Another interesting development. I just changed the spark plugs to NGK BPR6HS (apparently they are the same as the BP6HS, but with a resistor added -- I called NGK), which are the equivalents of the Boschs recommended for the B20B. Anyway, they are about 1/2 inch shorter than the ones the mechanic in Toronto had put in there last fall. All of a sudden, pretty much no ping at all.

Is it possible that the plugs which were in there, being too long (if they were) made the combustion chamber smaller, and so increased the compression, and so made the gas ignite earlier than it normally would with any given timing setting?

Greg

In addition to what Phil said, if your spark plug is much too long,
you have the effect of a much hotter plug because the threaded area
absorbs and retains heat rather than conducting it away to the head.
I'll bet the tips of the old plugs show signs of being pretty hot.
For example, carbon will not stick to a surface at over 700°F so if they
are sparkling clean they were at least that hot.

Don't ever hire a mechanic who doesn't know as much about your car as you do!
--
George Downs, Bartlesville, Oklahoma, Central US

Additionally, you have many many more sharp edges in the combustion chamber that give lots of nice places for heat to concentrate and cause pre-ignition.

Mike!

>Don't ever hire a mechanic who doesn't know as much about your car as you do!

Yes, I am starting to realise that. But at the time I was stranded in Toronto, 300 miles from home, and knew nothing about fixing cars. About half of the repairs I have had to do this spring and summer have been fixing the things which that guy in TO did, not having a clue about what he was doing. Oh well, $1000 poorer, but a little bit wiser.

Greg

I'd bet that there's a very high percentage of us here that had the same experience. Unless you're very fortunate to have a mechanic who really knows old Volvos inside and out -- and there are a few, but they're rare -- it's mandatory to learn to do your own work.

The good side of this is that 1) cars don't get much simpler to work on than old Volvos, and 2) you can get a lot of satisfaction from fixing the car right yourself.

"cars don't get much simpler to work on than old Volvos"

Some noteable exceptions being old VW Beetles and Citroen 2CV's. I think my riding mower is more complicated than they are!
--
I'm JohnMc, and I approved this message.

It's not the very slight increase in compression, but lighting off the charge in the wrong place that caused the pinging.