Compression Resuls

The number is the heat rating. The higher the number the cooler the plug. So the fact that you are using 5EY's means that your engine will be running "SLIGHTLY" hotter than with a 6. It doesn't really matter unless it would be wildly out.

In message , gazzafield writes

You have just answered a question I was going to ask. My engine has been running a touch cooler than normal ever since I changed the plugs in April- it only reaches "normal" mark when driven in a more "enthusiastic" manner. The plugs I used (can't remember the exact ones) were the only ones listed for my car (1999 Fiat Bravo 1.2 16v) in the Halfords plug guide. I think a new set of *proper* plugs will be obtained soon...

Cheers,

This numbering system is for NGK plugs only. I'm not at all sure how other manufacturers do their heat ratings.

I disagree. I found out about the numbering system many years ago when I ran a Suzuki X7. Anyone familiar with this motorcycle will be aware that it had a penchant for grenading itself from time to time. This was due to it getting really rather warm when being thrashed as Suzuki never made the cooling fins as large as thermodynamics demanded to keep the weight low.

I fitted cooler plugs to mine and the period of detonations decreased slightly.

There's an irritating nomenclature goes on though. A plug that runs cooler will reduce detonation, but many manufacturers refer to a cooler plug as one for a cooler engine, presumably from the days when you fitted progressively "hotter" plugs to your race engine as it warmed up.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Duncan Wood" saying something like:

The nomenclature makes sense if you refer to the plugs only. A hotter plug has a longer nose so the combustion heat takes longer to travel up it and so the plug runs hotter. This is good for self-cleaning, especially in a loosely-built or worn engine, where oil deposits carbonising on the plug nose might otherwise cause a problem.

Go too hot in plug rating though, and the problem is creating a hotspot and detonation.

Plugs that are too cool (stubbier nose, and easy heat transfer) will tend to get fouled more easily.

And what is a Hot Spot not? A Good Spot! Alwight!

Yeah... although... Im not too bothered about that dropping. I mean, it was high to begin with, so it could just have been de-carboned.

What Im worried about though, is the drop in compression on cylinder 1 in relation to the rest of the cylinders.

Someone suggested that it might be because there's a problem with the cams... then obviously remembering back to what one of the garages told me about worn cams... might have a winner (I hope).

Cheers Simon