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Please explain spark plug gap vs. tune

BAMF

Ready to race!
Location
Jacksonville, FL
I have seen it mentioned several times that it is recommended to reduce the spark plug gap from stock .032 to .028 to eliminate or reduce misfires on a stage 1 or 2 tune. I understand the theory behind spark plug gapping, but what I'm not understanding is what about a software modification causes increased misfires. The primary parameters that are being altered are timing and boost, correct? Does one of these changes cause misfires that can be corrected with a smaller gap? How many people are actually experiencing misfires after tuning that were not present before? Does it affect the majority and the smaller gap should be taken as an across the board measure by anyone getting tuned? Or is it only affecting a handful and regapping is an easy band aid method to fix (or cover up) the problem? The tuner I normally see this advice attributed to is not a company that inspires the utmost faith for me, which really makes me question the advice, but I'm open minded and curious to hear the reasoning behind it.
 

GIACUser

Master Wallet Mechanic
Location
SoCal
I have seen it mentioned several times that it is recommended to reduce the spark plug gap from stock .032 to .028 to eliminate or reduce misfires on a stage 1 or 2 tune. I understand the theory behind spark plug gapping, but what I'm not understanding is what about a software modification causes increased misfires. The primary parameters that are being altered are timing and boost, correct? Does one of these changes cause misfires that can be corrected with a smaller gap? How many people are actually experiencing misfires after tuning that were not present before? Does it affect the majority and the smaller gap should be taken as an across the board measure by anyone getting tuned? Or is it only affecting a handful and regapping is an easy band aid method to fix (or cover up) the problem? The tuner I normally see this advice attributed to is not a company that inspires the utmost faith for me, which really makes me question the advice, but I'm open minded and curious to hear the reasoning behind it.
Well performance tunes ultimately provide more power by pushing more fuel into the cylinder..... more fuel requires more air which is provided through boost which compresses more air into the cylinder.... this results in a charge (gas/air mix) of greater density. A greater density charge becomes more difficult to ignite. As you know (understanding sparkplug gap theory) closing the gap on a sparkplug causes the tip temp to rise. The goal is to find a gap that insures a complete burn for full power. Tuners typically test various gaps to find the best and then advise you what works best. It is quite normal to require a little tighter gap with a performance tune. What works best for a particular tune is usually what the specific tuner recommends and what they used in development of their tune.
 
Location
Pikeville, KY
Car(s)
Subaru
too tight of a gap will result in detonation and too big of a gap will result in less power, fouling plugs, worse gas mileage, and miss fires. go by what vw says for a stock tune but ive heard lowering the gap about 0.04 for every 50hp you add to help with misfires.

edit... tightening to an extent obviously. i forgot this is internet
 

Goingnowherefast

Go Kart Champion
Location
Walled Lake, MI
Most tuners recommend .028 so honestly id just stick with that if you are tuned.
 

BAMF

Ready to race!
Location
Jacksonville, FL
I get what you're saying, we're cramming more air and fuel into the same space... But wouldn't that be easier to ignite as it's compressed? The motor's compression ratio is unchanged, but with more mass (a denser mixture) the combustion chamber sees higher pressure, which should be exacerbated by timing advance, right? And an air/fuel mixture becomes easier to ignite as it is compressed, even going as far as self-igniting under high enough pressure (as in diesel engines.) So is the issue that it is harder for the spark to jump the gap when under higher pressure? Or in a more fuel-dense mixture? Not necessarily richer, but since air can be compressed and liquid fuel cannot, the fuel should be occupying more volume as the piston approaches the top of the compression stroke.

Thank you for the responses, I'm just trying to wrap my head around the logic, not argue. I'm not having an issue with misfires, on the contrary I just logged today and had no misfires without reducing the gaps. I realize that the gaps tend to open over time and it may become an issue later, but I was expecting to at least see occasional isolated misfires.
 

GIACUser

Master Wallet Mechanic
Location
SoCal
I get what you're saying, we're cramming more air and fuel into the same space... But wouldn't that be easier to ignite as it's compressed? (no, harder) The motor's compression ratio is unchanged, but with more mass (a denser mixture) the combustion chamber sees higher pressure, which should be exacerbated by timing advance, right? And an air/fuel mixture becomes easier to ignite as it is compressed, even going as far as self-igniting under high enough pressure (as in diesel engines.) (Dieseling is when things go wrong for us. We are always striving to control the heat genrated by compression)

Yes, your logic here is correct and it is one of the issues we fight to control when we are trying to increase power. The heat of compression can cause the charge to ignite before we want it to. So we take steps to prevent this from happening. We do things to cool the charge (intercoolers and water meth for example). We use higher octane fuel to retard its ability to ignite. Essentially we don't want to burn the charge until the perfect moment to get the most power, so we are looking for control.


So is the issue that it is harder for the spark to jump the gap when under higher pressure? Or in a more fuel-dense mixture? Not necessarily richer, but since air can be compressed and liquid fuel cannot, the fuel should be occupying more volume as the piston approaches the top of the compression stroke.

As mentioned above we do a lot to prevent the charge from burning up until we want it to burn. When we are ready for it to burn we want the burn to be immediate and complete. The gap of the plug has a direct relationship to the actual temp of the spark plug tip. It will take a better spark to overcome the cooling and higher octane fueling (if used). Also a denser charge is not easier to ignite with a spark. Greater density of charge will require a hotter spark.


Thank you for the responses, I'm just trying to wrap my head around the logic, not argue. I'm not having an issue with misfires, on the contrary I just logged today and had no misfires without reducing the gaps. I realize that the gaps tend to open over time and it may become an issue later, but I was expecting to at least see occasional isolated misfires.

I didn't think you were trying to do anything but understand. I was asking the same questions at one point. I am not a tuner and hopefully others will correct any of my statements and add to the information about how we use spark plug gaps as part of a performance tune.
 

Strieg

Go Kart Champion
Location
Central Cali
Car(s)
2008 GTI BPY
I've never gapped a plug...and never had a problem.

My car is over 400whp. I dunno what the big deal is.

What plugs are you using? What are they gapped at?

I had misfires when I went s2+. After gapping misfires went away. (Temporarily, as coils also went bad.) but yes, as GIACuser said, a more dense fuel/air charge is harder to ignite.
 

GIACUser

Master Wallet Mechanic
Location
SoCal
What plugs are you using? What are they gapped at?

I had misfires when I went s2+. After gapping misfires went away. (Temporarily, as coils also went bad.) but yes, as GIACuser said, a more dense fuel/air charge is harder to ignite.
This is an example from APR Stage III

Spark Plugs - APR's software is design for use with NGK BKR8EIX spark plugs gapped to 0.022" ±0.002" or 0.55mm ±0.05mm with a change interval of 10-15,000 mi or 16-24,000 km.

That's to confirm that each tuner has his spark plugs gap calculation for his software.
I don't think anyone is arguing that gapping a plug is a real thing. Plug manufacturers like NGK recommend it.

NGK writes about gapping.....

Since the gap size has a direct affect on the spark plug's tip temperature
and on the voltage necessary to ionize (light) the air/fuel mixture, careful attention is required. While it is a popular misconception that plugs are pre-gapped from the factory, the fact remains that the gap must be adjusted for the vehicle that the spark plug is intended for. Those with modified engines must remember that a modified engine with higher compression or forced induction will typically require a smaller gap settings (to ensure ignitability
in these denser air/fuel mixtures). As a rule, the more power you are making, the smaller the gap you will need.

A spark plug's voltage requirement is directly proportionate to the gap size. The larger the gap, the more voltage is needed to bridge the gap. Most experienced tuners know that opening gaps up to present a larger spark to the air/fuel mixture maximizes burn efficiency. It is for this reason that most racers add high power ignition systems. (we are using stock ignition) The added power allows them to open the gap yet still provide a strong spark.

With this mind, many think the larger the gap the better. In fact, some aftermarket ignition systems boast that their systems can tolerate gaps that are extreme. Be wary of such claims. In most cases, the largest gap you can run may still be smaller than you think.
 

DerSock

Ready to race!
Location
:D
What gap is recommended for stage 2? I have .032 and its been running great. I hear .028 gives people issues...
 

GIACUser

Master Wallet Mechanic
Location
SoCal
What gap is recommended for stage 2? I have .032 and its been running great. I hear .028 gives people issues...
Well stage 2 is not really a standard you can recommend one gap generically for "stage 2". Many variables, and each tuner does things a little differently.

Best to follow what your tuner recommends but if it runs "great" don't touch it, just enjoy.
 

Strieg

Go Kart Champion
Location
Central Cali
Car(s)
2008 GTI BPY
What gap is recommended for stage 2? I have .032 and its been running great. I hear .028 gives people issues...

That's reverse for me... I'm running PFR7Bs and they came gapped around .032. Misfires ensued. Checked tuners recommendation and they said between .026 and .028. Gapped to .028 and all better.
 

DerSock

Ready to race!
Location
:D
That's reverse for me... I'm running PFR7Bs and they came gapped around .032. Misfires ensued. Checked tuners recommendation and they said between .026 and .028. Gapped to .028 and all better.
Yeah iono... I have APR software, for their "high output" vehicles they use .028 and for anything up to stage 2+ they use stock plugs (which is .032 gap right?)

You can see Arin's response about halfway down:
http://www.golfmk6.com/forums/showthread.php?t=32176&page=4

No misfires yet after a week of stage 2, but I'll definitely be watching closely.
 

zrickety

The Fixer
Location
31709
Car(s)
09 GTI
I gapped to .030 as it will get bigger over time. No issues yet.
 
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