Performance question

[rwarre]

Have been reading the threads on engine performance. My question is does a 180hp engine running at 2350 rpm and a 150 or 160 hp engine running the same rpm indicated the same true air speed at same given altitude, temp.

[aerhed]

They would if the props were identical. Otherwise, no.

[jmbrwn]

Flew a two ship to OSH. I'm running 150hp O320. Flew with buddy who has 180hp IO360. He throttled back to 2350rpm to fly with me. I was turning 2500rpm. We were at around 8500' to 9500'...can't remember which as we changed altitude several times.

[Mike S]

Originally Posted by rwarre

Have been reading the threads on engine performance. My question is does a 180hp engine running at 2350 rpm and a 150 or 160 hp engine running the same rpm indicated the same true air speed at same given altitude, temp.

On the same airframe, with the same fixed pitch prop, I would say yes.

Too many possible variables in your question to answer without a lot of qualifications in the answer.

[David-aviator]

It is my theory BSFC (brake specific fuel consumption) is about the same with all Lycoming engines, i.e., a 0235 burning 6 gph produces about the same HP as the 0360 burning 6 gph.

That being a fact (?) maybe, the limiting factor in comparing one Lycoming to another is its rated HP. The prop matters of course, but set the same fuel flow leaned out and generally the HP and performance well be similar.

Allan Barrett could confirm or shoot down my BSFC theory, he has determined it with many different Lycoming engines on their dyno.

[rwarre]

In some of the discussions those who have a 180 horse say they throttle back to get better fuel consumption. If all the configurations are the same whether you have a 180 or a 150, it seems to me that fuel consumption, as well as speed would be the same. Any other factors to consider?

[Low Pass]

Consider fuel delivery. Carburetion versus fuel injection. Big difference typically.

[Bob Axsom]

Why would a person with a 180 HP engine select a prop that is designed for a 150 HP engine? When you are talking about performance you try to optimize the combination of components to produce the most thrust. I maintain the statistical records for the Sport Air Racing League which now contains 1373 individual flight records obtained from airplanes going all out for speed. The RV Red class is for 320 cu. in. and under the RV Blue Class is for 360s. The Excel files are sorted by race, speed, class, aircraft type and pilot. If you are seriously trying to compare performance based on equivalent data you can go to www.sportairrace.org, click on records and stats, click on leagues statistics then click on the specific files you want and they will be downloaded to your computer. Then you can study them or post process them any way you want to highlight the areas of interest to you.

Bob Axsom

[Sam Buchanan]

Originally Posted by rwarre

Have been reading the threads on engine performance. My question is does a 180hp engine running at 2350 rpm and a 150 or 160 hp engine running the same rpm indicated the same true air speed at same given altitude, temp.

Randy,

The real-world answer to your question is "no" if I understand your question correctly. The 360 and 320 are each going to be flying a prop with different pitch. The 360 @ 2350 rpm (let's consider each plane with a fixed-pitch prop to keep it simple) will have a coarser pitched prop than the 320 because it develops more power at 2700rpm (rated power rpm for each engine) so the prop needs to absorb more power.

Prop pitch in this example determines airspeed. At a given rpm, the 360 prop is taking a bigger bite than the flatter pitch 320 prop which results in more speed at that rpm. To make it even more interesting, the larger engine often spins a larger diameter prop than the 320. And to really make it interesting, the 360 more often than not has a constant speed prop (variable pitch) which complicates your simple question.

Look at Vans fixed-pitch prop recommendations to see this play out. The 150hp engine often spins a prop with 77" of pitch, the 180hp can handle 83" of pitch. More pitch results in more speed at a given rpm.

[RV10inOz]

I love these threads, the font of many OWT's

David

It is my theory BSFC (brake specific fuel consumption) is about the same with all Lycoming engines, i.e., a 0235 burning 6 gph produces about the same HP as the 0360 burning 6 gph.

That being a fact (?) maybe, the limiting factor in comparing one Lycoming to another is its rated HP. The prop matters of course, but set the same fuel flow leaned out and generally the HP and performance well be similar.

On the lean side of peak EGT the same fuel flow will be about the same delivered HP, of course depending on prop and RPM some losses are different but almost not relevant to the discussion.

On the Rich side of peak, this is not true. It is Mass airflow that will be the driver, so RPM and MP and the volume per minute matter.