Bryan Wood
Well Known Member
I'm hoping that somebody can clear up a few questions about power settings and how density altitude affects them. From struggling to read the small charts that are in the Lycoming manual I'm finding that the numbers are not accurate with the RV. With the Fab air box and the ram effect of it the charts never have lines for the extra manifold pressure that the RV's see for altitudes over about 4k feet. How, or what are the rest of you referencing for power settings, or do you just pick a manifold pressure and rpm and simply go with it? I've been using the rule of adding the manifold pressure to rpm/1000 and setting according to the rule I read in the RVator several years ago. If they equal 42 your at 55%, 45 your at 65%, and 48 nets 75%. Later I adjusted this because playing with the charts the 0-320 seemed to come in closer to 43,46, and 49 equaling 55,65,and 75% power. Anyway I've been using this to roughly guess % of horsepower, but was wondering if there are any charts that have been worked up on excel for the 0-320. I have charts from a Grumman Cougar that used the 0-320's and Hartzel combination, but since they couldn't pull the manifold pressure that we can with the fab they are not any good if running full throttle over 5 or 6k feet.
Question: First of all, my engine is carburated and therefore I have no interest in trying to run LOP. In fact because of the carb I try to avoid it. Here is where a question comes in. From what I've read detonation is a serious problem at high power settings or above 75%. The general concensus is that you can't make enough horsepower up high to have a detonation problem and if 8K feet is the magic number or close to it where a normally aspirated engine cannot make over 75% I'm assuming that I can lean to roughness at full throttle and 2700rpm at 8K' (theoretical) and then richen to smoothness. The lycoming manual lists this way as a proper way to lean if there is no egt in the plane. Now, for my question. Let's say that I'm leveling off at 5500' and the outside air temp is around 80' at that altitude. This makes the density altitude just over 8K' so can I be full throttle and 2700 and lean to roughness safely? Using the rule of adding the manifold pressure to the rpm seems to place the output above 75% because the manifold pressure is coming in above 25" at 5500'. In reality I don't run the engine this way, but was trying to demonstrate my question. At 5500 or 6500 feet I wouldn't thing twice about running full throttle and 2300rpm. This comes in between 48 and 49 on Van's rule and appears to be around 75%. Do you use density altitude or indicated altitude to figure power settings? I've noticed that the CAFE foundation tests show performance at a couple of altitudes, but that they use density altitudes. I don't recall the numbers exactly right now, but it seems they were around 6K to make the test runs for a 8K density altitude on the 9a test.
Here is another question, but this one kind of assumes that all the assumptions that I've made above are wrong. This could well be the case, as well as this next one. This is why I'm asking. Maybe a better way of looking at this power thing for the RV is that with the extra ram effect from the fab the highest altitude that we can make 75% is around 9 thousand or more feet. At 2700 rpm and around 22" the 48 or 49 rule can still be met. In other words, are we cruising around at over 75% continuous at 8K' with everything pushed forward? Again, can I hurt it by leaning to roughness?
Now for the next reach... If an engine is rated for say 160hp like the 0-320D1A is, then the 75% power yields 120hp. From the charts it looks like every jump in manifold pressure of 1", or every increase in rpm by 100 the horsepower goes up by around 3%. This translate to about 5 hp for each increment. Does this mean that with the fab an 0-320 will really be making around 167hp with the extra 1 1/2" of manifold pressure over an installation in a production plane? If so, am I running the plane to hard by running slightly over rated horsepower for the engine, or is it okay to run 75% of whatever it will give me? I admit it here publically that I really don't understand how this works. Please feel free to jump in. The more I try to get a handle on understanding this the more I can see why so many people just run 23 sq, or 24 sq.
For those of you that might be willing to tackle this but need fuel flow information here is a very typical flight scenario for my plane. 6500' and 2300rpm with a 7gph burn at 24.5" of manifold pressure.
Thanks, I'll be eagerly awaiting an explanation. Right now it's as clear as mud!
Regards,
Question: First of all, my engine is carburated and therefore I have no interest in trying to run LOP. In fact because of the carb I try to avoid it. Here is where a question comes in. From what I've read detonation is a serious problem at high power settings or above 75%. The general concensus is that you can't make enough horsepower up high to have a detonation problem and if 8K feet is the magic number or close to it where a normally aspirated engine cannot make over 75% I'm assuming that I can lean to roughness at full throttle and 2700rpm at 8K' (theoretical) and then richen to smoothness. The lycoming manual lists this way as a proper way to lean if there is no egt in the plane. Now, for my question. Let's say that I'm leveling off at 5500' and the outside air temp is around 80' at that altitude. This makes the density altitude just over 8K' so can I be full throttle and 2700 and lean to roughness safely? Using the rule of adding the manifold pressure to the rpm seems to place the output above 75% because the manifold pressure is coming in above 25" at 5500'. In reality I don't run the engine this way, but was trying to demonstrate my question. At 5500 or 6500 feet I wouldn't thing twice about running full throttle and 2300rpm. This comes in between 48 and 49 on Van's rule and appears to be around 75%. Do you use density altitude or indicated altitude to figure power settings? I've noticed that the CAFE foundation tests show performance at a couple of altitudes, but that they use density altitudes. I don't recall the numbers exactly right now, but it seems they were around 6K to make the test runs for a 8K density altitude on the 9a test.
Here is another question, but this one kind of assumes that all the assumptions that I've made above are wrong. This could well be the case, as well as this next one. This is why I'm asking. Maybe a better way of looking at this power thing for the RV is that with the extra ram effect from the fab the highest altitude that we can make 75% is around 9 thousand or more feet. At 2700 rpm and around 22" the 48 or 49 rule can still be met. In other words, are we cruising around at over 75% continuous at 8K' with everything pushed forward? Again, can I hurt it by leaning to roughness?
Now for the next reach... If an engine is rated for say 160hp like the 0-320D1A is, then the 75% power yields 120hp. From the charts it looks like every jump in manifold pressure of 1", or every increase in rpm by 100 the horsepower goes up by around 3%. This translate to about 5 hp for each increment. Does this mean that with the fab an 0-320 will really be making around 167hp with the extra 1 1/2" of manifold pressure over an installation in a production plane? If so, am I running the plane to hard by running slightly over rated horsepower for the engine, or is it okay to run 75% of whatever it will give me? I admit it here publically that I really don't understand how this works. Please feel free to jump in. The more I try to get a handle on understanding this the more I can see why so many people just run 23 sq, or 24 sq.
For those of you that might be willing to tackle this but need fuel flow information here is a very typical flight scenario for my plane. 6500' and 2300rpm with a 7gph burn at 24.5" of manifold pressure.
Thanks, I'll be eagerly awaiting an explanation. Right now it's as clear as mud!
Regards,
