spark plug gap

[Goopie 18]

Roush 427R 10.25 cr. MSD 6AL control, MSD master blaster coil. New MSD cap/rotor. MSD suggests 0.050-0.060 for this compression ratio. Roush suggests middle 0.03's. Suggestions?

[Dwight]

I'm interested in the answer to this question.

Dwight

[Rwillia4]

+1 to an answer with reason.
and what heat range plug

[mreid]

When you are really trying to wringe every last HP out of an engine, spark plug indexing and gap tuning become important. However, you have to identify your ideal operating RPM and tune for that range. Frankly, for the variety of driving you are apt to do with your car, I would go with .045.

[Got the Bug]

I've got a 402 Roush with basically the same set-up. A gap of .040 - .045 was recommended to me for a good idle and power.

[RET_COP]

Why big gaps. They don't offer a whole lot and increase the chance for a misfire especially if the system is not up to firing across resistance. .035 to .040 will work, always has for me.

[mreid]

Well, because too small a gap on a higher compression ratio with a high energy ignition (we're not talking your old car points and coil with current resistor set up) will shorten plug life significantly. And I do mean significantly as the plug consumes itself, particularly with a multispark, high energy box on an engine that spends most of its time below 3000 rpm (point above where msd box reverts to one spark per plug per cycle). Conversely, too large a gap will cause misfires, particularly at higher rpms where the charge cycle is shorter causing less energy from the coil.