btstarcher
Gold Member
OK, what are the specs for a motor to have acceptable compression? Is it all cylinders within 20% tolerance? 10%? I read a few different opinions on another forum, some said 5%, and one said 28.5 PSI.
Compression test question
- Thread starter btstarcher
- Start date
I honestly don't remember off the top of my head. Just make sure to do it after getting the engine up to operating temperature, and remove every plug before starting to test each cylinder. Crank until the pressure needle stops rising. Then, after you have your numbers, compare the % variance between your highest and lowest measurements.
Hopefully someone else can come in here and tell you specifically what % variance you want to be within, or what actual numerical difference in psi is the "approved" range.
btstarcher
Gold Member
It's already been removed and will be sold. I just need to see if it's within spec. 190, 180, 190, 167. I'm pretty sure that's in reverse order; #1 is low I think. Also there was no wet test done, or leakdown, but I believe the leak is in the head. It has nearly 200k miles on it and never was checked for valve clearance.
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Popular Mechanics: "Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings." Also "For a cylinder below 100 psi, pour 1 teaspoon of engine oil into the plug hole and retest. If the reading jumps, the piston rings are worn. If not, think valve problems."
Dummies book author: "The highest and lowest shouldn’t vary by more than 15 percent. If one or more of the cylinders reads well below the rest, use a trigger-type oil can to send a good squirt of motor oil down the spark plug opening, and retest the compression of that cylinder with the gauge. If the reading is the same, the valves either are worn (and letting pressure escape) or are out of adjustment. If the reading rises dramatically after you insert the oil, you probably need new rings on the piston in that cylinder. If the pressure recorded by the gauges is less than 100 psi, the cylinder definitely isn’t mechanically sound."
Linn-Benton Community College: "Analyze your test results. Your cylinders should be close to the specified compression pressures specified by the manufacturer. More important is even compression between cylinders. If the lowest cylinder is more than 20% less than the highest cylinder it can cause the engine to run rough.
The First Puff should be at least 50% of the fifth puff on that cylinder. If the first puff is well over 50% of the final reading, but the final reading is low, that cylinder is likely leaking past the valves.
If two adjacent cylinders have low compression it is often caused by a head gasket that is leaking between these two cylinders. If there is a very low first puff, and it gradually builds to an overall low pressure, that cylinder likely has bad rings or a worn cylinder wall. You can confirm this with a Wet Compression Test."
Engine Basics: "
* Record the reading for each cylinder. If any of them vary 10 percent or more from each other a problem may exist in one or more cylinders. If the variance is greater than 10 percent, specialized testing equipment may be required to fully diagnose the problem.
* If all cylinder readings are within 10 percent of each other, no further testing is required and compression is considered optimal."
Mazda shop manual (page 01-10B-12 and 13): "Compression
Standard: 1,280 kPa {13.05 kgf/cm2, 185.6 psi} (250 rpm)
Minimum: 896 kPa {9.14 kgf/cm2, 130 psi} (250 rpm)
Maximum difference between cylinders: 196.1 kPa {2.000 kgf/cm2, 28.44 psi}"
[doublepost=1456713191][/doublepost]Only problem is that everyone says to run it with the motor at normal operating temp. We tested a guy at our last install day that wasn't even close to meeting the specs cold, but was decent warm. But based on the numbers you posted it sounds like it's in spec cold.
Dummies book author: "The highest and lowest shouldn’t vary by more than 15 percent. If one or more of the cylinders reads well below the rest, use a trigger-type oil can to send a good squirt of motor oil down the spark plug opening, and retest the compression of that cylinder with the gauge. If the reading is the same, the valves either are worn (and letting pressure escape) or are out of adjustment. If the reading rises dramatically after you insert the oil, you probably need new rings on the piston in that cylinder. If the pressure recorded by the gauges is less than 100 psi, the cylinder definitely isn’t mechanically sound."
Linn-Benton Community College: "Analyze your test results. Your cylinders should be close to the specified compression pressures specified by the manufacturer. More important is even compression between cylinders. If the lowest cylinder is more than 20% less than the highest cylinder it can cause the engine to run rough.
The First Puff should be at least 50% of the fifth puff on that cylinder. If the first puff is well over 50% of the final reading, but the final reading is low, that cylinder is likely leaking past the valves.
If two adjacent cylinders have low compression it is often caused by a head gasket that is leaking between these two cylinders. If there is a very low first puff, and it gradually builds to an overall low pressure, that cylinder likely has bad rings or a worn cylinder wall. You can confirm this with a Wet Compression Test."
Engine Basics: "
* Record the reading for each cylinder. If any of them vary 10 percent or more from each other a problem may exist in one or more cylinders. If the variance is greater than 10 percent, specialized testing equipment may be required to fully diagnose the problem.
* If all cylinder readings are within 10 percent of each other, no further testing is required and compression is considered optimal."
Mazda shop manual (page 01-10B-12 and 13): "Compression
Standard: 1,280 kPa {13.05 kgf/cm2, 185.6 psi} (250 rpm)
Minimum: 896 kPa {9.14 kgf/cm2, 130 psi} (250 rpm)
Maximum difference between cylinders: 196.1 kPa {2.000 kgf/cm2, 28.44 psi}"
[doublepost=1456713191][/doublepost]Only problem is that everyone says to run it with the motor at normal operating temp. We tested a guy at our last install day that wasn't even close to meeting the specs cold, but was decent warm. But based on the numbers you posted it sounds like it's in spec cold.
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Hmm, so thinking about this a little more and comparing Mazda's numbers to percentages... 28.44/185.6*100=15.3 so mazda is allowing 15% from the high end. But if you were playing the "barely good enough" game and had a cylinder at 130 and one at the then highest allowable of 158.44, the percentage would be 28.44/158.44*100=17.95% of the highest cylinder or 158.44/130*100=121.8 or 22% more than the lowest cylinder. I like Mazda's way of specifying it because percentages depend too much on specifics of the wording which is easily misinterpreted.
btstarcher
Gold Member
Performed a leakdown, and it was leaking 8psi from 80psi, and air seemed to be coming mostly out of the intake manifold, with some air coming out the exhaust manifold. So we're at least going to check all clearances and get new buckets as needed, and possibly going to remove the head and clean the valves as well.