Ok here we go, ive been looking for about an hour now and haven't found a answer that I'm comfortable with.

So I have a mildly built 351c on a 125 shot. I am looking for the correct spark plug before I spray the car. This thread is about SPARK PLUGS only, I have the car set up for nitrous, (timing, fuel pressure, etc...)


Im currently running an E3.42 plug, which cross references to an autolite ap25 and ap26 plug. Now if I am correct, the ap25 is a stock heat range plug so I should be going one step colder. Before you go turning this into an E3 thread also know ive been running them for 3yrs on just motor and wont run anything else, and I know how to read plugs and have compared many different plugs against them in my car.

So two main questions, do I need to run a colder plug on just a 125 shot, because I have seen people not?

Also is the ap25 a stock heat range plug, if so what is a step colder in autolite, ngk, or any other that I can use to cross reference.

Thanks in advance and hoping some people who have been spraying for awhile will chime in.

 

one step colder than a stock 4 heat range is better. the charts suck because you can not have one heat range match two other heat ranges so who knows what your heat range actually is compared to stock.

the bottle increases your compression. when you have over around 10:1 compression you should go 1 step colder.

this will reduce cylinder and plug tip temps by around 180 - 200 deg f.

 

Autolite 3924 is the common plug for 14mm SBF heads on a n/a build. So 1 heat range colder on the equivalent plug for the heads you have.

Head manufacturers typically advise a heat range 4 on n/a engines because they tend to run higher cylinder temps and compression than stock. So a 3 on 125 shot should be fine.

It's better to start off 1 heat range colder than what you think, rather than 1 higher. A colder range will just foul the plugs; too high can cause preignition, which will destroy the engine on nitrous for sure.