Originally Posted by Box

I'm not meaning to sound rude, but you do not understand volumetric efficiency, or combustion pressure. The reason the supercharged engine produces roughly 100 more horsepower than NA engines is due to increased combustion pressure due to volumetric efficiency, even though the compression ratio is less in the SC engine. Basically, the more air/fuel mixture in the chamber, the higher the combustion pressure.

From NGK, "As an example, when you raise compression or add forced induction (a turbo system, nitrous or supercharger kit) you must reduce the gap (about .004" for every 50 hp you add)."

Concerning your information on the default gap, NGK tells you when the question is asked if you should change the gap of a newly purchased plug;

"A spark plug part number might fit hundreds of different engines from many different manufacturers. Although the NGK factory will set the gap to a preselected setting, this may not be the right gap for your particular engine. The incorrect plug gap for your engine can contribute to a high rate of misfires, loss of power, plug fouling, poor fuel economy and accelerated plug wear. It is always best to check the gap against the manufacturer's specifications. If adjusting the gap on fine wire or precious metal plugs such as platinum or iridium, be very careful not to apply any pressure or prying force to the fine wire center electrode or insulator as they can be damaged. The gap should be adjusted by only moving the ground electrode."

The correct gap for the AJ33 is what Jaguar specified, (0.035~0.039) not the plug's default shipped gap of 1.1mm (0.043)

Originally Posted by rf69

As I have been informing of my spark plugs replacing (V8 2004 Jaguar XJ8, with 126,500 miles) OEM from Factory NGK Iridium, I drove the Jag to Texas from Florida (1,120 miles one way from where I live ) and no issues at all so far, I don't notice any changes in power or driving since I replaced the old 8 NGK iridium OEM from factory in which the gap was 0.043-0.044 when I took them out. I'm wondering if I did right in gaping the new ones at 0.040 as Per the specs from The OEM NGK for the 04-09 XJ8 states that's the gap ( which only 2 out of 8 were with the 0.040 gap from factory) I did gap the other ones (6) that were at 0.045. My question is if there is a range from 0.040 to 0.45 ?
Or if I should have installed the new ones at the same gap that the old ones came out with ? Some one said in this forum that the correct gap was to gap the new ones to be installed at the same gap that the old ones come out with, So I am kind of confused now because the old ones never misfired or gave me any issues, I just replaced then because the specs from factory states to be changed at 100k miles.