ddgonzal wrote:
Quote:

USA BPR5ES-11
Canada MPG BPR5ES-11

The only real difference between these two plugs is that one of them is factory gapped to 1.1mm, so it would be intended for engines with the high energy electronic ignition. Thats what the -11 means, .... 1.1mm gap.

I personally recommend either Platinum or Iridium plugs in the equivalent heat range.
These plugs use a much finer centre electrode & the expensive metals are used to prevent them being quickly eroded away by the combustion processes.

The advantage of the fine centre electrode is that it requires a lower voltage to fire across a given gap at a given compression pressure. This means that a larger gap can be used at the previous voltages & with a high output coil that .044" [1.1mm] gap can be opened out to somewhere much closer to .050" [1.25mm] This will start to push the [high energy] coil closer to its maximum output causing heat buildup, but as long as the coil gets some cool air flowing over it, then it will perform flawlessly.
Big gaps mean big sparks that will ignite amazingly lean mixtures.

There was some discussion about "squish" earlier, but no explanation as to what it was & why it is good, so here goes.

Back in the mid 1920's a very clever engineer by the name of [Mr.] Ricardo in the UK discovered that detonation was caused by a combustible charge of fuel actually exploding after being ignited by the plug that was usually placed well to the side of the combustion chamber.
Remember that back then he was dealing with side valve [L head or valves in block] engines. The flame front would burn across the mixture, creating a pressure wave in front of it that further compressed the unburned fuel charge until the pressure rose high enough for the remaining unburned fuel to spontaneously combust [explode, This is what caused the knock noise]
Remember also that the fuel back then was hardly better than refined kerosene so the octane rating was quite low.

He found that if he redesigned the head to cover about 2/3rds of the piston crown with a very small clearance between the piston & the head [gasket thickness only] then the charge that was being pushed upward by the piston would jet, or spurt, at very high speed into the actual combustion chamber which was located over the valves alongside the cylinder itself.

This very small space between the piston & the head was termed the 'squish' chamber because this is essentially what it did & the resultant spurt of gas caused a very high degree of 'swirl' in the combustion chamber. At this critical moment the plug fired the mixture & the resulting flame front was mixed into the fuel charge at high speed causing it to burn before it had a chance to detonate.

The net result was that compression ratios could be raised from about 3.5 or 4 to 1 up to as high as 5.5 to 1 & Chrysler Corp even raised this to about 6.5 to 1 when the optional alloy head was used in the mid 1930's
It wasn't untill 1955 that 'super' grade fuel was introduced into Australia & compression ratios could be increased even further. It is interesting to observe that the 1955 Dodge Hemi had a compression of 7,6 to 1 but the following year, 1956, saw this rise to 9.25 to 1.

The same process is still used today.
The piston 'squishes' some of the fuel charge into the combustion chamber causing swirl & this can help to prevent detonation in modern engines that have even higher compression but use the lower cost grades of fuel available today.

If you want to run big compression numbers in a street engine, then paying close attention to the squish areas of the combustion chamber will provide benefits & allow the maximum use of a little more compression & a little more spark advance, both of which will yield performance benefits from a given grade of fuel.