Magnetos, Slick vs. Bendix
 

Wanted to know if anybody has some field experience using both Slick 4300 or 6300 series magnetos vs. the Bendix 1200 series.
Looking at running LOP at high altitudes with an IO-540 Lycoming engine on an RV-10.

They both make sparks equally well. Ignition strength is largely determined by magnet strength and a perfect E-gap setup. Mag spark voltage increases with RPM. The faster the magnet spins, the higher the voltage. The coil winding ratio is chosen to provide a healthy spark at a typical RPM. Slicks gained popularity over Bendix when Bendix part prices reached a level where you could buy a new Slick for less than overhaul price on a Bendix. Now Slick parts have risen to a point where you can hardly overhaul one for less than a new mag. You can still overhaul a Bendix for less than new, but just barely. Both manufacturers are ripping us off on parts (because they can). The new point cams in the slicks are trash and they won't sell the cam alone. You have to buy a complete point set. I just changed them on a pair of 63xx's on a O-470. 450 hrs since new and they had worn to a point that one wouldn't even break points anymore. Owner called back to say he thinks he gained 20-30 HP and knocked off a couple hundred foot takeoff roll. If you have newer Slicks with a few hundred hours I'd suggest a cam inspection. New ones look like ABS plastic and are junk. Slick should be ashamed (or indicted).

At one time, I had the data on secondary coil voltage for both Slick and Bendix mags. My cursory look this morning at the John Schwarner's site revealed that a Slick coil can produce up to 20,000V at 250F in the secondary. IIRC, the secondary voltage voltage in a Bendix coil was considerable higher. That being the case, the Bendix would provide a much faster and powerful spark.
There are many more qualified individuals on this site, but having worked on and operated the Bendix -200 and -1200 series, and the Slick 4300 series mags has made me true to the Bendix mags.

I like Bendix mags better
 

I think they both spark very well when in good condition.
Bendix mags seem to have much better build quaility to me.

Bendix mags can't spark any faster than Slicks...electricity will travel the same speed coming from either brand in my opinion. Just under the speed of light I'm told.

As to the voltage, that's determined by the spark plug gap or rotor gap whichever is greater. When the coil colapses voltage builds until the spark jumps. Wider gap = more voltage. Turn a mag slow with a wide spark plug gap and it may not jump across the gap. (not enough voltage).
Once it jumps the gap the voltage does not continue to increase.
At least that's the way I understand it. Somebody tell me If I'm all wet.

Mark

Maags
 

I have an A & P friend who says (not so jokingly), that he'd rather have a marginal Bendix than 2 "good" slicks;)

You're right about the voltage depending on the gap. There are some high voltage limiting devices that work on that principle. You're mostly right on the voltage not creating a "faster" spark. ONE of the parameters that affects voltage rise rate ("slew" rate) in a mageto coil (or any inductive circuit), is the maximim voltage the mag is capable of producing if it doesn't find a gap to jump. A mag with a higher open-circuit voltage will reach the level required to jump the spark gap slightly (micro- or nano-seconds?) before one with a lower maximum voltage. I doubt however that that small difference in timing would make much difference in engine performance.

What a higher voltage mag will do is be more likely to jump a given gap. A fuel-air mixture is an insulator, and the higher the pressure, the more voltage is required to jump the gap. I really don't have a feel for how much voltage is requred vs. compression, but I'd bet a dedicated googler could find out...

Cool
 

Thanks for the explanation Longranger!

My Experience with Slick vs Bendix
 

Yep...Thanks, Miles.
I do believe that voltage is a function of the number of windings in the secondary circuit. The spark plug gap has nothing to do with secondary voltage output of the mag.

Mark,
The voltage is a function of the number of windings in the secondary coil. The more windings, the higher the voltage. Spark plug and rotor gap have no bearing on secondary voltage capability, only the amount of voltage required to jump the gap. If the secondary voltage increases faster and to a greater level, the quicker the gap is jumped.
Additionally, it is the field in the primary coil that is collapsed by opening the points (hence why properly set e-gap is important) and this builds the voltage in the secondary circuit through induction. A mag will not spark across a wide gap when turned slowly because of there being insufficient voltage in the primary due to slow rotating magnet speed. Of course, you are correct that gap has a function in when a spark will occur, but it is all a function of the internal workings of the mag.
To the OP...If I had a set -1200 series mags available, they would be on both my planes! I had a Bellanca 17-31 with an IO-540 and -1200 mags. Good starts (even hot!) ALL the time, and never one failure over 8 years.