Can you help me understand something ?

Darren S · #1 08-26-2010, 09:56 PM

Hi group,

I've had my "new to me" -7 for a month. It has an IO-360, Constant speed prop. It also has an Advanced 2002 EIS. I live in Western Canada and my home field is about 3400 feet.

I typically cruise at about 5500 - 6000 ASL. At this height I get 160 knots cruise at 22" MP and 2350 RPM. Fuel burn is 9.5 gph when leaned to 100 degrees ROP. Peak EGT is around 1450'ish so I shoot for EGT's in the 1350 range. All right in line with the POH.

This morning I took a little foray into the mountains. On the way home I climbed up to 9500 feet to clear some peaks. At this altitude I was full throttle at about 21" MP so I turned the prop till the RPM was 2250. I leaned to peak (engine starts to stumble) then richened to 100 degrees ROP, as per usual. EGT's were in the low 1200's and my fuel burn was in the 10.5 gph range.

Here's what I need a little help with. Are these typical numbers ? Is my process of leaning to peak and then richening to 100 degrees ROP correct ? Why such low EGT's at 9500 ? Why such high fuel burn at 9500 ? At 9500 I don't think I could get EGT's of 1350.

I know this is elementary to some of you out there. If you could share your wisdom, I would appreciate that.

Thanks,

Darren

L.Adamson · #2 08-26-2010, 10:19 PM

I just looked at one of my 9500' pics. MP just under 20, 7.6 gph, and prop set 2300 rpm. I must have the throttle pulled back a bit. EGT on #3 is 1290 F. I'm only able to measure #3 at this time. I usually lean to rough, and then about three turns rich. This gets me in the ballpark & plugs always look good, and the engine runs smooth. Most of the time, my EGT on #3 will usually be between 1300 & 1325.

L.Adamson ---- RV6A

Greg Arehart · #3 08-26-2010, 10:58 PM

EGTs are, in part, a function of the probe location, so I wouldn't use exact numbers for comparison to another engine. Seems to me that you're burning way too much fuel. If I'm cruising at 8000 ft (airport is 5000, so I can't easily cruise there and usually I'm higher than 8000) I typically run ~2400 rpm, ~7.5 gph fuel burn, when leaned out. My leaning procedure is to lean through peak to about 50 degrees lean on the leanest cylinder. Typically, this results in maybe 10-20 rpm drop. Much further lean and the engine starts to run a bit rough. My temperatures, for what that's worth, are typically in the 1400 degree range at peak, and my probes are 6 inches from the port. At higher altitude, peak EGT is usually lower, and at a couple thousand feet they will run 1450 or a slight bit higher.

I'm not sure why your EGTs are so low at altitude, other than possibly you are far too rich. I know that if I richen the mixture, it doesn't take much to increase fuel flow dramatically (I can easily see 9-10 gph and 1200 EGT at 9000 feet if I'm way too rich).

Caveat is that I have a FP prop, but I don't see any reason why that would be significantly different than your CS in cruise mode.

Hope this helps.

greg

Darren S · #4 08-26-2010, 11:23 PM

Thanks for the help so far.

So it seems that it is normal for EGT's to be lower at higher altitude. Also, it appears that I am running pretty rich.

I don't think I want to go 50 degrees lean of peak. I understand that some folks prefer this method but I think my limited understanding may be detrimental and I may end up wrecking something.

I was shooting for 20" MP and 2250 RPM because the POH showed this as 55% power. This is also why I cruise at 22" MP and 2350 RPM as this is 65% power.

I also didn't necessarily have to run WOT. I guess I could have backed off but then I wouldn't be getting 20" MP so would I set my RPM to 2150 or go higher to 2400 ???

I was aiming for the 100 degrees ROP as I do at 6000 feet.

So...... I guess now I'll search to find what MP, RPM settings others are choosing at different altitudes.

I don't mean to make a mountain out of a mole hill. Just looking for a good, economical setup that is "engine friendly".

Thanks again,

Darren

scsmith · #5 08-27-2010, 12:33 AM

At those low power settings, you can't hurt the engine no matter what you do. Normal Lycoming procedure results in running just about right on the peak, and thats Ok at those MAP and RPM. I'm not saying its the best point to run the engine, but you can't hurt it there.

Don't be afraid to go lean - the engine loves it.

My EGT's seem to be sensitive to altitude and ambient temperature. I have not recorded enough points to understand the trends, but at the same MAP and fuel flow, I see some variation in EGT based on altitude and OAT.

Andy Hill · #6 08-27-2010, 01:37 AM

If all else allows, things are more "efficient" oversquare e.g. we cruise at 2300RPM / 25" [180HP FI / C/S / RV-8]. I find it easier to establish LOP in that regime as well. It gives at lower levels ~145-150KIAS on ~8.3USG/hr, and the Dynon says ~71%-72%. That is with 2 x Mags and FI, EI would clearly be more efficient.

There are as many opinions about "oversquare" / "overboosting" as posters on this forum

But the authoritive manuals I have seen indicate going to 4"-5" oversquare is fine, and a rule of thumb of max 2" is fine, even conservative.

Reducing RPM also tends to reduce EGTs.

Andy
RV-8 G-HILZ

DakotaHawk · #7 08-27-2010, 08:50 AM

This is the best series of articles I've ever read about leaning and engine management. I've been running my engine based on these articles and have been very happy with the results.

http://www.avweb.com/news/pelican/182179-1.html

C-FAH Q · #8 08-27-2010, 10:04 AM

Also, make sure your fuel flow meter has been calibrated. They all are calibrated at the factory, but matching recorded fuel used with actual fuel burned may give you a slightly different k factor number for the fuel flow meter and how it reads in your particular application.

Guy Prevost · #9 08-27-2010, 11:25 AM

Just another datapoint. My Lycoming manual for my parallel valve IO-360 says I can run at peak below 75% power. I usually run lean of peak, but basically below 75% power you are not going to hurt anything.

jtrusso · #10 08-27-2010, 01:20 PM

Quote:

Originally Posted by DakotaHawk

This is the best series of articles I've ever read about leaning and engine management. I've been running my engine based on these articles and have been very happy with the results.

http://www.avweb.com/news/pelican/182179-1.html

Here's a link to a whole series of Deakin articles. I can't recommend these enough. All the myths about LOP and "over square" operation are dispelled and with facts and backed up with real world data.

http://www.avweb.com/news/pelican/182544-1.html