25 or 20 TDC?

[LettersFromFlyoverCountry]

I just finished with Mike Busch's excellent advanced leaning webinar from EAA (I'll write up notes and share them on the blog later), but this one section caught my attention.

Here's what he said:

Lycoming originally specified that all of their IO-360 engines be timed at 25 degrees before dead center and they had lots of problems with high CHT. It?s a company that traditionally has said ?don?t worry about CHT? but eventually these high CHT were manifesting themselves in detonation damage so Lycoming respecified all timings to be 20 before TDC instead of 25.

That surprises me because I think I bought my engine in 2008 from Mattituck and I'm pretty sure (I'm not out at the hangar) that it still has a data plate for 25. I've heard nothing about this before that.

Can anyone enlighten me on this assertion?

[Mike S]

Caught that too, but he did not address it as related to 540s.

I have wondered if my high CHT issue could be due to timing.....

Sent Busch an email, hope he answers.

[Bill Dicus]

My IO-360M1A by Lycon with one Slick mag and one Lightspeed III was set for 20deg BTDC for the EI. This was, I thought, for the 10:1 compression. You engine experts out there - should it be even less? Thanks in advance for any input!

[az_gila]

From the latest Lycoming Type Certificate Data Sheet for IO-360 engines-

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/cb8eb266e5683e5b86257a09004d8f38/$FILE/1E10%20Rev%2024.pdf

* All models except IO-360-A1B6D and -A3B6D have optional timing of 20°BTC

For all O-360 engines it seems set at 25 degrees

http://rgl.faa.gov/Regulatory_and_Gu...286_Rev_21.pdf

[LettersFromFlyoverCountry]

Looks like it was revised in May. Wish I'd known about it. Guess it's time to retime the mag. How would this affect Lightspeed if at all?

[RV10inOz]

Timing at 20DBTDC will move Theta PP to the right or to higher numbers of DATDC, and in doing so will lower the peak pressure. This is the reason CHT will lower.

With the Lightspeed and other EI's they will keep firing off at anywhere up to the max available. You can work out ways to reduce the spark timing of these EI's.

The HP output will most likely not vary much if anything at all and the observations should be a higher EGT value and lower CHT values.

The downside might be less effective LOP ops, so if you are going to make a change do so in small amounts or be prepared to settle somewhere in-between.

[RV6airplanePilot]

from Superior, a parrallel valve model, has Compression of 8.5, right on the lightspeed instructions borderline. I have dual Lightspeed ignition. I set it up at 25 advance and the CHTs were a little high. I changed the timing to 21 before and the CHTs came down from 380 in cruise to 360. Similar improvement in climb. No loss in cruise speed. Re your question on timing Lightspeed, I believe it is more important than the timing of the mag. If your CHTs are on the high side it is worth the time to change to about 20 advance and check the results. My timing change was done be removing prop and moving trigger plate, not fun but worth it to me.

[IowaRV9Dreamer]

Originally Posted by LettersFromFlyoverCountry

Looks like it was revised in May. Wish I'd known about it. Guess it's time to retime the mag. How would this affect Lightspeed if at all?

The May revision was only minimum fuel grade (see the revision bars). The timing thing is decades old. I thought it was only a 200hp (angle valve) thing.

[LettersFromFlyoverCountry]

I gotta go out to the hangar. I could swear the dataplate on my engine date 25 degrees. New engine.

Update: I've written up my notes from the webinar here.

[az_gila]

Originally Posted by LettersFromFlyoverCountry

I gotta go out to the hangar. I could swear the dataplate on my engine date 25 degrees. New engine.

Update: I've written up my notes from the webinar here.

This is in the 2000 revision of the O-360 operator manual and seems to be serial number dependent -

On the following model engines, the magneto spark occurs at 20 BTC. Consult nameplate before timing magnetos.
Models Serial No.
IO-360-A Series (Except -A1BGD) L14436-51 and up
IO-360-C,-D Series (Except -C1C,-C1F,-C1CG,-C1D6) L14436-51 and up
IO-360-C1C,-C1F L13150-51 and up
IO-360-C106 L14446-51 and up
LIO-360-C1E6 G1064-67 and up
AIO-360-A1A,-A1B,-B1B G220-63 and up
HIO-360-C1A,-C1B G14436-51 and up
IO-360-C1C6 All Engines
IO-360-J1AD, -K2A All Engines