LOP fuel burn variation

[NickM20F]

‘67 M20F

Pretty new here so excuse me if this is a silly question or been asked before a bunch of times- I have not been able to find any posts about it:

I try to run LOP whenever I’m flying for more than 30 mins or climbing up past 6/7k. Don’t really understand well enough how to do the “big pull” and am a little fearful to try getting to LOP when I’m over 65%. I always use my edm900 and lean find to get to 5-30 deg lop (my spread isn’t that tight). 

For some reason I will burn 8-8.5 gph at these settings (~~20 LOP 65% 7-10kft) on some days, and on other days I will end up at 10gph. Is this normal? From everything I read on here it sounds like this is not normal.

It’s probably about a third of the time, I go up and she settles closer to that 10gph. It was really striking to me this week in particular because I flew on back to back days with very similar weather conditions and altitude. On Thursday I was getting 8.5, and on Friday 10. So yesterday I tried about 4 times to re-lean, make sure I wasn’t just getting my peak egt wrong. Kept leaning further and further until I got to 8gph, and started to get really serious power reductions and 100+ lop egt readouts on the EDM.

When I’m on a 10gph day she cruises a bit faster too, maybe 144, where I’ll cruise closer to 140 at 8.5gph. 

Any advice here? This normal for y'all? Doesn’t bother me much just curious what might be going on. No other issues with the bird, flies fantastic, generally can’t tell any other performance differences between an “8gph day” and a “10gph day”, other than I’ll go from 17mpg to 14/15 mpg on the EDM. 

Thanks, 

-Nick

[Crawfish]

@NickM20F Just as a starting point when LOP % power is determined by fuel flow. I believe for your engine the calculation is GPH*14.8= horsepower. While LOP you throw out the book Book calculations based off of MP/RPM. 

 

So for your flights 

8.5*14.8=125.8 HP or 63% power. 

10*14.8= 148HP or 74% power 

that would explain your difference in TAS. 

 

As long as your CHT + oil temp are in an acceptable range either one of these power settings is fine. Although at the 74% power you might want to be a little further lean of then 5 degrees. 

 

an often talked about operating procedure for LOP is WOTLOP, meaning Wide open throttle Lean of peak. How to do this is get to your TOC leave the throttle full forward. Set RPM where you want it and pull the mixture back to your desired power. 

 

(Quick caveat, Your MP has to be above what would give you the desired LOP power setting meaning if the book says you need 25"MP at 2400RPM to achieve 74% power and you are only making 24"MP & 2400RPM pulling the mixture back to 10GPH will have you slightly ROP) 

 

I've attached a video that explains this pretty well. 

 There are more indepth videos done by Mike Busch but this is a great starting point. 

[Ragsf15e]

There will be some variation based on altitude or temperature (throttle should be full in).  If altitude and temperature are similar, the ff to get lop should be relatively close.  Was your rpm the same?  I ran my F lop all tge time.  Usually at 2500rpm.  If you leave it at 2700 from the climb, that would definitely increase ff.

[MikeOH]

My $0.02: Don't overthink this.  If you are above 5,000 you can't hurt anything.

With my '70 M20F, I climb at WOT, 2700 rpm, and start leaning at around 3,000 to keep sea-level EGT.  Once at cruise I back off to 2500-2550 rpm, and pull the mixture to 8.5 to 9.0 gph.  That's it.  I will monitor CHTs to make sure they're cool.

[TheAv8r]

4 hours ago, NickM20F said:

For some reason I will burn 8-8.5 gph at these settings (~~20 LOP 65% 7-10kft) on some days, and on other days I will end up at 10gph. Is this normal? From everything I read on here it sounds like this is not normal.

It’s probably about a third of the time, I go up and she settles closer to that 10gph. It was really striking to me this week in particular because I flew on back to back days with very similar weather conditions and altitude. On Thursday I was getting 8.5, and on Friday 10. So yesterday I tried about 4 times to re-lean, make sure I wasn’t just getting my peak egt wrong. Kept leaning further and further until I got to 8gph, and started to get really serious power reductions and 100+ lop egt readouts on the EDM.

When I’m on a 10gph day she cruises a bit faster too, maybe 144, where I’ll cruise closer to 140 at 8.5gph. 

Hey Nick! 

Few things to think about:

1. The fuel flow gauge might not be accurate - the way to check this is compare what the fuel totalizer is saying is USD against what you actually put in the airplane. If it's considerably off, then it could be the gauge or the actual instrument itself (which is basically a tiny little fan that counts pulses).
2. You will see SOME variance in fuel flow because of density altitude, pressure changes, etc. I would not expect that variance to be 1.5gph at the same altitude and power setting though, more like .2 - .5 on the IO-360.
3. Make sure your RPM setting is the same, when we're pulling that prop back in cruise we are taking a more efficient bite of the air which gives us comparably the same speed but better fuel flow, so if you were running 22/2500 rpm on one flight and 22/2400 rpm on the next, that 100 rpm would show marginal speed difference but you would see LOP fuel flow variance. 
4. The 4kt speed difference could indicate being leaner, but it's close enough that it could honestly be a lot of things and that might just be the conditions of that particular day or slight thermal/downdraft activity changing the TAS. My E will do 142kts some days, 145 kts other days at the same power and fuel flow setting because of these variances. 
5. Check your GAMI spread (pull your logs and put them into Savvy Analysis), it's the fuel flow delta from the time the first cylinder peaked to the last cylinder peaked. If you have a big GAMI spread, you COULD have a clogged fuel injector. The JPI 900 is giving you the comprehensive LOP setting for the engine, looking at each cylinder and basically averaging it out and giving you that "-20" number. If you have 1 cylinder that's way off from the others, it could be way lean while the others are not, and skew that -20, so while you're at "-20 LOP" by the computer, in reality, 1 cylinder is 80deg LOP, the other 3 might be right at or slightly above peak, which would result in higher fuel flow. Just something to rule out.

If you want to go up and mess around with it, feel free to shoot me a text  .