anybody running MP ahead of prop RPM?

[rbridges]

I was always told this is a no-no, but I was reading an article in the recent EAA magazine (with the oshkosh controllers on the cover). The author mentioned that WOT was most efficient and that running a lower prop RPM reduced friction loss. He said keeping MP" below RPM/100 was an old wives tale. It makes sense about less friction with lower prop RPM, but I don't know if what he is saying is true about running high MP with lower RPM not being a problem for the engine. He showed a graph with reduced EGT when configured as he suggested.

What's everyone's input on this?

[jetdriven]

Ah, the old "oversquare" argument. Your own POH and the Lycoming Operators manual shows power tables up to 5-7 inches "oversquare".

If you notice that 75% power at, say 24.5" and 2500 RPM is also found at, say, 26" and 2200 RPM. But the lower RPM has a lower fuel flow. I have found the CHT rises slightly with lower RPM.

Someone asked me what my plane did at "25 square" and I couldnt answer them. For the entire first year (100 hours) the throttle was at the firewall and the mixture LOP. RPM for % power and noise.

[Hank]

Many POHs shows this in the Performance Tables, but I generally prefer higher RPM for speed. At cruising altitude, MP is limited anyway. For short hops and approaches, I typically run 23"/2300 down low [~3000 msl]; advance to 22"/2400 for mid-level [~4500 msl] although I tried 24"/2200 once just to compare fuel burn; generally by 6000 I run 2500 and (WOT - enough to move the needle) since I don't have fuel injection.

My Owner's Manual shows 24.5" available at 5000 msl, decreasing to 22.5" available at 7500 msl, so I don't worry about it. It's an available choice for turbo planes--Bravo operators routinely run MP of 24-30" at altitude with RPM 2200-2400. For me, on the ground at home I have just over 28" and it goes down as I climb and hovers around 20" at 10K.

Besides, my tach is redlined from 2000-2250. At 7500-10,000 msl, I'd have to be 2000 or below. No thank you.

[rbridges]

thanks guys. I may try running a little oversquare and see what my CHTs do.

[KSMooniac]

You run oversquare for every takeoff and climb.... there is nothing wrong with it! Turbo guys run oversquare all day long as well. I only reduce throttle when slowing down for a pattern or approach, otherwise it is always wide-open.

[danb35]

He showed a graph with reduced EGT when configured as he suggested.

I agree with the consensus here--"oversquare" operations are just fine. However, reduced EGTs are meaningless.

[jetdriven]

I agree, the EGT value means nothing in this case. The flame inside the cylinder is the same temperature. The EGT value is lower becasue the valve opens 1100 times a minute instead of 1250 times a minute. The probe has less hot gases hitting it in a same time interval and the there is more dwell time with the valve closed, hence the "lower" EGT.

[aaronk25]

Like jet driven I fly with the throttle, full forward and the only time I reduce it is when I descend below 2600 feet as when lean of peak (and way over square ) In order to slow the plane down to gear extension speed it's necessary to pull the throttle back to 18.5 inches which is just above gear warning horn.

It's probably running at 120 lean of peak when I'm at 2600 feet full throttle 8.8gph when I bring the throttle back and at 2300-2400 rpm and 27 inches of manifold pressure that's way over squared.

Here is the thought of the day..leave the throttle (airvalve) all the way forward as air is free gas isn't and the more air available for combustion the better your mileage will be. Only increase the prop above 2250 when you need more speed run the prop the lowest speed possible to achieve your required speed needed. Sorry touched on lean of peak operation .

[N201MKTurbo]

Actually, At lower RPMs the fuel charge has more time to burn in the cylinder, therefore more heat is delivered to the cylinder and less is available when the exhaust valve opens. Which gives lower EGTs. A timing change would have the same effect at constant RPM.

I have a turbo 201 and routinely run 29 inches and 2400 rpm at 9 GPH. This gives me 150 KTS down low and 170 KTS up high. for 21.7 MPG at 195 MPH. I wish my truck would do so good!

[rbridges]

thanks again for the info. I've had several people tell me not to push manifold ahead of prop.

[jetdriven]

Ask them to show you in print. To borrow a phrase, "In God we trust, all others bring data."

[N201MKTurbo]

P51 Mustang Max power 3000 RPM 67 in Hg!

[carusoam]

All hail the P51 and the Acclaim...

Byron,

Would the internal cylinder pressure (ICP) increase as well during over-square ops.

Turbo drivers put up with worn cylinders as a cost of going fast with high(er) ICPs. Ovation drivers, not so much. I don't think ICP data and limitations have been widely collected, yet....

Your thoughts? Or did I miss something...

-a-

[jetdriven]

I think all else equal, the oversquare engine has higher ICP, such as pulling the prop to 2200 from 2500. A LOP oversquare engine has the same or less ICP as a "Square" ROP engine for a given % of power. The "mean ICP" is the same, same HP, but the "peak ICP" is less. Below 75% power, ICP are below any value that can damage a Lycoming IO-360 engine. At least thats how I understand it.

[rbridges]

I think all else equal, the oversquare engine has higher ICP, such as pulling the prop to 2200 from 2500. A LOP oversquare engine has the same or less ICP as a "Square" ROP engine for a given % of power. The "mean ICP" is the same, same HP, but the "peak ICP" is less. Below 75% power, ICP are below any value that can damage a Lycoming IO-360 engine. At least thats how I understand it.

but you're still saying it's fine to oversquare ROP?

[jetdriven]

Oversquare at the power settings listed in the Lycoming Operators manual are all below limits. Running inside the "red box" such as leaner than 200 ROP at full rated power etc can cause harm. Below 75% power you can run where you want, provided it doesnt run too hot. Your engine is oversquare for every takeoff and is certified that way with no time limit.

[201er]

Ok... but is there harm in ramming the throttle in when the RPM is set pretty low (like 2000-2300RPM)? They always say to put the prop forward first so the prop is prepared to take the power.

[jetdriven]

Mike, ramming the throttle forward is the problem, not the prop RPM.

[Hank]

Mike,

What's the red stripe on your tach? Sure, red line is 2700, but don't you have a 'no continuous operation' zone? Mine is 2000 - 2250, and Rob and I both have Cs. Unless Rob has an IO-360 to go with his J cowl? I don't remember, Rob. But oversquare by itself is not a problem.

[rbridges]

Mike,

What's the red stripe on your tach? Sure, red line is 2700, but don't you have a 'no continuous operation' zone? Mine is 2000 - 2250, and Rob and I both have Cs. Unless Rob has an IO-360 to go with his J cowl? I don't remember, Rob. But oversquare by itself is not a problem.

Hank,

I've got an O-360. Thanks for the answer.

[Parker_Woodruff]

People who fly Cessna 172s fly wide open throttle all the time on takeoff and throughout climb. Many of these airplanes have an IO-360 or O-360. All of a sudden they go fly something that has a Manifold Pressure gauge and they think there is something theoretically different about the engine. Just because it has a MP gauge doesn't mean a thing.

What I want to know is how someone came up with the idea that inches of manifold pressure x 100 somehow has a linear relationship with RPM and should never be exceeded on the MP side.

I flew my IO-360 wide open throttle all the time, except when I needed to be slow. The TSIO-360 stays at wide open throttle (39" until you get up in the low-mid FL200s)... until cruise, where it settles in nicely at 31-32" and 2500-2550 RPM.

[jlunseth]

I am turbo and "oversquare all day long" as someone said. Typical cruise would be 2450 and 28 - 30" MP. About the only time I am "square" or "undersquare" would be on short final with the prop all the way up to 2700 (or whatever it will make at reduced MP), and the MP somewhere around 17. However, the compression ratio on my turbo is less than that on an NA, so not really a fair comparison.

[bd32322]

I fly a J model (non-turbo). If you are running LOP - you can oversquare all you want too - use it in the descent to get RPM down to 2000 or less if the engine permits and keep a high MP and the cylinders toasty and save on gas.

[aaronk25]

I wonder when were (m20j) at 2000-2200 lean of peak, or Rop peak for that matter at a high engine power % at a low RPM if it could contribute to cracking of the case? Each time the cylinder fires it puts out more leverage because more air is in the cylinder along with more fuel, and results in higher interanal cylinder pressure, so i wonder if the increased vibration is a contributing factor of cracking of IO-360 cases.

I can feel the vibration of the power strokes much more when rpms are down and just wondering if the vibration is bad? Probably not good for instruments, gel coat on the cowl ect? Thoughts? Thats why I typically never run less than 2250 at high power settings, such as 9.8-10 gallons per hour. There isnt much power stroke vibration at around 7gph our so (low power settings).

The 6 cylinders are much smoother at 2100 than a 4 cylinder. So I wonder if there is more cost per hour because of the increased vibration from low rpm high load operation and if so if it is offset by the .3gph fuel savings by running the prop slower?