Low RPM Drop with Slick Mags - Normal?

[krw5927]

I recently installed a new Titan O-340 Stroker (9:1 CR) in my 9A.

I've noticed on run-up at 1700 RPM that when grounding left, then right magnetos, I now only see a ~20 RPM drop each side. The engine is making plenty of power and running really smoothly, and EGTs are behaving as expected (all EGTs climb when operating on only 1 mag). I'm just not used to such a low drop.

My previous O-320 with 7:1 pistons would drop 90-100 rpm when switching off a mag. There's no difference in magnetos - I simply swapped the Slick 4370 and 4371 mags from the old engine to the new one. Timing was spot on with both as well. I did buy new harnesses though, and I'm using REM37BY plugs that the O-340 shipped with instead of REM40E plugs on the previous engine.

Could the low mag drop simply be due to the higher compression ratio? Do the longer-nosed REM37BY plugs have anything to do with it?

[Neal@F14]

The 37BY plugs do help, but your new harnesses are probably helping more.

My 8.5:1 O-320 with Slick mags and UREM37BY plugs (450 Hrs on the engine & mags, 200 on the plugs) typically has a 25-50 rpm drop when doing the mag check.

[alpinelakespilot2000]

At ~1800 rpm, I tend to see 30-40, so your 20 at 1700 is not surprising. Clean plugs plus new install is probably going to make for small drops.

[krw5927]

Good to hear that this is pretty normal. In all the planes I've flown I've never seen a mag drop so small. Good sign that everything is pretty healthy! Clean plugs, newer harnesses, (and even a Pmag) dropped more on my aged 7:1 O-320 E2D.

[mahlon_r]

Originally Posted by krw5927

I recently installed a new Titan O-340 Stroker (9:1 CR) in my 9A.

I've noticed on run-up at 1700 RPM that when grounding left, then right magnetos, I now only see a ~20 RPM drop each side. The engine is making plenty of power and running really smoothly, and EGTs are behaving as expected (all EGTs climb when operating on only 1 mag). I'm just not used to such a low drop.

My previous O-320 with 7:1 pistons would drop 90-100 rpm when switching off a mag. There's no difference in magnetos - I simply swapped the Slick 4370 and 4371 mags from the old engine to the new one. Timing was spot on with both as well. I did buy new harnesses though, and I'm using REM37BY plugs that the O-340 shipped with instead of REM40E plugs on the previous engine.

Could the low mag drop simply be due to the higher compression ratio? Do the longer-nosed REM37BY plugs have anything to do with it?

If it were mine, I would double check the ignition timing. If it was advanced out of spec, the mag drop will get smaller. 20 rpm is a very small mag drop normally closer to 50 and maybe as high as 75. If the timing is Ok, no worries, but I would check.
Good luck,
Mahlon