I've got a 406 that was originally a 71 400 4bolt main from a 71 Impala. I long ago converted to HEI ignition with an Accel HEI Supercoil (yellow wire version). Accel's tech support web site in a HEI conversion FAQ advises to use the factory initial timing and a plug gap of .045". I also had a mechanic tell me HEIs needed a plug gap that size. Can you tell me what the initial timing specs including rpm were for this engine? I have not been running that big a plug gap although I've never been satisfied with the performance. Does this sound reasonable or should I take the many other mods to the engine into account when chosing the plug gap (using AC R44TS plugs)?

With a bigger plug gap I doubt you'd see any more performance. Plus, it could tax the ignition system if you went much more. .045" is a good gap, I use it on all HEIs.

If I were you, I'd advance the timing as much as you can until it pings or is hard to start or heats up too much. The best thing to do would be to set up a timing curve, but that takes some time and effort.

Aftermarket modules can help a lot too, as the stock modules were't that great to begin with.

Try tuning your carb a little to squeeze out every ounce of factory HP it can produce.

Other than ignition, what have you done to the car/engine?

Fried_Guy, here's my combo:
Here's my combo:
71 400(+.030) block and 71 76cc heads. Cast flat tops with 2 valve reliefs per piston. Rings unknown. Comp Cams Magnum roller tip rockers with hardened pushrods (no guides).Comp Cams 268H 218/218@.050 w/.454 lift cam. Edelbrock Performer (not RPM) intake with Edelbrock Perf 750 carb w/elec choke. Stewart hi-flow water pump. Dynamax ceramic coated headers with 1 5/8' primaries. Torque Tech 2 1/2 " exhausts w/straight crossover. HEI with Accel Supercoil, MSD module and Crane adjustable vacuum advance, set tp 12 deg initial timing,32 deg total. Believe curved to all advance in by 2800rpm. TH350 trans with Transgo shiftkit. 2:73 open 12 bolt rear.

That really seems like a daily driver combination. The cam is a tad small for a performance 400, as well as the intake, headers and exhaust. Your timing seems fine, but you can never tell by just numbers, the best thing is to test and tune it.

If you want real performance out of that engine, put in a bigger cam and the RPM manifold. The headers and exhaust won't hurt too much as long as this isn't for more strip than street. Your rear hears need to be changed too, those are grandma gears. Of course, this is just MY opinion. ;)