Manifold Pressure with a Borer 82/44 Prop

[Darrel Starr]

I have been trying to break in my engine by running "hard", I thought, at 2550 rpm.
This is a 160 hp 0-320-B2B with Leading Edge exhaust and an 82/44 Borer prop. Recently I installed a manifold pressure gauge and was startled to find that 2500 rpm results in only 21 in. Hg manifold pressure which is about 63% power.
In discussing this with Darrell Bolduc at Bolduc Aviation Specialized Services, Inc, he told me that I must run 24 in. Hg to seat the rings. That turns out to be 2750 rpm, 83% power. So I am running at that rpm until the rings finally seat .... if I didn't already screw it up. Just thought I would publish the following info so that others with Borer props will see the manifold pressure / rpm relationship also and not make the same mistake.

 

[SC3CM]

Would it be better to just throw a cruise prop on for break in?

 

[Darrel Starr]

Yes, a 74/56 prop would work better for breakin but I don't have one.
However, I also discovered that running rich -- mixture pulled out only about 1/2 inch, just enough to get the EI scanner to move from cyl to cyl ( coldest EGT at 1125) oil temp is about 200, ambient at 80F. So I can run for hours like that.

 

[mvivion]

Um, Darrel, isn't the red line for that engine 2700 rpm, not 2750?

And, how are you getting that thing to turn up that high anyway? Constant dive? Sounds to me you need more pitch if you can run that much rpm in level flight.

MTV

 

[Darrel Starr]

It will run 2830 rpm flat out. A 45 or 46 pitch might be a little better match but the Borer prop is certified for the Super Cub only in the pitch range of 40 to 44 inches. So 2750 is not quite full throttle. However, I am running around at 105-110 mph at that setting!
Just checked with Darrell Bolduc today again about 2750 rpm. That does not alarm him -- ' says run it, can't hurt it at that setting.
Remember the Leading Edge exhaust is supposed to bump the hp up to 173 so maybe that is why it will top out over 2800 rpm.

 

[Darrel Starr]

One other observation, under the conditions stated in the chart, 2700 ft and 80F ambient temp, the 82/44 borer will not demand enough manifold pressure at rated rpm, 2700, to pull rated hp in straight and level flight. the prop demand curve crosses the 2700 rpm line at about 125 hp -- 78% power. Of course, pull it up into a climb and the manifold pressure & hp goes up but not in level flight. That surprised me.
I would be interested in similar results for a SC with a 74/56 prop but it takes someone with a manifold pressure gauge to record similar information. If you have that combination, I sure would like to see the answer just manifold pressure at 2700 rpm with elevation and ambient temp would be enough.

 

[sierra bravo]

And yet thousands of engines have been successfully run in and operated with Borer props. Including mine. You're manufacturing a problem where none exists.

Oil consumption stabilizes, compressions are good, it's broken in.

 

[Darrel Starr]

On the other hand, I know some folks revert to the original 74/56 prop for breakin.

 

[Darrel Starr]

Mike Vivion has been PMing me with dire warnings about running at 2750 so I will back off to 2700 rpm. Thanks Mike for your concern.

 

[mvivion]

Darrel,

I think the first thing I'd do, if you haven't already done so, is use a stroboscopic tach checker to verify the accuracy of your tachometer. That'd have to be a pretty strong O-320 to actually be ABLE to run a 44 pitch prop at that rpm. I'd almost bet your tach is in error on the high side by 50 to 100 rpm....just a thought. That too will give you false percent power indications.

Mike

 

[sjohnson]

... That'd have to be a pretty strong O-320 to actually be ABLE to run a 44 pitch prop at that rpm. I'd almost bet your tach is in error on the high side by 50 to 100 rpm....just a thought. ...

Hmm, my O-320 will overspeed during an easy climb at full throttle with a Borer 43 pitch. 44 is not that much more. My tach reads 100 rpm high at 2700 rpm indicated, but I'm taking that into account.

 

[Bugs66]

2750 seems hardly worth worrying about for short amount of time. Don't they run them at Reno 3500+ RPM?

 

[Carey Gray]

The sky doesn't begin to fall at 2750.
Might be over thinking this one.

 

[Darrel Starr]

I have an EI Electronic tach which I believe is accurate.

 

[ron]

I have seen this so many times,the more engine instruments the more worry to the operator.The original cubs did not have manifold pressure gauges and egt's .Go back to basics,run it hard for the run in and ENJOY it

 

[Darrel Starr]

But, but but Ron ... I'm an Ungineer, I live for data!

 

[aktango58]

full flaps, nose up, get it behind the power curve, and run her... you will develop heat!

I agree on the tmi

time to cover the gauges.

 

[ron]

Darrel I understand

 

[gpepperd]

When my 0-320 was freshly overhauled with 10:1 pistons by Ly-Con in California, Ken told me to run it up to 2900 rpm full throttle @ level cruise once in awhile for break in and it would do it! He said that was still not much rpm for an engine and no problem to run it up. My 1st break in attempt didn't work so well because I thought 2500-2550 was working it enough. That was not the case. I had it rehoned and ran it 2600-2700 and it broke in great. I did not have a manifold pressure guage to go by but know it was loafing @ 2500 to 2500 range.

 

[8GCBC]

Darrel:

Is the engine new, reman, overhaul, cylinders new (or chromed)? Sometimes engines need to be rehoned because of the builder not the pilot. My new engine was fine in about 5-20 hours (oil, compression all good). I had a PenYan that trashed in about 25 hours! PenYan stood behind their cylinders. That was when I swore off chrome at any price.

Hopefully you will find a conclusion with prop/RPM/MP and THEN go out there and go strip bagging!

 

[cgoldy]

Darrel,

Don't listen to Ron. He's nothing but a hypocrite. I have word he is putting a full on dynon in his cub!


Never trust Newzealanders. They steal your fish and your women.

 

[ron]

DO I KNOW YOU !For info that those who are geographically ignorant Australia is a small island to the south of New Zealand.

 

[cgoldy]

Darrel,

Please note that Ron did not deny the installation of the dynon!

 

[skywagon8a]

Darrel,
Here is some food for thought. Your engine underwent certification testing at 103% of rated rpm. That would be 2781rpm. Or if it was intended for a helicopter it was tested to 105% or 2835 rpm.

PART 33—AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES[/h][h=5]§ 33.49 Endurance test.[/h] top(a) General. Each engine must be subjected to an endurance test that includes a total of 150 hours of operation (except as provided in paragraph (e)(1)(iii) of this section) and, depending upon the type and contemplated use of the engine, consists of one of the series of runs specified in paragraphs (b) through (e) of this section, as applicable. The runs must be made in the order found appropriate by the Administrator for the particular engine being tested. During the endurance test the engine power and the crankshaft rotational speed must be kept within ±3 percent of the rated values. During the runs at rated takeoff power and for at least 35 hours at rated maximum continuous power, one cylinder must be operated at not less than the limiting temperature, the other cylinders must be operated at a temperature not lower than 50 degrees F. below the limiting temperature, and the oil inlet temperature must be maintained within ±10 degrees F. of the limiting temperature. An engine that is equipped with a propeller shaft must be fitted for the endurance test with a propeller that thrust-loads the engine to the maximum thrust which the engine is designed to resist at each applicable operating condition specified in this section. Each accessory drive and mounting attachment must be loaded. During operation at rated takeoff power and rated maximum continuous power, the load imposed by each accessory used only for an aircraft service must be the limit load specified by the applicant for the engine drive or attachment point.
AND
(d) Helicopter engines.....
(6) A 15-hour run at 105 percent rated maximum continuous power with 105 percent maximum continuous speed or at full throttle and corresponding speed at standard sea level carburetor entrance pressure, if 105 percent of the rated maximum continuous power is not exceeded.

 

[mvivion]

Darrel,
Here is some food for thought. Your engine underwent certification testing at 103% of rated rpm. That would be 2781rpm. Or if it was intended for a helicopter it was tested to 105% or 2835 rpm.

PART 33—AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES[/h][h=5]§ 33.49 Endurance test.[/h] top(a) General. Each engine must be subjected to an endurance test that includes a total of 150 hours of operation (except as provided in paragraph (e)(1)(iii) of this section) and, depending upon the type and contemplated use of the engine, consists of one of the series of runs specified in paragraphs (b) through (e) of this section, as applicable. The runs must be made in the order found appropriate by the Administrator for the particular engine being tested. During the endurance test the engine power and the crankshaft rotational speed must be kept within ±3 percent of the rated values. During the runs at rated takeoff power and for at least 35 hours at rated maximum continuous power, one cylinder must be operated at not less than the limiting temperature, the other cylinders must be operated at a temperature not lower than 50 degrees F. below the limiting temperature, and the oil inlet temperature must be maintained within ±10 degrees F. of the limiting temperature. An engine that is equipped with a propeller shaft must be fitted for the endurance test with a propeller that thrust-loads the engine to the maximum thrust which the engine is designed to resist at each applicable operating condition specified in this section. Each accessory drive and mounting attachment must be loaded. During operation at rated takeoff power and rated maximum continuous power, the load imposed by each accessory used only for an aircraft service must be the limit load specified by the applicant for the engine drive or attachment point.
AND
(d) Helicopter engines.....
(6) A 15-hour run at 105 percent rated maximum continuous power with 105 percent maximum continuous speed or at full throttle and corresponding speed at standard sea level carburetor entrance pressure, if 105 percent of the rated maximum continuous power is not exceeded.

Land at the conclusion of said test, the engine is torn down for inspection....

Folks,

I agree that running one of these engines at 2750 isn't likely to harm anything in a short run. The point is, by doing so, you would be violating the engine manufacturers and airframe manufacturers stated limits.....and advertising it on the Internet.....

MTV

 

[Darrel Starr]

Here is the history. The engine was built up with new Superior narrow deck cylinders & now has 220 hrs. Three of the cylinders seem OK -- 75/80 compression & dry plugs. But cyl #2 never quite seated properly with 65/80 compression & carbon on the plugs. So 35 hrs ago, I took #2 off & had Bolduc rehone it & install new rings. I have been running mineral oil since then but only at 2500 rpm. Now that Darrell Bolduc has pointed out that I need to run it at 24 in. MP, I am hoping I won't have to take #2 off again. Have run 2.5 hrs so far at 2750 but #2 plugs still wet & consuming the mineral oil at the rate of 1 qt/5 hrs.

 

[skywagon8a]

Is there excess clearance in the valve guides? This could draw in excessive oil thus lubricating the cylinder walls and preventing the rings from wearing in. Try doing a wobble check on the valves.

 

[Darrel Starr]

No other issues. When Bolduc had #2 apart, they checked everything. The cylinder is right on size & before they honed it, they didn't think it was glazed either. I'm sure they checked the guides too.

 

[sierra bravo]

Lycoming specifies full power for take-offs and cruise operations at 65% power or greater for break-in although they imply that 70-75% power is better. They define the break-in period to be 50 hours. There is no reference to manifold pressure and for 70 years Cubs have been broken-in using fixed pitch props and no MP instrument. I've broken in a few engines and never needed more than 5-6 hours to see oil consumption stabilize and that was with steel cylinders. The nickels took less than that.

Sometimes cylinders are just bad and honing doesn't fix them.

Are cylinder pressures higher at 2700 rpm than at 2500? Same compression, same fuel mixture, right? For most guys to achieve 2700+ rpm they need to hold level cruise where that same plane in a full power climb may only see 2500 rpm. In which instance is the engine working harder? I understand the concept of varying the rpm during the initial hours of break-in but to automatically assume 2700+ is better than 2500 or 2550? I'm not sold on that.

 

[Darrel Starr]

21 in. Hg MP @ 2500 rpm, 63% power, 24 in. Hg @ 2750 rpm, 83% power. More MP means the rings are forced out harder against the cylinder walls & therefore more likely to seat. Some engines no doubt will seat the rings @ 21 in. Some won't.