Was wondering since I have rebuilt my '81 engine and increased the compression ratio from 8.5 to 9.1....should I increase the spark plug gap from .045 to say 0.50 or so? I decreaed the head volume by about 1 cc and had the block zero decked, so total volume is less than what it used to be from stock.

What you say??????

 

You can play with plug gaps all you want, it won't make one bit of difference in performance.

 

My thinking (being an electrical engineer - ) is that with a larger gap a higher voltage is required to arc; higher voltage = larger spark = better burn = more power...????

Course I could just be too much of an engineer at times......

 

Yep. Its an engineer thing..Lol. I have read that when you have a higher performance coil that they say to open it a bit. But the reality of it is its not going to do anything noticeable.

 

EE here, too. (I used to design ignition modules, ECMs and PCMs). Theoretically, you are basically correct in your thinking, but the real world won't actually show any difference.

(Remember, energy, as stored in the coil, will be used up at the plug. Energy, in this case, is Voltage X Current X Time. If you increase the gap voltage, you will end up reducing the Time that the coil is able to supply the Current.)

 

It's a question for a thermodynamics engineer. Get out the general energy equation and see where the size of the spark makes any difference. The spark is there to start an explosion. If you start the explosion, what difference does it make if you have a big spark or a little one.

Do an experiment and post your results here. You never know unless you try.

-Mark.

 

Yep, an experiment would be best, but how to conduct such and measure the results.....waaayyy too much for a home experiment I think.

I will have to do a search and see if there are any results out there in cyber world.....

Also, in regards to the coil current and time: not sure I agree with that as the time between spark will be the same regardless of gap distance; now it will be longer in comparison between a smaller one and larger one, but not in relation to itself....at the same RPM of course. Besides the magnetic field is developed so much quicker than the engine is. However I have not looked into if the coil is in saturation or not, which has a longer recovery time...so that could be a factor.

Far too much thought here.....besides....there is alot involved in pulling all the plugs out just to regap...think I will wait for new plugs and try it then.

But great discussion.

 

Too much EE and not enough ME. Larger spark ≠ better burn.

A fire (of any type) does not burn hotter or cooler dependant on what lit the flame.

 

Yes and no. There are still the same number of BTUs available from the fuel regardless of how you light it. I'm guessing we probably agree there. My point is that the more (percentage-wise) air and fuel molecules you can light off initially (assume a very large gap for discussion), will result in a larger initial flame kernel, and an attending shorter burn time in crank degrees, requiring less timing advance. The less advance an engine requires, the more thermally efficient it is (more work out for the same BTUs in).
And, I assume, if the combustion occurs in a shorter time, there's also the possibility for less heat to be lost into the coolant, resulting in a slightly hotter average combustion temperature.
As I mentioned earlier, what should be a theoretical improvement isn't always noticeable in SOTP performance measurements.

 

Re-read what I wrote (not what you interpreted!), and then let me know if you still disagree.

 

Since the energy is a finite amount, if voltage increases, then time must decrease....in order to stay proportional.

Bottom line....I agree with what you say; now that I re-read it.

And I also agree that a larger spark will setoff a larger flame front resulting in a quicker burn rate compared to a smaller spark. If all the fuel could be ignited at one time, instantly, the power delivered would be greater than a travelling flame front.

Although still not practical.

 

Incorrect and that's not what was said. The dreaded preignition/pinging we all avoid is a result of all the fuel burning all at once (virtually exploding)- too much cylinder pressure at the wrong time. No motor can withstand such violent pressure extremes. A controlled, travelling flame front at the correct moment is much more desirable. The thinking above was that a 'bigger' spark would cause the flame front to travel faster. Possibly, but then the cylinder pressure would rise prematurely with the requirement to retard the timing to compensate. Net gain= zeroas peak pressure remains the same

Less heat loss to coolant due to reduced time from ignition to peak pressure? Let me know the results of your experiment.

 

Gettin' off the subject a bit, I think. The man just wants to know if he should open up the spark plug gap. And the answer is....NO....not unless he has upgraded the coil/ignition system to provide a "hotter" spark and more electrical energy. End of story.... {you can make the gap bigger, but the spark just won't happen if you go too large for the ignition system design}

 

...


Your after performance? then .035. You dont care about performance and worried about emissions, then everything else is satisfactory from ..040, to whatever......

 

After you pick yourself off the floor, perhaps you could consider a slight analogy, that being what happens when a lean mixture burns slowly. It allows more of the combustion energy (heat) to be exposed to more area of the cylinder wall as the piston travels down the bore. A very inefficent use of the BTUs in the fuel. The less you can expose the flame front to the "cold" cylinder area, the less heat lost.

 

I think the OP has an HEI system that could handle a bigger gap, but as stated before- it will not change performance at all (in the real world, not in Theory where some people have taken up residence)

 

Here is my take on it as a result of playing with my 427ci small block. I used to gap my plugs at .035" and be done with it. I have changed the gap before on mild builds and it did nothing.

The spark plug gap and the electrical / electronic devices ( distributors, multiple spark boxes ) that produce the voltage for the spark becomes dramatically more important as you move up the engine food chain.

Once you get into heads that flow 320cfm the fill the cylinders better and 11:1 cr you will notice a power increase as you increase the plug gap. Mine was sluggish at .035", it was smoother and stronger at .040" and it really started to thump at .045" a real noticeable seat of the pants power gain as well the exhaust was louder. I then went to a hotter plug and gapped it at .040" and there was an improvement again.

I was running a MSD digital E curve distributor by itself and today I added the MSD 6al box and again a nice power increase by the bigger spark the box puts out. If you read the literature on the MSD6AL box they do not even recommend it for anything under 400HP.

So in summation I must say I used to think that a match dropped into a gas can will produce the same explosion as if you put a blow torch to the gas can. And this thinking was backed up by experimenting with mild builds. But this analogy falls apart when you get into the higher HP small and big blocks. They become very finicky as I am finding out and have to be tuned right

One thing I forgot, once you get above about 11:1cr then you have to start reducing the spark plug gap again as too much cylinder pressure makes it hard for the spark to jump anything much over .035" so up to 11:1cr you can play around. One thing not discussed enough above is the pressure of the gas to be blowed up

 

Interesting response. I would suspect that ignition requirements will tend to vary between builds.

 

I find that my engine runs much better if I turn the key to the on position first.

 

When I wrote that igniting all the fuel at once would yield the most power.....remember I also wrote...not practical...and it is not for all the reasons above.

A bigger or hotter spark is not going to increase the speed of a flame front....it would just ignite more fuel at one time. And I was thinking that this would provide a better, more complete burn within the chamber....and so I asked the question. Yes the car does have an HEI with a so called higher output voltage coil....that was how the distributor came...and the only way to use this higher voltage is to increase the gap. Will I feel any benefit.....from what was posted here....no. But this sure was a good discussion.