Hi, all,

This question isn't about Corvettes specifically, but there are some pretty knowledgeable guys on this forum, so I thought I'd ask here. And be forewarned: it's a question about spark plugs, so it's gonna turn into a peeing contest sooner or later! OK, here goes.

I have been looking into spark plug heat ranges, and have noticed that before 1975 or so, the recommended spark plugs for chevy engines was the 44 heat range, 43 for the real high performance engines. (AC Delco plugs) After 1975 or so, the shop manuals say 45. My theory is that the low compression and relatively retarded ignition timing that the smog engines used to reduced peak combustion chamber temp. (in order to reduce emissions) made the move to a colder spark plug necessary. (EDIT: I meant to say hotter, sorry about any confusion.)

I have not increased the compression on my 1980 L48 engine, (yet!) but I have bumped the timing up. Initial timing 16*, 36* all in at 2800, 16* of vacuum advance on top of that. Also, Edelbrock Performer manifold and block-hugger headers with 2 1/2" dual exhaust and free flowing mufflers. (I plan to upgrade to Edelbrock E-Street heads and Stan's headers, eventually.) I would not be surprised to find out that my combustion chamber temp. is a bit higher than a stock engine. Would anybody care to comment on whether or not it would be worth my time and effort to try a colder plug? And what are some of the signs that a colder or hotter plug is called for, if any? Does the heat range of the plug have anything to do with the engines performance, or is it just the longevity of the plug? Feel free to add anything that you think is relevant, thanks,


Scott

 

(in order to reduce emissions) made the move to a colder spark plug necessary.


I think that you have it backwards. The general rule is to change a heat range colder for every 100 additional hp. You will be fine staying with the same heat range. When a plug is too hot you get motor run on after you shut the ignition because the tip is glowing. Another sign of too hot is electrode erosion

http://www.championsparkplugs.com/gl...t=char&imgID=8

 

Yes, yes, a 45 is a hotter plug than a 44, sorry, just a simple word mistake, but it does change the whole meaning of the sentence, doesn't it? Thanks for pointing that out.


Scott

 

Well I had to read what you said a couple of times to make sure I wasn't screwed up!

 

Based on what earlier engines had compression wise versus plug heat range, when you get to 11:1 and above, use 43, and between 10:1 and 10:75, use 44.

 

I have two solid roller 7500 rpm red line motors one is 11.2 and the other 11.7 compression. I run mid heat range Autolite Platinium or Bosche multi tipped platinium. I put in some kind of cold heat range racing champion plug and they fouled out in a short period of time. My motor cold ran terrible, but when warmed up and a few long Wide open blasts around the track would clear everything out and be fine. When I threw them away they looked black and wet.

There are alot more variables other than compression ratio and total HP

iron heads don't X-fer the heat like aluminum heads. Also a monster cooling system would requirer hotter plugs to keep from fouling out.

 

I do not disagree with what you are saying, i was simply pointing out what stock engines and cars had in them heat range wise versus compression as a guide line.

 

A 45 is a good plug for both pre- and post-HEI for normal street use.

 

For those of us old enough to remember these cars when they were new NOBODY ran 43s on street- far too cold. 44s or 45s were the 'hot pick'

So much for theory.

 

I believe that heat range fouling is also a likely condition of mixture richness.... so in the days when cars idlled rich, R43 was too cold a plug. FWIW, I used R43s in my stock '85 sbc for 15 years and never had fouling problems. My understanding is the heat range applies to WOT at peak rpm and is expected to be about 150*F per range, so no big difference at "normal" conditions (plugs a lot colder than at WOT). I had advanced timing set up on an LG4 set to pass emissions tests!