Here is some interesting reading.
https://resources.savvyaviation.com...30 1300 F7 Benefits of Running Oversquare.pdf
Using the free app is way easier, correlates directly to the book, handles all the calculations and chart tracing for you.
AircraftPower by James Petty
https://apps.apple.com/us/app/aircraftpower/id718023187
iOS only, but that’s probably not a limitation for most.
Basically, you can choose any MAP/RPM combo you want (that your prop restrictions will allow) that gets you the percent power you want.
Combined with the Savvy info above (assuming no prop restriction), let’s say 22/2400 gave you 75% power at whatever altitude. Using the chart/app, you see that 25/2000 also gives you 75% power. You should choose the over square, low rpm setting vs the under square, high rpm, for the host of reasons in the Savvy presentation.
If pressure had been in PSI, inH2O, millibars, pascals....
If rotational speed basis had been radians/x, hertz, ....
Would this myth have even been started since the associated numbers wouldn't conveniently line up? The validity of the over square myth was disproved a long time ago.
Side question; did "prop forward/throttle back" come from trying to avoid the over-square mythical danger zone? Avoid potential overspeed from sloppy governors?
I still do it mostly from muscle memory. It also keeps any related questions away.
thanks, as I read that chart you can run essentially any amount oversquared to a RPM as low as 2,000. Anybody else read this differently?
Dan, Exactly. Thanks for adding some background and data for other types.
It is interesting how the other (non-390 charts) have an angle at the right of the chart and the -390s line is pretty much vertical.
I am just wondering (the technical/mechanic side) what is the harm if manifold (air) is more than RPM by the greater number in these charts.
I assume the air/fuel mixture is correct, lets say at 28" and the RPM is 1600. What happens in the engine that causes harm?
So for that reason, assuming you weren't just picking 2 numbers out of thin air as an example, you shouldn't run continuous below 2000rpm at any MAP.
I am learning a lot in this area myself, so I could be off, and defer to much greater minds
If pressure had been in PSI, inH2O, millibars, pascals....
If rotational speed basis had been radians/x, hertz, ....
Would this myth have even been started since the associated numbers wouldn't conveniently line up? The validity of the over square myth was disproved a long time ago.
I am just wondering (the technical/mechanic side) what is the harm if manifold (air) is more than RPM by the greater number in these charts.
I assume the air/fuel mixture is correct, lets say at 28" and the RPM is 1600. What happens in the engine that causes harm?
larry, Good way of explaining it, thank you. Both the bicycle and 70's car example make the light bulb in my head go on much better, so thank you.Improper gearing (i.e. high output/low RPM with short gears) is hard on the engine mechanical components and is also more detonation prone.
Hop on you bike and set up the gearing for a good compromise in speed and pedal effort. Then upshift two gears and try to maintain the same speed without accelerating. Won't take long for your legs to hurt from the exertion even though maintaining that same speed with the correct gears didn't hurt your legs. Same thing happens in crankshaft, connecting rods, bearings, etc. The bigger issue is detonation. High load is detonation prone. Did you drive in the late 70's post leaded gas, before the designers figured out how to deal with the lower octane? if you drove up an incline and your transmission didn't downshift correctly or you didn't force it to downshift with throttle, you would hear marbles rattling in the engine. That is detonation caused by excessive load. They called it "Pinging" at the time, but it is detonation.
Larry
They called it "Pinging" at the time, but it is detonation.
It seems to me that "LOAD" is accepted as the cause of the detonation. Maybe it can be replaced by torque.
Why would a high torque promote detonation?
Possibly the detonation occurs due to the fixed advance which allows, at low engine speeds, the pressure in the combustion chamber to build up before top dead centre, giving the mixture time to detonate.
Imagining the propagation speed of the flame constant, it is not, it depends on pressure, temperature, air/fuel ratio, etc., to avoid detonation you have to reduce the advance as the revs decrease or, this seems to me the main reason for the pressure limit at low engine speeds, reduce the mixture pressure during the compression stroke to reduce the flames propagation speed.
If the engine doesn't have electronic ignition which reduces the advance I would respect Dan's redline.
Claudio