COPs, plugs and the Gap

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Telco

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Thought I'd throw this out. I know the gap is .060 over on the plugs, and the COPs fail regularly. What are the thoughts on perhaps the factory gap is too large? It occurred to me that other cars, particularly higher compression vehicles and forced induction vehicles run smaller gaps. It's pretty standard for example, to change the gap from .060 to .040 or even .030 on Chevy engines when you force induce them.

My thought is, perhaps the LS spark plug gap should have been .050 or .045? I know GM uses .060 on their HEI systems, but the coils on GM vehicles are quite a bit more robust than the coils on the LS. A large gap is harder on weaker coils, so the COPs might last longer or perhaps even not fail at all with a smaller gap.

Makes me no difference either way if it's a good idea or not, but if it helps solve the problem of these cars eating coils like potato chips you'd have more money to make... other... performance... <ssss> mod-i-fi-caaa-tions...
 
Telco said:
Thought I'd throw this out. I know the gap is .060 over on the plugs, ...
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Well, this does not match with what I think I know. Where did you get that figure?

Gen I V6: 1.3mm to 1.4mm (0.052 to 0.056")
Gen I V8: 0.99mm to 1.09mm (0.039" to 0.043")
Gen II V6: Having trouble finding this, but should be the same and gen I.
Gen II V8: 1.0-1.1 mm (0.039"-0.043")

I set mine to 1.0mm.
 
Is that even a factor when the spark plug is more or less in the center of the cylinder?
 
That was off the top of my head, for some reason was thinking 1.0mm and .060 was the same. Nevertheless, was wondering if taking 10 to 20 percent of the gap off (making the gap smaller) might benefit the COP lifespan. The larger the gap, the harder the coil has to work to jump the gap, the hotter it gets. Reducing the gap a bit might help them last longer.

So far as indexing the plugs, if I were part of a multimillion dollar race team looking for fractions of a horse, perhaps. On a street car, never even entered my mind. Didn't know there was any sort of myth about plug indexing.
 
Telco said:
Didn't know there was any sort of myth about plug indexing.
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The myth is that is makes a big difference. On a multi-million dollar race team that might get you 1% more HP or so. On a daily driver regular street car you won't even see the difference. It's mostly one of those bragging things you do. Plus with a modern engine the spark plug is in the top of the head anyways so there's no where to aim it really. Indexing was more for old iron where the plug was slanted and off to the side of the head. You wanted the spark to be facing the valves and top of the chamber. That way the fire ball ignited in the center of the head and burned all the way down the stroke, instead of partially burning by the cylinder wall and then wrapping around to get to the chamber. Was a more efficient burn.
 
I always gap a hair smaller than factory,since gap increases wuth mileage on the plugs. If it takes a .046" gap,I'll make it a TIGHT .044" maybe. Never had any problem doing this.
My older brother,though,argues that I should use a wider gap than factory for gas mileage. I don't agree since the gap usually ends up wider later. don-ohio :)^)
 
Personally, I try to stay within the tolerance band. Bigger gap = higher voltage = more stress. Smaller gap = higher current = more stress. You want the gap that exactly matches what the coil was designed for. That give the greatest power transfer and the least stress on the coil.
 
Not quite. A coil is going to convert X voltage/amps to Y voltage/amps. A simple 10 to 1 winding will convert 12V at 10 amps to 120V at 1 amp. Once it's converted to the higher voltage and lower amperage, it's converted. The spark plug gap doesn't perform a voltage/amperage conversion, it acts as a resistor with the gap's width determining its resistance. What causes the stress to the coil is pushing said voltage across an open air gap. A tighter gap should be easier on the coil because it's not pushing across as wide a gap.

The main negatives to a smaller gap would be fuel ignition and charge burn as a smaller gap makes a smaller ignition kernel. The smaller kernel might not allow the entire fuel charge to burn. I imagine someone making dyno runs could check this, or if no dyno was available adjusting the gap and measuring responsiveness and fuel economy would tell.
 
joegr said:
Personally, I try to stay within the tolerance band. Bigger gap = higher voltage = more stress. Smaller gap = higher current = more stress. You want the gap that exactly matches what the coil was designed for. That give the greatest power transfer and the least stress on the coil.
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Wider spark plug gaps require more voltage from the coils secondary windings to ionize the gap. Higher voltage output from the secondary windings requires more amperage draw through the primary side of the coil windings. This increased amperage draw results in more heat in the coil. Heat is the #1 enemy of the V8 LS COP's. That is why I selected fine wire Iridium center electrode spark plugs - their design requires less voltage to ionize a given gap width than other common spark plug electrode designs. Then I gaped them at 0.038".

BTW, conversely narrower spark plug gaps reduces the amperage draw though the coils primary windings.
 
And with a smaller gap you can get multiple-firings since the gap is smaller. You spend less time charging the coil before you can get a spark and more time generating that spark.

In addition to what Telco said, the other drawback to small plug gaps is fouling. If the gap is too small you could build up too much carbon on it which could short it out or not give a big enough spark to ignite properly.

It's all a balancing act. But plug indexing when the plug is already top dead center of the head is mostly useless.
 
I am going to try the .039 and see how that burns with a new set of COP's. I dont mind changing plugs and trying different gaps.

"Maybe try a magneto later....and let the rough end drag."
 
Kumba said:
The myth is that is makes a big difference. On a multi-million dollar race team that might get you 1% more HP or so. On a daily driver regular street car you won't even see the difference. It's mostly one of those bragging things you do. Plus with a modern engine the spark plug is in the top of the head anyways so there's no where to aim it really. Indexing was more for old iron where the plug was slanted and off to the side of the head. You wanted the spark to be facing the valves and top of the chamber. That way the fire ball ignited in the center of the head and burned all the way down the stroke, instead of partially burning by the cylinder wall and then wrapping around to get to the chamber. Was a more efficient burn.
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The ideal is to face the electrode gap to the intake charge (intake valve) to take advantage of the inrush to ignite as much of it as possible. There was a time when spark plugs were gasketed to make the seal at the head. Now mostly I see, excepting my garden tractor, they are beveled. You get sets of differing thickness copper gaskets so that the plugs could be tightened to index to the intake valve. It works, when I was racing my Triumph I and others saw a 2-3% gain in HP and more importantly in torque. Also a more complete burn as witnessed by the lower amount of soot at the tailpipe.
 
Wicked Ace said:
The ideal is to face the electrode gap to the intake charge (intake valve) to take advantage of the inrush to ignite as much of it as possible. There was a time when spark plugs were gasketed to make the seal at the head. Now mostly I see, excepting my garden tractor, they are beveled. You get sets of differing thickness copper gaskets so that the plugs could be tightened to index to the intake valve. It works, when I was racing my Triumph I and others saw a 2-3% gain in HP and more importantly in torque. Also a more complete burn as witnessed by the lower amount of soot at the tailpipe.
Click to expand...

That depends more on the piston and head chamber design. Indexing is more about pointing the plug gap to where the bulk of the compressed charge is going to be. If the engine has domed or dished pistons then when the piston comes up the bulk of the charge will be pushed to the largest volume of area in the head. Sometimes that's the intake valve since it's usually the biggest valve, sometimes that's the exhaust valve in order to help direct air out of the cylinder. Chamber shape also matters as a heart-shaped chamber will contain the charge in a smaller area then a big round oval chamber.

With flat top pistons there's less moving the charge around as much as just compressing it. Coupled with 4-valve heads the chamber shape is usually symmetric which means it's just pushing the charge to the center of the head. The LS pistons also have a small relief about the size of a quarter in the top of the piston which helps to keep the bulk of the charge centered over the piston. The biggest intake charge is going to be in the largest area when compressed. By cutting the small relief in the piston, the biggest area is going to be the top center of the head and the lowest dish of the piston.

So, there's not really anything to aim the spark plug gap at with a 4V head with flat-top pistons. The charge is being compressed and pushed to the highest volume area of the head which is where the spark plug anyways.

A 2V head gets much more benefit from spark plug indexing due to one valve being significantly bigger then the other. This causes a bigger space in the head chamber to be around the intake valve. There is not as much of a valve size difference in a 4-valve head so the chamber is more uniform. The charge is also more evenly dispersed since two small valves blend in the chamber instead of one large valve which can reasult in one area being leaner then another.

Indexing can't hurt, but I wouldn't expect to see a difference in a modern 4-valve street car.
 
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