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[Hoist!]

We ride these HD TC bikes with 88 inch to over 131 cubic inch displacement. All kinds of engine setups with HP and TQ all over the place. They use many different maps, and have a wide range of fuel consupmtion. I rarely ever hear of spark plug selection coming into play. These engines all seem to use the same plug. Unless we're missing matching plugs as part of the equation? There's not many factors. Diameter and reach are fixed. But heat range can be selected. What will determine the optimum heat range for the plugs? Does a bone stock 88" with 55 HP use the same plug as a 120HP 110, or a 150HP 131?

This might be a stupid question, and no, I haven't researched it. Just thought I'd throw it out there for discussion. I found this to be the best source for research anyway!

Hoist!

[resqdoc]

Been wondering the same thing. Other groups of mechanical enthusiasts are fanatical about this topic.....

[CVOMOE!!]

Check the plug to see how it is burning.  Adjust hotter or colder depending on whether or not it is running lean or rich.  Shouldn't matter much if the bike is Dyno tuned.  I use the cross referenced NGK plug and it works great.

Moe

[Twolanerider]

There's a few options.  Be careful when varying from the base fitment though.  Some plug manufacturers a larger number means a hotter plug.  Some it means a colder.

[Hoist!]

There's a few options.  Be careful when varying from the base fitment though.  Some plug manufacturers a larger number means a hotter plug.  Some it means a colder.

That's the standard matrix. I'm trying to understand what determines the plug you need in the first place. Is a plug for 88/96" good for infinite size engines? Does size even matter in determining spark plug type in the first place? An engine won't run without them, but there's some mysery behind them. We just put them in without questioning.

Hoist!

[rednectum]

hottr plugs are to burn off contaminates. colder plugs are to allow the plug to live in its environement.

[Twolanerider]

That's the standard matrix. I'm trying to understand what determines the plug you need in the first place. Is a plug for 88/96" good for infinite size engines? Does size even matter in determining spark plug type in the first place? An engine won't run without them, but there's some mysery behind them. We just put them in without questioning.

Hoist!

Howie, I never found any kind of guide as you're suggesting.  Something that said:

"If adding X pounds of compression and Y percent of cylinder pressure when firing Z degrees BTDC use a plug varying from stock by XYZ heat range."  (don't take literally, made up just to illustrate there is no readily available equation, resume regularly schedule posting)

Never having seen or heard of such a thing, and not expecting to because the variables are mind numbing, I always tune plugs by sight.  Pull them, read them, change accordingly, rinse and repeat.  Once the looks is where you want it just keep using that until something else changes.

[grc]

Assuming no physical change to the head that would require a different thread diameter or reach, and assuming no change to the head or piston that would require a shorter tip projection for physical clearance, and assuming no secondary ignition changes that would affect the spark gap, the only characteristic you may need to tweak is the heat range.  The tip of a spark plug needs to run within a certain range of temperatures (approximately 900 - 1500°F if I remember correctly).  Too cold, and you get carbon deposits and fouling.  Too hot, and you get preignition and rapid electrode erosion.  Fortunately, most modern plugs will perform adequately over a fairly broad range of combustion chamber temperatures.  Those temperatures are affected more by riding style than actual piston displacement, with the highest temps coming at the point of maximum torque output.

The best way outside of the lab to determine if you need to try a different heat range is through good old fashioned plug readings.

Jerry    

[rednectum]

looks like grc summed it up nicely.

[SneakyPete]

"If adding X pounds of compression and Y percent of cylinder pressure when firing Z degrees BTDC use a plug varying from stock by XYZ heat range."  (don't take literally, made up just to illustrate there is no readily available equation, resume regularly schedule posting)

Don, you forgot applying A coats of micro polish, B coats of Spider and C coats of pro polish will reduce the drag coefficient proportional to the angle of the dangle.

Howie, open the door in the garage, you are sniffing too much Glare! 

[Gecko]

The best quote I've heard is, "spark plugs are basically a nail with some insulation around it, and of the two the insulation is more important".  It's kind of like gasoline - if it's good enough there isn't much to be gained by switching.

[SPIDERMAN]

One of the things I'm big on is indexing my plugs. For those of you who don't know what that means, it is aiming the flame kernel from the plug at the intake valve. Another thing I USED to do, but don't anymore was to gap my plugs .005 over spec. It's an old drag racers trick that while making the motor start hard creates a nice fat spark. The trouble is these new ECM's read the increased demand on the coil and retart the ignition so you are defeating the purpose. I find that 0.038-0.042 works fine. Anything larger and you get the aforementioned effect. Beyond that, I am big fan of split or dual electrode plugs. I also like the SE Triple Platinum as they seem to hold up really well. To answer Howie's orginal question I think the hot / cold range plug went the way of the dinosaur with modern ECM/EFI. I think that as I mentioned with the overgapping trick, the ECM would adjust for any attempt to outsmart it. Of course I could be wrong 


B B

[rednectum]

One of the things I'm big on is indexing my plugs. For those of you who don't know what that means, it is aiming the flame kernel from the plug at the intake valve. Another thing I USED to do, but don't anymore was to gap my plugs .005 over spec. It's an old drag racers trick that while making the motor start hard creates a nice fat spark. The trouble is these new ECM's read the increased demand on the coil and retart the ignition so you are defeating the purpose. I find that 0.038-0.042 works fine. Anything larger and you get the aforementioned effect. Beyond that, I am big fan of split or dual electrode plugs. I also like the SE Triple Platinum as they seem to hold up really well. To answer Howie's orginal question I think the hot / cold range plug went the way of the dinosaur with modern ECM/EFI. I think that as I mentioned with the overgapping trick, the ECM would adjust for any attempt to outsmart it. Of course I could be wrong 


B B

i hear ya. used to close gap to fine tune advance!

[TORKER07]

OK Guys, Here is some more info to ponder.
Stock Plugs HD 6R12 = 40.6K ohms reading from tip to tip.
SE Trip Plat 7.97K ohms.

Wouldn't different resistance values have an effect on the ION sensing for the knock retarder?
What effect would it have if any?

I tried Champion plugs but it knocked like crazy and I removed them after about 100 miles. The resistance values of these were not matched to stock either. I just figured they were fooling the ION sensing circuit.

Chime in

TORK

[old wrench]

OK Guys, Here is some more info to ponder.
Stock Plugs HD 6R12 = 40.6K ohms reading from tip to tip.
SE Trip Plat 7.97K ohms.

Wouldn't different resistance values have an effect on the ION sensing for the knock retarder?
What effect would it have if any?

I tried Champion plugs but it knocked like crazy and I removed them after about 100 miles. The resistance values of these were not matched to stock either. I just figured they were fooling the ION sensing circuit.

Chime in

TORK

Ohm that plug, not many people do this, I do new out of the box. The most consistent ones I have found are the MP4164 Autolite spark plugs they will range from 4k to 7k and hold that value hot. The stock Champions in the bikes will go from 15k to 40k+ and will increase with heat, dont get me wrong Champion can build a good plug they make the SE Trip Plats that hold there ohms fairly well, they build them too there customes specs, I have just always had better luck with the Autolites.

The ECMs are programed to see a firing voltage range on each plug if you get out of this range it can give you a secondary ignition code, on a low ohm plug that fires easy you may have to increase the gap .002 over spec to achive the firing voltage the ECM wants to see.

Reading a spark plug on the newer ignition systems and injected engines can be a little different, I always look 3 threads deep in the plug to see the color of the ceramic around the electrode. I buy extra spark plug gaskets, the copper ones at different thickness and index the plugs, its a little extra time but most of the time it gets you a 1hp gain from the cylinder, the gasket not only seals but it is the main grounding point for the plug. I dont not use any type of never seize on my threads, just make sure the threads and seating area is clean, use a good sparkplug socket with a good rubber insert, torque the plug, and clean any finger prints off the ceramic top, a lite coat of dielectric gel in the boot.

As to the the question, When does spark plug selection come into play? I dont go by cubic inch of the motor, more on the comp. ratio, ignition system and what type of fuel is being used, street motor or strip motor, Brands will make a difference in more or less hp  and torque on the dynos.

Just my 2 cents on plugs, there is alot more as a matter of fact a whole book could be written on do's and dont's and types of spark plugs.

If you dont have good ignition you cant tune the engine