Run-Up Mag Drop?

[Guy Prevost]

Originally Posted by Kevin Horton

As others have said, the over 200 mag drop when selecting EI off sounds excessive. I've got one LSI EI and one mag. I see around 150 drop when selecting the EI off.

For troubleshooting, it might be interesting to disconnect the manifold pressure input to the EI. Plug the end of the line that goes to the engine, so you don't have a vacuum leak.

Now the EI will think you are at full throttle, and it won't advance the spark, so it should fire at the timing angle, which should be the same as when the mag fires. With both mag and EI firing at the same angle, you should have similar mag drops on each. If not, that may point to one or both being timed wrong, or possibly some weakness with the mag.

Good ideas Kevin, I'll definitely try that test. I won't quite be comparing apples to apples though, since even with the EI manifold input disconnected, I will still be getting the effects of the 8000+ feet density altitude measured at the sensor location.

G

[DanH]

<<about 160 when I shut off the electronic ignition. Now that summer temperatures are driving the density altitude at my home airport towards 8000', I'm seeing a 200RPM drop when I shut the Lightspeed off.>>

Guy's report is interesting because it illustrates something I had never fully appreciated prior to flight testing some recent carb development; mag drop appears to be dependent on mixture.

Guy, you might want to do an experiment of your own. Set runup RPM, then lean for max RPM before you turn off the Lightspeed. Repeat with partial leaning. Neither will hurt your engine; your runup is probably well below 65%.

If the drop is greatly lessened, chances are your midrange is just too fat for high and hot. Perhaps the great big spark gap on the Lightspeed side does wonders for lighting less-than-optimum mixtures.

[pauldan181]

Guy
I spent a couple of months last summer flying my 8A out of KAEG and can confirm what Dan is saying. Per the advice of an experienced high DA pilot, I would start my runup at full rich to 1700 RPM then lean until 1800, do the ignition check and prop cycle, leave the mixture there and take off. I have a similar engine setup to yours ( lightspeed, mag and Hartzell). I saw a big rpm drop with the LS off until I started leaning. On some days the DA was 9000 ft. the mixture would be about 1/4 of the way back.

If you get a chance, say hi to John (Vyking)

Paul Danclovic
Jamestown NC
RV-8A N181SB

[gereed75]

I have a similar set up - LSE II with a standard mag and WW 200RV prop. I routinely see virtually zero drop, mag off; and 160-180 (sometimes 200) RPM drop LSE off. The drop seems less when well leaned and/or well warmed up.

When brought to the attention of my local A&P, we re-timed the mag with virtually no change.

[rvmills]

Saw the very thing Dan, Paul and others are discussing (mixture impact) today during run-up in Reno (DA about 7-8K).

I have a very similar set-up (IO-540, 1 LS, 1 mag). Like others, I typically see a 150ish drop with LS off, almost no drop with mag off. I normally lean pretty agressively during taxi out, then do what Dan and Paul described during the run-up (lean to peak RPM).

Today I had a 200 RPM drop, and realized I hadn't really leaned the way I normally do. I re-leaned, let 'er run a bit at run-up RPM, and things when back to the normal drop. Given that the numbers are very close to what you described in the original post, I hope it's another good data point.

One thing I wanted to ask as a follow-on Q (and if it's too much thread drift, no worries, just disregard ):

I'm always in search of the best Hi DA mixture for T.O. Normally, after I lean as above for run-up, just before blasting off, I give it a turn in (richer) for T.O., just for a little more margin from the red-zone...even though up as hi as 7-8K DA the power output may not be enough to cause detonation problems...just playing it safe. I also look at climb CHTs and EGTs to make sure they're not to hot, and my FF (ball park measure), to make sure I'm not too lean and toasting things.

Paul, I saw you said to lean for max run-up RPM and TO with that. Are there any other parameters you look at to make sure you're in a good mixture range? Any other suggestions from others? Just want to be nice to the motor, while not leaving HP on the table uneccesarily! Thanks!

Cheers,
Bob

[Guy Prevost]

Originally Posted by pauldan181

Guy
I spent a couple of months last summer flying my 8A out of KAEG and can confirm what Dan is saying. Per the advice of an experienced high DA pilot, I would start my runup at full rich to 1700 RPM then lean until 1800, do the ignition check and prop cycle, leave the mixture there and take off. I have a similar engine setup to yours ( lightspeed, mag and Hartzell). I saw a big rpm drop with the LS off until I started leaning. On some days the DA was 9000 ft. the mixture would be about 1/4 of the way back.

If you get a chance, say hi to John (Vyking)

Paul Danclovic
Jamestown NC
RV-8A N181SB

First some background. I have historically just pulled about 1" of mixture for my runup and takeoff, and this served me well in the rentals I used to fly. For those of you about to squawk about this, I have landed at an airport with a field elevation of less than 4500' exactly once in the last 15 years. That was at Mesquite, TX two months ago in my RV. DA of 8k-10k is not uncommon in the summer.

I spent some time this weekend, doing timing checks, cleaning plugs and injectors, and generally inspecting the plane. There were no squawks, but my fiddling could have changed something to influence the data noted below. Before installing the cowling I pulled her out and did a runup using the method described above. I did not get a full 100rpm rise on this cool morning, but I did lean for maximum RPM. RPM drop on the Lightspeed alone was now ~20 RPM instead of 40. RPM drop on the mag alone was down to ~80 instead of 160-200.

Thanks for all of the helpful advice!

Guy

[pauldan181]

Originally Posted by rvmills

Paul, I saw you said to lean for max run-up RPM and TO with that. Are there any other parameters you look at to make sure you're in a good mixture range? Any other suggestions from others? Just want to be nice to the motor, while not leaving HP on the table uneccesarily! Thanks!

Cheers,
Bob

I didn't mean max RPM, I could have leaned further and gotten closer to max power. I just used 100 RPM increase at runup as a nice round number that kept the mixture on the rich side.

Guy
I took a look at your website and see you have throttle body of some sort so it's kind of apples and oranges. I have a stone simple carb. Your setup probably compensates for altitude better and atomizes fuel better so your not as sloppy rich to start as I was. Have fun finding the sweet spot for your airplane.

Paul Danclovic
Jamestown NC
RV-8A N181SB