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Old 09-06-2012, 11:58 PM   #1 (permalink)
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spark plug heat range effect on fuel econ

lately i have been surfing the internet to find general mods, applicable to all makes and models that are in the (or close to) realm of hens teeth. my friend has a 4 cyl wrangler that we have modded for better power and econ. we did the 4.0 throttle body swap and the duratec injector swap and he is getting 22-23 on the highway.... anyway back to the point, i have been looking at ignition tweaks other than the standard indexing and gap expatiation and i came across an article that compared how heat ranges other than factory could be beneficial (albeit in circle track racing). anyway here it is

Heat Range Selection – there are basically two theories on choosing a plug’s heat range for an engine and the tuning that goes with it.
1. Choose as hot a plug as possible – this choice has been the approach for many years and is justified by the thought that you eliminate any low RPM fouling and stumble, and that the hotter plug will light the flame faster at all RPM resulting in a increased burn rate. Advocates of this idea don’t mind cooling the plug by adding more fuel, (richening the engine), and decreasing timing for the race. Most of the supporters of this theory are drag racers, where fuel economy isn’t so important and a slight denotation can be caught before any damage is done.
2. Choose as cold a plug as possible – this is a fairly new idea but is gaining a lot of popularity among the oval track and road racers. The approach here is to run a cold plug coupled with a lean mixture and sometimes an increase in timing. This choice eliminates the possibility of the plug limiting the air/fuel ratio and ignition timing by becoming a pre-ignition point thereby allowing the tuner the ability to find “best power” in both of these cases. Some reports are that fuel mixtures much leaner, and timing higher than previously thought possible have been run with success. This can be a benefit where fuel economy is an issue. Other benefits to the colder plug are that it is more sensitive to tuning changes as not so much of the fuel is burnt off by the insulator heat, also with the increase in compression ratios and subsequently cylinder pressures the colder plugs have provided some insurance against pre-ignition/denotation and are probably much more in-line with the proper heat range for the cylinder temps. In my opinion the only concern with this approach would be in the low RPM situation where there could be the possibility of misfires. However most of the racing done today is at a relatively high RPM and most racers could probably use a cooler plug with no ill effects and the possibility of some benefits, just be sure you have enough ignition.

my thoughts are that it may actually work, and a hotter range or projected plug may be of benefit on vehicles with ecu controlled timing advance and air fuel ratio....and a hotter plug is only good for a 70c jump in plug temp, many vehicles could take up to two steps of heat range before tip temps become an issue.

also thoughts on expanding the gap on finewire precious metal plugs for a larger spark kernel? all the literature i have read states that plugs like ngk G power plugs take less voltage to light off, i was thinking maybe a 6-8 thou over stock jump may help economy
thoughts?

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Old 09-07-2012, 01:38 AM   #2 (permalink)
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It seems like a lot of cars are factory tuned to accelerate and idle smoother than smooth more than for MPG.
If you advanced the timing and lowered the idle,increase the spark gap on cold plugs & boost the coils output i think, it would run more responsive to throttle input possible requiring less throttle to get it moving.
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Old 09-07-2012, 05:57 AM   #3 (permalink)
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But how about on the vehicles with computer controlled timing and fuel? The way I see it It's not going to benefit from the colder plug. with a hotter plug, you get a faster flame front, a more complete burn, and hopefully the ecm will learn and lean the mixture. At the very least you should net a little bit more power, and have to use less throttle to compensate
Or my ideas completely off base?
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Old 09-07-2012, 01:17 PM   #4 (permalink)
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Normal spark plug (electrode) temperature is around 1,000ºF, while arc temperature is vastly HOTTER; so, guess which one actually ignites the A/F mixture (short of knock)?

Flame propogation, however, is more a function of the gasoline composition, its octane value, the compression ratio, and A/F mixture movement (swirl) and homogenity (mixture evenness).

Some SAE documents on this subject:

SAE 690018 - Inlest Manifold Water Injection for Control of Nitrogen Oxides--Theory and Expedriment.

SAE 700081 - Spark Plug Design Factors and Their Effect on Engine Performance.

SAE 710832 - Effect of Compression Ratio, Mixture Strength, Spark Timing, and Coolant Temperature Upon Exhaust Emissions and Power.

SAE 710835 - Effect of Engine Intake-Air Humidity, Temperature, and Pressure on Exhaust Emissions.

Last edited by gone-ot; 09-23-2012 at 01:21 PM.. Reason: added (electrode)
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Old 09-07-2012, 01:40 PM   #5 (permalink)
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Quote:
Originally Posted by hansj3 View Post
But how about on the vehicles with computer controlled timing and fuel? The way I see it It's not going to benefit from the colder plug. with a hotter plug, you get a faster flame front, a more complete burn, and hopefully the ecm will learn and lean the mixture. At the very least you should net a little bit more power, and have to use less throttle to compensate
Or my ideas completely off base?
Not off base at all, its all up for conjecture, i see your point. Sounds good.
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Old 09-07-2012, 01:49 PM   #6 (permalink)
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Quote:
Originally Posted by Old Tele man View Post

SAE 690018 - Inlest Manifold Water Injection for Control of Nitrogen Oxides--Theory and Expedriment.

SAE 700081 - Spark Plug Design Factors and Their Effect on Engine Performance.

SAE 710832 - Effect of Compression Ratio, Mixture Strength, Spark Timing, and Coolant Temperature Upon Exhaust Emissions and Power.

SAE 710835 - Effect of Engine Intake-Air Humidity, Temperature, and Pressure on Exhaust Emissions.
I went for a look for a paper, it seems they are selling the info, with all the links i was finding.
Specifically the SAE 700081 - Spark plug design factors and their effect on engine performance.
Thanks in advance if you can post a link to it.
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Old 09-09-2012, 07:50 PM   #7 (permalink)
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ecomodded -- I'll scan the copy I have if you give me somewhere to e-mail them to.
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Old 09-10-2012, 12:16 AM   #8 (permalink)
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i have found this snippet about plugs in the wikipedia page about spark plugs,

The central electrode is usually the one designed to eject the electrons (the cathode) because it is the hottest (normally) part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces (see thermionic emission

any thoughts about this section? what i have infered is that the hotter the material the easier it is to emit electrons. i am however not an expert in thermonic emission and dont know if there is a point where increased temps dont make for better electron flow.

also, i have sent a pm and would like to see the SAE 700081 paper if at all possible
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Old 09-10-2012, 02:32 PM   #9 (permalink)
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It doesn't really matter how easy or how difficult it is to emit electrons. As long as there is a spark being generated that is capable of igniting the fuel-air mixture that is around the plug, you don't really need anything else.

There is no magic bullet, no magic set of plugs you can swap in and see a 50% increase in MPG over a good fresh set of standard plugs. Or even a 5% increase.

-soD
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Old 09-23-2012, 12:28 PM   #10 (permalink)
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I'll disagree with you Dave.
IF the engine is misfiring at light load/lean mixture due to too cold an electrode temperature, a plug heat range change may help get more consistant ignition and improve fuel mileage. I have accomplished that in the past on carbed engines leaned out and lightly driven, hotter plugs improved driveability and fuel mileage.

Here it is, you have fresh mixture sitting between the plug electrodes. It is compressed, adding heat to it, all you need to do is trip that fuel over the ignition threshold energy level. Even a weak spark will do it, but wait, if the electrode is cold, mixture lean (low load) and compression low (throttle plate near closed) there just isn't anything to sustain that little kernel of flame and it goes out. Again and again. The losses in fuel and power, and resultant increased throttle opening can be substantial.

Now, IF you have the light load mixture near perfect (toward lean) and the throttle near closed due to light load, you CAN get more consistent ignition if your plug temperature was up near 400f-500f because you installed hotter plugs. Will this improve economy? Darned right it will.

Is there any risk? YES! If you run at full power (WOT) for any length of time you risk spark plug temperatures over 500f, plug damage, preignition, detonation and all the damage that comes with it. So usually plug temperature is set by full power needs.

Modern engineers know all about this. This is the main reason we have such high powered ignition systems these days. Arc weld the darned thing alight! So in most cases, little to be gained by installing hotter plugs IF EVERYTHING ELSE IS PERFECT.

I got this post mixed up with another one that I answered about plug types. So you might find some kernels (that is a flame front pun!) of information.

Steve

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