I've got a new Goodwrench 350, mild Crane cam, Performer intake, Q-jet in my '75. With the old stock motor I always ran AC R45TSX plugs with .060" gap (stock specs) and never had a problem, always ran strong. I'm still using the original stock HEI ,which has never let me down, on the new motor and am running AC R44TX plugs, also at .060 gap. This motor also runs like a champ, healthy and strong. GM only used the .060 gap in '75 for emissions and then went to .045 for HEI's after '76. Here's the question: I've always thought that the bigger the spark gap the more power; why did GM switch? Everyone on this site seems to say .040-.045" gaps are best for performance with HEI. Would I get better performance if I switch to a "smaller gap"?

Here's the question: I've always thought that the bigger the spark gap the more power; why did GM switch?

My guess is that they did it because it's harder on the coil and can cause an earlier failure rate.


Everyone on this site seems to say .040-.045" gaps are best for performance with HEI. Would I get better performance if I switch to a "smaller gap"?


It's possible if you have the stock coil.

On my friends Camaro bracket car we were hunting for ET and I opened up his plug gap from .045 to .060 and he dropped a full tenth. I really couldn't believe it did that.His ignition was all MSD with the 6A box.

I'm still using the original stock HEI ,which has never let me down, on the new motor and am running AC R44TX plugs, also at .060 gap.


I'd swap those out for a set of NGK V-Power YR55 plugs if you plan on staying with the .060 gap.
I didn't see one with a gap of .045 so I guess you could close the gap up to.045 on the YR55 or get a set of the YR5's and open then up fron .035 to .045.V-Power's are very good plugs and are very easy to read for heat range.

Whats the new engine's specs? The higher the compression the colder plug you should use. IMHO AC plugs are the best plug $$ for $$. Seeing how you have a stock coil I would gap those 44's between 35 and 40 thousands. Also keep in mind that the stock coil won't keep up above 5500 rpm (or there abouts) Aftermarket ignition is the very first upgrade you should do, it will help you 10 fold down the road with other add on's.

A bigger gap requires higher voltage.
If you haven't got the coil voltage to spare, a wider gap will hurt instead of help power.

You aren't going to hurt anything by trying a .045 gap on your plugs.

Whats the new engine's specs? The higher the compression the colder plug you should use. IMHO AC plugs are the best plug $$ for $$. Seeing how you have a stock coil I would gap those 44's between 35 and 40 thousands. Also keep in mind that the stock coil won't keep up above 5500 rpm (or there abouts) Aftermarket ignition is the very first upgrade you should do, it will help you 10 fold down the road with other add on's.

This car isn't going anywhere near 5000 rpm!.....4300 tops...it's not a racer....The motor is the universal Goodwrench, rated at 8.5 CR although I've read it's actually 8.4, cam specs 204/216, .427/.454 , stock coil. I've noticed that the GM recommended plug for '75 was R44TX with .060 gap and from then on in the late seventies it changed to R45TS with .045 gap. This new motor is probably more like a '79 motor so I may try changing the plugs out and see how it goes.