Spark Plug Gap

[dfarr67]

Early 90's Chev 350- small cap HEI called for 0.035in gap (I think). Using any of the newer Iridium's and such- is 0.040in a reasonable gap?

[F-BIRD'88]

It may not be better overall.
Is it nessessary?

Increased spark plug gap size puts more work on the ignition system and will promote arcing and crossfire in the cap and or wire to wire and or wire to ground.

Increased arcing under the diz cap promotes carbon burn tracking on the cap and rotor. Once the carbon track is established it becomes ever increasing easy path to ground (misfire crossfire).
So large sparkplug gaps must be taken with a grain of salt. short term gain (if) vs service life... and potential engine damage effects.

[F-BIRD'88]

Big spark plug gaps from the emissions era 70-80's. cars were an attempt at increaseing combustion efficiency and thus reducing exhaust emission mainly at idle and low speed low throttle angle and when engine is cold.
"warm up". The cylinder charge density (cylinder pressure) is very low at these tested parameters for low car emissions as the priority.

Not the same as optimized spark plug gap for better WOT engine operation.. (Racing)--- Different priority...
Reguardless of the sexy advertizing.

Your mileage may vary.

[BOOT]

Only one way to know for sure, track test it and if it slows down then it's too big. Larger gap gain VS loss from cylinders not firing.

That said my past research shows that a fine tip plug should allow for a bit larger gap, If your current gap is already optimal but I've never tested it.

[jeff swisher]

.035" or tighter for all my stuff.
Like was stated it is easier on coils etc.
No need for wide gaps. I have tried them and gained nothing.

My 1995 corolla calls for .032" gap and you can see carbon tracking misfire lines on the spark plugs at that gap.
I tried 5 different brands of plugs in it.
At .025" no more misfires and the idle gets much smoother.

If the gap gets to .037" it idles very rough.
It is a rough idling engine anyway do not need to help it in that respect.

As we all know we will try anything in this field and you should try different gaps and see what happens.

I still have some original HEI moduels with GM cast into them that still work.
Those I have are from the late 70's and that .035" I feel saved them from dying.

[plovett]

My limited experience says it is better to err on the small side for gap. Very little or no negative effect with a smallish gap most of the time. Very little or no positive effect with a larger gap most of the time, and sometimes a noticeable negative one.

I run around 0.035" on my newer fuel injected cars as well as my old hot rod which has an MSD ignition box, coil, distributor, etc. I have tried wider gaps up to around 0.045" and noticed rougher running in some conditions.

Plus, a tighter gap allows some room for wear without the gap increasing too much.

I don't doubt that there are some applications that benefit from wider gaps, but I would say most don't.

JMO,

paul

[Circlotron]

I found going from standard 35 thou in a mostly stock engine...
60 thou modest improvement in idle and general running smoothness
80 thou no further improvement
100 thou same but after stopping at lights for 20 seconds idle deteriorated possibly because plug cooled off.

I’ve been using 60 thou for over 40 years with no problems that others have suggested.
It costs nothing to try, so maybe it might help in your particular case.

[David Redszus]

The voltage required to fire a spark plug is determined by the chamber temperature, pressure
and spark plug gap.

Let us assume a chamber temperature of 400F and a pressure of 100 psi. Then:

.035"...12.7 kV
.045"...15.8 kV
.055"...18.8 kv

However, if chamber temperature is increased to 600F at the same pressure, then:

.035"...10.9 kV
.045"...13.2 kV
.055"...15.6 kv

During actual operation, chamber temperatures and pressures can vary substantially,
and so will the required firing voltage. Under select conditions of temp and press,
the required voltage can be in excess of coil output and a misfire will result.

[dfarr67]

aluminum AFR vortec heads (gasketed plug) with older ignition- I believe the pregap on vortec plugs is around 0.060in. At 0.040 there is roughness- but it may be the cam and EFI tune. With these headers, they are not spark plug friendly. I'm using NGK Ruthenium(?) I think the Iridium ran smoother.

87-95 5.7L TBI motors - .045 gap
96-00 5.7L vortec motors - .060 gap

[MadBill]

...

Let us assume a chamber temperature of 400F and a pressure of 100 psi...

David, is that with or without fuel, as it significantly lowers the voltage requirements?

[steve cowan]

Madbill,
congratulations on the 15000 plus posts so far

[RDY4WAR]

aluminum AFR vortec heads (gasketed plug) with older ignition- I believe the pregap on vortec plugs is around 0.060in. At 0.040 there is roughness- but it may be the cam and EFI tune. With these headers, they are not spark plug friendly. I'm using NGK Ruthenium(?) I think the Iridium ran smoother.

87-95 5.7L TBI motors - .045 gap
96-00 5.7L vortec motors - .060 gap

I bracket race a '93 LT1 5.7L which is similar to the two you mentioned but with factory aluminum heads and reverse cooling. I ran NGK TR55 .060" gap at first, but would get occasional misfires if the ambient temp fell below ~50*F. I switched to TR5 .040" gap and it runs the same ET and MPH as with the TR55s and no misfires in the cold.

That's with the stock EFI, coil, opti-spark, and OEM replacement wires.

[BOOT]

I bracket race a '93 LT1 5.7L which is similar to the two you mentioned but with factory aluminum heads and reverse cooling. I ran NGK TR55 .060" gap at first, but would get occasional misfires if the ambient temp fell below ~50*F. I switched to TR5 .040" gap and it runs the same ET and MPH as with the TR55s and no misfires in the cold.

That's with the stock EFI, coil, opti-spark, and OEM replacement wires.

Had a sim experience with one car, certain factors make the gap appear larger.

[David Redszus]

...

Let us assume a chamber temperature of 400F and a pressure of 100 psi...

David, is that with or without fuel, as it significantly lowers the voltage requirements?

As usual, you raise a very valid point. How does required voltage change in a running engine?

One factor that determines required firing voltage is ignition angle.
For an engine with an SCR of 13, and DCR of 10.4, the compression temperature and
pressure will change as follows, with required firing voltages. (All with a gap of .030")

-40 deg....84 psi...457F....9.1 kV
-30 deg...128 psi...573F..11.7 kV
-26 deg...166 psi...625F..14.1 kV

Normal driving can and will produce similar changes in required firing voltages.

Air/fuel ratio will have a linear effect on required firing voltage. Either too rich or
too lean will increase the required firing voltage depending on the type of fuel
and its cooling effect. But typically, the range of mixture ratio is much less than
the changing chamber conditions.

The posted required firing voltages are based on pure air, not fuel vapor or
relative humidity factors.

Seems like every engine should have an on-board recording scope.

[MadBill]

Madbill,
congratulations on the 15000 plus posts so far

To quote Elvis: "Thank you. thank you very much" Steve.