Spark Plug Question

[Dickie]

My XR600R factory service manual says that there are 2 plugs recommended. The stock one is an 8 and the "long distance riding" one is a 9. Does it really make a difference which one you use? I have the stock 8 plug in now but I ride mostly higher speeds and longer distances. Would the change to the long distance plug be noticeable/worth it?

[martinfan30]

My XR600R factory service manual says that there are 2 plugs recommended. The stock one is an 8 and the "long distance riding" one is a 9. Does it really make a difference which one you use? I have the stock 8 plug in now but I ride mostly higher speeds and longer distances. Would the change to the long distance plug be noticeable/worth it?

the 9 is a colder plug. its recomended to avoid hi cyl. head temps on long rides.

[HawkGT]

Changing the plug's heat range should have no practical effect on the operating temp of the engine. What it DOES change is the operating temp of the plug itself: colder plugs shed heat faster, hotter plugs conduct heat slower. Colder plugs are recommended for continuous high speed riding to help guard against an overheated plug.

[martinfan30]

Changing the plug's heat range should have no practical effect on the operating temp of the engine. What it DOES change is the operating temp of the plug itself: colder plugs shed heat faster, hotter plugs conduct heat slower. Colder plugs are recommended for continuous high speed riding to help guard against an overheated plug.

apparently ive been mislead for a long time, by UTI, toyota, etc.

[Dickie]

What happens if you get an overheated plug?

Possible stupid question, but if I get the colder plug, I can just put it in with no other adjustments/jetting correct?

[Denn10]

you can use either withour changing jetting, but remember there is a difference between say summer and winter, where you prolly dont get optimal performance but use what the manual says for the plug for the type of riding your gonna do.

[HawkGT]

apparently ive been mislead for a long time, by UTI, toyota, etc.

If I'm the one that's been misled then I'd like to know about it. ? I've looked carefully at this topic before though. Heat ranges are all about changing the operating temp of the plug itself, not changing the temperature of the engine. If you can show me something credible that indicates that the heat range of the spark plug will have a meaningful (or even measurable) impact on an engine's operating temperature then I'm all ears.

It doesn't make sense that a colder plug would lower cylinder head temps. A cold plug moves heat from the end of the plug into the cylinder head faster than a hot plug. That's why it's a cold plug: it gets rid of it's heat and dissipates it into the cooling system. How would more easily transferring heat through the plug and into the head lower cylinder head temps?

A spark plug doesn't create heat--it just provides a path for heat to move from one place to another. That might lead one to think that a plugs heat range might change engine temps by modifying the engines ability to absorb the heat created during combustion and allow more heat to escape with the exhaust gas. And it might--barely. But the change will be very, very small. Again, the source of the engine's heat comes from combustion, not the plug. If a plug is of a hotter heat range and therefore transfers less heat, most of the heat it could be moving but isn't will just transfer into engine through a path that doesn't involve the plug. And the amount of heat we're concerned with here (~80 deg C of plug tip temperature) is only a tiny fraction of the total heat in the system. Even a hot heat ranged plug is still moving a heck of a lot of heat.

All I have on hand to specifically corroborate that heat ranges have next to nothing to do with engine temp is this article written by Gorden Jennings for the long-defunct Cycle Magazine: http://www.strappe.com/plugs.html

"...A "hot" plug does not make an engine run hotter; neither does a "cold" plug make it run cooler..."

I've found other sources that support my comments (including HERE, HERE, and HERE) but I'm reluctant to put any weight in Internet sources with unidentified authors and/or no references).

[martinfan30]

that is some good reading hawk, thanks! i guess it is one of those things that was told and just never questioned it. i stand corrected sir.

[Dickie]

Good info, I have been wondering about this for a while now. I am glad I brought this up.

So based on the info above the primary benifit to using a colder plug is to prevent the plug itself-not the engine-from overheating, if that is true then what happens if you get an over-heated plug? I assume that the plug eventually fails?

[HawkGT]

Good info, I have been wondering about this for a while now. I am glad I brought this up.

So based on the info above the primary benifit to using a colder plug is to prevent the plug itself-not the engine-from overheating, if that is true then what happens if you get an over-heated plug? I assume that the plug eventually fails?

The danger of an overheated plug is that it can cause a pre-ignition condition. Basically, if the plug gets too hot it may become a source of ignition--not from sparking but from just being way too hot. The last thing you want is to have the A/F mixture ignited by something at the wrong time in the piston's stroke.

[Dickie]

Hmmm, well I suppose I should get the colder plug then. Right now I am riding 80% street.

[HawkGT]

Hmmm, well I suppose I should get the colder plug then. Right now I am riding 80% street.

I'd probably substitute "should" with "can". ? Unless you've got engine work that's producing a fair amount of extra power (heat!) or are really running the hell out of the engine (think: racing), there's probably little reason to need a colder plug. However, I doubt you'll notice much if any ill effect from running a colder plug (unless it's way too cold). Probably the worst thing that could happen from running a too cold plug is that it will build up more combustion deposits than it really should. In my experience with Honda RFVC engines you've got to be pretty darn rich to actually foul a plug to the point of it not firing at all. But a too cold plug will lower that tolerance. And, unless you occasionally really run the hell out of the engine and at least sporadically produce enough heat to actually warrant the colder plug, it may build up enough deposits to reduce the spark energy by some amount.

If you must know for sure which heat range plug best suits your engine you can test a plug and "read" the side electrode to gain some insight into how the plug is doing as far as heat.

[martinfan30]

The danger of an overheated plug is that it can cause a pre-ignition condition. Basically, if the plug gets too hot it may become a source of ignition--not from sparking but from just being way too hot. The last thing you want is to have the A/F mixture ignited by something at the wrong time in the piston's stroke.

basically it can become a glow plug.

[Dickie]

interesting, I am not sure how to read a plug other than the usual "white is too lean, black is too rich, brown and toasty is just right." How do you read an electrode?

I am not doing any racing and my set up is a Mikuni Flatside and a Supertrap with 7 rings in.

[HawkGT]

First, a general warning: IMO, the complexities of accurate plug reading is often underestimated. Although there are established techniques for using plugs to evaluate a/f ratios, ignition timing events, abnormal combustion, and plug temperature, there's such variety of engines, operating conditions, and fuels that it can be hard to apply the successful techniques used in one case to another case. In other words, it's hard to say: OK--perform a plug test by doing this, this, and this; pull the plug and look at this and this. You should see X, Y, and Z and here's what that means. IMO, a certain amount of knowledge and experience is often needed to draw meaningful conclusions. Plus, the techniques most often used by the casual enthusiast are so oversimplified they loose much of their usefulness (reading a plug for a/f ratios is the big offender). I suppose the usual "brown paper bag " type of standard is useful for many. But for fine-tuning performance engines, that standard has just as much placebo value as anything else. JMO, of course. That caveat aside....

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The side electrode will change color slightly at some point along it's length. It might go from one shade of gray to another shade of gray or from gray to tan or something like that. The color isn't what's important here. It's the location of the "line" that can be used as a guide for heat range. The red line in this photo would be a normal location for the line on a plug of appropriate heat range. Typically the color change should occur about half way down the length of the side electrode--this often corresponds to about the middle of the bend on a projected tip plug like pictured. If the line is close to the free end of the side electrode that indicates a too cold plug. If the line is close to the base of the side electrode, that's an indication of a too hot plug.

This method usually works but I have pulled plugs where it was pretty tough to decide where the hell the line was exactly. In my experience, sometimes the line appears so subtle I wonder if it's there at all! If reading the color change on the ground strap isn't getting you anywhere then you can always look to the center electrode for evidence of overheating. Look at the edges of the center electrode's tip for signs of erosion. A good magnifying glass and a strong light goes a long way here.

Practically all plug tests I'm aware of should be performed using new plugs. Pulling out an old plug that has even an hour or two of run time might clue you in if there's a major problem (you can look for gross plug erosion, peppering, and the like). But generally you should warm up the engine on an old plug, get yourself in a place where you can perform a WOT pass (for main jet a/f evaluations, timing events, or heat range), and quickly install a brand new plug (you don't want the engine to cool down--where gloves when changing the plug!).

With the new plug installed immediately take off--getting to and holding WOT as soon as you can. The idea is to not let the plug see anything other than full load, WOT conditions. Old plugs are a confused jumble of all different operating temps, throttle positions, loads, etc. For many evaluations running up through the gears and holding WOT for 5 or 10 seconds is all it takes to "paint" a plug with information that you can read. In some cases a bit more time (less than 1 minute?) can make the plug easier to read.

Sometimes experimentation is needed to pin down exactly what kind of test will produce the information you're after.

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All that said, there's no need to make this too complicated unless you really have a thing for detail-type testing. Does the manual actually say "long distance riding"? I wonder what they mean by that? Every Honda manual I have says "Extended high speed riding". Assuming the XR600R is similar to every other Honda I've had experience with, unless you've got a hi-comp piston and/or advanced ignition and/or you REALLY ask a lot of the engine (racing or running at or very near WOT for mile after mile), the showroom stock plug will be fine. If you're so inclined: put a new one in, perform a plug test, and take a look. If you see something that makes you think the plug is overheating, run the colder plug. If you run the engine hard and just assume error on the side of caution at the expense of unnecessary plug deposits, run the colder plug whether you decide to test the standard plug or not.

$0.02

[martinfan30]

thanks , that really cleared things up. you a pro wrench?

[HawkGT]

thanks , that really cleared things up. you a pro wrench?

Nah...I'm just an enthusiast with too much free time.

[Dickie]

QUOTE:

"Does the manual actually say "long distance riding"? I wonder what they mean by that? Every Honda manual I have says "Extended high speed riding". Assuming the XR600R is similar to every other Honda I've had experience with, unless you've got a hi-comp piston and/or advanced ignition and/or you REALLY ask a lot of the engine (racing or running at or very near WOT for mile after mile), the showroom stock plug will be fine."

You are probably correct; "Extended High Speed Riding" may be the correct wording from the shop book. I am not doing much more than 50 miles at about 50MPH so as you say the stock plug is probably fine. I did one trip that was 200miles round trip but staying at the speed limit and with a break or two I am not sure that even qualifies as "extended high speed riding." I might perform the procedure you outlined about just out of curiosity, at any rate, this is all very useful info and I appreciate the help!

[martinfan30]

ive ridden my xrl at 70 mph for 1-2 hours at a time,, no prob. and that is with 15/48 gearing. no hotter than 260. stock plug.

[robert68]

Has anyone heard of direct hits plugs?