Spark Plug Length 1147

[Greta]

In doing all I can to minimise misfire in recent wet weather I decide to change my plugs on the MKII 1147cc Spitfíre. Which? - Go to NGKs comprehensive list https://www.ngkntk.co.uk/index.php/downloads/


BP6HS  - 1/2 inch clearly listed. Order these from my TMS and pull the plugs I've got. To discover That the car has been running fine in the summer on longer 3/4 inch plugs - so why fit the shorter length plugs recommended by NGK?

The longer plugs don't appear to have been hitting the piston crown but do have soot on lower threads ( assume these have been exposed ). Am told the shorter plug will lower Compression Ratio.

Please advise pros & cons and make recommendations

[Greta]

Champion also show shorter plugs - but I'm sure there must be some here that can advise a good plug for both performance and importantly reliability.

[JohnD]

There's length, and there's 'hotter and cooler' plugs.
A 'hot' plug has less of the insulation around the electrode in contact with the body of the plug, so that heat flows away slower.  See pic.
A longer plug would have the same effect, see your comment, "The longer plugs don't appear to have been hitting the piston crown but do have soot on lower threads"      The tip has been hotter than the base.

But I note your other comment that it has been misfiring in wet weather.
Suggest you investigate that, rather than the plugs if they work fine in the dry.    WD40 is excellent for excluding water, and a spray onto clean plug leads, dizzy, coil ad associated wiring might be enough.

Occam's Law!   Or, KISS!
John
PS a plug that projects into the combustion chamber less will occupy less volume, making the chamber slightly bigger, and the CR smaller.    But the difference in volume will be a fraction of a milliliter, and the change in CD insignificant.

[Greta]

I appreciate this is all "old hat" to the experienced but as I find it frustrating on other forums to find only half an answer thought I'd post this pic as a possible useful guide to others .... And in an attempt to encourage other 1147 engineers to post their experience.

[Nick Jones]

Nothing whatever to do with heat range.  Just a question of wall thickness of the head casting.

The 1200 engine is slightly unusual in that it uses short reach plugs by design.  These have 14 mm thread x 1/2" long. The long reach (M14 x 3/4") one also fit but, as you note, the extra 1/4" of thread at the inner end soots-up and might potentially cause problems with removal if left a long time.

Use the correct short reach ones.  The effect on compression ratio is negligible.

The correct heat range equivalents should be available from whichever make you favour.  I prefer Bosch or Denso myself.

Nick