o360 180hp upgrade

[JOhnH]

I was reviewing the performance section of my POH and thought the numbers may not be correct since have the aforementioned upgrade. (The plane had the upgrade when I purchased it). But when I checked the manual supplement that came with the STC, it said "no change" for the performance chapter.

The primary changes appear to be for gross weight increase and W&B changes with a few other operational items.

I was surprised. I can see the TAS would be the same at a given RPM, but wouldn't the %BHP be lower at a given RPM? It seems that 50% of a 180hp engine would turn the prop faster than 50% of a 160hp engine.

Actually, all of that was a distraction to my original purpose. I wanted to brush up on %power ratings at various altitudes and RPMs. The manual kept referring to the "CESSNA POWER COMPUTER". Does anyone know where I might find one? The chart only goes up to Std Temp +20c (68f). Most of my flying is more like 80f to 90F+

Or better yet, is this something that could be coded into an I-phone App? (Jesse, are you reading this? I already have your WnB pro and E6B Pro. apps).

 

[Skylane81E]

Many times the STC holder will not spell out exact performance gains

 

[Pilawt]

I assume you're talking about an O-360 conversion in a C-172.

If the performance is "equal to or better than" the original, the STC holder doesn't have to publish revised performance charts.

For example, the Air Plains O-360-A4M STC on my C-172N has a new power setting table, but no new takeoff or cruise performance charts, because they are equal to or better than the original. It does, however, include a new landing distance chart, because with the higher gross weight, landing distance is now "worse" than before.

As to power settings, you likely now have a larger prop with greater pitch, and therefore takes more power to turn at a given rpm, compared to the original. So a given percentage of power will likely be at a more-or-less similar rpm than it was with the original engine-prop combination.

A 172M/N/P with a 180 hp conversion, 2550-lb gross weight and fixed-pitch prop is the functional equivalent of a stock C-172Q Cutlass. You can buy a manual for that model and have reasonably close performance data.

 

[JOhnH]

Thanks, that makes sense. They did swap the prop at the time.
And yes, it is in a 1977 Cessna172n and is a PenYan O360-A4A.

I assume you're talking about an O-360 conversion in a C-172.

If the performance is "equal to or better than" the original, the STC holder doesn't have to publish revised performance charts.

For example, the Air Plains O-360-A4M STC on my C-172N has a new power setting table, but no new takeoff or cruise performance charts, because they are equal to or better than the original. It does, however, include a new landing distance chart, because with the higher gross weight, landing distance is now "worse" than before.

As to power settings, you likely now have a larger prop with greater pitch, and therefore takes more power to turn at a given rpm, compared to the original. So a given percentage of power will likely be at a more-or-less similar rpm than it was with the original engine-prop combination.

A 172M/N/P with a 180 hp conversion, 2550-lb gross weight and fixed-pitch prop is the functional equivalent of a stock C-172Q Cutlass. You can buy a manual for that model and have reasonably close performance data.

 

[JHW]

if you just want a power setting table in electronic form, here is one in MS word with notes on temperature

 

[RussR]

The chart only goes up to Std Temp +20c (68f). Most of my flying is more like 80f to 90F+

Not sure if there's a typo there or what, but if (as in your profile) you're in Daytona Beach, at sea level, Std Temp + 20C = 35 C, or 95 F.

Std Temp + 20C would only equal 68F at about 7500 feet.

 

[JOhnH]

if you just want a power setting table in electronic form, here is one in MS word with notes on temperature

Thanks, but I don't have a manifold pressure gauge. I don't believe too many O360s with fixed pitch prop do. I never really looked into installing one, but I just might. Are there any comments on this?

Part of the reason I was researching this is because I don't believe I am getting full rated performance from my O360.

At 2400 rpm at medium altitude (3,000 to 6,000) and OAT from high 70s to low 90s, I am only seeing about 100 IAS and getting about 8 to 9gph.

This could well be because of not leaning enough. But when I try to lean more my CHT readings climb too high. (There are several other threads about 172s with this problem). My A&P has just done some work to try to fix that, but he has been trying for years with little success. But maybe this time!

By CHT readings too high, I mean that cylinders 3&4 stay in the high 390s to 400s with even mild leaning. If I run full rich, the temps are not as bad but still around 390+ and they go to about 425-430 during climb.

 

[Pilawt]

Thanks, but I don't have a manifold pressure gauge. I don't believe too many O360s with fixed pitch prop do. I never really looked into installing one, but I just might. Are there any comments on this?

We had a MP gauge installed when the engine swap was done -- with fixed-pitch prop -- and I really like it. You get a better understanding of what the engine is really doing.

The power setting chart Russ posted is good. I'll print one out and keep it in the airplane. But they gave us a short-cut method of power setting for the O-360-A4M: Add the MP in inches to rpm/100 (example 21" + 2400 rpm = '45'). If the number is '48', that's ~ 75% power; '45' ~ 65%; '42' ~ 55%. Not precise, but good enough for government work.

Here's the MP gauge and the cheat sheet in my airplane (this is in a climb):

Part of the reason I was researching this is because I don't believe I am getting full rated performance from my O360.

Has your tach been checked for accuracy?

 

[JOhnH]

We had a MP gauge installed when the engine swap was done -- with fixed-pitch prop -- and I really like it. You get a better understanding of what the engine is really doing.
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Interesting. My mechanic would probably prefer I remove the EDM700 and replace it with an MP gauge anyway. Then I would stop complaining about high CHT.

Has your tach been checked for accuracy?

Yes, and it was off so I had it replaced with a digital tach at the same time I installed the GNS530w and the FS 450 Fuel Flow gauge.

 

[JHW]

If you know the rpm is right then the engine doesn't matter, you only need to know what prop you have. Measure your prop diameter and twist. Don't believe the numbers stamped on it unless you've owned it since new.

 

[JOhnH]

If you know the rpm is right then the engine doesn't matter, you only need to know what prop you have. Measure your prop diameter and twist. Don't believe the numbers stamped on it unless you've owned it since new.

Good point. The prop was swapped when the first O360 upgrade was installed before I bought it. I guess I need to do some research on propellers too.

 

[JOhnH]

Not sure if there's a typo there or what, but if (as in your profile) you're in Daytona Beach, at sea level, Std Temp + 20C = 35 C, or 95 F.

Std Temp + 20C would only equal 68F at about 7500 feet.

hmmmm,
I typed that from memory and at the moment I don't have my notes. I will double check to see where I erred.

Edit

Ok, I checked my notes and I see what I did wrong. It was so stupid the best I can do is admit my error, but not say what I did. But with a little arithmetic it becomes obvious.

 

[poadeleted20]

And yes, it is in a 1977 Cessna172n and is a PenYan O360-A4A.

The modified 172N you have is for performance purposes a C-172Q. The POH for that model is available from several sources on line and provides the exact data you want.

 

[JOhnH]

The modified 172N you have is for performance purposes a C-172Q. The POH for that model is available from several sources on line and provides the exact data you want.

Thanks. I'll get one.

 

[Graueradler]

A rule of thumb (don't know how good it is) is that 2 inches of prop pitch change corresponds to a 10 HP increase. If you increase the prop pitch 2 inches, it will take 10 more HP to spin it at the same rpm at the same air speed.

 

[83Golf]

I assume you're talking about an O-360 conversion in a C-172.

. It does, however, include a new landing distance chart, because with the higher gross weight, landing distance is now "worse" than before.

Not just the higher gross weight, more to do with limiting flaps from 40 to 30 degrees.

 

[Pilawt]

Not just the higher gross weight, more to do with limiting flaps from 40 to 30 degrees.

Quite true. I forgot about that. (In the Air Plains conversion, the engine and flaps are separate STCs. I have both, but never installed the flap limiter. So I still have 40° of flap and 2300 lb MGW.)