Bonneville America Forums Forums Technical Forums Performance Mods Hotter plugs

I read an article by a Triumph guy who said he goes to a hotter plug instead of a smaller jet when he's running a little rich. That way he doesn't loose any performance. If this realy is a better way to go, I would think it would have been posted on this site.

It's a common misconception that a "hotter" plug means a hotter spark.
What the term refers to is the plug's speed of heat dissipation. That is, a "cooler" plug will dissipate heat faster than a "hot" one.
It won't affect performance by burning the fuel more efficiently. Though a "hot" plug may not foul as quickly, your friend's bike is still running too rich, and as such is not running with top efficiency (and power).
Too rich is a fuel/air problem that requires a fuel system fix, not a spark problem.

Thank you. That makes sence.

I'm probably stepping off the deep end of my limited pool of knowledge, but you could run a hotter coil.

I always thought if you're running a hotter coil it allows you to burn more fuel. So wouldn't it depend on just how rich you are to begin with before installing the hotter coils? After all, isn't the air/fuel ratio measured through the exhaust output when on the dyno?

But like I sad, limited knowledge here.

The air/fuel mix and amount compression is far more of a determinate of how completely the mixture burns than the heat of the spark. The spark just starts the process, though a cooler spark lasts longer (measured in degrees of crankshaft rotation). A hotter spark can more easily ignite more difficult (leaner) mixtures.
The only thing you can measure at the exhaust is how complete the combustion is, and from that perhaps extrapolate the mixture if you know the other variables.
Air/fuel ratio would be determined from the jet size, air flow, altitude, barometric pressure etc., as well as oxygen content of the exhaust (that's how EFI does it).

The best investment I made was having a tuner set my carbs up with the aid of a dynamometer. No worry about plugs fouling, or running too lean. The carbs just function right. Well tuned carbs make this MC junkie smile. I've had both my rides dyno tuned and would do it again in a heartbeat. Saves time and money.

To receive a benefit from an ignition mod, such as a "hotter" coil, one needs to make a change to the plug gap. That is to say, increase the gap by several thou.

Run a search on this topic, as I've addressed this change, before.

NOTE: Wider plug gaps help burn the richer mixture, or, put another way, wider gaps (and greater energy to get the spark to reliably jump that bigger gap) can cause a well-jetted motor to run a little lean.

Oh, nitro burning drag motors no longer use magnetos. Those dual units you see on the front of those "Chrysler" engines are actually generators, which feed an electronic box (which is full of capacitors and such). When I crew chiefed my buddy's nitro funny car years ago, I used an MSD single plug ignition system and an on-board battery to run it. Biggest benefit seemed to be the easy starting, even when the engine was turning over slowly when the 36 volt starter batt. unit would drop off in charge. And yes, I ran large plug gaps. Those were the days of magnetos and .016" gaps, by the way.

TECH NOTE: Nitromethane is a conductor and once you fuel foul a plug, it's very difficult to get it to fire. High voltage and big plug gaps solved that problem!

Yeah i know mags have been replaced in drags, I was retro-referencing there .
I was wrenching at an Olds store when GM first came out with their high energy ignitions, 40K+ volts, .080 plug gaps, high resistance wires, once a plug was fouled it would not self clean like lower voltage points units would.
The hot spark is just too short of a duration to burn off the fouling.
The reason for the advent of the H.E. units was stricter emissions controls meaning leaner running engines.
Now, the old dual point units like Mallory would produce a hotter spark, but it would last longer too. I think the newer ignition units are do this electronically