I am looking for a calulation to determine appropriate spark plug gapping based upon cylinder pressure.

As we are aware, it is necessary to reduce the gap on your plugs (and drop a temp range or two) when inducing a turbo or supercharger and/or nitrous. What is not known, is exactly how much. There is a lot of empirical information, usually based upon success and failure more than thoughtful analysis.

As the cylinder pressure increases, what stresses are placed upon the ignition system to create an adequate spark and though the common adjustment is to shorten the plug gap, exactly how much is not defined anywhere with engineering accuracy.

If my compression ratio is 8.0:1, 8.5:1, 9.0:1, or 10.5:1 and cylinder pressure is then increased in one pound increments due to a blower, then what is my true cylinder pressure and when exactly does spark break down.

I realize there are several components other than a spark plug in the ignition that could be factors, but that opens this discussion up too wide to other variables.


I am told that if I have an 8#, intercooled LT1 with 9.0:1 CR on forged pistons, I should drop my heat range by one notch and decrease the spark plug gap from .050" to .035 or thereabouts. I have found (through hit and miss) that .030" works a lot better in resolving my 4500RPM hit, but I am now experiencing a 5,000+ RPM miss. So, just dropping another .002 or .005" may solve the problem, but I want to know how to calculate this properly so I don't have to keep pulling and adjusting and testing.

Is there a calculator available?

UD

 

Don't think the calcs would be of much use without knowing the capacity of the ignition system. Then you mix in differing electrode materials/diameters different indexing from cylinder to cylinder (if they're not indexed) and things get complicated in a hurry.

What's wrong with buying a good magnifying viewer and "reading" the spark plug to see which way you need to go?

-Mindgame

 

Here's my low-tech, shade-tree opinon on this, having built lotsa nitroused motors and a small handfull of boosted engines.....

Get a good ignition box and set 'em at ~.035.

Stock ignitions under high-squeeze applicatios run outta poop pretty quick. It takes volts to jump the gap (aginst high cylinder pressures) but it takes amps to keep the spark lit long enough to ensure good combustion.

Stock igintions can often produce the volts to jump the plug if you gap down (as you have done) but they often just don't have the necessary "oomph" to get the job done and keep the fires lit in the upper RPMs against a combo running a power adder.

No, just bolting on a better coil won't usually solve the problem.

 

So... Your variables are:

1. Spark voltage - the higher the voltage, the longer the gap or higher cyl pressure may be.
2. Cyl pressure - higher pressure needs more voltage or a shorter gap.
3. Gap - Longer gap means higher voltage or less cyl pressure.
4. (turbulence)

1st step would be to find out what kind of cylinder pressure's you're seeing. Engine Analyzer Pro may be able to give you a good estimate. Changing your timing will change the crank angle of your spark event, which will be a different cylinder pressure.

I suppose one could find out how the conductivity (resistance?) of air:fuel compressed at different pressures is. Will changing the mixture affect it? Knowing the voltage at the spark plug, you can calulate what pressure your gap will become "iffy".

 

Thanks for the info.

It still does not provide any resource for plugging in variables to calculate.

If GM, Ford, Toyota, et al can determine their ignition component requirements, it seems to me there is a real-world mathematical solution to this issue.

Popping plugs in and out and just pouring technology on the problem is a shotgun approach at best.

Years ago, computer programmers use to have to count their bits and bytes on every program, but now with gobs of disk and memory space available, they have become lazy. I see it in data networking and lots of other situations where the abundance of resources reduces the need to conserve. If a 1,000 Gazillion volt MSD-type ignition system were available for a few dollars and it didn't melt my ignition wiring, it would be a non-issue. But I am not convinced that is the solution to my problems.

So do you suggest just to pour money on the problem, replacing everything wily-nily that is in the path of the ignition spark, hoping for an overwhelming victory? Seems like a pretty poor engineering solution to a technical problem.

UD

 

It's not laziness. It's lack of access to the rather sophisticated test equipment you would need to actually figure out what's going on inside a running engine engine. And besides, ignition is not something you ever want to be "barely adequate." You always want it to be overkill becuase the various ignition components on the secondary side start degrading from the first time you start the engine. And conditions inside the cylinder vary considerably from one cycle to the next. What's going on inside the combustion chamber is far from a static model.

Here's one tool you could use that IS readily available.... an inductive voltage gague. Designed so it can read how much voltage is actually shooting down the wire to the plug. I've used them and they usually indicated that it only takes about 8K Volts to jump the plug gap at idle (voltage stops building as soon as the arc is established between the plug electrodes- after that it's all about amps). Do a quick throttle snap and it'll spike over 20KV. Spend some time with this little tool and you can actually learn quite a bit about what it takes to get the job done lighting the fires on a running engine.

Sorry I can't help with the math side of things. Not my strong suit.

 

I picked this PDF file off the web, there's lots of stuff in it . Hope it helps ya. The file is large 1.5meg.

http://www.sps1.com/spark.pdf

 

Just a guess, but I'd be surprised if even the OEM's "calculate" plug gap. I imagine they are quite empirical about it - trial and error.

I think Damon is right. Use a decent ignition, gap 'em at ~0.035" and go racing!

Rich Krause

 

What's the trade off? Fuel economy over ensured spark?

What if we were to gap all the plugs regardless of cylinder pressure
to 0.035"?

What is the harm? Would it cause premature wear to the electrodes?
Perhaps "bridging" if the gap is too tight?

I have used gaps between 0.040" and 0.055" with a MSD 6AL
and Blaster 2 coil, Accel 300+ wires and NKG UR5 plugs (one step colder)
on a first gen iron motor.

Cranking pressure is 235 PSI with an LOOSELY calculated static
compression of 11.2:1

All this on 92-94 pump gas with details in the signature link.

RE: Gap Setting Variance

I haven't noticed a performance gain, or fuel economy gain in over
7 years of driving this car and hundreds of passes down the 1/4 mile.

Am I missing out on something more?

 

The 0.035" referenced above is in the context of very high cylinder pressures from forced induction. Higher cylinder pressures require more voltage to jump the gap and if the ignition can't supply it you will have an inadequate spark. Most basic aftermarket ignitions can handle 0.035". Using a larger gap, if your ignition is up to it, sometimes makes more hp due to more rapid and complete combustion from the stronger/bigger spark. But 0.035" seems perfectly adequte with no real gains from a larger gap even if the ignition can fire it, at least for blower LT1 cars.

I have a pretty basic ignition (Opti, MSD6 box, MSD coil, stock wires) and have no spark related problems with plugs gapped to 0.035".

Rich Krause