Spark plug question for 76 Mercury 1150

[kungpaoshizi]

*Apologies, could a mod please put this in the Mercury outboard motor section

I see that the L76V is the suggested spark plug, but the guy I got the boat from put AC Delco V40FFM spark plugs in.
I'm familiar with in-depth physics, and I'm assuming this plug runs cooler than a regular plug because it can spark in a 360 degree pattern around the head...
Anyone know of details surrounding the two? I know when talking about engines you don't want to run too hot, but is too cold an issue for outboards?

Thanks very much!

 

[Aqualift]

Re: Spark plug question for 76 Mercury 1150

The V40FFM spark plugs are the equivalent replacements. NGK BUHW's work great too. These (L76V included) surface gap plugs are all on the cold side of the heat range.

 

[Texasmark]

Re: Spark plug question for 76 Mercury 1150

I am not a plug designer, but over the years I have messed with plugs in different engines. I have a semi modern Merc 90 and in the service manual it lists a SG as standard and a conventional gapped plug as an alternate but it is listed because it has a RFI filter coil imbedded in it for folks with sensitive electronics on board needing some RFI protection.

I have run both in my engine. After some fooling around I decided to leave the SG's in. Reason being the gap. The gap for the conventional is spec'd at .040 and my measurement of the SG came to .060.

The SG is a self cleaning plug and electrode wear is minimum. On the cooler running plugs in these engines, electrode wear is a no brainer anyway. I had none on my conventional gaps or the SGs.

From an energy perspective, The CDI has the ability to deliver a 40 kv pulse, in 4 microseconds as I recall. That's one heck of a lot of whoopie in a very very short time. That would occur in an open circuit but before the plug fires it is an open circuit so that is the shape of the leading edge of the potential 40kv pulse as it is supplied to the plug and rises over the plug gap.

Since the energy is stored (1/2 CV exp2) and used as the square of the voltage, the higher the voltage, the higher the energy but the energy level is the square of the voltage. So, for the energy contained in the spark at the time a .04 gap breaks and a .06 gap breaks you have a 50% increase in voltage (considering a linear kv/inch scale) but a 225% increase in energy in the spark at the firing time.

Therefore a surface gap has better than a 4x chance of firing a contaminated plug. Wink! I think OEM's are onto something as I know OMC and Merc used them for a lot of years. I had them in my OMCs and my Mercs. Don't know about any other brand.

The reason I started thinking about what I said above is one day I was troubleshooting a problem and happened to substitute a SG plug for the conventional. The difference in the spark intensity and snap caught my attention and I got to thinking about just why was that. Well, as I see it that's why.

On cool running, this is just speculation, but I think it is part of pre ignition reduction on being able to use the lower octane gasolines. Pre ignition can burn some bad holes in an alum piston. Have seen pictures of it, both 2 and 4 stroke engines.

My 2c,
Mark

 

[Frank Acampora]

Re: Spark plug question for 76 Mercury 1150

You don't need esoteric physics to understand spark plug heat range: Because surface gap plugs have the center porcelain insulator completely enclosed, it is not exposed to combustion heat and does not get as hot as a bent electrode plug. Again, since it is completely enclosed there is a very short path to the relatively cool heat sink of the head. Combined, these two factors make the surface gap plug a very "cold" running plug. Since outboard two cycle engines have a power pulse each revolution on each cylinder, the piston and cylinder does not have as long a time to cool. Combustion temps are a bit hotter and the engines are more prone to pre-ignition so the cooler surface gap plugs are used.

While they deliver a sharp, high voltage spike to the plugs, the down side is that the self-energizing, capacitor discharge ignition systems act very much like a magneto, delivering high voltage only at high RPM. At start, voltage is low and a fouled surface gap plug that will fire normally at high RPM may not fire to start the engine. That is why when you have a starting or stalling problem at idle, the FIRST thing you do is clean or replace the plugs.

However, a lot of OMC engines did use a bent electrode QL plug (I forget the actual number)

 

[merc850]

Re: Spark plug question for 76 Mercury 1150

This is how it was introduced in 1966

they went to Perma-Gap plugs later.

 

[Texasmark]

Re: Spark plug question for 76 Mercury 1150

Best I can tell the surface gap was part of the design of the CD ignition. Your historical accounting shows them and explains how they outlast conventional plugs back in '66. If that isn't a "permanently gapped" plug then I don't know what one is. In the picture you offered that plug looks just like the ones in my engine which are NGK BUHW-2 surface gap (Champion L76V) which are also used in a '77 7 1/2 hp Merc and surely a lot others. I guess they needed something to advertise in '70 and maybe figured we forgot about '66 or didn't notice.

Mark

 

[Faztbullet]

Re: Spark plug question for 76 Mercury 1150

Most Merc engines 1971-1980 came with AC plugs especially the Inlines and V-6s.....and all came with permagap plugs if it had a solid state ignition. The only ones to use a gapped plug where the coil/point ignition systems