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Pre Ignition and Detonation / Pinging / Pinking / Knocking

addo

Contented Peugeot Driver
I thought Roger's comments deserved better than being buried in a post that many might gloss over on their search for meaning:
lhs2.1 said:
think some clarification is needed.
Firstly, as far as I'm aware there is no combustion phenomena known as "pre-detonation".
For a gasoline fuelled IC engine there are two separate abnormal combustion processes, which are separate but can be interlinked. These are "pre-ignition" and "detonation". The latter is also known as "knocking" or "pinging" or "pinking".
Pre-ignition is when ignition of the fuel-air mixture in the cylinder occurs before the spark plug event. This ignition can be initiated by a incorrect heat-range spark plug, or incandescant hot-spots in the combustion chamber.
Detonation is when there is spontaneous combustion of the the fuel-air mixture in the combustion chamber ahead of the flame front progressing from the spark plug ignition. When this happens there is a rapid rise in local pressure causing a shock wave in the combustion chamber which may be heard as a knocking sound.
Detonation can result from too much ignition advance. too low an octane rating of the fuel, too high a compression ratio, and from pre-ignition. It is influenced by manifold pressure ie engine load, and if detonation is allowed to persist, the shock waves can damage piston crowns and ring lands.
For the engines we're discussing here, pre-ignition shouldn't be a problem assuming that correct spark plugs are used, and that there haven't been running problems in the past which may have resulted in excessive deposits in the combustion chambers.
Octane ratings of gasolines are really indicators of the detonation resistance of the fuel. Even the un-leaded 91 octane crap that we are served up today is far superior (in detonation resistance) to the fuels available when the Dees were launched. Consequently, there is scope to advance the ignition timing beyond the original OEM recommendation.
However, setting the optimum ignition setting is a trial and error process and relies on the ability of the operator to hear when detonation is happening ie when to back off the advance. Some people are good at hearing "pinking", others aren't.
My advice is to set the static ignition timing to the manufacturer's recommendation, find a nice clear road, roll at 60/70 km/hr and floor the accelerator in a high gear and listen for a tinkeling sound. If no sound, tweak the ignition advance and repeat until you hear it. Then back off the static timing a degree or so.
In a previous life I spent some years testing fuel octane requirements of cars, including verification of this for pre-production for most of the major Oz manufacturers.
roger
Nothing to add myself other than opinion. :wink2:
 

Alain

Member
Pinging problems

If pinging persists after a distributor timing adjustment (with a timing light)and the fuel quality is known it is usually a mechanical issue most likely being the distributor itself.
As an example i purchased a Renault R20 years ago in WA with an inheritant ping. The seller (mechanic) said that the head had been skimmed quite a bit and that was how it is. Knowing the quality of WA water and the reluctance of some owners to service correctly that was plausible and i put up with it for some time.
Eventually pissed off with this, i pulled out the dissy to find one of the counter weight springs disconnected.
Needless to say the Renault was totally restored power and driveability wise.
A new or low wear (low mileage) dissy totally transforms older pre electronic vehicles and should be the first place to look when chasing performance and driveability issues with on road fine tuning to fuel to follow.

Alain
 

JohnW

Too many posts!
Which is precisely why I bought a 123 distributor for the CX (carbie type) and have a Hall Effect device in a near new distributor in the R8. I totally agree about getting the distributor really right first, in other words. I was talking recently to a guy with many years experience with Strombergs in his business and he reckoned that 95% of complaints went away when owners took their advice to fix the spark first and then come back to fix the carbie.

I have found with the 123 in our CX that the onset of pinking is quite sudden as the static advance is changed. Rotate it a mm or two in the advance direction and it goes from no pinking to noticeable pinking (front window down) on the hill up to our house.

Reading the excellent article (Thanks Jo) it seems I have never heard pre-ignition and what I am hearing as a tinkling noise is actually detonation. Very interesting topic.
 

tobytronic

New member
@lhs2.1
"It is influenced by manifold pressure ie engine load, and if detonation is allowed to persist, the shock waves can damage piston crowns and ring lands."

Simply put, I've read about this sort of stuff almost every day since 1974. Yes, I am sad, but don't care. One bit.
So, yes, a superb post. It wasn't until 2007 that I started to understand the MPA's role in 'pinking'. Please write some more. Thank you.
I've just realised that I have resurrected an old post? Sorry, I'm new...
 

Artificer

Member
http://www.aussiefrogs.com/forum/re...t-all-ducellier-distributors.html#post1610700

Check this out near the end.
I time all older carburetor vehicles with a vacuum gauge for great near perfect timing results when using modern day fuels.

Mechanics trained @ Carlisle TAFE College W.A. in the 1970's & probably years prior to then, were taught engine tuning & diagnosis using these inexpensive very handy tools.

Give it a go! Don't use any vacuum advance port unless it is full manifold vacuum & they usually are not.
 

Artificer

Member
The above link didn't seem to work but this one should:
The timing with a vacuum gauge is right near the end
 

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