Increasing the Spark Plug Gap?

[Vincent Hill]

I "Know" this looks like an Easy Question, But does anyone have "Proof" of any advantage?

I have the Permatune Ignitor and set the gap at 0.050. Then when I had some problems I reset the Gap to 0.035. Now that I have fixed all of the real problems and know that the Spark was not blowing out at high RPMS, Someone please tell me what this larger Gap is really doing?

I know that a larger gap will better ignite a less than perfect Fuel Mixture than a small weak spark. I had Motorcycles back in the early 50 that had Magneto Ignition and the plug gaps were in the low 20's

WIll it really help me to better burn all of the fuel?

WIll this make me have More HP?

Better MPG?

[wwest]

As the compression level of the F/A mixture within the cylinder increases it requires a higher and higher voltage to initiate the starting of the arc across the gap. So worse case, assuming a proper mixture ratio, is at WOT and under a heavy pulling load, HARD acceleration.

Make the gap too wide under these circumsatnces and the voltage will "look" for other places to "arc".

[sailchef]

Not to hijack the thread.....

since we're talking about spark plugs....

Would "indexing" the plugs make a difference? For the average motor/daily driver type. I have the shims but have never had that kind of "play time" to install them. Just wondering.

[Vincent Hill]

W West, A Good Answer and one I totally understand but that one would make me want to Lessen the gap! When I was taking Automotive Electricity, they way it was described to me is Electricity is "Lazy" and Always looking for the Shortest route to Ground! This ties in exactly with what you said and is the reason that they finally developed Stick Coils.

Because the engine is Air / Oil Cooled and the Compression is relatively low, (Cranking pressures of about 150 PSI) I would not think the Porsche engine could stop the spark ?

As far as Indexing the plugs, For Drag Racing Motorcycles where Everything you can do is important, but now with Surface Gap plugs and Center fire plugs, No Indexing is needed or could make a difference. The Idea was to have the Open flame facing the intake valve. But if you think about it, the fuel is not fired until After it is closed and the Piston is near the top of its travel and the fuel is in a very small area so Pointing straigh down would be best!

[sailchef]

Thanks.

[911st]

My two cents.

Low compression, poor air fuel mixing with CIS, and potential for lean conditions at low preboost rpm can make for a mix that is not even and or dose not have as many fuel molecules densely packed together.

Increasing the spark plug gap can increase the chance there will be enough fuel in proximity to insure ignition.

However, at high boost it can be difficult for the spark to jump as far.

The AFR can speed up or slow down the rate of burn but I do not think the gap has much effect on that.

Also, CIS with first acceleration goes lean for a bit intill the metering plate can start to try to maintain AFR's. Thus, you might get better results from richening your idle AFR to about 13.5 as a richer mix is easier to light, burns faster, and burns cooler.

If you go to the MSD web site and look up one of the installation guides there is gap recommendations for there products relative to compression ratio. Of course this may not transfer to a non MSD but it should give the outer limits.

MSD and CIS seem like a perfect combination.

A stock compression 930 at .8 to 1 bar is about equal to a CR of 10 to 11/1.

[T77911S]

hp gains? only in that it runs good, or it runs bad.

here is a quote i pulled from a site about spark plugs.

Insufficient gap size will not only cause misfiring during idle, but will also misfire during cruise power with lean fuel/air mixture. This intermittent misfiring during cruise lowers the temperature of the insulator tip to such an extent that lead deposits forming on the insulators may not vaporize sufficiently to keep the tips clean.


too narrow is bad, too wide is bad. you have to find the range that works best for your car if you dont want to set them to spec. but as you have found out, every thing else has to be in perfect working order.
me, i would try going wide until it runs bad, then go back about half the gap width from spec to where it ran bad, thats just if i was really interested in finding out how wide i could go.
i run mine a little wider than spec, but have never dont any tests.
an excessive wide gap puts more of a strain on the ignition system too. you will find weak points.

[moneymanager]

Jacob's book "Optimizing Your Ignition" has a good explanation. Basically, you need fuel to burn very quickly. The time in milliseconds required for the flame ball to grow from the size of the sparkplug gap to 0.1inches, when it really begins to burn, is mostly determined by what the spark gap was in the first place. Obviously if the gap was set at 0.1", the so called "growth lag" would be close to zero. When it is at .033", milliseconds are consumed. This means more ignition advance is required, meaning that the piston is working harder to get to the top of the cylinder with ignition underway, which translates into less power, more roughness, etc. But widening the gap only works if you have an ignition in first class condition.
On my car, with an MSD, going from .032 to .050 gap makes a noticeable difference in smoothness.

[wwest]

LEAN A/F mixture in a high compression engine = DETONATION = DESTRUCTION.

Like it or not the fuel is used to COOL the mixture, that's why you ALWAYS lean an air-cooled N/A aircraft engine just a tad to the RICH side. So the leaner the mixture is, to a point, the EASIER it will be to begin the ignition, flame front spread, under compression.

A richer mixture, WOT/acceleration, will mean a cooler mixture along with the increased density, making it harder to ignite the mixture, higher spark plug voltage just prior to spark jump.

[wwest]

Originally Posted by moneymanager

Jacob's book "Optimizing Your Ignition" has a good explanation. Basically, you need fuel to burn very quickly. The time in milliseconds required for the flame ball to grow from the size of the sparkplug gap to 0.1inches, when it really begins to burn, is mostly determined by what the spark gap was in the first place. Obviously if the gap was set at 0.1", the so called "growth lag" would take virtually no time. When it is as .033", milliseconds are consumed. This means more ignition advance is required, meaning that the piston is working harder to get to the top of the cylinder with ignition underway, which translates into less power, more roughness, etc. But widening the gap only works if you have an ignition in first class condition.
On my car, with an MSD, going from .032 to .050 gap makes a noticeable difference in smoothness.

"..Basically, you need fuel to burn very quickly.."

No...NOT!!

Except in diesel engines.

A nice even rate of flame front spread, spreading, results in a nice smooth downward force on the top of the piston. Detonation ALWAYS results from VERY QUICK burning of the mixture, compression brings the ENTIRE mixture volume to the point of ignition, DETONATION, simultaneously.

In theory the most return from the burning of the fuel would occur at the mid-point of piston travel due to greater angular motion of the crank. Which is one of the reasons why these new DFI engines are yielding such improving performance overall, FE and HP. A second (and third, etc.) spray of fuel can occur just as the piston approaches the mid-point of the power stroke.

[stlrj]

Unfortunately a larger plug gap also places a greater stress on your distributor cap, rotor and ignition cables with a higher chance of failure if these components are not high energy ignition (HEI) rated. For instance, HEI ignition caps have a larger diameter, resulting in a larger air gap spacing between individual ignition posts to prevent crossfire. The rotors are also correspondingly larger. HEI ignition cables do not use copper as they too easily conduct to the nearest ground and carbon core cables are far less prone to shorting.

Regardless, a bigger spark kernal as a result of a larger spark gap is always a good thing provided the rest of your ignition is up to the task.


Cheers,

Joe
74 911 w/ 86 3.2 , plugs gapped at 0.060" with HEI ignition components and stock DME.

[big911fan]

Good thing we're not making this complicated by talking about the effect that a fast burning fuel (Regular) vs a slow burning fuel (Premium) has on this topic!

Let the fun begin...

[Vincent Hill]

I am enjoying this! I have seen "Regular" Racing Gas! In the Winter time I drop from Premium to Mid grade to make it start easier. In the Summer, I actually want a little less efficiency until I can make sure the Oil temp will stay cool.

I did not know but it makes good sense that the farther the terminals are spaced in the Distributor Cap the less chance for jumping the Gap.

The Permatune says I can use a 0.060 Gap. That "Sounds" Good but for me all that means is under ideal conditions that is "Possible". My feeling is that if I can use 0.040 I am better off than I was with an 0.030 Gap.

I understand MSD (Like shooting the Bird with A Shot Gun vs the Rifle) Too many hits and the Bird will end up being worthless!

In other words, with the engines we have and the plug wires we are using, What is the largest gap we should be using to get the most out of our engines for daily use.

Last, speaking of this, there are Kits now available to replace the Coils and Spark Plug wires on my Motorcycle. Has anyone made a "Stick Coil" conversion for the 911?

[911st]

Why does a rich mix burn cooler and a lean one hotter?

I thought a lean mix was harder to ignite and burned slower and a richer easier to ignite and burned faster. That is unless it is way to rich.

Isn't that what the choke does?

My point of reference is a 930 will not run well at idle at 14.7 AFR. Bump the AFR to 14/1 and things are good.

I could very well be wrong on this.

[wwest]

The leaner the mixture, to a point, the more likely is the probability of detonation, "self-ignition" due to the heat of compression.

Ever hear of a carburator throat freezing up...? That's the result of the vaporization of fuel thereby chilling the carburator throat to the point that airborne moisture freezes at the coldest point.

Even if a lean mixture doesn't "self-ignite" it might still be heated so close to the point of detonation that once spark ignited the flame front will propagate very/too rapidly. That's called spark knock and can be somewhat overcome via retarding the spark timing.

[911st]

I understand that all bet's are off it we detonate.

I understand that a richer mix may not be as prone to detonation (because it runs cooler?).

However, not all motors running lean detonate.

If we do not detonate or pre-igniting then why dose a lean mix run hotter?

Also, I thought a lean mix is harder to ignite. Not easier. Is that not the case?

I thought a lean mix burns slower and a rich mix faster, baring detonation.

Thx.

[T77911S]

Originally Posted by wwest

"..Basically, you need fuel to burn very quickly.."

No...NOT!!

Except in diesel engines.

A nice even rate of flame front spread, spreading, results in a nice smooth downward force on the top of the piston. Detonation ALWAYS results from VERY QUICK burning of the mixture, compression brings the ENTIRE mixture volume to the point of ignition, DETONATION, simultaneously.

In theory the most return from the burning of the fuel would occur at the mid-point of piston travel due to greater angular motion of the crank. Which is one of the reasons why these new DFI engines are yielding such improving performance overall, FE and HP. A second (and third, etc.) spray of fuel can occur just as the piston approaches the mid-point of the power stroke.

if you are saying you DONT want the fuel to burn quickly, i dont not agree. you want it to burn fast, but controlled. the slower thr fuel burns, the better chance of detonation due to heat and compression. also the slower it burns, the more advance you have to run, increasing the chances of detonation.
part of the this burn time is controlled by combustion chamber and piston dome shape. a flatter piston dome and combustion chamber decreases the burn time, thus allowing for less spark advance.
here is a very good article from an engineer for the northstar.

Engine Basics: Detonation and Pre-Ignition - ClubWRX Forum - Subaru Impreza WRX and STi Community and Forums

[T77911S]

Originally Posted by 911st

I understand that all bet's are off it we detonate.

I understand that a richer mix may not be as prone to detonation (because it runs cooler?).

However, not all motors running lean detonate.

If we do not detonate or pre-igniting then why dose a lean mix run hotter?

Also, I thought a lean mix is harder to ignite. Not easier. Is that not the case?

I thought a lean mix burns slower and a rich mix faster, baring detonation.

Thx.

detonation and preignition are 2 different things.
detonation happens AFTER the plug has fired, preignition happens BEFORE the plug fires. you can live with a small amount of detonation for some time, preignition will instantly kill a motor since it happens even more advanced of what is likely an already advanced timing.

a lean mixture does burn slower, thats the problem. a lean mixture burns slower, so you may think i need to advance my timing to allow it burn, but that just adds to the problem. we have all heard our cars detonate from too much advance. and you have already said a lean mixture makes for a hotter combustion chamber, thus increasing the chances of detonation, so you would never want to advance an already lean burn condition? besides, more advance does not always equal more power.

[WIL84911]

I know this worked for my old beetles. Tried it with my 3.2 (.035 inches copper plugs 3.2 motor) and I see it still works though I noticed my low range speed suffered a bit (slight bearable hesitation which does not bother me at all) but she's so much fun to drive. I noticed the acceleration improvement specially at the top end.

[Vincent Hill]

Originally Posted by WIL84911

I know this worked for my old beetles. Tried it with my 3.2 (I think I did .2 gap) and I see it still works though I noticed my low range speed suffered a bit (slight bearable hesitation which does not bother me at all) but she's so much fun to drive. I noticed the acceleration improvement specially at the top end.

??.2 What? Inches? MM?