Lycoming cam wear power loss

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PiperCruisin

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I just had to overhaul my o-320. It had pretty severe inlet cam wear which impacts 2 of the 4 cylinders. I measured that it lost 0.15 inches. It felt very low on takeoff performance, and highish fuel burn (9.5 vs 8).

What would you estimate my power loss would have been?

Took off from a relatively long backcountry strip and was flying down the canyon for quite awhile.
 

Dan Thomas

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I just had to overhaul my o-320. It had pretty severe inlet cam wear which impacts 2 of the 4 cylinders. I measured that it lost 0.15 inches. It felt very low on takeoff performance, and highish fuel burn (9.5 vs 8).

What would you estimate my power loss would have been?

Took off from a relatively long backcountry strip and was flying down the canyon for quite awhile.
Power loss maybe 10%.

When I was instructing we taught partial-power takeoffs to simulate engine trouble or high density altitude. Right after liftoff the instructor would reduce the RPM by only 100; it was amazing how much performance suddenly disappeared. And it's amazing how, later on, owners would sometimes comment on how much better their airplanes flew, and it was usually after doing the mag inspections and reinstalling them at just the right timing to the engine. The internal inspection usually included new points and getting them set so the E-gap was just right. And sparkplugs cleaned, gapped and tested under pressure. I liked to replace a bad plug set with new Tempest REM37BY plugs wherever they were approved for the engine. That plug has extended electrodes that get the spark farther out in the mixture, rather than so close to the plug well that might be harboring spent gases. And they don't foul easily, either.

Some folks think a spark is a spark. Nope. A HOT spark, at the right point in the cycle, gives improved performance.
 

proppastie

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in the TCD (type certification data sheet) for your aircraft there is a minimum static rpm for fixed pitched aircraft......course that would be a sea level standard day.
 

Dan Thomas

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in the TCD (type certification data sheet) for your aircraft there is a minimum static rpm for fixed pitched aircraft......course that would be a sea level standard day.
I found that the static RPM didn't vary a lot with altitude and temperature. With denser air, there're more prop drag but more power to overcome it. The opposite is true with less dense air. A very humid day would reduce it a little; water vapor displaces air and its oxygen.
 

PiperCruisin

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in the TCD (type certification data sheet) for your aircraft there is a minimum static rpm for fixed pitched aircraft......course that would be a sea level standard day.
My static rpm was about 2200 or a little less. Even checked the tach (only off about 20 rpm at 2000 rpm). The type certificate says 2250 to 2350. Compressions were all good. Cam lobe was almost round.
 

PiperCruisin

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I found that the static RPM didn't vary a lot with altitude and temperature. With denser air, there're more prop drag but more power to overcome it. The opposite is true with less dense air. A very humid day would reduce it a little; water vapor displaces air and its oxygen.
I was thinking similar.

New engine has the higher compression pistons and the Powerflow exhaust. Hoping for much better performance. I had an other plane, same make, model and year. It performed much better. Also worked on the rigging and adding gap seals. Very extensive "annual".
 

TiPi

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If you have exact before & after static WOT rpm numbers (similar location conditions, same prop), the power loss/gain can be claculated quite accurately: Pdelta = sqrt(rpm1^2/rpm2^2)
rpm1 or rpm2 can be either depending on if you want to calculate power loss or power gain.

EDITED (got that one wrong):


the correct formula is:
P2=(rpm2/rpm1)^3*P1
 
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Victor Bravo

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Also worked on the rigging and adding gap seals. Very extensive "annual".
IMHO gap seals are a likely bigger deal in climb than they even are in cruise. VG's work really well on a lot of back country type airplanes, but IIRC you had a Cherokee or something??? Running the CG back to 85-90% will noticeably increase the performance, at the cost of having to fly the airplane a little better. Some that energy used to make extra downforce at the tail can be put back into lift. Do you have the Art Mattson stuff on your airplane?
 

Dan Thomas

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Wow, .150" of intake valve lift on 2 cylinders. That had to been around 1/2 of original. I'd say you had maybe ~80% of the rated power available. Very misleading because the engine still runs smooth.
It's worse than that. He said .150" off the cam. Since the rocker amplifies the pushrod movement to the valve, the loss in valve travel would be greater.
 

Dana

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.15 inches is huge, and would be an alarming account of metal flakes in the oil chewing up other parts of the engine... hard to imagine it not being caught sooner. That sounds like years of undetected wear. Are you sure it wasn't .015?
 

TiPi

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\[ \sqrt{\frac{a^2}{b^2}}=\frac{a}{b} \]?????
sorry, I got that wrong :( I trusted my memory!

the correct formula is:
P2=(rpm2/rpm1)^3*P1

eg 75% power is achieved by reducing the WOT rpm by 10% (9.14% exactly).

a 100 rpm drop (2,300 to 2,200) on this engine would result in a power loss of
Power loss = 1-(2,200/2,300)^3 = 12.5%
 

Dan Thomas

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sorry, I got that wrong :( I trusted my memory!

the correct formula is:
P2=(rpm2/rpm1)^3*P1

eg 75% power is achieved by reducing the WOT rpm by 10% (9.14% exactly).

a 100 rpm drop (2,300 to 2,200) on this engine would result in a power loss of
Power loss = 1-(2,200/2,300)^3 = 12.5%
And that would depend on a particular torque curve. Engines vary in that. RPM is only one factor here.
 

Dana

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And that would depend on a particular torque curve. Engines vary in that. RPM is only one factor here.
It's correct for a fixed pitch prop, the power required to spin it varies with the cube of rpm.
 

speedracer

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Here's a pic. of one of two cams I've personally flattened out. They both soldier on now as towel racks in my shop. When the first one was happening I noticed a distinct loss of power but didn't know why. The engine was running smoothly so I kept on flying trying to figure out what the problem was. The longer I flew the greater the power loss, a noticeable difference in only 30 minutes. The second time it happened I caught it much sooner. And yes, both times the two lobes were the shared ones which makes perfect sense. Also, both cams are O290 "narrow lobe" cams.
 

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Dana

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Wow. I'm having a hard time picturing a scenario that chews off that much of a cam without other bad things happening.

For that matter, I'm having a hard time picturing a scenario that chews off that much of a cam, period. The tappets would likely be toast as well. I wonder if it was defective heat treat from the factory?
 

Dan Thomas

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Wow. I'm having a hard time picturing a scenario that chews off that much of a cam without other bad things happening.

For that matter, I'm having a hard time picturing a scenario that chews off that much of a cam, period. The tappets would likely be toast as well. I wonder if it was defective heat treat from the factory?
Lycoming long had cam problems. I think the cams themselves might have been carburized and case-hardened, and once corrosion sets in on the lifter and cam, the metal disappears quickly. That skinny little line of contact between the cam and lifter face is under terrific pressure, and is probably the most highly loaded surface in the entire engine.
 
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