Acceptable mag drop?

A forum for the proverbial airport bum who just wants to talk about anything and everything related to flying. Introduce yourself here !!

Post Reply
User avatar
bullojm1
Chief Rivet Banger
Posts: 1357
Joined: Sat Mar 25, 2006 6:37 pm
Location: KDMW
Contact:

Acceptable mag drop?

Post by bullojm1 »

The engine on my RV-7 has always been strong. Never has it hickup'd or caused me any concern. However, my ground runup has always been less than ideal. I always hear of people who have very little mag drops when they do their runups. Here is what I run into.

I run my engine up to 1800 RPM after my oil temps are 100+ F and CHT's are 200+ F. My left mag check is first (Slick impulse coupled). My mag drop is down to 1600-1625. My right mag check is for my e-mag, with a drop down to 1675-1700 RPM. The mag check is done at full mixture, and the engine continues to sound like it is running fine. Both e-Mag and slick mag are timed properly.

Am I worrying about nothing?

Thx!
Mike Bullock
http://www.rvplane.com
RV-7 | Superior IO-360 | Whirlwind 200RV
Garmin GTN650 | GRT Dual Sport SX EFIS
Status: FLYING!

lancef53
Class D
Posts: 376
Joined: Sun May 20, 2007 8:34 pm
Location: Portland, North Dakota

Post by lancef53 »

Do you warm up the engine at full rich? Does it get any better if you lean it? 175-200 rpm seems kind of high.
RV 8
First Flight--4-3-2010

User avatar
hydroguy2
Class B
Posts: 1307
Joined: Mon Sep 10, 2007 11:13 pm
Location: Townsend, Montana

Post by hydroguy2 »

I tto have heard and seen the stories of now drop when switching off a mag

BUT

I have Emag and Pmag combo. almost full rich run-up 1800rpm... I don't recall exactly which one is which, but I think my Emag drops about 40-60. and the Pmag drops 120-150. I've timed it, swapped plugs, swapped wires, retimed it, talked with factory only to find out "yep, some of them do that"
Brian
Townsend, MT

User avatar
Brantel
Class B
Posts: 1029
Joined: Wed Aug 30, 2006 3:23 pm
Location: Newport, TN

Post by Brantel »

Mike,

I have a Pmag right and an impulse coupled Slick mag left.

I get less than 10 rpm drop when running on the Pmag only.

I get 40-50 drop when running on the Slick only.
Brantel (Brian Chesteen),
RV12-IS, #121606, N912BC - Building Now!
RV10, #41942, N????? - Project Sold
RV-7/TU, #72823, N159SB - SOLD

speedpants
Class G
Posts: 16
Joined: Wed Dec 29, 2010 9:39 pm
Location: KPYM
Contact:

Post by speedpants »

Mike,
I have had this problem in the past with some arrows and senecas with the IO-360's your high end factory mixture setting is most likely too rich, try this for an experiment, warm the engine normally, then when it comes times to perform your mag check set the engine rpm to 1800 and then lean the mixture back as far back as you can with out getting a rise then perform the mag check the drop should go down, we had a arrow do this same exact thing when the mixture was full rich at 1800 rpm, it would drop 200-250 per mag but never be rough, as soon as the mixture was pulled back the drop went down to 50-75. Let us know how you make out.
The best aircraft mechanic is one you only see once a year

User avatar
bullojm1
Chief Rivet Banger
Posts: 1357
Joined: Sat Mar 25, 2006 6:37 pm
Location: KDMW
Contact:

Post by bullojm1 »

Thanks for the advice guys. I warm my engine up with the mixture pulled back in order to not foul the plugs. My runup is done at full mixture. I will try a runup with the mixture pulled back and report back with my results.

Just out of curiosity, if this test proves to give me less of a mag drop, is there such a thing as my mixture being too rich for a FI engine?
Mike Bullock
http://www.rvplane.com
RV-7 | Superior IO-360 | Whirlwind 200RV
Garmin GTN650 | GRT Dual Sport SX EFIS
Status: FLYING!

speedpants
Class G
Posts: 16
Joined: Wed Dec 29, 2010 9:39 pm
Location: KPYM
Contact:

Post by speedpants »

Mike, there is a thing of being too rich with a Fi engine, but in normal conditions you would never know because you always pull the mixture back in cruise to get your lean of peak or rich of peak senerio. If the test results show what i think will happen you have 2 options, 1) leave it alone, or 2) send the fuel controler out to a facility and have them re-jet the high end. There is no down fall or issues with leaving it alone and living with it. At the flight school we sent the controler out and had it re-jetted...but then again we have 100's of students a year fly it so getting the same squack over and over again got annoying.

Hope this helps,
Mitch
The best aircraft mechanic is one you only see once a year

Spike
Chief Rivet Banger
Posts: 4013
Joined: Thu Oct 14, 2004 8:40 pm
Location: Baltimore, MD
Contact:

Post by Spike »

This is an interesting question. Ultimately what we are asking (I think) when doing a mag check is if the combustion is degraded by the expected amount when turning off the second set of plugs. That would indicate to us that the remaining mag is working properly because it only let performance drop by that expected amount. The PMags are designed to be more efficient in their timing, etc. I would think that this is a tough apples to apples comparison to make because of their performance.

However what you have proven is that the on only the PMAG the system is fully capable of running the engine by itself and that it is better at doing it than the normal mag. So if you are absolutely sure that the traditional mag is no longer firing the plugs then I would say you are money.

Spike
http://www.rivetbangers.com - Now integrating web and mail!
Current Build: 2 years into a beautiful little girl

User avatar
bullojm1
Chief Rivet Banger
Posts: 1357
Joined: Sat Mar 25, 2006 6:37 pm
Location: KDMW
Contact:

Post by bullojm1 »

speedpants wrote:Mike,
I have had this problem in the past with some arrows and senecas with the IO-360's your high end factory mixture setting is most likely too rich, try this for an experiment, warm the engine normally, then when it comes times to perform your mag check set the engine rpm to 1800 and then lean the mixture back as far back as you can with out getting a rise then perform the mag check the drop should go down, we had a arrow do this same exact thing when the mixture was full rich at 1800 rpm, it would drop 200-250 per mag but never be rough, as soon as the mixture was pulled back the drop went down to 50-75. Let us know how you make out.
I tried this on Sunday and the Mag drop were much better. I don't recall the exact numbers, but the Slick mag dropped 75 RPM or so, and the EMAG dropped about 20 RPM. Makes me feel much better! Thanks for the advice!
Mike Bullock
http://www.rvplane.com
RV-7 | Superior IO-360 | Whirlwind 200RV
Garmin GTN650 | GRT Dual Sport SX EFIS
Status: FLYING!

Post Reply