Compression: too good to be true?

[Steve Brown]

80/79
80/80
80/79
80/78
Is this possible?

engine has about 250 hobbs

[n5lp]

Originally Posted by Steve Brown

Is this possible?...

It seems quite unlikely. I wonder what compression checker you are using? I had a buddy who was bragging about his compression, all 80/80. Turned out he didn't know about the needed orifice in the compression checker.

[Steve Brown]

Originally Posted by n5lp

It seems quite unlikely. I wonder what compression checker you are using? I had a buddy who was bragging about his compression, all 80/80. Turned out he didn't know about the needed orifice in the compression checker.

on second check 80/80 was more like 80/79.5

the orifice is in. when I "pressurize" without being in a cyl, pressure is about zero as air rushes out

[Mark Burns]

Originally Posted by Steve Brown

80/79
80/80
80/79
80/78
Is this possible?

engine has about 250 hobbs

I've seen 80/80 before. I usually will call it 80/79 so it's more believable

I just helped with an annual on a Decathlon with just about the same amount of hours on it that had very similar readings.

Mark

[Mark Burns]

Steve,
Your post (#3 in this thread) is titled .060 orifice.

Someone correct me if I'm wrong but I think it should be .040


AC 41.13-1B states:
"Engines up to 1,000-cubic-inch displacement: 0.040-inch orifice diameter, 0.250 inch long, 60-degree approach angle.
Engines in excess of 1,000-cubic-inch displacement: 0.060-inch orifice diameter, 0.250 inch long, 60-degree approach angle."

Mark

[gasman]

There should be a shut off valve at the outlet end of the gauge body right where the line connects that goes to the cly. Close that valve and crank up the knob to 80 and read the gauges. Do they read the same? If the left gauge reads lower that the right, then you will get high numbers.

[Steve Brown]

Originally Posted by Mark Burns

Steve,
Your post (#3 in this thread) is titled .060 orifice.

Someone correct me if I'm wrong but I think it should be .040


AC 41.13-1B states:
"Engines up to 1,000-cubic-inch displacement: 0.040-inch orifice diameter, 0.250 inch long, 60-degree approach angle.
Engines in excess of 1,000-cubic-inch displacement: 0.060-inch orifice diameter, 0.250 inch long, 60-degree approach angle."

Mark

One the body it says 0.060" orifice. I need to check part numbers to see if that's what I actually got.

If so, I would expect the drop to be about 2.25x greater with the 0.040 orifice. That should still be no problem, but I need to actually make the right measurement.

I'll be working with my mechanic tomorrow to finish up an annual condition inspection. I'm sure he's got the right one.