Thread: Spark plug gap

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I have a quick question on spark plug gap. I'm running a 350SB with a mild cam, HEI ign and ACDelco R45TSPlugs. Can anyone help with the proper gap? Even the parts store is not sure if it's .035 or.045. I even contacted ACDelco and they told me to ask the parts store on the application? Any help would be greatly appreciated!! Thanks....

Use the wider gap, 0.045. HEI stands for High Energy Ignition....
It takes more energy to jump a wider gap and thus, the energy in
the spark is more....plugs were gapped narrower in the last century
because the available energy was less and therefore, for a reliable
spark, the gap had to be smaller.

The spark plug gap dictates the energy delivered....not the
coil because only enough energy to jump the gap is required.
Of course, changing the gap changes your timing so adjust
accordingly.

Thanks for the info on the plug gap. Now all I have to do is play with the timing.

In trying get info on the same problem I have read the following:

1. Increased gap tolerates leaner mixtures with less misfire.
2. Increased gap requires more energy
3. Increased compression dictates a smaller gap or more energy due to the density of the air.

Opinions are welcome. Seems like a crap shoot in a way but if it were that critical, I think a lot of DIY's would have stumbling engines. Best way IMO would be to gap it how you think best (based on feed back or whatever) and then pull the plugs and read them after a few weeks and make adjustments accordingly.

yup denny has it... to elaborate on the higher RPM part, your coil has to charge a magnetic field to spark, and the wider gap has higher resistance and so it demands more juice to get the spark across it.... the more juice required the more powerful magnetic field is generated, the higher rpms make it to a point that the coil cant get a fast enough charge to dictate a powerful spark..... thats why all newer cars are going to coil over plug... and also why i'm working on converting my race engine on my mustang to coil overplug.... if only i could afford magnetos like top fuel cars use.... that'd be fun....cept then i'd need a fuel cutoff switch

Good info, except this part....

Of course, changing the gap changes your timing so adjust
accordingly.

How in the world does this happen? ...the timing dictates when the distributor sends the energy to the wire that fires the plug, so gap has no effect on when this happens.

Its a resistance thing. The same applies for cable length.

Also here is another thought, change your cam duration and you also change your spark timing, I am talking amount of time for the spark to jump the gap here ?

not so much... the cam duration will change valve time but it wont change the electrical circuit, it will somewhat change cylinder tempature and wether the valve is fully closed when your plug is firing, which is ok as long as the valve will be closed by the time full combustion occurs, but fuel combustion only takes 4ms, under normal compression, which is like nothing but is also why higher and higher performance applications require more compression and more combustable type fuels....

remember one thing the GM HEI units where designed initally to use a gap up to .060 inches and the gap can be used to change the timing up to 2 degrees (either direction ) as when I went to school for briggs this is how we adjusted the timing on those little motors as the magnetos are fixed to the motor frame and when you ran kero it was crutial ,

[QUOTE=thesals]not so much... the cam duration will change valve time but it wont change the electrical circuit,

Think abouty that statement for a minute. I agree a longer duration cam will definitely close the intake valve at a later time.... when the piston is further up the bore. Since the piston is furthur up the bore, what does this do to cylinder volume ? It decreases it because compression cannot happen untill the intake valve is shut. So with less volume of fuel air mix to compress the dynamic compression ratio is lower. Subsequently if you put a shorter duration camshaft in the dynamic compression ratio will be higher. So these pressure variations will have an effect on spark timing due to the density/ resistance to the compessed air at time of ignition. It will not be great but the variance is there.

Originally Posted by lucforce

Its a resistance thing. The same applies for cable length.

Speaking of resistance.Another area that is often over looked is spark plug it's self.
These platinum plugs are a much denser material making it much more difficult for the spark to jump the gap.
In the dirt mod. we still run the $2 NGK plugs.

When HEI came out in1975, they set the gap at .060, which turns out was a bit wide....by 1977, they set it at .045.