Spark plug question - resistor or non-resistor type

[Moody Blue]

1971 Merc 800. Manual specifies Champion L78V plugs (non-resistor) but after reading some info on the Champion website it seems like the QL78V (resistor type) may be better suited. The "Q" designation is a resistor type plug and according to the tech info they are used with CDI type ignition systems to suppress RFI interference. The '71 Merc uses a CDI ignition.

Any first hand experience or knowledge about these plugs?

What about NGK vs Champion? Any preferences?

 

[Chris1956]

Re: Spark plug question - resistor or non-resistor type

I could never find a Merc endorsed substitute for the Champ L76V plug. Other brands of the L76V work fine.

 

[achris]

Re: Spark plug question - resistor or non-resistor type

I hate chump plugs (way too many DOA), and only use NGK. The plug you need for that engine is NGK BUHW-2. The 'Q' in the chump number is an inductor, not a resistor. The are much more expensive, and you should only use them if you are suffering from a bad case of radio interference...

Chris.....

 

[sschefer]

Re: Spark plug question - resistor or non-resistor type

You only need a resistor plug on digital EFI systems. Everything else uses a standard plug. You can play around with different brands of plugs. Just make sure that they do not execeed the length of the chumps or the heat range. Surface gapped plugs (BU8H or BUHW) are designed for use with flat top pistons. You should not use them on a engine with domed pistons.

 

[Chris1956]

Re: Spark plug question - resistor or non-resistor type

I beg to differ, NGK BUHW are recommended for many Direct Charge Mercs. DC Mercs have the eyebrow-topped pistons.

 

[sschefer]

Re: Spark plug question - resistor or non-resistor type

I beg to differ, NGK BUHW are recommended for many Direct Charge Mercs. DC Mercs have the eyebrow-topped pistons.

Can you give me a year and HP of an engine that has these eyebrow-topped pistons. I'd like to do a little research. Thanks

 

[Moody Blue]

Re: Spark plug question - resistor or non-resistor type

How about a '71 Merc 800. Recommendations for plugs are NGK BUHW-2 or Champion L78V and both are surface gap type. Pistons are not flat topped.

I've got the NGK's installed now but will try the Champions next week for kicks. Have not had any interference problems so will stick with the non-resistor plugs.

 

[achris]

Re: Spark plug question - resistor or non-resistor type

Can you give me a year and HP of an engine that has these eyebrow-topped pistons. I'd like to do a little research. Thanks

All the in-line 6 engines (90/115/135/140/150hp, 1970-1987) have crossflow type pistons, and use surface gap plugs (see attached picture). The same pistons and plugs were used in the small in-line 4, 45/50hp from 1970-1996. Also the big 4 cylinder 65/75/80/85 from 1970-1988.

Chris........

 

[sschefer]

Re: Spark plug question - resistor or non-resistor type

How about a '71 Merc 800. Recommendations for plugs are NGK BUHW-2 or Champion L78V and both are surface gap type. Pistons are not flat topped.

I've got the NGK's installed now but will try the Champions next week for kicks. Have not had any interference problems so will stick with the non-resistor plugs.

Interesting, it definitely has the high dome and it is calling for the BUHW-2. NGK. That's odd too. Oh well... Just when you think you've done all the research the Merc Monster jumps out at you.

Here's the specs on your plug if you want to try something different:

Traditional Spark Plug, 14mm Thread Size, 12.7mm (1/2") Reach, 13/16" (20.6mm) Hex Size, Gasket Seat, Non-Resistor, Solid Terminal Nut, Surface Discharge, Tungsten Center Electrode, .071" (1.8mm) Gap