Lycoming O-320 power settings for break in

[Ted RV8]

Wasn't there a write up in the RVator about percentage of power calcs? I can't seem to put my finger on it at the moment, so maybe someone else will remember the specifics.

As I recall you added the manifold pressure and the RPM hundreds. Example 23" manifold and 24(for 2400 RPM) equals 47 which equated to "X" percentage of power. There was a chart for the various combined numbers and the percentage of power it yielded.

Does anybody else remember this. I'll keep looking in previous RVator's and post it if I find it.

Ted

[vlittle]

Hi Bevan. The nose-gear service bulletin requires that the nose wheel pants always be in place to help prevent the nose fork from digging into the terrain and folding over.

V

[Steve Sampson]

It is difficult to remember but here goes:

Add MP in inches to RPM in hundreds.

eg 23" + 2500RPM = 48

Then if the answer is:
42=55%
45=65%
48=75%
51=85%
54=95%

[whifof100ll]

Originally Posted by Ted RV8

Wasn't there a write up in the RVator about percentage of power calcs? I can't seem to put my finger on it at the moment, so maybe someone else will remember the specifics.

As I recall you added the manifold pressure and the RPM hundreds. Example 23" manifold and 24(for 2400 RPM) equals 47 which equated to "X" percentage of power. There was a chart for the various combined numbers and the percentage of power it yielded.

Does anybody else remember this. I'll keep looking in previous RVator's and post it if I find it.

Ted

I read this years ago and have used it as a rule of thumb in many A/C's. It it works pretty well. The way it was written referenced an index where a change of 3 equals 10% power.

Add the first RPM/100 plus mp and get a "power index". A power index of 48 = 75% power. For every change in index of 3, change the power estimate by 10%.

24 squared gives a power index of 48. Say you are running 2200RPM and 23" mp. That gives a power index of 45, three less than 48, so the power estimate is 65%.

Say you have an FP prop and develop 2150RPM on the takeoff roll at 29.5 inches. That gives a power index of 51, 3 above 48, so that would be 85% power.

[Ted RV8]

That's one heck of a memory Steve!! You are exactly correct.

Found the RVator, August 1997. In was in an article that Van wrote regarding "Real World Performance" relating to claims being made by some aircraft companies, magazines and alternative engines. Excellent reading!! Those contemplating alternative engines for thier planes should read the whole article. Still valid information today, over ten years later.

Here is the part on "Determining Power."

"For the normally aspirated Lycoming engines we are using, the following formula provides quite accurate results.
Manifold pressure (inches of mercury) + RPM (in hundreds) = the percentage of full power shown below. (This is a simple approach to determining power output. For more precise power determinations, refer to the Lyc. Operations Manual for your engine.)

54 = 95%
51 = 85%
48 = 75%
45 = 65%
42 = 55% For Example:
22 in. MP + 26 hundred rpm = 48 = 75%
23 in. MP + 25 hundred rpm = 48 = 75%
24 in. MP + 24 hundred rpm = 48 = 75%

Note that although the rpm varies by 200, the percent power remains the same because of the varying manifold pressure. The lesson here is that with a fixed pitch prop, rpm alone does not an accurate indicator of power. The pitch of the prop determines how hard the engine has to work (Manifold pressure) to spin the prop at a given rpm. Different props turning at a given RPM will require different amounts of power."

I used to carry a chart of this in my fixed pitch RV-6. Found it to be very usefull.

Ted

[Kevin Horton]

Originally Posted by Ted RV8

22 in. MP + 26 hundred rpm = 48 = 75%
23 in. MP + 25 hundred rpm = 48 = 75%
24 in. MP + 24 hundred rpm = 48 = 75%

Just keep in mind that in reality the power produced by a given MP and RPM also depends on the altitude (the power goes up with altitude). So the above formulae are only approximate. If we also consider the effect of temperature, the power values from the above formulae could be out by 5 to 10%.

[Steve Sampson]

Originally Posted by Kevin Horton

Just keep in mind that in reality the power produced by a given MP and RPM also depends on the altitude (the power goes up with altitude). So the above formulae are only approximate. If we also consider the effect of temperature, the power values from the above formulae could be out by 5 to 10%.

Kevin, help me with that please because it is not intuitive.

I can see that as the air gets colder there is more mass of air coming into the engine, therefore more fuel, therfore more work done. But how does the engine 'know' the altitude changed for a constant manifold presure? I can see the exhaust gets out a little easier, but we are not talking about that are we? I presume it is a small effect.

The prop will have less air to work against, but the c/s unit will just adjust the pitch to make it a bit harder.

What am I missing?

[Kevin Horton]

To be honest, I don't claim to completely understand the mechanism by which power varies with altitude. But if you look at piston engine power charts from Lycoming, Continental and Pratt&Whitney, they all show a significant increase of power with altitude at a constant RPM and MP. I think the lower exhaust back pressure at altitude improves the scavenging of burnt gases during the intake stroke, and this leads to a power increase. The lower pressure in the crank case may also help ring sealing. There may be other reasons too.

If you look at the power chart for the O-360-A series engines, and see what power it predicts for 2700 rpm and 21" you should get approximately the following (assuming standard temperature):

0 ft 67%
4000 ft 72%
8000 ft 76%

Note that the rule of thumb quoted above says that 2700 rpm and 21" would produce 75% power.

[Steve Sampson]

Kevin, having mulled it over all afternoon, my guess is exhaust back pressure and low sump pressure aside (which I presume are minor) the effect is because the standard temperature drops with altitude. No more no less. It makes sense for Lyco to present its curves against altitude - that is what the pilot knows about - but in fact the real issue is the colder air into the engine. (Standard temp drops with altitude doesnt it?) The engine has no idea how far above the ground it is.

You know much more about these hings than I, so if you have another tought I would be interested.

[Kevin Horton]

The Lycoming power charts do have a formula to correct the power for temperature, but the magnitude of this correction is way too small to account for the change of power they show with altitude. For example, on an O-360-A, at 2700 RPM at 21" MP, at a constant 5 deg C, the Lycoming power chart, plus correction for non-standard temperature, shows:

0 ft 68%
4000 ft 72%
8000 ft 75%

So, assuming that Lycoming knows how the power of their engine varies with temperature and altitude, the explanation is something other than the effect of temperature.