Compression Tester Problems

[Dave Russell]

Just a quick note on compression tester calibration.

I was recently doing a quick compression check on a new engine. I ran through the cylinders the first time & was getting a pretty even 190 psi. In the middle of a second test, the gage failed to hold the readings, leaked down.

I replaced the Schrader type valve in the tester with a standard valve stem. The readings dropped to around 135 psi. After much studying, I found that valve cores are not all the same. Collecting a few different types of valve cores, checking the compression tester readings, & measuring the actual tension required to open the various valve cores, I found the following.

Valve cores marked with a red band, standard tire type:
Pull tension - 56 ounces.
Compresion gage reading - 135 psi.

Valve cores marked with an orange band, usually used on air conditioning applications:
Pull tension - 2 ounces.
Compression gage reading - 185 psi.

Valve cores marked with a white, clear, or black band:
Pull tension - 1 ounce.
Compression gage reading - 190 psi.

In checking around, I found one local Carquest dealer who looked up & ordered valve cores for Lysle compression testers. Part number STL20100, black marking band. They work as expected. Compression readings back to the original 190 psi. The orange band air conditioning type cores are very close to correct & easy to obtain. I find that the air conditioning type valves actually hold pressure more reliably than the "special" Lysle valves.

I'll bet I'm not the only one to be perplexed by this problem, or to
buy a new compression tester when the previous one failed to read
properly. It's pretty obvious when you realize that the gage can only
read what is "left over" after it goes through the valve core.
D

 

[Keoke]

You mean you didn't know what them little bands was going on about Dave?---Keoke-- /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif


Well Dave, I kinda stumbled on to it too. I was given a MAC'S guage that did not work correctly. I purchased a solid brass Marsh 0-to-600 PSI gauge and put on it it still didn't work correctly. Soldered up a rigid fitting to get rid of the flex line still didn't work right. Borrowed my friends Snap-on and swapped the air valve Bingo! it worked like a charm read the little color band and went and got some at the industrial supply house, Been good to go ever since.--Keoke- /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif

 

[Dave Russell]

Nope, I didn't. It took quite a bit of digging to find out also. I haven't even looked closely at a valve core for 10 years. I was aware that some bicycle cores had no spring at all, FWIW. Just thought maybe I wasn't the only one that has had the problem. So be it!
D

 

[66ejag]

Doesn't a compression reading of 190 psi indicate a compression ratio of 13:1 ? I would expect the reading of about 135 psi (9:1) to be correct.

Ed

 

[AweMan]

Realisisng everyone wants thier tools to work properly.
Isn`t the percentage difference between cylinders what we are looking for? Seems to me as long as the guage reads the same numbers on each test weather it be 135 or 190 is of little consequence. BTW I would LOVE to have an engine that has 190 P.S.I. in all cylinders. {You might need an aircraft bump motor to turn it over}. /bcforum/images/%%GRAEMLIN_URL%%/lol.gif And some real good high octane fuel to make it run decent. With exception of some real high compression nitro/alcohol race engines a friend of mine builds. I`v NEVER seen one that has that much compression.
Kerry

 

[Dave Russell]

Ed & Kerry,
Here is a partial from an article I did for one of the mags.
---------------------------
"What do the readings mean -
The exact compression readings can vary quite a bit from engine to engine & even on the same engine under different test conditions. Some of the variables are - - exact compression ratio, condition of the engine, calibration of the gages used, cam timing & valve lash setting, starter cranking speed, engine temperature, throttles open or closed, & altitude of test site. Because of these variables it is hard to determine an exact number for "good compression". Generally a difference in readings of more than 10% to 15% is considered cause for question. Cylinder to cylinder differences of a given engine are not subject to to these variables & are thus the most reliable indicator. However if an engine has very low compression averages even with little cylinder to cylinder variation , it can still be worn.

Altitude can have a marked effect on the compression readings. Atmospheric pressure at 6,000 feet is about 80% of sea level. This means that an engine which reads 160 psi compression at sea level will only read 128 psi at 6,000 feet.

An interesting aside-
A recently asked question;
A healthy Healey has compression readings around 165 lbs. If that is taken at face value; that means the compression ratio is 165/14.7 ~ 11.2:1 This is much higher than the published numbers. That air must get really warm when you "squish" it.

Your calculation is correct for "isothermal" compression which assumes that no heat is gained or lost during the compression cycle & if the actual compression ratio is the same as the rated ratio. - See below. The formula for "adiabatic" compression takes heat loss or gain into consideration. Eg. as the air is compressed it is heated by the compression & the final pressure is higher than would be predicted by isothermal compression. The variables are rather large & the difference between the two calculations can vary by a factor of almost two to one in the final pressure. The pressure rise due to heating is an expotential function. (more heat = more pressure = more heat = more pressure). The exponent for air will vary from one with complete heat loss to about 1.4 with no heat loss.

Rated vs actual compression ratio ---------
To complicate things , at low rpm such as cranking speeds --
The actual engine compression ratio is never the design calculated ratio because in reality the piston cannot begin to compress anything until the intake valve gets closed. If the intake valve were to be completely closed at BDC, which by design it never is, the actual & rated compression ratios would be the same. At higher rpms & thus higher intake gas velocities, the late closing of the intake valve is offset by the gas inertia in the intake tract & the engine can approach or exceed it's design compression ratio.

For a hypothetical engine with 9/1 compression ratio, Assume that the actual mechanical CR is 6/1 due to late intake valve closing. We can calculate that the pressure for isothermal compression would be 14.7 * 6 = 88 psi. With an adiabatic exponent of 1.14 this would give a compression pressure of 165 psi."
--------------------------
Here is an on line compression ratio calculator that gives both static & dynamic compression ratios based on intake valve closing point.
https://kb-silvolite.com/calc.php?action=comp
--------------------------
"Whether heat is gained or lost during compression depends on several things such as how quickly the compression takes place (starter & engine cranking speed), temperature of the engine during the test (is the air in the cylinder hotter or cooler than the air compressed in it) , & a few other variables. The end result is usually that the measured compression pressure is usually higher than would be calculated by isothermal compression."
----------------------------
My particular engine has a static CR of 9.7, dynamic is 7.7. A pressure calculation, 14.7 psi atmospheric pressure X 7.7 comp ratio = 113. With an adiabatic exponent of 1.14, this comes to 219 psi. I don't believe my readings of 190 are out of line though lower than theoretical. I do live at 2800 ft elevation & would expect the readings to be lower than at sea level. Three different compression testers, with correct valve cores, all read about the same.

Kerry,
MY gear reduction starter turns it over just fine.
D

 

[Keoke]

/bcforum/images/%%GRAEMLIN_URL%%/savewave.gif
----------------------------Keoke-- /bcforum/images/%%GRAEMLIN_URL%%/lol.gif /bcforum/images/%%GRAEMLIN_URL%%/lol.gif /bcforum/images/%%GRAEMLIN_URL%%/banana.gif

 

[roscoe]

Some of us have been using differential compression testers on aircraft for a long time. There is no reason for not using them on any 4 cycle engine. You get way more information from them and the cranking and valve closing issues become moot. You pressurize each cylinder, in turn, sitting at TDC (all valves closed on the compression stroke) to 80 psi. You then measures the "leakdown" through a calibrated orifice between the 2 gages (one reads the 80 psi you dial in, and the other reads what the cylinder actually holds due to leakage in rings, or valves, etc.). Since the cylinder remains pressurized and stabilizes, you can hear if it is leaking out the exhaust (at tail pipe), intake valve (at carb. intake), or through rings (at breather tube or oil filler). You see if there is a significant difference between cylinders as with a direct compression test. The down side is that you have to get each cylinder at TDC on compression so that the pressure you pump in doesn't push the cylinder down. The testers can be purchased for about $70. Be vary careful, I'm sure that somewhere there has been a mechanic killed by an airplane prop turning due to pressurizing a cylinder that was just past or just before TDC. DO NOT leave a wrench on the crank nut while pressurizing if this is how you are turning the engine to find TDC! You get results that are expressed as X psi/ 80psi. In the airplane world of small engines if the result is 60/80 or lower we usually pull a cylinder and fix it, or wack the valve stem with a mallet which can allow a piece of debris on a valve to be blown out by the air pressure in the cylinder. We do the test hot for the best results. It is always better to approach TDC in the direction of normal rotation in order that the rings sit the way they normally would during running.

 

[AweMan]

I guess Living at 4,500 Ft. above sea level and ONLY testing my own 8:1 to 9:1 engines is the reason I have never seen 190 P.S.I. . To make even a 10:1 engine run decent up here 89 octane just isn`t going to do the job. My original point was measuring for percentages between cylinders. Weather your tester is reading 190 or 150 P.S.I. isn`t the precentage difference going to indicate a weak cylinder or cylinders just the same? Seems to me a 20% difference weather it be with a guage reading 190 or 150 P.S.I. indicates a problem.

{I reaslise the compression tester guage needs to be able to consistantly hold what ever pressure the piston pumps into it without leaking off.}
Kerry

 

[Keoke]

/bcforum/images/%%GRAEMLIN_URL%%/iagree.gif

Yep, you be talking about the leak down tester I got one of them by Proto too. But I only need it if the compresion tests go askew.---Keoke

 

[Keoke]

/bcforum/images/%%GRAEMLIN_URL%%/iagree.gif


Yep AweMan, ifin you got a 20% difference you got a problem. OTOH, if you get 10 PSI on all the cylinders you gotta bigger problem me think! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif Unless you have the wrong valve in the gauge-Keoke- /bcforum/images/%%GRAEMLIN_URL%%/laugh.gif

 

[Dave Russell]

Some of us have been using differential compression testers on aircraft for a long time.

Hi Roscoe,
The "other part" of my mag article was an in depth coverage of differential compression testers & their use.

As I said at the beginning of this thread, this was a QUICK check to see if all was well with a new engine. It led to discussing problems with "compression gages". Differential compression testing, or cylinder leak down tests, was not relevent to the discussion so I left it out. If the quick test had shown something suspicious, I most certainly would have next used the more difficult method.

BTW - Automotive race engines are often rebuilt if they show more than 5% leakdown. The aircraft spec is pretty liberal.

Kerry, You are absolutely correct.
D