Spark Plug Gap vs Coil Voltage

[Five Oh Brian]

After searching, reading, and searching some more, I'm still left looking for info that I hope someone can help me with.

I have a Vortech V2 blower. With stock spark plugs I had plenty of spark blowout. Colder/copper FRPP 3V0 spark plugs helped immensely, but I initially ran them gapped as delivered (about .040). Car ran a best of 11.85 @ 113.5 in the 1/4 mile set up like this.

Several months ago, after reading plenty of posts (via search) stating that blower cars need a tighter gap, I installed a fresh set of plugs gapped down to .028. This made the car run smoother at initial tip in for daily driving, but did nothing for WOT at the dragstrip. In fact, my average trap speeds went down about 2 mph even though the weather and track conditions have stayed constant.

So, I bought hotter coils (rated at 42K volts vs the factory 20K volts). With the spark plugs still at .028 I found a little improvement (with trap speeds going back up to normal). However, my average ET's are still just a little off.

My question: Will the higher voltage coils allow me to run a wider spark plug gap? I've noticed that Ford's factory supercharged cars emission labels call for plug gaps of .032 to .035, so I suspect that I need to widen my plugs' gaps a bit and the hotter coils should prevent spark plug blowout. Thoughts please?

 

[dysan]

On my e-force set-up with the 3" pulley I'm around 11-11.5psi of boost and I'm gapped at .032 working perfectly. .028 is pretty tight unless you're running some high boost close to 20psi I would guess. I still have my stock coils as well in my car.

I would say that if you're under 15psi of boost put the stock coils back on and get some plugs in there gapped at .032.

 

[JeremyH]

Agreed. For your boost level .028 is overkill. Gap to .032. And see how she does.

I was having spark blowout at 12psi with my autolite ht0's gapped to .032 so i went to.028 and it runs better and has seen 15psi no problem. I have aftermarket coils as well.

And yes in theory increasing spark voltage should allow you to run a wider gap, but I wouldnt rely on it.

 

[cycosarge]

I had massive spark blowout with my KB set up even with colder plugs. I eventually went with Granatelli 60KV coils and all is better. I am gapped at .035

 

[RRRoamer]

Let's start at the top on what is going on to fire that spark plug, then we can see what happens with "hotter" coils and smaller and larger plug gaps. There is a LOT of misinformation out there and the companies that make these aftermarket coils don't help much.

So how does the coil/plug work?

1) Before the spark plug should fire, the primary side of the coil is grounded, causing current to start to flow through the primary side of the coil. Because the primary is an inductor, it takes a "while" for the current to build to it's peak. The ignition driver's job is to start charging the coil at the right time so the current peaks out right when it is time to fire the plug.

2) Fire the plug... That ground on the primary side is opened (disconnected from ground). The "problem" is the primary side of the coil is an inductor. It will NOT allow the current flow to stop instantly, so the voltage across the primary starts to build, rapidly. The primary side is tightly coupled to the secondary, but the secondary has MANY more turns for each turn of the primary, so the voltage on the secondary rises MUCH faster than the voltage on the primary. Something has to give...

3) Something gives... Usually, the something that gives is the spark plug gap. While there is a lot of pressure in the cylinder, the gasses are already at a high temp, so the voltage across the spark plug gap is enough to to ionize the gasses in the gap, which makes them conductive, and the plug "fires".

4) IMPORTANT POINT: Once the spark plug gap ionizes, the secondary starts to conduct because the circuit is complete and starts to dump energy (from the primary side of the coil) through the spark plug gap. And the VOLTAGE across the gap STOPS rising! In fact, it starts to collapse as the current starts to flow! If you have a "1 MILLION VOLT COIL!!!!!" on there, it will STILL only get up to about 20,000V if that is all it takes to ionize the spark plug gap! PERIOD.

5) The rest of the energy stored in the electromagnetic field is "dumped" through the plug gap, depleting the coils stored energy and (hopefully) firing off that cylinder for a nice, clean power stroke.

Summary: The voltage required to fire the plug is ONLY determined by the conditions in the cylinder (which includes the spark plug gap). The voltage of the spark will ONLY go as high as required to ionize the spark plug gap. Period. A "hotter spark" does NOT correlate to "higher voltage".

So why do FI engines have problems firing the plugs? Simple, the higher the cylinder pressure, the more voltage is required to ionize the spark plug gap. FI engines have a LOT more cylinder pressure, so they require more spark plug gap voltage.

Now, you can address this problem a couple of ways: 1) Install coils that have a higher turn ratio (aka: high voltage coils) so they can develop higher voltage on the secondary side, or 2) close the spark plug gap so it doesn't take as much voltage to fire (common option with the "0.028 inch" crowd). But there are problems with both approaches...

Let's say you close up the spark plug gap. The problem is, there is less exposure of the spark to the air/fuel mixture, which can lead to slower ignition which can lead to less efficiency (or another way of saying you are not getting everything out of your engine that you could). But it is a cheap solution...

Let's say you buy the super duper high voltage coils instead. High voltage, standard spark gap, max efficiency, etc. What more could you ask for, right? The problems can often start with the coils themselves. The problem is two fold: space and cost.

Remember that "higher turns ration" thing? That simply means for each turn of wire on the primary, you need additional turns on the secondary coil to generate that higher voltage. Which means more wire.

Problem: More wire takes up MORE space. If they want to stuff these new coils in stock size coil packs, they need to do something about it. Normally, the "solve" this part of the problem by using smaller size wire. Smaller wire, less space, right? But smaller wire has a higher resistance, so we won't get as much current flow through an equal length of this smaller wire. And Damn! The wire is LONGER, which means it would have higher resistance even if it was made of the same wire size. But it's smaller, so we are double hammered.

So, if they go this route, it simply means we will have LESS energy at the plug even if we are finally able to get our FI engine to fire reliably. Less energy means weaker spark, which means less efficient firing of the engine. Same problem we had with just closing down the spark plug gap....

Of course, this would all be easy to evaluate if the coil manufacturers would simply include electrical specs on their coils (turn ratios, primary wire size, secondary wire size, inductance, etc) so we could actually compare REAL measurements instead of "45 THOUSAND VOLTS!" or "60 THOUSAND VOLTS!". Marketing crap versus real information.

And remember, you generally want to run as wide a plug gap as you can to maximize the engine efficiency.

 

[Five Oh Brian]

Very good info and I think I'm starting to get the idea. Thanks to all for the info. My Vortech is a standard, non-cooled blower making no more than 11 psi of boost (at redline). I knew my car was down on power with the tighter gap at .028 as indicated by lower trap speeds. I just didn't know how to cure it.

I'll keep the 42K volt coils, but widen the gap back up to about .035. If I'm understanding correctly, the car will only use as many volts as required to ionize the wider gap and fire the plugs, then dump the excess voltage. If the wider gap (and enough volts to prevent blowout) makes combustion more efficient, then my trap speeds will reflect that increased efficiency.

Next track day is in two weeks. Can't wait to get out and see. Just need to see 113 mph trap speeds again and I'll be relieved. Thanks again for the insights and education.

 

[JimIII@JDM]

Just so everyone knows we have a customers 2010 SuperCobra Jet that makes over 950 RWHP with w 4.0 whipple and runs about 28 psi boost. He has run 8.50 in the 1/4 at over 160 mph and this has all been done on the stock OEM ford coil packs, same that come stock on any 05+ mustang. There is no need for aftermarket coils!!!!

Having the right heat range spark plugs, with the proper gap is all you need. In a case like above we would recommend HTO's at .030" This is the same all the way up to our 10 sec street cars and we only tighten the gap up to .028 if we are making over 600 RWHP. Idle quality is nice and smooth, we've never had an issue otherwise.

Most of the aftermarket coil packs burn out with in a few thousand miles, the stock units will last nearly 100K miles if they do not get any moisture damage.

 

[Five Oh Brian]

Just so everyone knows we have a customers 2010 SuperCobra Jet that makes over 950 RWHP with w 4.0 whipple and runs about 28 psi boost. He has run 8.50 in the 1/4 at over 160 mph and this has all been done on the stock OEM ford coil packs, same that come stock on any 05+ mustang. There is no need for aftermarket coils!!!!

Having the right heat range spark plugs, with the proper gap is all you need. In a case like above we would recommend HTO's at .030" This is the same all the way up to our 10 sec street cars and we only tighten the gap up to .028 if we are making over 600 RWHP. Idle quality is nice and smooth, we've never had an issue otherwise.

Most of the aftermarket coil packs burn out with in a few thousand miles, the stock units will last nearly 100K miles if they do not get any moisture damage.

The consensus appears to be that I gapped my plugs too small at .028, but I gotta say that doing so (even with the stock coils) really made the car run smoother at tip-in and cruise speeds, and nearly cured the spark blowout. However, that was at the expense of peak power at WOT. The aftermarket coils did help with power at WOT (brought my trap speeds back up), the spark blowout is completely gone now, and the car runs silky smooth now at tip-in and partial throttle, so the aftermarket coils were definitely worth the $250 I spent, IMO. Now I just gotta run the car with the aftermarket coils and the spark plug gap opened up to about .035 and see how the car responds. I'm hoping the extra voltage is enough to light the plugs efficiently at the wider gap.

I know that many people feel aftermarket coils are useless, but as I've already spent the money on the aftermarket coils, I'll use them until they burn out. I kept the stock coils, so they'll go back on when/if the time comes.

 

[RRRoamer]

Five,

One other thing to keep in mind: the more voltage you have on the primary, the more energy you can store in the primary. The more primary energy, the better spark and higher voltage your coils can produce when they (try) to fire the plugs.

How to get this higher voltage to the coil packs? Run larger gauge wire to them (both from the relay that provides +12V to the coils and from each coil to the PCM) so there is less voltage drop before it gets to the coils. It's pretty simple and fairly cheap to do if you are handy with electrical systems and wiring.

Just keep in mind, make sure you do a RELIABLE job as your car doesn't run so well if the coils have no power...

 

[Five Oh Brian]

Five,

One other thing to keep in mind: the more voltage you have on the primary, the more energy you can store in the primary. The more primary energy, the better spark and higher voltage your coils can produce when they (try) to fire the plugs.

How to get this higher voltage to the coil packs? Run larger gauge wire to them (both from the relay that provides +12V to the coils and from each coil to the PCM) so there is less voltage drop before it gets to the coils. It's pretty simple and fairly cheap to do if you are handy with electrical systems and wiring.

Just keep in mind, make sure you do a RELIABLE job as your car doesn't run so well if the coils have no power...

Good advice; thanks. However, I'm quite a bad electrician. My teenage daughters love to laugh at me every time I electrocute myself doing any rewiring around our house (I've done it three times in the last few years). Larger gauge wiring would need to be installed by a pro if I want to stay alive.

 

[RRRoamer]

Good advice; thanks. However, I'm quite a bad electrician. My teenage daughters love to laugh at me every time I electrocute myself doing any rewiring around our house (I've done it three times in the last few years). Larger gauge wiring would need to be installed by a pro if I want to stay alive.

To quote Clint Eastwood, "A man has got to know his limitations."

 

[Five Oh Brian]

I emailed the coil manufacturer, explained my car's setup, and asked for a spark plug gap recommendation. They emailed back stating that their coils should easily support a gap of at least .038.

With the stock coils, a gap of .040 outperformed the .028 gap (trap speed was 2 mph better with .040 vs .028), so with the aftermarket coils I believe I can run at least the .040 again and maybe a bit higher (.045 perhaps, unless I get spark blowout). And, if I understand the theory presented by RRRoamer above, a wider gap is better, as long as there is enough voltage to ionize the gap to fire the plug.

Our next (and last) track event of the season is the 16th-17th of this month, so I've got one more shot at dialing this in before the season is done. Might have to get it back on the dyno to test out other gap settings after that. I hate doing trial & error stuff, but at least pulling/gapping the plugs is free (just my time spent in the garage doing it).

 

[JeremyH]

Let us know how it works out. I personaly wouldnt widen the gap anymore with just coils. Maybe a wire upgrade or boost a spark in addition.

Also the other thing to consider is you are increasing cylinder temps (chance for detonation/melt a plug) with the wider gap so be careful on that non-intercooled setup, food for thought, good luck.

 

[Five Oh Brian]

Let us know how it works out. I personaly wouldnt widen the gap anymore with just coils. Maybe a wire upgrade or boost a spark in addition.

Also the other thing to consider is you are increasing cylinder temps (chance for detonation/melt a plug) with the wider gap so be careful on that non-intercooled setup, food for thought, good luck.

I didn't know that wider gaps raised cylinder temps. I thought it was just allowing for a more efficient and complete combustion up to the point that the increased cylinder pressure from forced induction starting inducing spark blow out. I could see too wide a gap melting plugs, though, if the voltage required the fire the plug got too crazy high.

I guess I'll be a guinea pig and see what works.

 

[JeremyH]

I didn't know that wider gaps raised cylinder temps. I thought it was just allowing for a more efficient and complete combustion up to the point that the increased cylinder pressure from forced induction starting inducing spark blow out. I could see too wide a gap melting plugs, though, if the voltage required the fire the plug got too crazy high.

I guess I'll be a guinea pig and see what works.

Yeap its the increased cylinder temp(heat) from the wider gap that allows for that more efficient and complete combustion. Which in turn means more power.

Too much heat, before the spark starts combustion early = detonation. And vice versa, not enough heat from say too much fuel and the spark isnt hot enought to start combustion(flooded). But as long as you have the right air/fuel ratio and keep iats in check you should be okay.

On that note you may need to add a little fuel back with the wider gap. Which will couteract the higher temp as more fuel helps cool more but also running slightly richer takes away some power, it all seems to be give and take lol.

 

[Five Oh Brian]

Update. I checked the original gap I ran and it was .045, which is the gap FRPP sets them at. That is the gap that I ran my best 1/4 miles runs (as quick as 11.85 @ 113.5, with several runs over 113 mph). So, with the extra voltage from aftermarket coils, I figured I'd try .045 again (up from .028, which hurt my trap speeds by about 2 mph). Our next track day is next weekend, so I'll report back with my trap speeds (anything 113 and higher would be a good sign).

The only issue I had before with the .045 gap (with stock coils) was occasional spark blow out, with a slight stumble at tip in and slight misfire at partial throttle under load. The tighter .028 gap with stock plugs worked good on the street, but not on the track. Now, back to .045 and hotter coils, it's all good. No stumble, no misfire, no problems. And there's noticably more torque with a healthy bark of the tires on the 3-4 shift on dry pavement (automatic transmission!).

 

[VTXFrank]

Back in the days of distributors and even points, the whole MSD distributor and high energy coil was to get your BB 460 to run a .55 gap that would use all the gas the very inefficient carbs were delivering. That big ass gap with a high voltage spark would mean a much better combustion cycle of the intake charge. But that gap size started dropping when TBI, then Tuned Port, then Fuel Injection as we currently know it came out. With a Supercharger, I would be terrified to run a gap of .45 without some serious voltage going through those plugs. The kind of voltage that make a set of plugs only last for a few thousand miles and make pre-detonation a real concern. All it would take is to burn the spark plug electrode down a couple thousandths and you could easily burn a hole in your piston(s).

 

[Five Oh Brian]

Back in the days of distributors and even points, the whole MSD distributor and high energy coil was to get your BB 460 to run a .55 gap that would use all the gas the very inefficient carbs were delivering. That big ass gap with a high voltage spark would mean a much better combustion cycle of the intake charge. But that gap size started dropping when TBI, then Tuned Port, then Fuel Injection as we currently know it came out. With a Supercharger, I would be terrified to run a gap of .45 without some serious voltage going through those plugs. The kind of voltage that make a set of plugs only last for a few thousand miles and make pre-detonation a real concern. All it would take is to burn the spark plug electrode down a couple thousandths and you could easily burn a hole in your piston(s).

Well, I certainly don't want to melt electrodes, but I do want to find that sweet spot where the gap is most efficient. The coil manufacturer tells me that the coils I'm running (42K volts vs the factory 20K volts) can support at least a .038 gap with my set up, and my best trap speed and ET happened with a .045 gap (and stock coils), so I'm hopeful that going back to .045 will result in my best ET's again, plus, at least 6 months between plug changes.

 

[bmaxwell]

I am curious to see how this all works out because I am looking into getting aftermarket coils but after reading that they are pretty much useless im not too sure anymore.

I was checking out Weapon X coils and they claim a 20 to 25 rwhp gain so that sounded pretty sweet but then to think they may burn out pretty quick and all that doesnt sound too sweet anymore. Anybody used Weapon X or GMS coils and had any problems??

 

[JeremyH]

I am curious to see how this all works out because I am looking into getting aftermarket coils but after reading that they are pretty much useless im not too sure anymore.

I was checking out Weapon X coils and they claim a 20 to 25 rwhp gain so that sounded pretty sweet but then to think they may burn out pretty quick and all that doesnt sound too sweet anymore. Anybody used Weapon X or GMS coils and had any problems??

The hp gain claims are bullshit, they may show a few hp which is most likely do to having failing stock cops. Just like putting in new spark plugs it can show hp on the dyno, but its just becasue the old ones are well old.