Question: spark plugs

[mrmuzz]

changing plugs on a 1979 with a L-82 has accel HEI super coil with headers w/ manual trans previous owner said motor was rebuilt and has about 200 miles on it the plugs i pulled out are AC R45TS8 gap is .80 book calls for R45TS gap .45 can somebody explain why one would use the big gap

 

[Vettehead Mikey]

Not a clue and no real reason to run such a big gap. 0.045" is correct.

 

[redvett]

Most auto MFR's did the large gap at that time for fuel economy and emissions. A service bulletin to change to .045.

changing plugs on a 1979 with a L-82 has accel HEI super coil with headers w/ manual trans previous owner said motor was rebuilt and has about 200 miles on it the plugs i pulled out are AC R45TS8 gap is .80 book calls for R45TS gap .45 can somebody explain why one would use the big gap

 

[Vettehead Mikey]

Most auto MFR's did the large gap at that time for fuel economy and emissions. A service bulletin to change to .045.

The gap recommended by GM when HEI first appeared was 0.060". This was well within the capability of the system but it was soon discovered that the HT wires did not have sufficient insulation and the spark was leaking to ground. The gap was reduced to 0.045" as you mentioned.

HEI was introduced for two reasons

1) stays 'in tune' for longer periods of time as compared to points and coil

2) the spark was strong enough to fire a larger gap, this being required to ignite the extremely lean mixtures of the early smog engines.

 

[kpic]

There was a moment in time when GM believed they could violate certain rules and discovered they couldn't which prompted a service bulletin.

To Vettehead Mikey's 2 points, I'll add a third. At one time the GM HEI was the only factory ignition (with appropriate wires) which could keep up with a Jacobs ignition. Any other OEM ignition required the purchase of Jacob$ coils. Quite a compliment to GM.

As the spark plug's fire ball must expand to a certain size before combustion begins; the large gap shortens the time. I forget the real number so using .12" a .045 gap requires 2.67 time constants and a .06 requires two. As Vettehead Mikey said it supports emissions. With a spark box, it helps performance.

 

[LLC5]

The gap recommended by GM when HEI first appeared was 0.060". This was well within the capability of the system but it was soon discovered that the HT wires did not have sufficient insulation and the spark was leaking to ground. The gap was reduced to 0.045" as you mentioned.

HEI was introduced for two reasons

1) stays 'in tune' for longer periods of time as compared to points and coil

2) the spark was strong enough to fire a larger gap, this being required to ignite the extremely lean mixtures of the early smog engines.

There were GM vehicles in the mid '70's that had a spec of 0.080" plug gap and it looked huge and not normal, possibly not Corvette though, I cannot specifically remember which vehicles required that 0.080" gap. I remember setting the gap and wondering if it was a real number, after a few calls yes it was. It was later reduced to 0.060" and then settled at 0.045", but initially it was a huge 0.080".

 

[mrmuzz]

mrmuzz

Thanks for the replies one less thought i gotta think about, it makes sense now!Have a good day

 

[mk's78]

Just a quick???

Very interesting post regarding spark plugs etc..

My ? is...........I am running R45TS set to .045................Are there any other plugs on the market that would be better for my 78L-48 engine?


Thanks in advance for your guidance and input...

mk'78

Mike

 

[mr.beachcomber]

Very interesting post regarding spark plugs etc..

My ? is...........I am running R45TS set to .045................Are there any other plugs on the market that would be better for my 78L-48 engine?


Thanks in advance for your guidance and input...

mk'78

Mike

I've used AC Delco Rapidfires for years with excellent results, i.e., long life, smooth idle, low emissions when tested. Using the AC Delco Parts Finder and plugging in your Vette's info, the recommended plug is a Rapidfire #2.

 

[Vettehead Mikey]

Very interesting post regarding spark plugs etc..

My ? is...........I am running R45TS set to .045................Are there any other plugs on the market that would be better for my 78L-48 engine?


Thanks in advance for your guidance and input...

mk'78

Mike

In a word, no. Essentially, plugs either fire the mixture or they don't. Super-duper zoomy space age plugs made of unobtanium costing a fortune might sound great but they don't function any better than a $3 AC Delco piece.