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Old 09-17-2010, 04:38 PM
nomocom's Avatar
nomocom nomocom is offline
Join Date: Jun 2009
Location: Caldwell ID
Posts: 264
Default Pelican's perch #43

Originally Posted by N941WR View Post

Under heavy load, such as at full power takeoff, most EI's limit the advance, 24 degrees in your example, to preclude pre-ignition.

At high altitudes and lower power settings, such as at cruise, most EI's will allow for greater advance settings.

The idea is that there is less fuel & air in the cylinders when operating at lower MAP's and it takes a longer time for the flame to propagate across the entire cylinder, so they can start it burning sooner.

At high power settings (MAP), the fuel/air mixture will burn faster because there is more fuel/air. Thus the EI's delay when to ignite the mixture until closer to TDC (Top Dead Center). If the mixture burns too fast or too far before TDC, the maximum pressure could be well before TDC, which you hear as engine destroying pinging.
I suspect you mean to preclude detonation? Easy to get the two confused, but if ignition wires are routed correctly and the timing chip not scrabbled, then its really detonation we are worried about, not so much pre-ignition.

IMHO, the best description of what goes on in our Lycoming cylinders is by Deakin, "Detonation Myths" Probably a good read in terms of understanding what is at stake with our choice of timing advance.

Deakin also points out that you won't hear pinging in an aviation big bore, as the frequencies produced are linked to the diameter of the chamber, thus the noises are lower frequency, and close to the usual noises coming from the engine.

I appreciate his detailed explanation of the different levels of detonation coupled with the timing and pressure maps of the cylinder. Even if you've got heavy detonation, don't expect to hear it, you might see really high CHT's if you are paying attention and have a probe in the offending cylinder. Otherwise, when you have the severe detonation long enough, the burnt valve or hole in the cylinder may be the first clue.
1990 RV-3 (now apart, upgrades in the works)
1959 C172 O-360
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Old 09-21-2010, 01:23 PM
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nomocom nomocom is offline
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Location: Caldwell ID
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Default Hmmmm.

Originally Posted by nomocom View Post
The EI commander can show timing advance so this would be easy to do and would be a benefit to the community here.
I'm hearing nothing but silence on coming up with a timing map . Have I stepped on toes here or???
1990 RV-3 (now apart, upgrades in the works)
1959 C172 O-360
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Old 05-26-2014, 07:11 PM
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flyingriki flyingriki is offline
Join Date: Sep 2008
Location: California
Posts: 697

Looks like this thread died out some time back.
I overhauled a 150 hp O-320 to, hopefully, around 180 HP or so with more compression and Ly-Con cylinders and indexed cam. What should I do with my P-mag? I cut the jumper to run smoother on 7:1 compression. Figure I need to reconnect at 9.5:1. Any other suggestions? Was hoping to upgrade to two EI but didn't get there.....

Updated post for accuracy!

Last edited by flyingriki : 05-26-2014 at 08:32 PM.
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Old 05-26-2014, 08:20 PM
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RV10inOz RV10inOz is offline
Join Date: May 2008
Location: Brisbane Qld. Aust.
Posts: 2,304

My suggestion is at 10:1 CR you do not want advance of any great amount. My thoughts on this are best not made in this post, so you could read between the lines.

I would strongly urge you to research the mag timing of high compression engines and make an educated decision.

David Brown

DYNON Authorised Dealer and Installer

The two best investments you can make, by any financial test, an EMS and APS!
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Old 05-26-2014, 08:24 PM
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N941WR N941WR is offline
Join Date: Jan 2005
Location: SC
Posts: 12,887

This is a tough question because of your high compression pistons. I hope someone else jumps in with experiance and comments.

With the jumper the timing starts at about 26? BTC. Without the jumper it starts around 31? BTC.

It seems to me that with your pistons, you might want to try moving your timing in the other direction.
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
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Old 02-02-2020, 06:52 PM
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rleffler rleffler is offline
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Location: Delaware, OH (KDLZ)
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Resurrecting a really old thread........


Now that the PMag 200s are shipping, does the EIC32 support them? If not, is there a plan to?

Bob Leffler
N410BL - RV10 Flying
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Old 02-02-2020, 07:56 PM
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Carl Froehlich Carl Froehlich is offline
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Recommend you look at the six cylinder pMag install manual. For this pMag you set the maximum advance during install - in other words it is not the binary ?jumper in or jumper out? option.

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