - I didn't suggest that. I suggested having the spark later causes ignition later, which then allows the piston to get to TDC before the event occurs, thus reducing the possibility of ping/knock. Hot spots can be a HUGE contributor to pre-ignition. If no hotspots, the likelihood goes way down. Lower octane fuel just adds a whole other dimension to it. Nov 15, 2016 at 10:48
  • 1
    @Pᴀᴜʟsᴛᴇʀ2 I put that in the wrong place; it was addressed to the OP not specifically to you or your response. I can't fathom how electrode gap would affect timing, unless it was a misfire. The speed of electricity approaching c is not comprimised by a tenth of an inch of extra "gap".
    – SteveRacer
    Nov 16, 2016 at 4:14
  • 1
    @method Sorry, I assumed "people retard the timing. They do this by either increasing the electrode gap..." was part of your post. I stand firm in debunking such a pile of wombat dandruff...
    – SteveRacer
    Nov 16, 2016 at 4:25

I am not sure about increasing the electrode gap but the rest I can answer.

Retarding the timing should not cause any damage to the internals of the engine. it is basically making the spark happen later in the compression cycle while advancing makes the spark happen earlier in the compression cycle. If your car has an electronically controlled ignition it may not be possible to do this.

The knock is caused by the expanding gas hitting the top of the piston while the piston is still rising through the compression stroke. Lower grade octane burns faster which is why you need to retard your timing to avoid this. As said above retarding brings the spark closer to the end of the compression cycle giving the expanding gas less time to hit the top of the piston.

The knock itself is caused when a separate explosion of fuel charge happens. Usually due to the expanding wave of exploding gas squeezing a second bit of not yet ignited fuel charge. This extra pressure causes this second explosion in the not ignited pocket of fuel charge. Slowing down the wave by using higher octane, or changing the timing so there is less time till the piston starts moving back down, will fix this problem. This can also be caused by hotspots in the engine.

here is an excellent explanation of detonation vs preignition.

in your comment the second part is closer to the truth. basically the wave of burning fuel charge will hit the piston pushing it backwards causing a ping / knock sound. Most often this is accompanied by extra fuel burning due to heat and pressure outside of this wave which causes an extra buildup of pressure.

also the mixture is always ignited before TDC as the shockwave of the burning fuel expands it will take a little bit of time. During this time the piston is still travelling up. by the time the pressure wave of the explosion starts hitting the top of the piston the crank should be just past TDC.


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