Oem spec the spark plug gap at 0.040", Have my spark plug change a while ago and I gap my plug at 0.042" and it's running fine. However I was wondering if the coil on the XJR will be able to handle if I gap my plug to 0.044" or 0.046"? Or what is the max gap that I can safely gap the plug into without causing detonation into the redline?

On my Ford 4.6L engine spec is 0.054" however I've run my gap at 0.060" with copper plug and it likes it a lot. So wondering if anyone have any insight on supercharge 4.2 application?

Thanks.

 

I am not sure from your post why you want to move away from the Jaguar recommendation. However, 44 or 46 thou is not hugely different, being about 10% and 15% over the original setting. Well within operating limits I'd say. I may be wrong, though.

 

What's the effect on the 4.6L when running the wider gap?

 

Opening the gap has the effect of retarding the timing....loses power, I can't see the reason to mess with the factory settigns. They are engineered for the car by experts, no guessing! Yea!

 

Yes, because the coil takes a bit longer to build up the voltage to jump the gap. The ecm would at least try to compensate for that.

The wider gap yields a fatter, hotter spark when it is finally able to jump the gap.

However, if the gap is too wide, the coil will not be able to build up enough voltage to jump the gap.

But, the OP has seen a benefit on his Ford 4.6L, so knowing what the benefit is can translate into an application for Jaguars. And specifically why copper.

Tuners have always played with sparkplug gaps. A narrower gap was known to be useful for more reliable firing in a high compression, well maintained engine. A wider gap was more useful in a daily driver that was not as well maintained. The gap might also need a small adjustment to account for differences between one specific sparkplug and another, even if they are the same brand.

As for whether the factory spec is "optimal", there is good reason to believe that it is not right on the edge if there is good reason to believe that the factory spec includes some consideration of tolerance stackup. The thread in the X300 section by XJRengineer shows that there was indeed an allowance made in design for tolerance stackup as practiced in the design of the AI6.

 

Do thousandths of an inch make that much of a difference? .040 vs .044 for example

 

No.

Although it is true that a wider gap will require more energy for the spark to able to jump or ionize the larger distance, and that the coil will require additional time to store the energy to do so, the difference is immeasurably small and not visible as differences in ignition timing vs. crank and piston position.

This can be proven by taking old, worn out plugs with immense gaps that are double if not more so than the OEM new plug settings and observing when the plug fires using a timing light. If new, properly gapped plugs are then installed, there will be no difference in timing observed. I do not believe there is any device on our cars to automatically adjust for late or early timing.

As to the benefits of wider gaps or different plug material, I'll state simply that a spark is a spark is a spark- it either successfully initiates combustion or it doesn't. It's a yes/no proposition, with no variables. The ONLY function that the spark serves is to ignite the very very few droplets of vaporized fuel that happen to be in the direct path of the spark as it jumps from one electrode to the other. Those burning droplets then ignite their neighbours as the flame progresses in a controlled manner throughout the combustion chamber. Typical velocity of the flame front in a gasoline engine is about 60 feet per second. Despite the relative low flame speed, there is more than sufficient time for the fuel mixture to completely burn by the end of the power stroke, and in fact the phenomena of having a fixed rate of burn is why the ignition timing needs to be advanced as engine speed increases. Igniting more droplets with a larger gap or doing it with a fatter hotter spark essentially changes nothing.

It would be great to get more power by widening the gap or by installing miracle plugs but just as a gas cooking grill does not run hotter or colder depending on what was used to light the flame- it ain't going to happen.

 

During most of the range of operation, there may be little noticeable difference attributable to spark gap.

But, for certain operational usages, the spark gap will make a noticeable difference and is only one of the variables that can be manipulated in the spark plug.

For the "a spark is a spark" crowd ... sure why not ... throw in some used spark plugs from the lawnmower, snowblower, chainsaw, whatever it takes to get 8 plugs, mixed brands and range are ok, gap them by banging them on the garage floor, any gap between a dime and silver dollar ought to do ... if it idles like crap ... that's ok, it's firing ... most of the time.

Flame Imaging Studies of Cycle-by-Cycle Combustion Variation in a SI Four Stroke Engine

 

Please bear with me on my long post.

My 4.6L is 1996 Ford Thunderbird. Also have a 2003 Ford Crown Victoria P71(Police version). On the Thunderbird the ignition is the dual coil pack style which also employ waste spark type ignition, which means the same plug will ignited twice per power cycle once at TDC and another once is at exhaust cycle. The coil pack is dumping spark twice per engine revolution. Hence the reason why those Ford engine with those ignition type only like copper plug and not other platinium plug, iridium plug etc because it'll eat thru those plugs like crazy since platinum and iridium plug etc is not design for those type of working environment. Plus platinum is 12 times(if I remember correctly) more electrical resistance than copper. Countless dyno and test on that particular car platform forum which I once very active on always prove the copper is better suit on that situation. Only downside is that the plug need to be change out average about 15000 to 25000k miles, but it is easy enough that it's only about 30min or less job from start to finish for that Modular Ford 4.6L engine. On the Crown Victoria it use coil on plug ignition type like the Jaguar but I can remember as much detail as like the Thunderbird setup because I didn't spend enough time on it researching and I did forgot most of the more detail technical spec as those were many years ago.

My own experience that on the bigger gap to 0.060" in that setup yields better throttle response and slightly better torque and pulling power at the top end. Also from long discussion on that particular board also mention that bigger gap yields fatter spark which ignite the air/fuel mix better and faster thus result the power increase, which it does make sense to me but in this thread the timing retard effect theory also make sense to me, so it's another eye opener for me to trying to understand this better.

Now onto the Jaguar ignition setup which employed the coil on plug setup so which I believe it only fired once per revolution like most other regular engine design and the theory of coil takes longer to build up power to fire the plug which will slow down the ignition really make sense to me. So here I need to relearn what I know and I really thanks you guys for the new insight for me. I do understand that the higher the compression the smaller the gap is needed for the plug is able to fire hence why on the 4.6 NA vs SC application there is 2 different spec required. NA 4.6 required 0.052" and up gap while SC 4.6 call for anywhere from 0.038" to 0.044" gap depending boost and compression level on that particular Ford application.

At the first thread the reason I ask for max allowable gap level for the 4.2 SC application because I've always under the assumption based on previous experience that the igniton will provide the optimum spark ignition on the widest gap that will still fired without the spark being not able to jump the gap at highest compression level and RPM level.


Example like if at redline RPM 0.0440" gap the spark will be able to jump the plug gap and fired and then at 0.0441" gap the spark will not be able to jump the spark plug gap and not able to fire, then the most effiecient power producing gap will be set at 0.0440" gap.


Another theory that looks promising is spark plug indexing but I will save it for another thread to be discussed in the later date.

I'm sure there is some flaw in my reasoning in there somewhere and I am hoping that any of you can help me correct it or point me in the right direction and I really appreciate it.

 

Thanks for the details on the Ford application. Does the factory specify copper in those particular applications?

Don't let the implied longer rise time turn you off. Even *if* the ECM does not fine tune ignition timing all the time, it does have an analogue in that the anti-knock sensors come into play later than where it "thinks" they should because by retarding the ignition by a tiny bit, you are also presenting data that tells the ECM that you are running higher octane due to the fact that the anti-knock sensors are kicking in later. This in effect would be the same as a direct timing adjustment that counteracts an retardation of ignition timing due to any minor voltage rise time increase.

As a final observation, you can see that the Ford 4.6L SC gap spec @ .038-.044 brackets your Jaguar SC gap spec of .042. Now was the .060 for the Ford NA or SC?

BTW, a wider gap addresses some of the same factors as does plug indexing. Indexing is a pain for regular diy though.

 

Stock is fine, a wider gap needs a coil that can deliver enough voltage, especially under boost so you will not get a spark blowout. You can go a little wider before that will happen on a stock car, but I can't see the point as stock is already optimal.

 

Casper-

The presumption that a bigger spark results 'ignite(s) the air/fuel mix better and faster thus result the power increase' is false.

If it were 'better' this would result in more power and far more importantly less pollution- the OEMs would be all over this as a 'freebie' gain rather than spending millions of dollars on research to achieve the same or less results through alternative developments. As stated above- the spark simply ignites the atomized fuel in the path of the spark. These few droplets then ignite the rest of the fuel/air in the combustion chamber. By percentage- the spark probably ignites less than .0001% of the fuel, leaving 99.9999% not influenced or affected by the spark at all.

If it were 'faster' this would be the equivalent of advancing the timing as peak power is achieved by coordinating maximum cylinder pressure with piston position. Putting aside the fact that gas burns at a fixed rate of speed no matter what the source of ignition is, if there were gains to be had, the OEM or owner would simply advance the timing on the engine to achieve the same thing. Given that the engine already operates close to the detonation threshold under most conditions, any 'faster' combustion could possibly put the engine over the threshold and the knock sensors would put things back to square one. Nothing gained.

There's plenty of aftermarket plugs out these that promise wonderful gains based on the theories that you've presented- but each and everyone of them falls flat as has been proven time after time. No free lunch.

I remember when GM introduced the first HEI system back in the 70s which accompanied a change, at least initially, of plug gap from .035" to .060". All the hot rodders couldn't wait to toss the evil points and coils in the garbage in favour of the new system- only to find that it made NO difference whatsoever in performance or economy. GM subsequently changed the plug gap to .045" as it was found that although the system could produce enough energy to fire the plugs, the rotor caps and HT wires could not handle the amperage and were burning out breaking down in short order.

I did some reading on the waste spark ignition system. First- the system fires once per engine revolution, not twice as you stated. Second, I could find no reason to explain why iridium or platinum plugs would not survive well in such an engine as compared to copper plugs. Being that the 'wasted' spark is done at the top of the exhaust stroke with virtually no cylinder pressure present, very little spark energy is required to fire the plug. Under these conditions, erosion of the electrodes would be almost non existant, so the plugs should last as long as they would in a conventional engine with a non-wasted system. The fact that you're getting 15-25K miles out of the copper plugs for these reasons sounds about right.