in general, if compression ratio is increased will you want to use a hotter or colder spark plug as compared to lower compression ratios?

 

I was just reading a car craft online article on that today:

• Compression ratio: Increasing the mechanical compression ratio raises cylinder pressure, resulting in higher cylinder temperature. The higher the compression ratio, the colder the spark plug needs to be. According to Champion Spark Plugs, for normally aspirated, gasoline-fueled engines, a good rule of thumb is to go about one heat range colder for each full point in compression ratio increase from 9:1 through about 12.5:1, and two heat ranges colder for each point increase between 12.5:1 and 14.5:1. Beyond 14.5:1, 3-4 heat range reductions per point may be needed.

 

another fact, more CR requires more voltage or a narrower gap

 

Good info
What I do is run the coldest possible plug that doesn't foul. Most times I end up with the plug recommended by the manufacturer, but occasionally I end up a grade or 2 colder. The tables given by plug manufacturers are great (like the info above), but there's so many variables that the recommendation may apply to 95% of cases & you can be in the 5% occasionally (altitude, ambient temps, dynamic CR, condition of engine, fuel quality, does it burn oil, etc, etc). The biggest problem I have with plug grades is the manufacturer's methods of grading eg, Champion numbers increase, whereas NGK decrease. That I can remember, but do Champion increase for a hotter plug, or was that NGK? And what was it Bosch do again? Then, just to confuse even more, some people insists on calling them hard & soft grades If I'm feeling lazy (or skint) I'll look up the recommended grade & then fit one grade colder. If they foul then I fit a hotter one, but quite often they don't foul which makes me suspect that maybe manufacturers specify a hotter plug than required to prevent any possible fouling in some situations (ie the 5% of users)? If the engine is used for more than normal street use (a grocery getter) then it's worth checking each pot separately as sometimes different grades of plug can be used on the same engine eg. an air cooled triple can sometimes run a lot better with a different grade of plug fitted in the center cylinder (as it runs hotter than the outer ones). A particular engine (which I try to avoid) is infamous for punching holes through the center piston &, far worse, bending/ breaking the center rod (bent/broken rod + aluminium crankcase = one hell of an expensive mess). I don't know of any that have done this when fitted with the correct grade of plug on the center pot. Owners also remark on how well they run after changing the grade of the center plug. Another example (which applies to all engines, but possibly affects air cooled far more than water cooled?) is when running aluminium heads that have had a Helicoil fitted in a plug thread. The Helicoil, being stainless steel, conducts heat far less efficiently than the aluminium of the head, so any plugs fitted in Helicoiled threads are running hotter than those fitted in the bare aluminium. This makes a lot of difference on an engine & sometimes, if a Helicoil is required in a plug thread, I'll Helicoil all the plug threads so that the same grades of plug can be used & they'll all be running at roughly the same temp (it also prevents people from mixing the plugs up & fitting them in the wrong pots & then come bleating to me that it's not running right).
The above approach has worked well for me (so far!), but how do other people determine the correct plug grades?

 

Whats the downside to running a too "warm" plug?
I`m currently using a NGK BP6FS plug and they tend to foul up on me so i thought i would try a BP5FS plug which is a step hotter.

 

The only thing I can think of is that you may be closer to detonation or run on issues.

 

sure would be nice to cross reference this to actual part numbers. did that article have any part numbers associated with it?

 

No it didn't - here is the full article:
http://carcraft.com/techarticles/64378/

I have that confusion too, for some makers the increase number is colder, for others a decrease in numbers is colder, and it's very hard to find this information to translate what the numbers mean.

 

 

http://www.ngksparkplugs.com/techinf...31000&country=

 

Hi,
the downside to too warm a plug (or the most important thing to me) is that the plug tip can get too hot, leading to pre-ignition. I've read about plug tips melting & falling apart if too hot a grade is used, but in my experience (lots of it!), long before this happens a piston has been holed. When we had to change to unleaded fuel we found that our engines, that had run fine for 20 years or more on the same grade of plug, started feeling harsh & some were occasionally pinging, while others holed pistons What I found was that the governments advice of "For changing to unleaded fuel you need to retard the ign timing 2 degrees" may have worked for the majority of vehicles, but those of us running tuned engines with higher than stock CRs still had problems. It turned out that unleaded fuel caused the engines to run hotter &, even worse, super unleaded (which has a higher octane so we all used it) supposedly runs even hotter. Changing to colder plugs cured most (but not all) problems.
Champion plugs are graded so that the lower the number, the colder (harder) the plug. I was running N5 plugs for years with no problems & changed to 2 grades colder (N3). The N3 plugs could start showing signs of fouling when used for a lot of short journeys, especially in cold ambients, so I went up to N4 & they're fine.
If your plugs are starting to foul then changing to a grade hotter should solve it (but MAKE SURE that the A/F mixture is correct, the engine is healthy & that there's no other problems that could be causing it - plugs show what's happening in the cylinder but not always why. It's so easy to assume that the wrong grade of plug is fitted when something like a partially closed choke could be causing the symptoms). When you change the grade of plugs keep a close check on their condition, especially when it's run in unusual circumstances eg. a particularly hot summer. Always pay attention to anything that could be mild pinging. The amount of engines I had to rebuild due to holed pistons wasn't funny