RPM and MPG

UdoR wrote:

why do you need 33” for operation on the Lean side?

In summary: for power/speed.

If I want to maintain 16GPH (pwr level) away from peak EGT, I need high MP. Lower MP will either need lower FF or put me closer to peak EGT with subsequent higher CHT.

I usually do either 31" /15GPH (66%pwr, ROP that would be about 75-80% pwr depending on altitude and OAT but FF >22-25GPH) or 33"/16GPH (that is about 70% pwr) normally 2500RPM about 80 F LOP.

I like the fact that when LOP – the power is directly related to fuel flow.
It’s easy to calculate.
FF in USGal x 14,9 = HP for 1:8,5 compr.ratio
FF in USGal x 13,7 = HP for 1:7,5 compr. ratio (most of turbos)

Of course we need to be LOP wit ALL cylinders and that can be confirmed only by full engine monitor and is not always possible due to high “gami spread”.

Raven wrote:

FF in USGal x 14,9 = HP for 1:8,5 compr.ratio
FF in USGal x 13,7 = HP for 1:7,5 compr. ratio (most of turbos)

Yah, my Turbo has the normal breathing compression ratio, therefore 14,9 it is. A thing I really like, because I’m not addicted to flying high alone. I enjoy the resulting climb rates and that the Turbo is a game changer regarding weather avoidance, having oxygen built-in and so on; But I can easily cruise in 3000 feet with nearly the same MPG as in high altitudes, when circumstances permit. When getting high (referring to altitude of course) I’ll have to enrichen more to keep temperatures low, but that’s what it is. It fits my mission profile.

I’ll report back when I have some more to contribute here, regarding RPM and MPG.

Raven wrote:

FF in USGal x 13,7 = HP for 1:7,5 compr. ratio (most of turbos)

That is exactly what I use. 16GPH= > 219hp or about 70% out of 310 max rated hp.

I fly a late model Saratoga, normally aspirated IO540K.

Normal cruise is FL80-FL100. I started with 2300/21" @ 11.4USG/h. I have the original piper electronic box (plus all the analog round dials underneath it), with only single cylinder CHT & EGT. I am now using 2200/22 @ 10.4USG/h. This seems to work well at lower altitudes as well, i haven’t gotten high enough over the last year to the point where it wont maintain 22" anymore (about FL90+) to see how 2200 works at lower MPs.

I haven’t gotten around to installing a 6channel EGT/CHT, but at my cruise power settings (about 50% power) I am not anywhere near the edge. Spark plugs look textbook perfect when they are serviced.

I was thinking of installing a Surefly electronic magneto, now that the timing advance has been approved for the Saratoga. At lower MPs it will advance the timing up to 38 degrees. I don’t have any problems starting, I am purely thinking about fuel savings. Any opinions on this – not much info online about the fuel savings, most people just comment on ease of starting.

hammer wrote:

I haven’t gotten around to installing a 6channel EGT/CHT, but at my cruise power settings (about 50% power) I am not anywhere near the edge.

Flying below, but quite close to peak EGT in FL100 I see comparatively high temperatures even with only around 50% (or a bit below) power settings I use. In my example given above it is 47% power, and my hottest cylinder indicates 380°F in FL100, where all 6 are around 360°-380°F. In 3000 feet I can go to any reasonable power setting at 2000 RPM, I typically stay below 26"MP and the temperatures stay well below 350°F. In 3000 feet, depending on air density, I end up with indicated speeds around 140 to 147 knots, resulting in nearly the same MPG as in FL100 (only read from the fuelflow-indicator about 9 to 9.5 Gal/h, didn’t have the chance to fly longer legs in this altitude so far to be able to “double-check” the low-level FF at the pump station). But at FL100/120 it gets quite close to the maximum temperature I’d like to see (which is 380 or below). And my engine, in principal, is certified for 100 % continuous power without limitation.

The factory installed “CHT probe” is only ticking the green arc from below. It’s ridiculous. I have no idea when it is supposed to indicate normal “green CHT”, definitely too high for me. I know my Turboed engine does sustain higher temperatures, according to Lycoming the IO-540-R may go up to 435°F instead of 400°F for normal operation – but so far I don’t even think about eating up this margin.

I really recommend to set installing a 6 channel temperature probe far up on the list of things to install, whenever any form of deviation from book values for power settings is performed, and watching the single cylinder temperatures very closely in the beginning. Depending on the “GAMI spread” you can roast one cylinder (e.g. 5 on the lean side, one peaking EGT).

My 2 cents.

UdoR wrote:

you can roast one cylinder (e.g. 5 on the lean side, one peaking EGT).

Yes. And especially you will roast one that is about 50degF ROP – but you have no idea about it having just single probe CHT/EGT.
Unless you only fly below 60%PWR then it doesn’t matter.

UdoR wrote:

my Turbo has the normal breathing compression ratio, therefore 14,9 it is.

That’s great for you as your engine is more efficient (more power from the same FF). Is it turbocharged or “turbonormalised”? What kind of engine?

Raven wrote:

That’s great for you as your engine is more efficient (more power from the same FF). Is it turbocharged or “turbonormalised”? What kind of engine?

Piper named it simply “Charged”.

It’s turbonormalised, because sea level pressure is the limit.