ignation system modification.


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Well-Known Member
Apr 15, 2014
Hi guys
I often read about the dual ignition during auto conversion. Why does the car engine ignition system is not good (enough)during auto conversion for airplane use.?
Is it being modified for more safety, technical reasons or more?
What is included in that modification process?
and can I or auto engine mechanic do it easily?

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Nov 14, 2009
Rocky Mountains
For most auto conversions there is no technical/efficiency reason for adding dual ignition. It's added for redundancy. On certified engines it's mandated for it's safety factor but historically that wasn't the reason it was adopted. Aircraft engines developed to the point where the cylinder volume was so large that there was a significant advantage to starting the burn from two points.

Adding dual ignition to an auto engine is generally limited by the ability to add a second spark plug. Often there just isn't enough room, the water jacket is in thee way, or valve covers need to be modified, etc. Since spark plugs have such a high MTBF a reasonable imitation of dual ignition can often be designed using methods such as incorporating a automatic dual coil selector. Many modern auto engines use initiation such as coil on plug or waste spark. These systems are already semi redundant and need duplication of only the master control device to prevent single point failure of the complete system.
Last edited:


Well-Known Member
May 16, 2014
Clarksville, TN / USA
Also since modern auto engines don't use magnetos and instead power the ignition system via the battery and alternator it may be wise to incorporate redundancy into that system as well if your aim is to get as close to the "equivalent in-flight reliability" mentioned in the FAR 33.39 as possible. Even tho not required looking through the far requirement for certified engines can give an idea of what will be required of your auto engine conversion. Subpart c are design requirements and subpart d testing requirements for engines.