I have Edelbrock RPM heads on the engine block in my 79 vette. Myservice manual (GM) states the spark plug gap should be .045". I have the Edelbrock recommended champion rcy12c plugs. The champions do not gap to .045.(the electrode does not align with the tip. In a previous post I asked about replacement plugs and AC 41629 plugs were mentioned. My question is since I have these new heads with a smaller combustion chamber, do I keep the .045" plug gap and will these plugs gap to .045"or do I use a different plug gap?

 

Call the tech support number for the company that makes the spark plugs. Tell them about your engine, and they will either suggest different plugs or they will suggest a specific gap. Most spark plug companies will give you all the information you need based on your unique combination.

 

How can the head manufacturer specify the plug gap when the ignition system is what determines the gap.

HEI - .045
Non-HEI - .035

 

In my previous thread, I meant to mention that the 1979 service manual states the plug gap should should be .045. I did edit the original thread to include the above statement.

 

I actually said to contact the manufacturers of the spark plugs he wants to use. I like NGK, so when I need to pick a plug I contact NGK and tell them what I have and they suggest a plug and gap. I didn't say to contact his cylinder head manufacturers.

 

I wasn't implying that you did. Before oldguard edited his post, it sounded like the head manufacturer was specifying the gap.

 

Oh, whoops, I assumed you meant me cause it was right after mine. My bad.

 

BECAUSE, cylinder pressure determines the gap. Just run them at .035" and be happy.

 

I'll agree that extreme cylinder pressures could blow out a spark, but usually not with normally aspirated engines.

 

If you have good high energy system like HEI or any car with an MSD, with good secondary wires you can find some more power. You can open the gaps .010 at a time until power falls off and then back up .010. I routinely gap my plugs at .050" out of the box and never look back. When I was maxing out my '90 Corvette with stock igintion I had them at .060 before the power dropped off. The larger gap creates a larger kernal of spark, which should light off more localized fuel droplets. This cuts back on intermittent miss fires which happen all the time in engines, especially if is running rich.

You can do a simple stop watch test with a helper. Get rolling in second gear, floor it while your partner starts a stop watch. Have them watch the tach and when it reaches your redline have them click off (you watch the road). Repeat and as long as you do not have wheel spin you will get very repeatable results. Open up the gaps retest. This works great for all sorts of tuning - timing or carb changes. It tends to overshadow drivablity problems under light load.


If you have points all this is "pointless" because they will not tolerate much more than .035"

 

Interesting, I'm tinker with my 78 L82 because its not running right and a slight misfire. I'm pulling plugs and finding them set at .035 and blackened. I suspect it was running too rich before I had carb rebult.

I can reset gap and try the .050 and planning to clean them up with my blaster.

 

OK....good info. here.How do TI systems figure into this?

 

Tom,
call me a worry wart if you must but I fear the abrasive blasting grit can hide up inside the plug recesses and drop out into the cylinder when the plug is reinstalled. Be careful to get it all out or just buy new ones.

Maybe paranoid, maybe not.

-Mark.

 

I think that's the prevailing opinion nowadays. Back in the 60's when we used leaded premium, plugs would load up quickly driving in town, and sand blasting them was pretty much regular maintenance. Sparkplugs are so cheap, relatively speaking, that it's false economy to try to extend their life by blasting them, especially when the potential of harming the engine is there, IMHO.

Cheers,
Pete

 

Hmmm..."the electrode does not align with the tip" is this a split tip as in a split fire or rapid fire plug? As far as I can tell there is no improvement in performance over a single tip...it's just to extend the life of the plug. However, platinum plugs actually will improve performance if the performance is sluggish. I once had a Pinto that had one cylinder with a burnt exhaust valve (1/4 of the valve head was gone), and another cylinder with a cracked valve(about 1/16 of the valve head was gone). It could barely get up any speed going up a freeway onramp. I put in 4 new slightly hotter plugs and it did improve a little, so I went ahead and bought the bosch platinum plugs and that pepped that Pinto back up so that it was like new...no misses...and plenty of power. It didn't last forever though...the engine eventually just wouldn't keep running without a very high idle.

The reason platinum plugs help performance is because if there's only partial combustion then there will be left over oxygen and hydrogen in the chamber...when a molecule of hydrogen touches a molecule of oxygen in the presence of platinum they will combine to form water...the chemical term is explosive reaction. It requires very little platinum to make this happen...and none of the platinum is used up...the chemical term is that it's a catalyst.

A lot of engineering has gone into spark plugs, but so has marketing...and it's hard to figure out which is which. If the combustion chamber is designed well and there are no problems with it, and the proper air/fuel mixture is getting sucked in, and ignition timing is correct, then it really doesn't make any difference...if you are already getting 100% combustion then nothing can make that better. So, my advice is to just keep the plugs you have...in factory preset condition. If they don't make a spark then replace them with some that do.

 

Mark:

Well I had to to ahead and blast them...I think its genetic. Anyway, you're absolutely correct. As careful as I was to blast with tip downward, blast material got up into the plug. I blew but that wasn't sufficient. I clinked them together which knocked out considerable debris, blew again, clinked, used brake cleaner, dried, clinked and blew.

All of the plugs were set at .035, black and wet. I suspect that was contributing to the poor running performance. Tracked down another problem I've resolved (fuel leaking on return line at top of tank).

 

I was just trying to visualize why a plug can't be gapped to something as small as .045 If it causes the tip to move off the center of the electrode all the better. The spark is not shielded by metal. If you look at racing plugs they jump more from the side to the center because the tips are so short.