PA28R-200 MP/RPM/CHT/%HP etc what is right? IO360C

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xander75

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Jan 21, 2022
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xander75
hey folks,
a topic of much repetitive discussion I guess, but i was looking for some guidance on this.

I have this piper arrow ii with all kinds of speed mods (lowpresti/knots2u), electronic ignition system and a JPI, very helpful tools. the problem is the POH numbers don't match what I experience and was wondering that for the engine longetivity what are the most important values to obey by.

to make the subject a bit more complex, this thing has a hartzel STC prop that has a red arc between 2350/2550 at MP>24.

my questions are this :
1) I get at 60% HP 290/315/289/275 CHT OilPressure 62 OilTemp 175 at 5000ft with 24/2350
a IAS of 130 and a TAS of 140 with 8GPH. This plane regardless of altitude almost is very consistent in the CHT's at this MP/RPM (TAS gets higher with altitude). does this look reasonable?

2) for the red arc RPM section of the hartzell prop how sensitive is that? if I am at 2345-2355 as what the jpi indicates, am I in trouble or how close should I pay attention to it.

3) if I'd lower RPM and increase MP obviously you run at higher %HP is that advisable?

4) do you guys trust a JPI over the "legacy" tach and mp/fuel flow gauges? I noticed that the tach reports a bit higher rpm at higher rpm settings above 2300rpm.

was wondering if you guys have some experience to share. only owned her for 2 months so still trying to figure out a lot of things :) but wanted to make sure that i take care of the engine correctly. hence the Q of what kpi's are most important for the engine longetivity.

cheers!
xander
 
Sorry I can’t get you any advice because I don’t have any experience with a arrow2 until 2 days ago.
My buddy bought one about a month ago. Finally the weather got good enough for him to get it home this past Thursday.
We flew both of our planes Friday night to dinner with my wife and his girlfriend and was his first trip local. He flew it 5 hours to get it home, 2 hours hours IFR, the rest fight following.
Then I got to fly with him in it this morning. I flew it a little bit. It was bumpy mid morning.
He gave me my own POH to study so I would be more prepared to fly it, He is a check airmen.
I thought it was ironic that I saw your post tonight.

Good luck with your plane, I think it is great plane, wish I had one. 9388AAE8-D212-45CA-B39E-2A6A653A3ED5.jpeg
 
I'd trust the jpi over the mechanical tach any day.
 
Racerx said:
I'd trust the jpi over the mechanical tach any day.
Click to expand...
If you have an iPhone get the video tachometer ap for free. Uses phones camera to look at prop from cockpit and tells rpm
 
Or borrow/buy a laser tach.

I only recommend this because:
  • I have a friend with one.
  • I trust a point-made device over an iPhone app for no reason other than my own preferences.

I discovered my tach was off 100. When it registered 2,300 the engine was doing 2,400. Common weak-magnet issue for an older mechanical tach. Knowing this brought my birdie right to the book.
 
You can't do much to that engine at 55% (~8gph when nominally leaned); the whole thing is about your prop not playing well with the -C1C crank at said RPMs at said load (it was meant to play with the -C1C6). It's an inconvenient limitation, given 2400-2500 is very popular, but it's not an insurmountable problem.

24 inches ought to be plenty to do what you're doing down low, negating the need to worry about the RPM resonance.
upload_2022-4-24_11-28-35.png
As you can see, there's no real reason to be running anywhere close to 24 inches down low to pump out a measly 55% power, while maintaining the entire RPM range available to you. The only scenarios that are sketchy with your prop is 75% power below 5k, since it's gonna force you above 2550 and that's noisy. That's the part that's crap about that prop given the price (installation which you didn't eat the full cost of, so I suppose you can't complain that much lol). Again, not an insurmountable problem (aka fly at 65% power settings, or accept the noisy rpms). At the fuel flow you're targeting (8gph/55%), this whole thing is a non-issue.

Enjoy. It's an airframe that's treated me well on the money front, all things considered. I wouldn't do it again if I knew what I know now about recurring and moral hazard derived ADs, but hey, my username checks.
 
hey thank for that great info guys! this is very helpful!
few responses: that jpi has a backup battery so i am not too worried about it. good to hear you all also feel it is more reliable than that tach and that i am not the only one where the rpm is off by about a 100 at higher rpms :). in fact jpi just came back to me stating that it is considered a primary instrument and you can remove the mechanical tach plus MP/FF. mwah i'll keep it just in case.
ps yeah that prop came with it, wonder why the previous owner went with that one instead of one that doesnt have the restriction...

ok since i am running so low on the power scale, but still get very nice TAS, that is why the CHT's report so low, ok so be happy and shut up :).

yeah that wing spar, happened what, one time right? (is always too many of course), but yeah that is why i was looking for a low time airframe/non trainer.

say hindsight2020, on that power table, if I find a setting that is for instance lower RPM but higher MP that RGbeard confirmed is not an issue, is that wise or would you stick with that
chart?

thanks for all the guidance. whole new experience owning vs renting. it is having like a new daughter! crazy. awesome though :)

cheers!
xander
 
xander75 said:
if I find a setting that is for instance lower RPM but higher MP that RGbeard confirmed is not an issue, is that wise or would you stick with that
chart?
Click to expand...

1) You want to look up “lugging” the engine. I went through this question during my first few years of aircraft ownership. You can run the engine at whatever power settings is allowed by the POH, including the higher MP/lower RPM combination, all things being equal. However, one thing I notice not being equal is vibration. I decide to run at the RPM that results in the smoothest feeling operation.

2) You can probably trust your JPI tach reading more than a mechanical tach, but you can double check with a 3rd device to be sure. I verified my tech reading when I got the prop dynamically balanced.

3) What you should NOT trust is the HP reading. In my experience, the JPI HP reading is off by at least 5% at altitudes below 5,000ft, even after careful calibration. So, I trust the POH charts for that (below 5k’). In another forum, George Braly confirmed this limitation. Allegedly, suggestions to correct the HP equations have been made to JPI but refused.

4) It would not hurt to calibrate your MP reading as per the JPI manual. Mine was 0.2” off.

Edit:
PS Here is a book that I enjoyed reading very much that you may find helpful. “Fly the Engine,” by Kas Thomas

6795189C-E418-4385-8651-796191432A94.jpeg
 
Last edited:
Clip4 said:
Probably not the day you had a total electrical failure.
Click to expand...

Why would you care at that point. You are going to land and fix things. You don’t need a tach.
 
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