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Mag drop: LS vs Magneto

banzai37

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I am trying to make sure that the RPM drops I am seeing are correct for my setup:

10 SMOH IO-360A1A CS (8.5:1 compression), with a LS II and regular Slick mag. Doing a runup at 1800, I get 30-50 on the LS and 150-175 on the Slick.

Is this normal?

The engine was just rebuilt with new LS wires, LS plugs, and a new Slick mag. I kept the old Slick plugs (cleaned/gapped) and Slick ignition wires. Before the rebuild, the Slick drop was closer to 100, so I have been hunting for causes of the higher drop, but can't find anything. Prior to rebuild the engine had 10:1 pistons. I have timed and retimed the mags.

During the mag checks, all EGT's rise approximately evenly. The only other observation is that the #4 CHT is higher than the other cylinders (compared to the rebuild).

I have looked at other posts and can't find consistent reports of what mag drop I should expect. The LS manual says the Slick mag drop could be 100-200.
 
I think the drop is normal. I have found that aircraft with one traditional mag and a electronic ignition show a greater drop running on just the mag than a aircraft running two conventional mags. I suspect it’s the greater efficiency of the E mag carrying a bigger load during normal ops. My mag drop of 75 per side with dual Mags switched to almost zero and 150 after adding a surefly.
 
Rpm drop test is In one part for measuring timing differences. The ei is likely more re advanced at 1800 and therefore show a lower drop as that advanced timing is more optimal and therefore produces more power. Timing is a composite of both ignition systems and dropping one will impact the final timing and power output
 
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I've the same setup (well Bendix instead of Slick) on io-320. I get about 10 on the LS and about 100 on the Bendix. Your Slick drop seems high. Try disconnecting (and plugging) the vacuum line on the LS and re-test.
 
10 SMOH IO-360A1A CS (8.5:1 compression), with a LS II and regular Slick mag. Doing a runup at 1800, I get 30-50 on the LS and 150-175 on the Slick. Is this normal?

Did the rebuild include a servo overhaul, or an idle mixture adjustment?
 
Thanks for all of the quick feedback. The overhaul did not include the servo or idle mixture (they were untouched).
 
I've the same setup (well Bendix instead of Slick) on io-320. I get about 10 on the LS and about 100 on the Bendix. Your Slick drop seems high. Try disconnecting (and plugging) the vacuum line on the LS and re-test.

If I do this, then the LS timing will revert to 25d BTDC, right? This should cause an rpm drop and the LS will be more comparable to the Slick, correct? Good idea. I’ll try it.
 
If I do this, then the LS timing will revert to 25d BTDC, right? This should cause an rpm drop and the LS will be more comparable to the Slick, correct? Good idea. I’ll try it.

Not necessarily - I suspect timing advance is a function of both RPM and MP.

Carl
 
Do you kean for ground ops? If not you might want to try that. I never needed to in my rv but in mny Cardinal it makes a big difference.
 
I have dual LS on my O-320 and I do the runup at 1700 RPM.

Don't know if that would change anything. Just a data point.
 
...IO-360A1A CS (8.5:1 compression)...Doing a runup at 1800, I get 30-50 on the LS and 150-175 on the Slick... Before the rebuild, the Slick drop was closer to 100, so I have been hunting for causes of the higher drop, but can't find anything. Prior to rebuild the engine had 10:1 pistons.

Well, if no change to mixture, it follows the increased drop is a function of lower CR, an interesting point.

During the mag checks, all EGT's rise approximately evenly.

Normal. The effective ignition retard moves peak pressure further ATDC, so higher pressure at valve opening, higher EGT.
 
I missed the engine compression change. It’s likely the electronic ignition might have a base advance schedule still set up for the higher compression. When I ran 10 to 1 pistons 22 degrees was recommended. Current engine has 9.5 to 1 with 23 degrees recommended.
 
From the LS manual, "If fuel mixture setting is near optimum, there will be no significant RPM drop when
the mag is turned off and the engine runs on the PLASMA CDI alone."

I have been chasing this issue since a FI overhaul, I am now down to 0-10 rpm drop which is where I was prior to the overhaul.

I will be installing a LS to EFIS adapter to have the ability to see the timing of the LS on the EFIS.
https://www.aircraftspruce.com/catalog/eppages/L2Elightspeed10-06222.php
 
If I do this, then the LS timing will revert to 25d BTDC, right? This should cause a rpm drop and the LS will be more comparable to the Slick, correct? Good idea. I’ll try it.

No, you need to read the procedure Klaus publishes for verifying LSE timing with an old school automotive timing light with and without the LSE vacuum line connected.

The Slick does not know if the other ignition system is a mag or LSE if it's off - the Slick running on it's own doesn't know/care if any other ignition system even exists.
 
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Just a quick update to close this out:
- I adjusted the timing of the LS - it was a little retarded
- I flew 5 more hours on the engine (it's now at 15 SMOH)

Slick mag drop is now ~130 and the LS drop is 10. My EGT's also settled to be more even across all cylinders after 5 more hours of flight, so I think the engine may have been finishing its break-in.

Thanks for the support and advice.
 
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