Light Aircraft Association

Can anyone give advice on the life of REL37B spark plugs? My inspector has raised the question and I haven't an answer. They're used in a VW boxer aero conversion.

Champion? 0 hours is still too old.

NGK every time.

Depends on condition and type of ignition. More details please.

Rob, thanks for the reply.

Rectimo VW conversion with magneto ignition. Aviation type harness with screw on connectors, hence the aviation type spark plugs. I have been advised by a friend in the motor repair and tuning business that NGK are best but then I'd need to change the harness to accept car type plugs, which I don't want to do. Well, actually I'd like to be able to use low cost car plugs but I don't want to change the harness.

The condition of the plugs presently fitted is good and the engine runs just fine. I'd be happy to run them 'on condition' but, as I mentioned, my inspector has asked the question so I'm trying to find an answer.

Hmmm. Was that too much information, or the wrong information, or maybe no one knows.

This any good?

http://www.greatplainsas.com/scspkplug.html

Ivan

Ivan, yes I know about these, thanks.

I have in the past experimented with different leads and car plugs and suffered several plug failures including one on climb out and another whilst crossing the Grampians. Those got my attention! So at the moment I am where I am with good reason. The aviation harness works well and is reliable. The REL37Bs are expensive but I've never blown one so I'm content for now to stay with them but I don't want to be forced into replacing them on some purely notional timescale just to tick a box.

Are they massive electrode plugs or iridium wire?

If they are iridium wire they are supposed to outlast an engine with zero (plug) maintenance. They are also fiendishly expensive so I'd guess they are not fitted to a VW conversion.

All the massive electrode plugs I have seen have a circular centre electrode flanked by two earth electrodes. When new the centre electrode is circular. When the plug is worn the centre electrode ends up visibly oval, worn away where the sparks jump between the two points.
This wear is why you have to check and re-gap plugs - the gap really does get bigger over time.

Plugs should be changed around on an engine to even out wear, and to help stop them getting too crudded up.
For a 4 cylinder Lycoming the swap sequence is top 1 to bottom 4, and vice verca, and top 3 to bottom 2 and vice verca.
A mag chucks out 2 positive sparks and 2 negative sparks as it wanders around the firing order. One type of spark erodes the centre electrode and the other erodes the earth electrode(s). No I can't remember which way round it happens! Swapping 1 to 4 and 2 to 3 varies the plug getting only positive or negative sparks, I believe.
Moving them from top to bottom gives the ones moved from the bottom some time to clean themselves up as when you stop any excess oil always ends up on the bottom plug.
Lycoming's point of view on this is stated in http://www.lycoming.com/Portals/0/techp ... ouling.pdf: Point 2 states 2. Rotate top and bottom spark plugs every 25 to
50 hours. Top plugs scavenge (lead) better than bottom.

Easiest way to remember the order is 'rule of 5' and top to bottom. (1+4=5 and 2+3=5)

BTW, 6 cylinder Lycomings use 'rule of 7'.
I presume 8 cylinders would use 'rule of 9', but I don't actually know for sure.

The other maintenance check for a spark plug, particularly Champions due to the way the resistor is installed, is to check the resistance of the centre electrode of the plug, lead end to hot end. If it is above 5000 Ohms (I think, I'll have to look it up on the Tempest site) then the resistor is breaking down and you are not getting as good a spark as you should. Up to you whether to change or disassemble and clean the plug.

Thanks Rob, I had not heard of that resistor check so another thing to investigate.
In the meantime I have received information from Champion themselves, who say:

The REL37B "twin electrode" is not a true fine wire. Fine wire plugs use precious metal electrodes whereas the massive electrode and twin electrode plugs use nickel alloy material.
It is hard to put an hour limit on spark plug life because there are so many variables such as engine RPM & MP, gap setting, etc. These type of plugs are worn to maximum when the ground electrodes are eroded to half their original thickness and the center electrode begins to become oval shaped.  This is usually in the two hundred to three hundred hour range.

Now I don't think my engine operating regime is particularly harsh, a normally aspirated little VW boxer usually operated quite a way below wide open throttle, around 2700 - 2900 rpm (3400 static), so I'm hoping I can work towards the high end of that 200 - 300 hour range but with something I can measure, the thickness of the electrodes, I have some rational basis for establishing a 'life' for my plugs.