Impact of mag timing change

[hindsight2020]

New-to-me 28R-200 still at annual, hope to get it back by the end of the week/start of the next one. IO-360-C1C (angle valve 200hp). AP found the mag timing at 25* instead of the 20* as stamped on the engine plate. These engines come in both timing varieties and there have been Lyco SIs in the past which have called for retarding timing to 20*. Be that as it may, there it is stamped at 20*

I'm getting a little anxy because I know this means a reduction of engine performance, though it provides an increase in detonation margins, lower CHTs and theoretically easier starting.

My normal running parameters are exclusively at 55-65% power WOT and 2400RPM, 6k-14k msl. I never run 75% in cruise; only gain 3-4 knots for an additional GPH and I can't get 75% power at 2400 anyways at the altitudes I like cruising at (higher the better).

Should I expect any substantial decrease in cruise speed after annual? A 4-5 knot loss at 65% would constitute a substantial loss to me. I'd also be ****ed if this thing starts running hella rough above 10k. Currently it leans very well at altitude and as opposed to the theoretical difficulty an injected engine would have with an advanced timing, I haven't suffered any starting problems on mine on 25*.

Is 5* timing between 20-25 range really something that's going to be noticeable at all by me, or are we talking about the loss of a couple HP on takeoff at the top end RPM/power combo where the performance loss would be manifested? Cube root power rule leads me to believe I shouldn't see jack squat in cruise performance change, but interested in hearing y'alls experience. Thanks!

 

[N801BH]

New-to-me 28R-200 still at annual, hope to get it back by the end of the week/start of the next one. IO-360-C1C (angle valve 200hp). AP found the mag timing at 25* instead of the 20* as stamped on the engine plate. These engines come in both timing varieties and there have been Lyco SIs in the past which have called for retarding timing to 20*. Be that as it may, there it is stamped at 20*

I'm getting a little anxy because I know this means a reduction of engine performance, though it provides an increase in detonation margins, lower CHTs and theoretically easier starting.

My normal running parameters are exclusively at 55-65% power WOT and 2400RPM, 6k-14k msl. I never run 75% in cruise; only gain 3-4 knots for an additional GPH and I can't get 75% power at 2400 anyways at the altitudes I like cruising at (higher the better).

Should I expect any substantial decrease in cruise speed after annual? A 4-5 knot loss at 65% would constitute a substantial loss to me. I'd also be ****ed if this thing starts running hella rough above 10k. Currently it leans very well at altitude and as opposed to the theoretical difficulty an injected engine would have with an advanced timing, I haven't suffered any starting problems on mine on 25*.

Is 5* timing between 20-25 range really something that's going to be noticeable at all by me, or are we talking about the loss of a couple HP on takeoff at the top end RPM/power combo where the performance loss would be manifested? Cube root power rule leads me to believe I shouldn't see jack squat in cruise performance change, but interested in hearing y'alls experience. Thanks!

Retarding timing will slightly lower CHT's and raise EGT's a fair amount... Motor will have less torque...

With that said.. if this is a certified plane, you are locked into whatever the manual says/ or stamped on the mfg's tag... In an experimental, personally I would run it at 25 -27 .. as long as you are running 100LL ALL the time... Throw in some mo gas and 20-22 is alot safer...

To answer your question... if it has a constant speed prop then you will loose a little mph.. If it a fixed pitch prop bent to cruise and not climb then you will loose a bit more..

Bottom line.. if it starts good at 25 and does not kick back, I would say leave it at 25.. IMHO

 

[hindsight2020]

It is certified of course and the damn plate is stamped at 20* so the mechanic's hands are tied if one is to remain legal. It does have a constant speed prop. What's a little mph? 2-3mph? I can live with that. 10mph? hell no. I'll obviously go test it out and report back once I get it back.

Damn effing certified engines

 

[Velocity173]

Get ya some knots 2 U mods to make up for the loss.

 

[hindsight2020]

Get ya some knots 2 U mods to make up for the loss.

Its got them all already. flap/aileron gap seals, wing root fairing, K2U wingtips and gear well lobes.

I can bank at an honest 133-135 true at 9GPH above 6k. Around 130 at 8 GPH between 12-14K. I'd like to keep it that way.

Frankly I don't care about takeoff performance and top end speed. I do care about 65% cruise. My understanding is that timing changes become the most noticeable at high power settings, not across the entire power band. But I also understand it affects high altitude ops, which do affect my ability to lean smooth above 10k.

 

[flyingmoose]

Ok so this was addressed in SI1325A and for your engine you will need to find SB380 Also.
The TCDS says that the engine is a 25Deg engine(with a note for optional 20Deg). That being said, it would appear that someone over the years has compled with the SI1325A and made it a 20deg. With that you change the Impulse coupling and the data plate. You cannot just reset it to 25 after you have done that.

I suppose you could reverse the SI, change it back the way it was and resume using 25deg. That is an option.

Check your logs, I would guess its in there somewhere where they complied with it and what they did.

 

[Norman]

Has your A&P checked for a SB/AD that would change the timing to 25*? There may or may not be one. If not, you're stuck with 20*.

 

[Capt. Geoffrey Thorpe]

The torque vs. spark advance curve tends to be flat around MBT.

You appear to be assuming that you are retarded from MBT at 20 and not over advanced at 25 during a normal cruise. Do you have data to support that or is it just a guess?

 

[hindsight2020]

The torque vs. spark advance curve tends to be flat around MBT.

You appear to be assuming that you are retarded from MBT at 20 and not over advanced at 25 during a normal cruise. Do you have data to support that or is it just a guess?

I'm guessing at this point that my performance will decrease based on the anecdotal experience of others running 20* versus 25 on Lycos of this same vintage. The extent of that performance loss I've yet to flight test. If it's negligible I'll let it go. If it's not I'll go hunting for ways to revert the engine back to 25*. I agree with flyingmoose, the dataplate stamp at 20* was probably the result of the SI; the data sheet does have the -C1C engines as 25* originally. I'm gonna talk to the mechanic and see what has to be done in order to revert it if I'm not happy with the engine at 20*.

Like I said I don't care about the top end speed but it does hurt my efficiency and that's a sticking point for me. If I can't run my sweet spot anymore then what's the point, might as well go buy a fixed leg 182 and throw 13GPH ROP to go 130 while refilling my inflight beverage with the ice cubes coming from the carb ....TC

 

[hindsight2020]

Ok so this was addressed in SI1325A and for your engine you will need to find SB380 Also.
The TCDS says that the engine is a 25Deg engine(with a note for optional 20Deg). That being said, it would appear that someone over the years has compled with the SI1325A and made it a 20deg. With that you change the Impulse coupling and the data plate. You cannot just reset it to 25 after you have done that.

I suppose you could reverse the SI, change it back the way it was and resume using 25deg. That is an option.

Check your logs, I would guess its in there somewhere where they complied with it and what they did.

Excellent feedback! I'm going to talk to thew A&P this morning and go logbook diving for the SI. I wonder what would be required to run the engine back to 25* legally. Is it a change of impulse couplings when re-timing the mags? Now you got me wondering about the impulse coupling on it right now, since it doesn't give me problems starting currently with the 25* timing....

 

[flyingmoose]

Excellent feedback! I'm going to talk to thew A&P this morning and go logbook diving for the SI. I wonder what would be required to run the engine back to 25* legally. Is it a change of impulse couplings when re-timing the mags? Now you got me wondering about the impulse coupling on it right now, since it doesn't give me problems starting currently with the 25* timing....

If they complied with it properly then they should have installed impluse coupling LW-391429 on the LH mag and changed the plate. I don't see a reason yet why you couldn't change it back if you really wanted to. That is assuming you have impulse and not a retard breaker which is a bit different. Ill attach a copy of the body of the SI so you can scan through it. Also I think this is the older version of the SI that I have, so you will want to find a current copy to see what changes they might have.

 

[hindsight2020]

If they complied with it properly then they should have installed impluse coupling LW-391429 on the LH mag and changed the plate. I don't see a reason yet why you couldn't change it back if you really wanted to. That is assuming you have impulse and not a retard breaker which is a bit different. Ill attach a copy of the body of the SI so you can scan through it. Also I think this is the older version of the SI that I have, so you will want to find a current copy to see what changes they might have.

Spoke with the AP and he's gonna do some digging. I'm going to forward him the SI scans you posted. Saved me the search, thanks a ton!

We agreed to take it up with the 20* timing and if there's no appreciable difference in my operating performance, then we'll leave it alone. Otherwise if it becomes an issue for the way I run my engine, we'll dig into legally putting it back to 25*. I love it when this forum actually yields useful help!!

Thanks again moose.

 

[KSMooniac]

You'll notice it, but as mentioned above you should be able to legally turn it back to 25* with the right instructions and inspections. Just make sure to mark the data plate!

There is a similar conundrum in the Mooney world with IO-360 engines like mine (-A3B6D). It too started as a 25* BTDC engine. Mooney later went to the -A3B6 to get rid of the siamese magneto, and these engines got turned into 20* engines in the same SI flurry that got your engine. For some reason they overlooked the D engines, so they still come with 25* settings. I know of one Mooney owner that last year exchanged his -A3B6D engine for a factory overhaul -A3B6 (split mags) and it came as a 20* engine. Their performance was noticeably reduced, so he contacted Lycoming and figured out how to legally change it back to 25* to restore performance.

 

[hindsight2020]

Update: Flew the airplane from annual today with the new timing. Happy and elated to report that no change in cruise performance was noted. Albeit I was 18 gallons lighter than my baseline test runs from months ago, the cruise numbers came up pretty much identical for my power setting of 2400/WOT:

ind ALT/TEMP/8gph/9gph/10gph (KTAS): (29.78 alt setting)

5k /18*C / xx / 129 / 139
7.5 / 14 /132 / 141 / xx
9.5 / 08 /130 / 135/ xx
12.5/ 00/ 128 / 132/ xx
**12.5/ 00/ xx /135 / xx <==2600RPM WOT 9GPH (for curiosity's sake)

The 7500 reading was the only anomaly from the 25* runs. It showed a solid 6 knots true above my previous test runs, whereas the rest of the altitude runs were within one knot true from the baseline timing performance. Maybe I didn't quite get settled before I took the readings at 7.5k, 6 knot increase in performance with a retarded timing just doesn't fit the curve.

Climb rate was difficult to ascertain due to the fact I was light and the winds were howling, plus it was a high DA day today. My mission profile has the wife and baby plus luggage and full fuel, so my climb rate today wasn't a good metric to bother with.

As far as LOP operation, I was able to run 7gph at 12.5 with corresponding roughness at 6gph. Which was identical to its behavior at 25* timing. The 8gph setting at 7500 also was rough, which was also noted behavior in the 25* mag timing run numbers. For all intent and purposes the thing runs identical.

All things considered, I'm happy with the performance and elated I didn't lose out, other than the 6AMUs I just dropped to settle the annual/new-to-me first year mx bill. Oh well, it's just money.

 

[N801BH]

Glad you are happy....

Hard to believe retarding the timing 5 degrees didn't change anything tho..

Horsepower will be down and EGT's will be up...

 

[Ted]

Hard to believe retarding the timing 5 degrees didn't change anything tho..

When I played around with angle valve engines, I didn't find much difference, either. I think other aspects of mags at altitude kick in and are bigger factors at that point.

 

[philiplane]

The 20 degrees is required for this engine to maintain detonation margins. 25 degrees is too advanced when the engine is under full load. The peak cylinder pressure at 14 degrees ATDC is high enough on the 200 HP Arrow to require 100 octane fuel. The 180 HP arrow had 25 degree timing because the engine could tolerate that on 91 octane fuel.

There is no HP change with the advance timing versus the standard timing because the engine is not optimized for advanced timing. Advanced timing works best under light load conditions at cruise rpm, where it provides better economy since the engine has more time to complete the burn. Increase RPM and the timing has to be less advanced, not more. It's a function of flame travel during the ignition phase. Flame actually travels faster as RPM increases so it does not need more timing advance, it needs less. Long story shortened, excessively advanced timing will hurt your power output when you need it most.

 

[Geico266]

Retarding timing will slightly lower CHT's and raise EGT's a fair amount... Motor will have less torque...

With that said.. if this is a certified plane, you are locked into whatever the manual says/ or stamped on the mfg's tag... In an experimental, personally I would run it at 25 -27 .. as long as you are running 100LL ALL the time... Throw in some mo gas and 20-22 is alot safer...

To answer your question... if it has a constant speed prop then you will loose a little mph.. If it a fixed pitch prop bent to cruise and not climb then you will loose a bit more..

Bottom line.. if it starts good at 25 and does not kick back, I would say leave it at 25.. IMHO

I know several RV drivers that add E-Mags to their engines that advance the timing and they run mogas.

 

[Ted]

I know several RV drivers that add E-Mags to their engines that advance the timing and they run mogas.

That's because the margins to detonation in normal operation are enormous. I suspect they aren't running their engines at the conditions used for detonation testing.

 

[N801BH]

I know several RV drivers that add E-Mags to their engines that advance the timing and they run mogas.

My hangarmates RV-6 has a electronic ignition and the brain box has a feed line running from the intake manifold.. It constantly adjusts the timing advance based on manifold pressure and will not let it get too far advanced under wide open throttle, thus saving the motor from detonation..

 

[mondtster]

So, how do the guys that are messing with timing changes on their experimentals know where to set the timing? Are they measuring firing pressure to see if their changes are actually being productive or detrimental?

 

[Geico266]

So, how do the guys that are messing with timing changes on their experimentals know where to set the timing? Are they measuring firing pressure to see if their changes are actually being productive or detrimental?

Here is a link to the P-Mag.

http://emagair.com/Intro.htm

I know hundreds of guys running these on their Lycomings. You start out at what the engine was built for say 25 degrees and it retards (starting) or advances the timing as needed.

 

[mondtster]

Here is a link to the P-Mag.

http://emagair.com/Intro.htm

I know hundreds of guys running these on their Lycomings. You start out at what the engine was built for say 25 degrees and it retards (starting) or advances the timing as needed.

So who decides how much ignition lead is optimal and how do they determine that?

I'd be more inclined to use something like this unit that provides feedback:

http://www.gami.com/prism/prism.php