what heat range plugs should I use?
 

Based on my sig. below, what would be a good starting point for the heat range of spark plug to run.
I know every engine is different but I just want a good starting point and then I'll go from there after I read what the plugs indicate after awhile.

I think the old autolite # is/was 45. If all else fails just get plugs for 69 Mach I (351W 4bll) II'm asuming your using early windsor heads with the big plug.
Bill

Actually, the heads are '74. They are the 5/8 size(smaller plugs) and not the 13/16 size

Then a good starting point would be Autolite 25 which is stock heat range for 5.0. If you need to go colder then 24s I've always have had good luck with autolite but if your using another brand just cross the number.
Bill

It's not a 5.0 engine. It's worked over 351w

They both use the same plug size and to be on the safe side go with the 24s. If its to cold they will just load up. If you go to hot you run the chance of detonation

Well if #25 are for stock engines then a better starting point should be around #27-30 for what I have.
Remember, it's a 351w that's no where near stock.

You did help me though. Now I know where the base minimum starting point is. I think I'll start at #27 and work from there.

Sorrry I don't think I made it very clear. The higher the number the hotther the plug, lower colder. You want a colder plug not because of engine size but whats in it. You have higher compression and a hotter cam both will allow the clyinder to fill more. This usually means going to a colder (lower number) plug. You may even get away with using a 23. I hope this clears that up

I would recommend starting with 1 heat range colder than stock as well (24's). With your compression and iron heads you might still have to go one colder.

Ok, now I understand. I had my theory backwards.
Thanks