Automotive Spark Plugs

paul330 · Post by **paul330** » Wed Aug 24, 2011 7:50 am

Now that there are at least 2 aircraft on the register flying dual PMags, I wondered what the LAA Engineering view was on using automotive spark plugs which, for various reasons, may perform better with certain electronic ignitions.

mike newall · Post by **mike newall** » Thu Aug 25, 2011 12:55 pm

We have dual P-Mags on our RV7 - top are NGK plugs, bottom are Unison.

All work fine.

Next time, I will fit NGK's top and bottom - much cheaper.

Rob Swain · Post by **Rob Swain** » Thu Aug 25, 2011 1:31 pm

Time for a dumb question!

Are there 'car plugs' that actually fit aircraft engines, or are the installations that use them using adaptors?

And what about the potential for using car plugs with mags? It must be possible electrically as VW conversions use car plugs with Lucas SR4 mags, but what about other issues?

What is the 'approved' way to connect the HT lead to the car plug? Just push on ones like on a car or bike?

The chance to change to plugs that are about £25 for a set of 8 rather than £25 quid each should be jumped at. I'd be fitting new plugs at 50 hour services rather than messing about cleaning and regapping the aircraft style ones!

Nick Allen · Post by **Nick Allen** » Thu Aug 25, 2011 2:01 pm

When we got a new Limbach engine for our motoglider, it came with the latest Limbach sparkplugs, which are automotive in design (though come with a JAR form and a Limbach logo, so cost more than Halfords charge!). The harness supplied (Slik mag) indeed just has the rubber push-on "car-style" caps fitted. This is all certified kit, remember -- though as it's for a motorglider only one mag and four plugs is demanded...

paul330 · Post by **paul330** » Thu Aug 25, 2011 4:13 pm

As I understand it, the advantage of auto plugs with the PMags is you can get a larger gap and get a stronger spark - aero plugs just can't be gapped that big. Yes, they do need an adaptor. I can't see why auto plugs couldn't be used with a mag if they were gappped smaller for the weenie spark. Caps are just push on like a car. Apparantly, the only real issue is getting rid of the air around the cap seal so they don't pop off when you climb - known as "burping".......

Mike. Glad to hear that you are permitted to use auto plugs. I'm hoping to get approval for dual PMags on my -10 - still waiting for the 6 version - some time in the next few months I hope. Might get in touch for a little advice on the Mod application if you don't mind.

martin payne · Post by **martin payne** » Fri Aug 26, 2011 11:42 am

Jabiru engines use automotive plugs as standard, they use DR9EA 's with push on Car style caps, they also use Honda (lawnmower type) coils, and conventional distributors and rotors.

rogcal · Post by **rogcal** » Sat Aug 27, 2011 10:39 am

Just a word of warning on the use of automotive plugs.

Try and get the type that has a removable nipple and use a plug cap that fits over the threaded end of the plug.

I've had nipples come unscrewed and caps come off of nipples in flight, all due to vibration.

If you do have to use spark plugs with nipples, at least check them regularly (and the caps) for security. It's no fun having then come off in flight!

The type of cap that fits the threaded end of a plug is by virtue of its design far less likely to be affected by vibration and airflow.

I've invested in NGK plugs and caps and feel far safer in the air as a consequence.

Incidentally, all my plugs, leads and caps are unsuppressed to enable the HT impulse to reach the spark gap with no resistance to impede it.

Gives for a bigger spark and places less stress on the mags secondary windings.

Plays havoc with radio reception and transmission but shielding and bonding usually resolves this problem if done correctly.

Rob Swain · Post by **Rob Swain** » Wed Aug 31, 2011 1:21 pm

rogcal wrote:Incidentally, all my plugs, leads and caps are unsuppressed to enable the HT impulse to reach the spark gap with no resistance to impede it.

I'd agree with copper leads for mags, but I'd probably use resistive plugs to aid with RF noise suppression.
I'd use non resistive caps as the resistors inside the resistive/suppressed ones break down long before anybody thinks to replace the caps. Having the resistors in the plugs means they get changed as a matter of course as part of 50-100 hour servicing.
As to how much the resistors impede and reduce the spark (in a properly maintained system) you start getting into physics and ohms law but as I understand it the reduction is negligible due to the current and voltage involved.

martin payne wrote:Jabiru engines use automotive plugs as standard, they use DR9EA 's with push on Car style caps, they also use Honda (lawnmower type) coils, and conventional distributors and rotors.

I thought new Jab engines had electronic ignition, or at least breaker-less ignition.