School me on Spark Plug gaps

[tonkota]

I know next to nothing about spark plug gaps. I know what the FSM says.

What if they are opened more?

What if they are opened less?

When can they be opened more?

Does the gap depend on plug construction?

 

[ntsqd]

Ideal spark gap is sort of trial and error.

The bigger the gap, the larger the spark kernel which the old theory holds would be better for complete ignition. IF, and it is a very big IF the coil can fully Ionize the gap. If it can't then you end up with a weak spark or no spark at all. A smaller, fully ionized spark is better than a larger but weak spark. Too large a gap results in excess time required to ionize the gap, which is a loss of ignition timing advance. Coupled with the weaker spark kernel and you get less than stock performance.

The thing to remember is that the cylinder's pressure is quite a bit higher than ambient pressure (it had better be anyway!!) and that makes ionizing the gap much harder to do. Those old parts house counter top displays showing how much better so-and-so's coil worked than their competition was basically worthless because of the lack of cylinder pressure. The transition from ionizing in ambient pressure to ionizing in cylinder pressure is not direct and simple.

GM, in the very American tradition of "if a little is good, more is better and overkill is just enough", originally spec'd spark gaps of .060" when the HEI first came out. The coils were capable of ionizing this gap when everything was brand new. Apparently it didn't age well. They backed off the gap spec to .045" or less.

Similarly, say that you have a coil that can develop 45,000 volts to ionize the spark gap, but the worst case cylinder conditions only need 28,000 volts to fully ionize the gap. There would be no point in swapping in a Joe-Racer coil capable of 100,000 volts if the rest of the ignition system can only support 45,000 volts and the cylinder doesn't need it anyway. The voltage needed to strongly ionize the gap is all that the system will ever develop. Anything beyond is just excess capacity that made the people who sold you that Joe-Racer coil that much richer.

If when you install that Joe-Racer coil you also increase the spark plug gap, then you'll start to use more of that excess capacity. Beware though that if the cables, cap, and rotor can't contain that increased voltage that the increased gap will not get properly ionized because the extra voltage 'leaked' to ground. At present the practical upper limit for spark gaps appears to be about .045"

Always remember that electricity is lazy. It will always, always take the path of least resistance.

The trend in the last decade or so is to increase the duration of the spark rather than simply increasing it's voltage. This is much harder to do than just increasing the turns ratio of the coil. This is what the MSD's and other ignition boxes like them are, in part, trying to do.

Some of the alternatives to electric spark ignition are also pretty wild. Things like laser ignition and plasma ignition have been tested. The best is no ignition system at all. That is called "HCCI" or "Homogeneous Charge Compression Ignition". HCCI is basically detonation, but the air/fuel mixture is so thoroughly mixed that the detonation occurs at the exactly perfect point and the engine runs correctly. At my work we have achieved HCCI, as have others. No one has been able to do it consistently and repeatably. I suspect that by the time the technology exists to do so internal combustion engines as a whole will be archaic.

 

[Michael Hanson]

Gap as required by each engine,no problem.2 cents. Mike

 

[cruiser88]

When you go buy your plugs from the local advance crap ask for the gap size(on anything) and the gapper.....the rest is obvious.

There is some that is pre-gapped and cannot be gapped further

 

[Spook50]

When you go buy your plugs from the local advance crap ask for the gap size(on anything) and the gapper.....the rest is obvious.

There is some that is pre-gapped and cannot be gapped further

Those Iridium IX plugs (made by NGK) are a bitch to gap, but with patience and a pair of needle nose pliers I was able to get them dead on. I always gap the plugs in my 62 to 0.035" (factory spec is 0.032" but I've gotten slightly better economy on average with the larger gap. Any larger and the economy and performance both suffered). I also mark the plugs so that when I install them I can easily index them, and that makes for a smoother running engine as well. No difference with overall performance or economy by doing that though. Just more comfortable and a little better throttle response.

With the HEI controller and MSD Blaster 2 coil on the SL6 in my Plymouth, I gap them at 0.045" and it runs beautifully with its little one barrel carb

 

[klinetime574]

Great info from this thread already after just 5 posts.

I am running an HEI distributor and I am running my gap at .045. The 2F seems to like it. I had it at .050 and it was way too much. It ran awful.

 

[askeyey]

ntsqd's post is exactly why I love Mud and read every post. The wealth of information the Mud community holds and how freely it is shared still amazes me every day.

 

[Elbert]

gap is based on the type of ignition system and the timing. On a stock vehicle I would use what's referenced in the manual. On hot-rods and race cars.....it becomes a challenge.

 

[tonkota]

Wow! This is really great! I put in some plugs this weekend, just the standard NGK and I used a gap of 0.040". The idle is smooth but I notice that it is really low. I will take them out and gap at the recommended measurement. I was just testing it out I guess, I have a MSD Blaster2 coil in there.

Thanks!!!

Ideal spark gap is sort of trial and error.

The bigger the gap, the larger the spark kernel which the old theory holds would be better for complete ignition. IF, and it is a very big IF the coil can fully Ionize the gap. If it can't then you end up with a weak spark or no spark at all. A smaller, fully ionized spark is better than a larger but weak spark. Too large a gap results in excess time required to ionize the gap, which is a loss of ignition timing advance. Coupled with the weaker spark kernel and you get less than stock performance.

The thing to remember is that the cylinder's pressure is quite a bit higher than ambient pressure (it had better be anyway!!) and that makes ionizing the gap much harder to do. Those old parts house counter top displays showing how much better so-and-so's coil worked than their competition was basically worthless because of the lack of cylinder pressure. The transition from ionizing in ambient pressure to ionizing in cylinder pressure is not direct and simple.

GM, in the very American tradition of "if a little is good, more is better and overkill is just enough", originally spec'd spark gaps of .060" when the HEI first came out. The coils were capable of ionizing this gap when everything was brand new. Apparently it didn't age well. They backed off the gap spec to .045" or less.

Similarly, say that you have a coil that can develop 45,000 volts to ionize the spark gap, but the worst case cylinder conditions only need 28,000 volts to fully ionize the gap. There would be no point in swapping in a Joe-Racer coil capable of 100,000 volts if the rest of the ignition system can only support 45,000 volts and the cylinder doesn't need it anyway. The voltage needed to strongly ionize the gap is all that the system will ever develop. Anything beyond is just excess capacity that made the people who sold you that Joe-Racer coil that much richer.

If when you install that Joe-Racer coil you also increase the spark plug gap, then you'll start to use more of that excess capacity. Beware though that if the cables, cap, and rotor can't contain that increased voltage that the increased gap will not get properly ionized because the extra voltage 'leaked' to ground. At present the practical upper limit for spark gaps appears to be about .045"

Always remember that electricity is lazy. It will always, always take the path of least resistance.

The trend in the last decade or so is to increase the duration of the spark rather than simply increasing it's voltage. This is much harder to do than just increasing the turns ratio of the coil. This is what the MSD's and other ignition boxes like them are, in part, trying to do.

Some of the alternatives to electric spark ignition are also pretty wild. Things like laser ignition and plasma ignition have been tested. The best is no ignition system at all. That is called "HCCI" or "Homogeneous Charge Compression Ignition". HCCI is basically detonation, but the air/fuel mixture is so thoroughly mixed that the detonation occurs at the exactly perfect point and the engine runs correctly. At my work we have achieved HCCI, as have others. No one has been able to do it consistently and repeatably. I suspect that by the time the technology exists to do so internal combustion engines as a whole will be archaic.

 

[lehiguy]

On my old 22R I found that it ran better with the stock gap plus .005. I could feel the difference, sort of a "butt dyno" thing...

 

[ntsqd]

Can't go wrong with the OE gap recommendation on a stock engine. You might find more power or economy in trying different gaps, but it will want a pretty thought out scientific approach to doing so.

The more the engine is modded, the more this tuning will have an effect. Low stressed engines are not sensitive to large changes in the tune. Highly stressed engines are very sensitive to changes in the tune. With a non-stock engine, as Lehi says, it's a matter of trying things. Can probably use the stock recommendations as a starting point, but it wouldn't surprise me to hear that the best results were from quite a bit different from stock.

 

[Elbert]

classic automotive term:.... "butt dyno" that's a good one!

You guys should have seen the "redneck car lift"..... a friend and I had to use when the two post lift we were using ruptured a hydraulic cylinder and we were locked about half way up (about 4 feet off the ground). The cam mechanism locks the lift for safety but it also make it impossible to get the vehicle off the lift, since we had no power due to hydraulic failure... A large front end loader on the front and a forklift on the back (using the bumper as a lift point).... Very scary, I wish I had my camera but it was not to be....

The lift was located outside so we had limited access to the front and decent access to the rear.....or otherwise it would have been a royal pain in the a$$.

Got the truck up high enough to move arms out from under it.....then lowered slowly. I had a close encounter with the "pucker syndrom"....

I know a lot of stock vehicles are happy with the OEM settings... We you are dealing with high horsepower engines or race cars....that when the art of tuning really comes into play.... how to read the plugs...set the gap, and some folks index the plugs too. On a stock truck I just don't think much is gained by screwing with the plug gap.

 

[Mace]

The best is no ignition system at all. That is called "HCCI" or "Homogeneous Charge Compression Ignition". HCCI is basically detonation, but the air/fuel mixture is so thoroughly mixed that the detonation occurs at the exactly perfect point and the engine runs correctly. At my work we have achieved HCCI, as have others. No one has been able to do it consistently and repeatably. I suspect that by the time the technology exists to do so internal combustion engines as a whole will be archaic.

Mr Diesel though it was a good idea in 1892

 

[BG_TX]

Great thread...

I also mark the plugs so that when I install them I can easily index them, and that makes for a smoother running engine as well.

School me... what is meant by marking the plug and indexing them?

 

[ntsqd]

Mark on the outside where the ground electrode is. When you screw it into the head you use washers of different thickness' to set it to stop with the electrode in some particularly desired position. Sometimes that is with the electrode as far from the intake valve head as is possible. I'm not up on all of the ideal goals, just the basic mechanics of the process.

 

[Spook50]

Mark on the outside where the ground electrode is. When you screw it into the head you use washers of different thickness' to set it to stop with the electrode in some particularly desired position. Sometimes that is with the electrode as far from the intake valve head as is possible. I'm not up on all of the ideal goals, just the basic mechanics of the process.

Bingo. I set mine so the ground electrode actually "points" towards the combustion chamber though.

 

[BG_TX]

I never would have thought the position of the ground would matter, thanks for the explination. More hp, mileage, etc?

 

[Spook50]

I never would have thought the position of the ground would matter, thanks for the explination. More hp, mileage, etc?

It's just where the electrode is oriented in relation to the combustion chamber. There's no improvement in economy or performance (that I could feel anyway). It just runs a bit smoother as a result, especially at idle.

 

[rover67]

and with blown or turbo'ed motors the bigger the gap the better the ignition with the caveats stated above.. but the risk that the spark gets blown out by the incoming charge or won't light the charge due to the higher cylinder pressures. I ran stock or smaller gaps on turbo'ed motors because of this.

Don't knwo the theory, just that when the gap get's too big they miss under boost.