Sparkplug Heat Range

[Honda Hornet]

I accidentally bought a sparkplug (NGK D9EA) for my 1972 SL100 that is heat range 9 instead of the recommended heat range 8, but not practical to send it back. Since most of my riding is at higher speeds (40-55mph) I was thinking this may not be a problem. From what I read it seems that the higher heat range will take longer to get hot, but will be better at dissipating heat. For the $3, I might as well try it? Any cautionary concerns?

Thanks.

 

[dirtbkr188]

The 9 is actually cooler than the 8 - The heat rating of each NGK spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type.

 

[OLD CT]

dirtbkr is right. The only thing that could happen with a cooler plug is, it might foul out. If it does, put the 8 back in.

 

[Honda Hornet]

I am confused because my CT90 is speced for a 7 heat range, while my SL100 is speced for an 8. I would have thought that the SL100 runs hotter (as it is 11.5 HP and definitely has a higher top speed, while the CT90 is only 7 HP, and I assume the vertical cylinder cools more efficiently). I will try the 9, but keep the 8 in my pocket if the 9 does not perform well. Thanks for sharing what you know.

 

[Honda Hornet]

I am just adding that the SL100 seems to be at least as happy with the NGK D9EA sparkplug as it was with the D8EA. When I pulled the D8EA it was a little black and greasy, so I thought I should adjust things to be less rich. After a little work (I checked and adjusted valve lash, point gap, ignition timing, moved the throttle needle to the middle position (it was one notch toward the point - to make the fuel mix more rich for the middle portion of the throttle range), and added the new D9EA sparkplug and the SL100 is definitely running better. I put off cleaning the carburettor, until it shows signs of needed yet or a bunch more miles. Top speed is above 65mph with dirt tires on pavement. I will pull the sparkplug and check how it looks this week.

 

[Honda Hornet]

I am just adding that the SL100 seems to be at least as happy with the NGK D9EA sparkplug as it was with the D8EA. When I pulled the D8EA it was a little black and greasy, so I thought I should adjust things to be less rich. After a little work (I checked and adjusted valve lash, point gap, ignition timing, moved the throttle needle to the middle position (it was one notch toward the point - to make the fuel mix more rich for the middle portion of the throttle range), and added the new D9EA sparkplug and the SL100 is definitely running better. I put off cleaning the carburettor, until it shows signs of needed yet or a bunch more miles. Top speed is above 65mph with dirt tires on pavement. I will pull the sparkplug and check how it looks this week.

 

[OldGuy]

Here's my explanation on heat ranges...
As the heat Number rises, (7 > 8 > 9), think of it as indicating that the higher number plug can HANDLE a HOTTER environment. That means it can dissipate heat better allowing it to survive in a hotter combustion environment. But when you are swapping plugs in a given engine, the higher numbered plug's ability to dissipate heat better means the plug itself runs cooler.

 

[Honda Hornet]

Thanks OG, that makes sense to me, and how I had figured it to explain the difference between the CT90, which is tuned to be ordinary for durability, and the SL100, which is tuned to get the max power out of the piston. Since I am expecting to be riding at high RPMs (so the motor will be hotter) most of the time, I think the lower heat range will be better at keeping the plug cool in this environment, but I am keeping a D8EA on board, just in case.

 

[joel1234567]

"Top speed is above 65mph with dirt tires on pavement."

That might be the fastest sl100 ever built. You might want to double check that with a gps.

 

[Honda Hornet]

You are right to observe that I am using referencing the on-board speedo (and not much above 65, but it was not holding WOT and tucked - as I need to do on my CT90), and I will check with a GPS. Regardless, it is noticeably faster than my CT90 which has a speedo reading about 53mph at its maximum. I am not trying to hold the land speed record or anything.

It seems to have very low gearing at the lower end, so I was planning to install a one-tooth larger drive sprocket, just so I am not needing to be in third gear to get out of a parking lot. The ability to ride above 50mph for a bit seems to be a great benefit for touring on a small Honda, enough to make up for being much louder at idle and slow speeds (I have to shut off the motor when I am talking to border guards, when the quiet tickover on the CT90 is easy to hear over).