Zzz wrote:

soyAnarchisto wrote:pretty simple actually - more fuel going through a venturi lowers the temp (that's how carb ice is formed)…

I was under the impression carb ice is formed purely by virtue of the temp drop of higher velocity air moving into the vacuum of the intake and that the fuel is just along for the ride. Lower pressure = colder air and condensing water vapor which then freezes and chokes off the carb throat.

Does the fuel atomizing contribute to this temp drop too?

Didn't notice any answer to this question...yes fuel atomization contributes to ice formation.

Seems like a lot of pilots are running WOT. Are we talking WOT at a higher altitude, or WOT regardless of altitude. The book recommends a max cruise MP of 25" (I am at sea level and sometimes don't climb past 1000ft). Mike Busch goes as far to say that he operates WOT no matter what, and doesn't follow the books suggestion of 25" being the max cruise MP. Are people here running 26", 27", 28" MP at WOT in cruise? I just got a new engine analyzer and excited to be able to try some LOP flying. I am on floats, so I will be curios to see if I can get to the lower CHT's on the LOP without falling out of the sky.

Yes. WOT regardless of altitude. Because when running LOP, fuel is the limiting reagent. That is, fuel flow, not MAP, determines power output.

So, if you’re LOP at 3000’, 26”, and 11gph, you’re making the same power as you would be at 6000’, 23”, and 11gph.

I’m totally with you that at LOP operation Fuel Flow is the limiter for HP. And MP will effect how LOP you are running. For instance 26” at 11GPH is leaner than 23” at 11GPH.

My question isn’t so much about the max cruise HP, but the max over square operation. Continental recommends a max 24.5” for cruise MP @ 2200 RPMs. And a “Max oversquare” at 23” and 1900 RPM. I’m curious if anyone has experience running at a power setting like 26” and 2300 or Lower RPMs over the period of an engines lifecycle.

I have tried some LOP operation now on floats with the new engine analyzer. Operating at ~25” MP and 2300 RPMs @12gph. On floats I lose about 4kts compared to ROP operations while running at 4GPH less. I do have one cylinder that seems to be running a bit cool, and Oil temps are hovering around 180.

Embarrassing when my old post keeps popping up. I’ve completely changed my view on LOP. I think I was running too lean and just wallowing along at high AOA before with high drag if Bushwheels. Now I’ve cleaned things up, run 8.50x10 Dessers (25.5”), Voyager, Surefly. I’m getting 155mph true on 10.6 gph and love LOP operations. Huge fan… just to set the record straight.

Sky_Cowboy wrote:I’m totally with you that at LOP operation Fuel Flow is the limiter for HP. And MP will effect how LOP you are running. For instance 26” at 11GPH is leaner than 23” at 11GPH.

My question isn’t so much about the max cruise HP, but the max over square operation. Continental recommends a max 24.5” for cruise MP @ 2200 RPMs. And a “Max oversquare” at 23” and 1900 RPM. I’m curious if anyone has experience running at a power setting like 26” and 2300 or Lower RPMs over the period of an engines lifecycle.

I have tried some LOP operation now on floats with the new engine analyzer. Operating at ~25” MP and 2300 RPMs @12gph. On floats I lose about 4kts compared to ROP operations while running at 4GPH less. I do have one cylinder that seems to be running a bit cool, and Oil temps are hovering around 180.

Does your question pertain to ROP or LOP operation? If LOP, I’m not sure it is relevant as you note that MP is no longer controlling power in the same sense as when ROP. As you know, when LOP, increasing MP can decrease power as the mixture continues to lean. If ROP, you may consider re-posting the question elsewhere in the forum for more exposure.

LOP operation is where I am convinced it makes the most sense to operate, so for my purposes that is the envelope in which I am interested. This forum is the first where I have seen others talk about such over-square operation which is the point of my question. I understand that the TSIO-520 uses the same cylinders and same compression ratios os the IO-520, and cruise is 30" MP and 21 RPM. So it would seem difficult to overload the IO-520 at 28" and 2300 or lower RPM's.

Nonetheless, the IO-520 manual doesn't differentiate between LOP vs ROP over-square operation. Simply that 24.5" and 2,200 RPM is the Max recommended over-square cruise operation. In ROP configuration thats about 65% HP. In LOP, it's obvious that 65% HP is ~12.5 GPH. Is the target here max HP of 65-75% while maintaining CHTs below 380? Is there a limit to max over-square? And does anyone here have a large amount of time on the power plant to say "I did it at "XYZ" power settings, got to TBO or beyond, and everything seemed normal during an overhaul.

FWIW, I can tell you that John Deakin said and George Braley says that “oversquare” is an old wives’ tale.

ington6 wrote:Embarrassing when my old post keeps popping up. I’ve completely changed my view on LOP. I think I was running too lean and just wallowing along at high AOA before with high drag if Bushwheels. Now I’ve cleaned things up, run 8.50x10 Dessers (25.5”), Voyager, Surefly. I’m getting 155mph true on 10.6 gph and love LOP operations. Huge fan… just to set the record straight.

Ha! Try having 20 years of noob posts on here.

Good to have questions like this preserved for posterity though, and all the good info that accompanies them. And the follow-up "I stand corrected reply."

Io 520 is 81/2 to1 comp.ts10520 is 71/2 comp pponk 520 were certified 71/2 comp. they get higher tbo because the pistons are better built. They have a steel top ring insert in the groove for turbo application.the 0470u also has the same groove and get increased tbo

I see IO-550's on Cirrus's that are run exclusively WOT LOP go to TBO and beyond, but in a Cirrus you can't run less than about 2400-2500 RPM in cruise. (no blue lever)

I run my IO-550 in my 185 the same way most of the time. Usually dial the prop to 2450RPM and it seems happiest there.

I don't know if say 28" MP and 2,200 RPM would be hard on the engine, it kinda gives me the willies too.

Lately I have been wanting to run a lower RPM with a baby onboard for noise reasons, and 23 square LOP is just too low a power output for my liking with floats on. So I just run 23 square ROP when the baby is with and it allows a decent power output and decent fuel burn with a lower noise level. Running it like that uses approx 2 gallons more fuel on the 1:10 hr roundtrip to our cabin and back.

At 3,500 ft on floats, WOT, 2450RPM, 15gph I get about 125mph indicated
At 3,500 ft on floats, 23" MP, 2300RPM, 17GPH I get about the same

Temps are very similar, LOP a bit cooler.

Great discussion, I can add two things:

Years of over square power settings for us, 24”/2200. No issues whatsoever.

However, a new prop that’s smoother at >/= 2400 changed that technique. Now it’s 24”/2450 or 22”/2450.
Just as well, because TCM recommends >/= 2300 for most of the engines being discussed here. See below, cheers.

“No lower than 2300RPM”