Spark Plugs - Are all those fancy plugs that claim increased power/mpg worth it?
Mar 26, 2009, 09:51 PM
#1
Spark Plugs - Are all those fancy plugs that claim increased power/mpg worth it?
After seeing several questions on this topic I decided to address it once and for all.
First some background to substantiate my explanation. I have a Masters in Automotive Engineering from U of Michigan Ann Arbor and work full time as a combustion performance engineer. As such I have a reasonable understanding of spark plugs and their effect on combustion. The purpose of this post is to help clarify some of the key misconceptions about sparks and spark plugs and offer some of the real hard facts involved in a simple easy to understand way.
In a spark ignition engine, the Voltage used to create the spark itself has NO IMPACT on engine power. All the different types of plugs hotter/colder/iridium/multipoint etc all of them have the basic function of starting the combustion process by creating a flame kernel at the spark. Don't let them fool you into false claims. Here is why:
The power and torque generated during combustion is a factor of the speed and efficiency of the flame propagation as it spreads across the cylinder. In other words, how quickly and completely the starting flame kernel (initiated at the spark) spreads outwards to the cylinder wall burning the charge. This laminar flame speed is related primarily to the localized A/F around the spark plug as well as in cylinder turbulence (swirl, tumble, and squish) of the charge and has NO DIRECT RELATION to spark voltage.
That said, given a higher voltage, and a more powerful spark, it may be possible to widen the spark plug gap slightly. This will increase (very slightly) the size of the original flame kernel and given absolutely identical A/F and in cylinder turbulence compared with a smaller gap, the larger one might show a tiny gain in laminar flame speed based solely on the fact that the longer spark will allow a larger kernel and a slightly higher surface area in the developing front. Will it be enough to justify a bunch of money for fancy plugs? Not likely. In fact it is unlikely the gain will even be noticeable if there is any at all. More on this later...
So what benefit will a higher voltage spark have? More voltage makes the spark more RESISTANT to misfires because it will be harder to extinguish when running high boost levels and extreme charge mixing turbulence in the cylinder. In other words a stronger more powerful ignition system will not serve to increase engine power directly, what it will do is resist misfire better allowing the engine to be tuned to run higher boost and more harsh in cylinder conditions.
What is there to take away from this information?
Once you have a ignition system and spark plug setup that consistently ignites your fresh mix (no misfires) at every operating region without getting fouled up (due to overcooling) or degrading (due to overheating), you have won. At this point increasing the voltage to spark plug, upgrading plug wires, or changing the type of plug will do nothing beneficial at all. If you have that already, there are no improvements to be made.
When should you look to upgrade your ignition system? When you start to see consistent misfires on high boost or extreme operating regions.
I have a colleague of mine who is on this forum (VishnuNair) who has access to some testing equipment in his lab. I've discussed this issue with him ad nauseum and he mentioned he might try to do a quick test in the lab if he can get away with it. I asked him to post results in here if he is able to do so.
First some background to substantiate my explanation. I have a Masters in Automotive Engineering from U of Michigan Ann Arbor and work full time as a combustion performance engineer. As such I have a reasonable understanding of spark plugs and their effect on combustion. The purpose of this post is to help clarify some of the key misconceptions about sparks and spark plugs and offer some of the real hard facts involved in a simple easy to understand way.
In a spark ignition engine, the Voltage used to create the spark itself has NO IMPACT on engine power. All the different types of plugs hotter/colder/iridium/multipoint etc all of them have the basic function of starting the combustion process by creating a flame kernel at the spark. Don't let them fool you into false claims. Here is why:
The power and torque generated during combustion is a factor of the speed and efficiency of the flame propagation as it spreads across the cylinder. In other words, how quickly and completely the starting flame kernel (initiated at the spark) spreads outwards to the cylinder wall burning the charge. This laminar flame speed is related primarily to the localized A/F around the spark plug as well as in cylinder turbulence (swirl, tumble, and squish) of the charge and has NO DIRECT RELATION to spark voltage.
That said, given a higher voltage, and a more powerful spark, it may be possible to widen the spark plug gap slightly. This will increase (very slightly) the size of the original flame kernel and given absolutely identical A/F and in cylinder turbulence compared with a smaller gap, the larger one might show a tiny gain in laminar flame speed based solely on the fact that the longer spark will allow a larger kernel and a slightly higher surface area in the developing front. Will it be enough to justify a bunch of money for fancy plugs? Not likely. In fact it is unlikely the gain will even be noticeable if there is any at all. More on this later...
So what benefit will a higher voltage spark have? More voltage makes the spark more RESISTANT to misfires because it will be harder to extinguish when running high boost levels and extreme charge mixing turbulence in the cylinder. In other words a stronger more powerful ignition system will not serve to increase engine power directly, what it will do is resist misfire better allowing the engine to be tuned to run higher boost and more harsh in cylinder conditions.
What is there to take away from this information?
Once you have a ignition system and spark plug setup that consistently ignites your fresh mix (no misfires) at every operating region without getting fouled up (due to overcooling) or degrading (due to overheating), you have won. At this point increasing the voltage to spark plug, upgrading plug wires, or changing the type of plug will do nothing beneficial at all. If you have that already, there are no improvements to be made.
When should you look to upgrade your ignition system? When you start to see consistent misfires on high boost or extreme operating regions.
I have a colleague of mine who is on this forum (VishnuNair) who has access to some testing equipment in his lab. I've discussed this issue with him ad nauseum and he mentioned he might try to do a quick test in the lab if he can get away with it. I asked him to post results in here if he is able to do so.
Last edited by ktk; Mar 26, 2009 at 09:57 PM.
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Mar 27, 2009, 07:59 AM
#11
Choice of metals for spark plug tips revolves around several factors, but two of the key ones are:
1) longevity
2) heat transfer
Iridium is one of the metals used in spark plugs tips because it fulfills these two requirements quite well. It resists electrical arc erosion, (tendency of metal to erode away when subjected to repeated electrical sparks), and it has a very high melting point making it a good choice to use in applications (like spark plugs) where extreme temperatures must be withstood.
The ability to transfer extreme heat loads away from the tip also varies from metal to metal. If a plug transfers too much heat away from the contact points, spark plug fouling will be the result. Unburned or partially burned fuel will condense on the surface of the contacts, increasing their electrical resistance and reducing their efficiency and ability to generate a strong spark. These deposits can lead to hotspots the can in turn lead to preignition of the charge. On the other hand, a plug that doesn't transfer enough heat away from the contacts could melt when subjected to extreme temperatures (like a high boost 4G).
All of the various metal choices that are used in spark plug tips have pros and cons in this regard. But OEM will always tend to sacrifice a bit of outright performance (for any part), for longevity in a factory application for reliability,warranty, and running cost reasons.
Most of the factors revolving around metal selection have to do with spark plug contact conditioning and longevity. The actual metal itself has no bearing on engine power.
Last edited by ktk; Mar 27, 2009 at 08:01 AM.
Mar 27, 2009, 07:59 AM
#12
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Based on the manufacturing cost of the iriduim vs the standard plugs and the above stated info wouldn't the manufacturer be prudent to install standard plugs in order to save on output costs? After all, plugs are a wear item and not under warantee any how.
There has to be a benefit.
There has to be a benefit.
Mar 27, 2009, 08:06 AM
#13
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Sorry to post so quickly. Thank you for the response. No more Iriduims for me....40 bucks for plug replacement sucks. NGK BRE7 or 8s from here on out.
Thanks for the information.
Thanks for the information.
Mar 27, 2009, 08:10 AM
#14
Based on the manufacturing cost of the iriduim vs the standard plugs and the above stated info wouldn't the manufacturer be prudent to install standard plugs in order to save on output costs? After all, plugs are a wear item and not under warantee any how.
There has to be a benefit.
There has to be a benefit.
From an OEM perspective there is a lot more to warranty costs than that of the spark plug itself. OEM would have to pay for dealer visit, and diagnosis time and all that in addition to the price of the plugs. As a result it makes sense for them to recommend running an iridium plug whose slight 20$ increase in cost would pay for itself 10x over if it saved a trip to the dealer under warranty.
The small displacement high boost application is probably one of the WORST most demanding application for a spark plug out there. So OEM errs on the side of safety.
Mar 27, 2009, 08:56 AM
#15
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question ktk, I'm NOT a Phd or anything of the sort, I find that your information to be very enlightening (good stuff), now if I may ask you for your opinion.
1) what would be the limit or is there a limit before the stock coils have to be upgraded to meet the Boost demands to prevent misfiring?
2) wouldn't the Coil Over Plugs (C.O.P.) by design be better than the stock coils connected in series/parallel or is that just over kill and hence a waste of money?
1) what would be the limit or is there a limit before the stock coils have to be upgraded to meet the Boost demands to prevent misfiring?
2) wouldn't the Coil Over Plugs (C.O.P.) by design be better than the stock coils connected in series/parallel or is that just over kill and hence a waste of money?