Topic: Plug question

[feliz]

I did a search but didn't really come up with an answer to this. I've always used D8ES is my 750s and went to my dealer today to pick some up and was told NGK don't make them any more and showed me the catalogue to prove it. They looked it up and Honda recommends D8EA for these bikes, you probably all new that but I've been living in the past and somehow I've been coming up with the ES plugs, lucky I guess. The Honda dealer told me the only difference in the plugs is the gap, everything else identical. Can anyone confirm this for me, just want to make sure it's OK to use the D8EAs. Every manual I have says ES are the plugs. Thanks.

[City Boy]

The A plug is fine in your machine.There are many variations in this series.The important identifiers are the D,the 8,and the E.Everything else is secondary.I currently use the projected nose,DPR8EA version.I find this version stays cleaner and resists the misfire that results when you ask for max rpm after toodling around town.   Rock On

[feliz]

OK, thanks City Boy. I see you're using resistor plugs, I didn't think that would work?

[andy750]

D8EA will work very well - at least they have for me over the past 17 years...although now Im using DENSO iridium plugs which are also very good. D8EA can be bought at any auto store.

More info about DENSO plugs here..

http://forums.sohc4.net/index.php?topic=35791.0


Good luck!
Andy

[Bodi]

The resistor is supposed to make the spark last longer, the factory caps have 5K resistors built in and the R plugs also have 5K resistors. With both, you have 20K total resistance in each coil secondary circuit since they are dual fire coils. This is a bit much but not cripplingly high - the coil is going to make a spark somewhere regardless. If your spark plug cables are iffy and run close to metal, the spark may be there instead of through a plug and higher plug circuit resistance exacerbates any such problem. If you notice some problems at high throttle or high RPM you may want to look at new non-resistor caps and/or new cables and sleeves. Non-resistor plugs are an option of course but I'm having trouble finding them lately. Maybe an auto store would have them, but here in Toronto I have looked with no success. I think the Iridium plugs have resistors.

[Deltarider]

The resistor is supposed to make the spark last longer...

You've made us curious. Please explain.

[Bodi]

Called me out, eh?
I thought for quite a while, but can't explain it to myself. I went through a bunch of stuff until it looked as if calculus would be required. I know just enough calculus toi understand that I don't know any calculus.
I lookes around the web, and found this pretty good story of a guy experimenting with resistor plugs... I don't think his explanation holds water but he claims that observation proves the spark lasts longer with resistors. This is from an ultralight aircraft forum, so the ending does make sense.


A bit about resistance.

From: Rainier Lamers <Rainier@Questek.co.za>

Subject: Was: Spark plug safety tip

Date: Fri, 26 Feb 1999 16:35:56 +0200

Recently an interesting thread on this newsgroup discussed the relative merits of resistive plugs as used on many Rotax engines.

In order to get to the bottom of this Spark plug vs. resistance story I decided to do some experiments. I am a electronics engineer (when I am not flying :-), so I have the tools for the job.

First a coil was rigged for the job to create the high voltage required for the plugs. I used a standard car coil combined with an electronic interrupter/generator. Spark plugs with and without resistors as well as caps with and without resistors where obtained. The plugs where all equally gapped to 0.4mm. I used a scope to monitor the voltage at the plug tip as well as at the coil (via high impedance probe - scopes are expensive !). The result was quite interesting.

As expected, the voltage at the cable (before any resistor) rises at the same rate regardless of resistance until the point of firing. Thereafter however the picture changes.

The coil generates a certain amount of energy. This energy wants to go somewhere. At a voltage of about 7KV the plug fires (irrespective of resistance). Until that happens NO CURRENT FLOWS. Whether you have resistance or not does not matter. A paltry 5KOhms does not do anything when compared to the nearly infinite resistance of the gap itself (until the plug fires that is).

Once the plug fires the resistance comes into being. The coil cannot get rid of its energy in the shortest possible time due to the resistance.

This reduces current flow in the spark and it takes quite a bit longer until the energy in the coil has expelled itself via resistor and spark gap. This results in a longer spark. However the spark is weaker due to energy loss in the resistor.

The scope shows this very nicely and it also gives reason why the resistor helps to suppress RF. With the resistor changes in the rate of current flow are much less resulting in "flatter" edges on the voltage vs. time curve that the scope shows you. This implies less high frequency components of the signal in the cable feeding the spark plugs.

Conclusion: It is quite safe to insert a resistor. It will not stop the plug from firing at all. High values will however lead to a weaker spark -but it is going to spark, no matter what.

However, contaminate the plug with fuel (even just a little) and a high value resistor will cause the plug not to fire. 10K plugs as are used on some motorcycles now are about maximum I would guess.

I do not recommend you use resistance plugs or caps at all on inverted Rotax installations due to the possibility of plug contamination by oil and fuel.

As further experiment I increased the resistance value to see what effect this is going to have. I tried values 10K, 47K, 100K, 220K, 470K, 820K and 1M.

Even with 1M the plug (dry, not contaminated) still fired but noticeably weaker and longer (you can actually see it firing longer !). But contaminate this plug even slightly (a little moisture by exhaling onto it) and you get no spark.

Finally, a disclaimer: All of the above has been found out by a little experimenting. Use the results at your own peril. Don't get back to be with a heap of bent metal that used to be your plane claiming it is because I said you should put 1M resistors into your spark plug leads. I did NOT.

Rainier Lamers.

[upperlake04]

   It's been posted here many times that resistor plugs cannot be used. The 78F owners manual states the DR8ES-L (NGK) or X24ESR-U (ND) plugs are correct for the Canadian model. Thats what I have used (NGK) without problems.  Don't believe everything you read on the internet, but you can believe this.
   D8EA (NGK) or X24ES-U (ND) is recommended for the US model.

[TwoTired]

The resistor is supposed to make the spark last longer...

You've made us curious. Please explain.

When the coils fire, they create a spike voltage peak with a sharp rise and fall time and a fixed duration based on flux strength and the core mass.  A series resistor delays the rise and the fall thereby broadening the spark total duration.  In a fixed voltage delivery system, the resistor also removes some peak voltage, too.  However, in our SOHC4 collapsing field coil delivery system, the voltage rises until both plugs fire.  The resistors therefore force more voltage generation at the coils to created the spark, but the spark is now expanded in duration.  The same peak voltage is seen at the spark plugs with or without the resistors.  But, the duration of spark is increased when resistors are added.

Cheers,

[Mach-5(50)]

 the duration of spark is increased when resistors are added.


is this a good thing? forgive me for being technically challenged, but would a longer spark duration potentially create a more complete burn per combustion cycle?

[Bodi]

I don't think that voltage rise time can be affected by a resistor. With no current the resistor has no effect, and voltage will not rise above the ionization voltage where both plugs arc.
The problem I have understanding it is this: the coil has a given energy to get rid of, and will provide enough voltage to accomplish this. No resistor means high current and low voltage. With resistor means higher voltage and lower current. If these changes are proportional, then the power is equal and the time to dissipate the equal energy is also equal. This is all without an actual spark though... and resistance is all messed up with a spark plug in circuit because of non-linearity in the arc's effective resistance as current varies.
Using a fixed voltage source with added resistance power flow is reduced (there's an incongruity approaching zero ohms where power and current rise to infinity). With a fixed current source, adding resistance will increase power flow... again at zero ohms things get a bit impossible as although the fixed current flows there's no power because of the zero voltage.
The coil secondary is neither a constant current or constant voltage source though... what it is exactly is unknown to me.
Once the arc is initiated, things get more complicated. The arc has non-linear electrical characteristics but generally with low arc current the arc voltage will be quite high, maybe 1000 volts. With higher current the voltage may be as low as 100 volts.
This gets back to power. The power level (and therefore duration) will be the same for a 1000V 100mA arc and a 100V 1A spark. Practical experience, though, has shown that the 1A spark will perform better at igniting an engine's fuel/air mixture though.
Also important is time. That 1A spark is better in a motor but a longer duration spark is also better than a very short duration one.
Experiment has shown that about 300mA is about the lowest spark current acceptable. With 20K of plug and cap resistance in the secondary circuit (arc resistance becomes trivial) the coil voltage during sparking needs to be 6000 volts for 300mA.
A coil firing event is not all about resistance, just to make things more complicated. The rapidly rising voltage brings with it a lot of "AC" considerations: coil impedance is particular. The DC resistance of the coil is about 13K. The impedance affecting the fast rise time of secondary voltage is not going to be close to the DC resistance. Guessing the coil source impedance is vital to estimating the arc current with no resistor but I don't have any guesses. I think what happens must be that the arc voltage and current settle wherever the voltage equals the coil's voltage capability... determined by the current across its impedance (which changes depending on dv/dt). Adding a resistor should reduce the effect of the arc's non-linear EI characteristic but why that would lengthen duration...
SO I'm back to head scratching. The guy who measured it and found resistor plugs give a longer spark duration has the only answer I can understand right now.

[TwoTired]

When the points open, the stored energy in the coil's primary causes the magnetic field to collapse.  As these collapsing lines of flux cross the coil secondary windings, they generate a voltage in the secondary circuit, including at the spark plugs tips, that builds until the gases between the spark plug electrodes ionize, forming a plasma conductor.  Until the plasma is formed no current flows in the secondary.  (Note that both ends of any series resistor will be at the same potential before the circuit path is completed by the plasma arc.)
With no added secondary resistance, current flow is restricted only by the resistance of the circuit. Spark current swiftly rises and falls in shortest duration.  Adding a resistance to the arc event, slows the current rise and fall of that event and lengthens the duration of the spark event.


A longer duration spark event, allows more of the air fuel mixture to ignite at the beginning of the fuel burn, and usually more complete combustion over the entire power stroke.

Cheers,

[feliz]

I just thought it would boil down to Ohm's law (I=E/R) were if we double the resistnce we would half the current. Seeing as how a spark is a flow of electrons I assumed we would have half the spark (5000 + 5000 ohm=10,000ohm). Nothing is what it seems anymore.

When I learned that in grade seven physics I was still dipping the girl who sat in front of me pigtail in the ink well on my desk and I probably missed something important. For you young'uns, before ball point pens we used a pen with a nib and dipped it in an ink well for writing our ABCs. I guess I missed the part about coils and CB750s.

[andy750]

I just thought it would boil down to Ohm's law (I=E/R) were if we double the resistnce we would half the current. Seeing as how a spark is a flow of electrons I assumed we would have half the spark (5000 + 5000 ohm=10,000ohm). Nothing is what it seems anymore.

When I learned that in grade seven physics I was still dipping the girl who sat in front of me pigtail in the ink well on my desk and I probably missed something important. For you young'uns, before ball point pens we used a pen with a nib and dipped it in an ink well for writing our ABCs. I guess I missed the part about coils and CB750s.

Ink and pens with nibs??? When was this the 19th century? Where the "pens" really feathers and you dipped the tip in the ink?  Perhaps the gaslight went out and you missed the chalkboard drawing of the 750/coil diagram? 

cheers
Andy