[Captain_John]

I have searched the forum and found remarkably little information on this topic.

I have the Mattituck test cell data that shows a 26 degree advance, however the engine data plate says 25 degrees.

I checked the actual timing and it was a schoch over 26 as the run in sheet indicated.

So why am I looking at it?

When I use my G3i electronic ignition during high manifold pressure it makes a popping noise from the exhaust. It sounds like the same popping that we all get during a descent, but occurs at a totally different point in operation.

I figured that a good starting point would be to set the timing at nameplate and go from there.

In my search for the "proper" setting, I have heard many people using a varied range of settings.

What is the reason for this?

CJ

 

[LettersFromFlyoverCountry]

Mahlon had a perspective on this in the summer of 2013:

"The 20 BTDC timing change for the IO-360 only applied to 200HP slant valve engines. It was not applicable to the parallel valve engines. To me the 8.5:1 and 9:1 parallel valve engines with mags should be timed to 25BTDC. If you have modified compression ratio higher then that or run Mogas continually then you could go to 20 BTDC. Timing with electronic ignition was generally set to 20 BTDC to limit the max advance the timing would go to during cruise power. You can experiment with timing and will get varying power and temp output but besides those items, you change the fuel requirements and detonation margin for the engine. So be careful when you experiment, keep good data, evaluate it well and change the timing in small amounts, gradually, to be safe. BTW, I am a firm believer that Lycoming cylinders running chts in the 425 or slightly more area will last just as long as one running at 300- 350 CHT. Years in the engine overhaul business bears that belief out."

 

[Captain_John]

Thanks Bob!

This makes me wonder if I should retard it further, as he says that engines with EI should be set to 20?

Anyone have any thoughts or comments?

Repucci, you always have an opinion on everything! What's your take? Don't tell me to get Pmags, either! Lol

CJ

 

[N941WR]

My opinion doesn't always mean good advice CJ.

Let's see...

Moving the timing to 20* BTDC means more more of the unburnt fuel-air charge will exit the exhaust valve. This will also result in a drop in CHT's and an increase in EGT's.

Moving the timing the other way means more of the charge is burned in the cylinder, raising ChT's and lowering EGT's.

That is standard stuff.

What I don't know is if the popping sound is a result of the fuel burning in the exhaust or the sound of the higher pressure wave exiting the cylinder.

Try adjusting the timing a few degrees either side of 25 and see how the popping changes.

Dan Horton and the others may have a thought on this as I simply don't know.

 

[vic syracuse]

Exhaust leaks

My IO-540 built by Mattituck has one mag and one Lightspeed, set at 25 BTDC. I don't think the popping has as much to do with timing as it does with mixture and throttle position. Also, exhaust leaks can cause popping.

Vic