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20 or 25 degree mag timing?
- Thread starter Steve
- Start date
gmcjetpilot
Well Known Member
Well 25 deg BTDC is standard but........
Mags are fixed timing. However when you install them you can set them any where you want. STANDARD timing is 25 degrees BTDC (before top dead center). Typically you retard timing to something less than 25 deg (eg 20 degree) for things like high compression pistons, turbo chargers or using lower octane gas. If you have a stock Lyc 320/360, not turbo charged with AV-gas than 25 deg BTDC is the standard spec for most Lycs.
Mags are FIXED timing, they never change. Where ever you install them, that is where timing stays. That is the base timing setting.
Ignitions on **automobiles and after market electronic ignitions for planes will advance the timing at lower power. However they also have a base timing setting, or the min advance when the extra advance has not kicked in (usually at lower engine power).
The lower the power the more you want the spark to be advanced. At low power the flame in the combustion chamber slows shown, so you need the timing to advance or spark earlier, before top dead center (BTDC). This alows all the fuel to burn and do work. If the timing is too late (too close to BTDC) than a lot of fuel does not burn and just gets blown out the tail pipe.
You can have timing TOO advanced and that will damage an engine. Lyc has found 25 degrees BTDC is a safe number for all power settings. Of course high power is the most critical.
The reason fixed 25 degree timing works on planes is because plane engines work in a very narrow power ban. So they settle on a compromise timing that works safely at high power and is OK for typical lower power settings. However when you start to fly real high and power is down below 70% or into the 60 % power range, the timing is starting to hurt your power and fuel economy more. This is where electronic ignition shines, it can advance timing up to 40 deg BTDC. That helps economy and adds a little power as well.
At high power you don't want the timing too advance because you can cause "Pre-Ignition" which can increase the combustion pressure and temp. In general if you have high temps retarding the timing helps.
FOR MOST LYCS ENGINES 25 deg BTDC is OK at FULL power. HOWEVER is you use poor quality fuel you may want to retard the timing (ie 20 BTDC). However using Av-gas and stock compression 25 deg is fine.
For cars they will retard the BASE timing a few degrees when they add power adders like turbo chargers, superchargers and NO2.
Advancing timing in ANY engine has to potential of increasing power at high power settings but it can also distroy and engine.
**Autos in the old days used fly weights and springs and vacuum advance. Now all ignitions are pretty much electronic. Some race cars still use the old fashion mechanical advances systems.
Well NO, only on installation, than its fixed all the time.Steve said:
Mags are fixed timing. However when you install them you can set them any where you want. STANDARD timing is 25 degrees BTDC (before top dead center). Typically you retard timing to something less than 25 deg (eg 20 degree) for things like high compression pistons, turbo chargers or using lower octane gas. If you have a stock Lyc 320/360, not turbo charged with AV-gas than 25 deg BTDC is the standard spec for most Lycs.
Mags are FIXED timing, they never change. Where ever you install them, that is where timing stays. That is the base timing setting.
Ignitions on **automobiles and after market electronic ignitions for planes will advance the timing at lower power. However they also have a base timing setting, or the min advance when the extra advance has not kicked in (usually at lower engine power).
The lower the power the more you want the spark to be advanced. At low power the flame in the combustion chamber slows shown, so you need the timing to advance or spark earlier, before top dead center (BTDC). This alows all the fuel to burn and do work. If the timing is too late (too close to BTDC) than a lot of fuel does not burn and just gets blown out the tail pipe.
You can have timing TOO advanced and that will damage an engine. Lyc has found 25 degrees BTDC is a safe number for all power settings. Of course high power is the most critical.
The reason fixed 25 degree timing works on planes is because plane engines work in a very narrow power ban. So they settle on a compromise timing that works safely at high power and is OK for typical lower power settings. However when you start to fly real high and power is down below 70% or into the 60 % power range, the timing is starting to hurt your power and fuel economy more. This is where electronic ignition shines, it can advance timing up to 40 deg BTDC. That helps economy and adds a little power as well.
At high power you don't want the timing too advance because you can cause "Pre-Ignition" which can increase the combustion pressure and temp. In general if you have high temps retarding the timing helps.
FOR MOST LYCS ENGINES 25 deg BTDC is OK at FULL power. HOWEVER is you use poor quality fuel you may want to retard the timing (ie 20 BTDC). However using Av-gas and stock compression 25 deg is fine.
For cars they will retard the BASE timing a few degrees when they add power adders like turbo chargers, superchargers and NO2.
Advancing timing in ANY engine has to potential of increasing power at high power settings but it can also distroy and engine.
**Autos in the old days used fly weights and springs and vacuum advance. Now all ignitions are pretty much electronic. Some race cars still use the old fashion mechanical advances systems.
Last edited:
Radomir
Well Known Member
Here are two resources that should help.
First, see Mahlon's post at this link (scroll down to it):
http://www.vansairforce.com/community/showthread.php?t=8078&highlight=impulse+coupling+angle
Second, Sacramento Sky Ranch has a nice article that will help better understand what's all that lag angle about:
http://www.sacskyranch.com/eng16.htm
HTH
First, see Mahlon's post at this link (scroll down to it):
http://www.vansairforce.com/community/showthread.php?t=8078&highlight=impulse+coupling+angle
Second, Sacramento Sky Ranch has a nice article that will help better understand what's all that lag angle about:
http://www.sacskyranch.com/eng16.htm
HTH
Harvey
Well Known Member
Steve,
You need to find out more about that option. It sounds to me that they are asking which lag angle in the mag you want. Mags don't come 20 or 25 degrees - that's where YOU set the ignition timing, generally 25 degrees Before Top Dead Centre on No. 1 cylinder. From some previous postings, I gather that if you have a Sky Tech PM starter motor and a wood prop, the preferred lag angle is 20 degrees to help prevent kick-back on start up. But I would be asking the engine supplier for more information, and whatever he suggests, go with that.
BTW - ignition timing has virtually no effect on 'pre-ignition' which is caused by a glowing ember of carbon or overheated spark plugs. It does have some bearing on 'detonation' though.
Best Rgds
Martin in Oz
You need to find out more about that option. It sounds to me that they are asking which lag angle in the mag you want. Mags don't come 20 or 25 degrees - that's where YOU set the ignition timing, generally 25 degrees Before Top Dead Centre on No. 1 cylinder. From some previous postings, I gather that if you have a Sky Tech PM starter motor and a wood prop, the preferred lag angle is 20 degrees to help prevent kick-back on start up. But I would be asking the engine supplier for more information, and whatever he suggests, go with that.
BTW - ignition timing has virtually no effect on 'pre-ignition' which is caused by a glowing ember of carbon or overheated spark plugs. It does have some bearing on 'detonation' though.
Best Rgds
Martin in Oz
Perhaps this is a question for Mahlon, but what is the recommended (or commonly used) timing for running 92UL? From what I've read, it seems 25 BTDC is fine as long as it's running 8.5:1 (or lower) pistons, not turbo'ed and your EI doesn't get crazy with the advance at low power settings.
Opinions?
Opinions?
az_gila
Well Known Member
Other way around....
Martin, I believe it is the other way around.... all O-360s have a factory timing (not lag of magneto impulse coupler) of 25 BTDC.
A 20 degree lag could fire the cylinder before TDC and cause a kick back...
A 25 degree lag cannot do this, since the impluse coupler would retard the first spark to TDC...
Hence a 25 degree lag Slick magneto Impulse coupler could be considered safer....
Check the earlier posts from about a week ago....
NOTE -- IO-360's are mixed in timing....
gil in Tucson
Harvey said:
Martin, I believe it is the other way around.... all O-360s have a factory timing (not lag of magneto impulse coupler) of 25 BTDC.
A 20 degree lag could fire the cylinder before TDC and cause a kick back...
A 25 degree lag cannot do this, since the impluse coupler would retard the first spark to TDC...
Hence a 25 degree lag Slick magneto Impulse coupler could be considered safer....
Check the earlier posts from about a week ago....
NOTE -- IO-360's are mixed in timing....
gil in Tucson
Harvey
Well Known Member
Thanks Gil,
I can see how setting the ignition timing point at 25 deg BTDC on a 25 deg lag angle magneto would allow the plugs to fire at TDC on start up. Now setting the same 25 deg ignition timing point with a 20 deg lag magneto would .....aaah, yes ! It would retard only 20 degrees, hence the timing point would actually be 5 deg BTDC. Excellent. Thanks.
Martin in Oz
I can see how setting the ignition timing point at 25 deg BTDC on a 25 deg lag angle magneto would allow the plugs to fire at TDC on start up. Now setting the same 25 deg ignition timing point with a 20 deg lag magneto would .....aaah, yes ! It would retard only 20 degrees, hence the timing point would actually be 5 deg BTDC. Excellent. Thanks.
Martin in Oz
az_gila said:Martin, I believe it is the other way around.... all O-360s have a factory timing (not lag of magneto impulse coupler) of 25 BTDC.
A 20 degree lag could fire the cylinder before TDC and cause a kick back...
A 25 degree lag cannot do this, since the impluse coupler would retard the first spark to TDC...
Hence a 25 degree lag Slick magneto Impulse coupler could be considered safer....
Check the earlier posts from about a week ago....
NOTE -- IO-360's are mixed in timing....
gil in Tucson
