Mag Drop

Larry Lowenkron

Larry Lowenkron

Member
Dave, if you have a 200 rpm mag drop, things you might check, after external timing are:
Spark plugs, ignition leads, internal timing, messed up parts inside the mag.
 
Regarding mag drop on Franklins, they will always have more drop than a continental or lyc ,all other things being equal, because the plugs are so far apart(1). The retarding effect of single ignition is greater. If mixture is off much, flame speed will be slowed and even greater mag drop will be had.

So, if mag drop seem excessive but smooth, it is more likely caused by somewhat retarded timing. This is A&P 101 stuff. Keep the timing on the high side of the spec. Your exhaust valves will love you for it and you will get a teeny bit more power. Also, in case of a mag failure, the significant power reduction that will occur will be less.

(1)On a Stinson Franklin, all plugs are 180 degrees apart on the bore - 1 bore diameter or 4 1/2 inches. On a Bellanca Franklin 3 cylinders have plug spacings of the same 4 1/2 inch and on 3 cylinders the plugs are 90 degrees apart or about 3 1/2 inches. In this latter case, The flame has to reach all the way across the bore to complete combustion. - I don't like this feature because the effective timings of the left and right side of the engine are different. This all has to do with the plugs being located in the suburbs on a Franklin and the flame taking a while to get downtown where most of the mixture is. (at 20 or 30 feet per second)

r
 
dtreid said:
Mag drop is very smooth, and does not change much between the two mags.
Click to expand...
Well, very good news. I would carefully check the mag timing. Sometime those timing scratches on the Franklin prop flange are hard to use accurately with the cowl stuff there. Maybe a TDC finder and a degree wheel will be more practical along with a buzz box to set the timing accurately.

Way back when, cellophane from the pack of Luck Strikes (that everyone had in their shirt pocket or tee shirt sleeve) was used to set point timing. The position where the cellophane loosened established point opening. Now we use buzz boxes to detect point opening. There is a degree or two difference between the methods. To compensate, a degree or so has to be added when using a buzz box to get the cellophane timing that likely established the spec for the engine in the first place. Then, because timing tolerance is generally plus or minus 2 degrees, give yourself those extra 2 degrees as a reward for such a precise job you did. :D At 7:1 compression ratio our Franklins are no where near detonation limit with any fuel I know of. So, I advocate setting the mags on a Franklin little bit on the high side for safety and an all round better running engine.

True story: A former student bought a nice Stinson. Later I ask how he was doing with his airplane. He said he totaled it in New Mexico during a cross country at night. He lost a mag and the airplane would not maintain altitude on one mag. To make matters worst, the new pilot added flaps for "more lift". He could not see a thing and slowly descended into the high country trees and sage. Luckily, he didn't get a scratch.

Was that Stinson's timing on the low/late side? Could be. The point is Franklins lose considerable power running single mag. Minimizing this by keeping the timing on the high side is a good thing.

Dave, good luck with your mags.

r
 
I have always had a L mag drop of at least 150 rpm maybe closer to 175 rpm.

3 mechanics including Dan T have checked it 6 ways from sunday.

:?:

Leads, plugs, and timing. Nobody finds anything wrong.

Other mag is 50 rpm drop.

Mags were re-maned by Bendix 200 hours ago.

All I can think is the point gap or e-gap aint quite right.

Bugs me, but it is with-in specs ( barely).

Oh well.
 
Well this was a first. I installed a Slick mag kit on a TriPacer replacing the original Bendix mags circa 1960. After topping with new Titan nickel we fired up to check for oil leaks and mag drop. No oil leaks but the left (impulse) mag had a 300 rpm drop! I fussed with this for hours and then called Slick tech support. We found that the mag was the wrong rotation but had the proper data plate installed. The guy I talked with was pretty sharp. I could get the thing down to 150 drop by retarding the timing but I knew something was not right. The secret was no matter where you moved the left mag the timing light did not move from the original setting. Another return to Aircraft Spruce, thankfully they are very easy to deal with. Lynn the crate :shock: Yes his name was Joe!
 
I've talked with the guy from Slick before, after I bent the timing pin. You're not kidding he's sharp. I think he was there when slick was new and now just handles warranty and diagnosing.
He sent me a current manual, with a new pin and rotor cap for FREE! He was just happy that I was an IA that actually confessed to screwing up. Apparently most people that do that return the mag for warranty repair and claim they didn't do anything.
 
I think you're talking about Joe Logie. He is at Champion Aerospace now (who now owns/supports Slicks.) I talked to him a couple of months ago regarding a plane we have at school. He was great! Got back to me right away, and even called back when my cell phone dropped the call.
I must say, I look inside a Slick, and then look inside a Bendix or even Eismann, and think the Slicks look puny, but they work! I'm not complaining! Have been keeping my old Eismanns as a back up, but think I'll just rebuild them and sell them...when the spirit finally moves me.
 
I got a good accurate job on the timing finally. It was set about 30 degrees and I was able to move it to 32.5 degrees. I haven't flown it yet, but could I expect any power difference with this change in timing?
 
dtreid said:
I got a good accurate job on the timing finally. It was set about 30 degrees and I was able to move it to 32.5 degrees. I haven't flown it yet, but could I expect any power difference with this change in timing?
Click to expand...

I would expect a little increase in static RPM. To interpret this, an increase of 25 RPM near 2500 RPM is a 1% increase. For some reason static RPM might vary a little from day to day, but with that in mind, a 1% increase in rpm will mean a 2% increase in torque and a 3% increase in power. So, that would be about 4 1/2 HP with a Franklin.

You are just about spec now on the timing for a 165, so let us know how it goes.

r
 
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