Electronic Ignition And Plug Gap

[blazerguy 90]

On a lot of cars and trucks with HEI ignitions (the distributer with the coil in the cap) the plug gap .40 .45, why is the plug gap smaller on TBI engines with remote coils, (350 TBI plug gap is .35) would anything be gained by gapping the plugs at .40 ?, more power or better gas mileage ?, ive always wondered about this. any thoughts on this ?.

 

[13lazer]

.060 gap

if your coil can push it the larger your gap, the larger your spark, the more of your fuel ignites. the disadvantage is your plugs will wear out faster. the performance catalogs also sell indexing washers to point your electrode at your intake valve to make the best use of the spark you get. i was amazed at the difference when i did these things the first time.

 

[blazerguy 90]

I have upgraded the coil to a 49000 volt max output coil and I have an accel ignition module, with these upgrades I was thinking of setting my plugs at .40 to see if I will gain anything from these upgrades, better performance, gas mileage, or what ever.

 

[diesel4me]

wide gaps..

Some GM's used plugs with .080 gaps in 1975,and some other years.(Olds and Buick mostly).They cut back the gap to .060 or less in later years after they discovered they were flooding easily upon cold starts in feezing temparatures,and that the spark tended to jump right thru the insulation on the wires,or over the porcelain on the plugs under a heavy load..

Bigger gaps are better at more complete combustion,but can limit top end performance,unless you have a "super" hot coil and wires..Most chevy motors plugs are gapped at .045 with stock HEI..I never tried those "indexing" washers,but I have known dirt track guys that do,and say they do make a difference..

 

[dyeager535]

I'd suspect the gap change is a result of the redesign of the intake port and the sort of flow they were getting in the cylinder. If GM could improve emissions with plug gaps, guaranteed they did.

Stock GM coils will easily push a .060 gap in a low compression engine. As mentioned, other makes that use the exact same HEI setup ran widely varying gaps, Olds sometimes at .080 to .060 on 307's 350's 403's and 455's (IIRC) while Chevy was bouncing around from .060 .045 and .035 on the small blocks alone. I noticed no difference when I went from .035 to .050 on my SBC 350.