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cjbouldin

Active Member
Hi all,

Recently had my conditional inspection by my local AP/IA. He called and asked whether the mag timing should be set at 25 degree's or 20 degree's. I have an ECI 0-360 A1A with 9:1 pistons with Slick mags. The engine doesn't have a data plate for timing references. Historically it's been set at 25 degree"s. He called ECI and they said that with 9:1 pistons the timing should be set at 20 degree's. So that's what we did.

Now, flight testing indicated a slightly slower TAS (167 kts vs 172 kts) but slightly cooler cylinder head temps (320 vs about 340). This is after about 1 hour of flying. I'm not concerned about running with a slightly higher cylinder head temp but hate to lose 5 kts. I will re-check the performance but was wondering if anyone has a different opinion or different experience's.

Regards
 
Most Lycomings I work on are 25 degrees BTDC. I don't do much with high compression, though. If it was running fine at 25, and temps aren't too high, then is change it back.

This from a Mooney forum:

I noticed that Jetdriven mentioned that he had his timing changed from 20 deg. BTDC to 25 deg. with better performance noted, especially LOP. The below info from the type certificate for the I0-360 A1A ends with the note that 25 deg. is spec, with 20 deg optional. Pretty weird, huh? So, how is your timing set, why, and what effect do you see? Anyone else changed timing and has before and after?

http://mooneyspace.com/topic/3877-io-360-a1a-what-mag-timing-20-or-25-deg-btdc/

Another question that comes to mind is, what was your actual timing when your mechanic checked it?
 
Most but not all 8:5:1 Lycs are 25*BTDC as Jesse notes - O-235-L2C is one exception. To complicate matters, Lycoming changed the timing spec on some models. I know Klaus/LSE recommends O-320 and O-360 be mag timed at 20* if CR ≥ 9.0:1. I have 9.2:1 and run at 23* after years of running it at 27*. It runs cooler at 23* but no cooler at 20* so 23* it is on my engine and airframe set-up. Experimental aviation, right :D
 
Thanks guys

The Mooney discussion about the SI involving the impulse coupling change out is news to me. Frankly, nothing was mentioned about the parts exchange in the L mag.
Honestly, I like the performance at 25 degree's.

I think on Monday I'll give Ken a call at Lycon and get his opinion.

Regards
Chris
 
Chris,

My understanding is that the timing is retarded by 5 degrees for higher compression engines to reduce the possibility of detonation at high power, for example take off at sea level.

You might want to think about installing electronic ignition instead of the mags.

I have 9:1 compression and dual Lightspeed ignitions which have a base setting of 20 degrees. I have a display which shows the ignition advance and I get 20 degrees for a full power, sea level takeoff and about 27 degrees up higher at cruise power.

Fin
9A
 
When you see what happens to the ICP(internal cylinder pressure) with respect to timing, and HP output you start to realise the importance. I think ECI said 20 degrees for a god reason. Sure the LOP ops will be a bit better at 25 but if it were me I would be following their advice and at the most the outer edge of the tolerance, so if they publish 20 +/- 2 be accurate at 22.
 
cjbouldin said:
Hi all,
He called ECI and they said that with 9:1 pistons the timing should be set at 20 degree's. So that's what we did.
Click to expand...

There could be a reason for the suggested 20 degrees.

Here's a thread that I posted back on 09-16-2012 about compression ratio.
http://www.vansairforce.com/community/showthread.php?t=91036&highlight=compression+ratio

I?ll be flying the Supercharged SA750 into KOSH and Russ will flying in his Supercharged 7 through the week, just hanging out on the field having fun. I will be park over @ the IAC aerobatic aircraft. I would like you to stop by to meet in person and show you my ride and see yours. Talk about performance and other dyno fun stuff. My cell 303-906-6846
See ya there
7557660588_5afc3d8a3d_m.jpg
 
Just to be clear, I have an O-360. It will absolutely not run LOP without running rough. That being said, with healthy ROP (19.5 gal/hr) flows on takeoff detonation margins are increased. I find it interesting that Lycoming indicated that 25 deg or 20 deg was optional with 8.5 pistons. I suspect they'd recommend 20 deg with the 9:1 pistons. The engine has 380 hours at 25 deg. flying at sea level temps from 0 C. to 37 C. with no indications of cylinder/valve issue's. I boroscope at every oil change.

The electronic ignition is an interesting solution but I'm reluctant to make any significant changes ($).

Regards,
 
G3i Ignition said:
There could be a reason for the suggested 20 degrees.

Here's a thread that I posted back on 09-16-2012 about compression ratio.
http://www.vansairforce.com/community/showthread.php?t=91036&highlight=compression+ratio

I?ll be flying the Supercharged SA750 into KOSH and Russ will flying in his Supercharged 7 through the week, just hanging out on the field having fun. I will be park over @ the IAC aerobatic aircraft. I would like you to stop by to meet in person and show you my ride and see yours. Talk about performance and other dyno fun stuff. My cell 303-906-6846
See ya there
7557660588_5afc3d8a3d_m.jpg
Click to expand...


Thomas

Interesting analysis. I guess your point is that manufacturing tolerances are so sloppy and variable from piston to piston that we could be getting anything from 8.5 to 9.5 pistons when requesting 9:1 pistons? I find that tough to swallow. It almost sounds like you actually got 9:1 pistons instead of 8.5 pistons. In other words, a shipping boo boo?

The difference between 9.1:1 and 8.5:1 is 12cc. I can't imagine errors such as that.

Makes me want to perform the same test. Thanks.
\

Regards
 
No, it wasn?t a shipping mistake on the piston P/N, or sloppy piston tolerances. I would have like it to be that easy of a solution. Multiple - 8.5 to 1 pistons were blue printed to check tolerances, all test pistons were correct on wrist pin to piston top measurements. This what keep me from making KOSH last year, high CHT?s and detonation. Further investigation was required. The cylinders were removed and blue printed. The cylinder results were performed on a bench fixture with a test fixture piston that referenced off the base flange of the cylinder to duplicate each cylinder cc volume @ TDC. The correction for me was going with a lower compression ratio piston for my specific application.
 
Mattituck had to remove a little material in 3 cylinder head combustion chambers to equalize the CR in all 4 cylinders. All 4 pistons (not Lyc but aftermarket) were exactly the same above the wrist pins so the variation in my engine was the brand new cylinder/head assemblies (ECI)

My O-320 and the IO-360 both run LOP very smoothly - especially at full throttle above 15,000.
 
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