VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Compression ratio vs Spark plug heat range

Hi,
Does anyone have any thoughts/ideas on the relationship between engine compression ratio,and the required heat range of spark plugs?

No I have not thought of the possible relationship of Compression ratio to plug heat range, but I will now! I usually go with the GM reccomended plugs, well with the '28 I use a Champion plug with the correct reach and heat range, since the only non Gm engines I have are B&S, and a Force outboard and a couple of Evinrudes, I also go with the manf. on those.

Well, I have been thinking about the relationship of compression ratio to sparkplug hear ranges. Since the heat range is based upon the rate a sparkplug tip will dissapate combustion heat and a cold (low range) will dissapate the heat faster, resulting in a cooler tip, unless the plug is remaining hot long enough to cause pre-ignition I really don't right away see any relationship that would effect performance or plug life, it is still something that would be good to know for sure.

Generally speaking Chevrolet used a one step colder spark plug in the higher compression premium fuel engines.The type of driving would determine the correct heat range more than the compression ratio.Colder plug for high speed driving - hotter for low speed.Also the correct air/fuel ratio would be important.

Gene, that seems a logical idea, to use a cooler plug in a high performance engine, more horsepower in the same C.I. engine would have to develop more heat per power stroke and hence would need to disapate more heat from the plugs even with premium fuel which at that time meant higher octane. I guess if I raise the compression ratio of the 28 engine to 5 or even 6, with a dome added to the original pistons or some F0RD Model A rods maybe I will have to go to maybe an 80 octane gasoline, and no more "white gas"? Ha!

"Listen to the roar of that Chevy four!"

Hi Mr Mack & Chevgene,
Thank's very much for your replies to my topic.I mainly raised the idea to see how the relationship between compression ratio and spark plug heat range would dictate the selection of the correct spark plugs.
I realise that the forum topic of the right plugs for a 4 cyl Chev has probably been flogged to death by now,but any fresh ideas are always welcome.
I've raised the compression ratio in my '28 truck engine to about 6:1,and have had a set of
AC 18 spark plugs in it for many years.A lot of the "experts?" tell me that these plugs are in too high a heat range(they're supposed to be for agricultural or stationary engine use)and that it will pre-ignite and cause missing,but to date,no problems.
Anyway,thank's for your ideas,much appreciated.

WIZARDSMITH278 I use a Champion that has the correct reach and it also is an industrial plug. I haven't had any pre-ignition with unleaded gas. I think that hot spots caused by carbon deposits in the chamber (head) are the most usual cause for pre-ignition in the old fours.

What method did you use to raize the compression ratio on your 28? With the way the 28 combustion chamber is made, I think that the sparkplug reach and gap are what counts, just my opinion. Any opinion from you?

Hi MrMack,
The way I used to raise the compression in the truck motor was to TIG weld a prepared disc of 5mm aluminium plate to the top of each piston.
The preparation cosisted of a large chamfered hole in the centre of the disc,and a large chamfered edge on the outside diameter.
Then the disc was clamped to each piston,and the disc totally welded to the crown of the piston in the centre,and fully welded around the the top edge of each piston.After cooling,the piston's were shot peened to counteract any distortion due to the heat from the TIG welding process.
"ozjim",a mate of mine out here,tells me the standard "combustion chamber" depth is about
1 1/16" deep,whereas,mine is only 3/4" of an inch deep.This, apart from raising the compression ratio,has made the engine much more responsive,and more importantly,more fuel efficient.Where I could only get at best 17 -18mpg before,I can now average around 22-25mpg.
This may seem to be a lot of work,but combined with changing the diff ratio to 4.166:1,I now have a truck that I can comfortably use on long tours,without getting left behind,as used to happen to me all the time.The old truck can't outrun a good tourer or roadster,but at least I can keep up with anyone I travel with.