MG Restoration

The Compression Check

Les Bengtson

There are a number of tests which may be performed that will give you an idea of how well your engine is functioning. Some of these require expensive test equipment, costing far more than most of us can afford if we are non-professional mechanics. Electronic engine analyzers and four gas emissions testers cost thousands of dollars and even the humble CO (carbon monoxide) tester costs almost $200. But, MG's, even the later models, were relatively unsophisticated automobiles, the types which have been tuned, quite successfully, with unsophisticated and inexpensive test equipment. One of the most useful pieces of test equipment is a compression gauge, used to perform an engine compression check.

There are two basic forms of compression gauges-the ones which are pressed into the spark plug hole and held in place during the test and those which have a short cord with a metal plug attached, which screw into the spark plug hole. The latter may be used by oneself, while the former normally requires an assistant to turn over the engine while the test is being performed. Both types have a pressure release valve which means that the gauge holds the reading until the valve is released. This allows time for you to write down the reading for each cylinder before moving onto the next cylinder.

Performing the compression test is fairly simple. It is usually performed on a hot engine because that is the state that the engine operates in most of the time. Both the engine block and the pistons and rings will expand when heated and the most accurate indications of engine condition are obtained with the engine at operating temperature. This does not mean, however, that you cannot perform a test on a cold engine and get useful data, it simply means that the data collected may be slightly less accurate than when the engine is at operating temperature. To perform the test is fairly easy, especially on the older models which do not have air pumps.

First, run the engine up to operating temperature, then shut it down. You are now working on a hot engine and hot spark plugs. Be careful not to burn yourself. The first step is to remove all of the spark plugs and lay them out in the order removed. You might find it a good idea to use small pieces of masking tape and label the spark plug wires as you remove them. Most of us know that the number one plug lead should be in about the 2 O'clock position and the distributor rotates counter clockwise. With the firing order of 1-3-4-2, we simply hook the wires up in that order when we put them back on. Start by installing the wire coming out at the 2 O'clock position on number one plug, then move to the left terminal and install on number three plug, the one below it on number four plug and the remaining wire on number two plug. Still, labeling the wires can save some confusion when learning the process.

After the spark plugs are removed (using either a socket having a foam rubber insert or a standard, deep well socket and using a rag to remove the plug after it has been loosened), the coil needs to be de-activated on most cars. This is because the later model cars have a combined ignition switch and starter switch. This means that the later cars energize the coil whenever the starter is turned. The coil can build up a charge of rather high voltage that has no where to go, which, in some cases, may damage the coil. There are a couple of ways to fix this. The easiest is to simply ground out the coil lead which goes to the center terminal of the distributor cap. Disconnect the coil lead and either peel back the boot on it and lay the bare terminal against the body or use a wire with crocodile clips on both ends to jumper the coil lead to a good ground. On cars having points type ignition systems, simply disconnect the wire that runs between the coil and the distributor at the coil terminal. On the older cars, such as MGA's and some of the Midgets and Sprites, you have both an ignition switch and a starter switch. In that case, you simply do not turn on the ignition switch and turn the engine over using only the starter switch.

Before you turn the engine over for the first test, you lay out a piece of paper, a 3"X5" index card is excellent, numbered from one to four (or one to six for an MGC), running down the side of the card. You will note the compression reading for each cylinder next to its number. Across the top, make two columns-dry and wet.

The next step is to insert the gauge into the number one spark plug hole. Then, crank the engine over for five or six revolutions while holding the throttle open. If you have a plug type gauge, have an assistant turn the engine over while you hold the gauge in place. Take your reading from the gauge, release the pressure in the gauge and move to the next cylinder until all are tested. This is known as a dry compression check. There is also a wet compression check which, depending on what readings you get, you may wish to perform.

The wet compression check is a follow up check useful to determine the condition of the piston rings. Some perform it every time they perform a compression check, others do not. I perform it when I see lower than expected readings, especially if it is only one cylinder. The common instructions are, "pour one teaspoon of oil into the cylinder and test". Frankly, I always spill oil all over when I try to pour it from a tea spoon into a spark plug hole and, then to compound the problem, always get caught trying to sneak the wife's teaspoon back into the house. Then, I learned a better way. Get one of the pump type oil cans and find out how many strokes of the plunger it takes to fill one, level teaspoon full. Then, you can insert the spout of the oil can and depress the plunger the required number of times without making a mess. Insert the oil into number one spark plug hole and repeat the compression test on that cylinder, note your reading under the "wet" column and repeat for the other cylinders. The wet compression check will, in most cases, be higher than the dry compression check. This is because the oil creates a better seal against the top of the piston rings and the cylinder bore. The LOWER the difference in readings between the dry and wet checks, the better the condition of the bores and rings.

You now have one or two sets of readings for each cylinder. What do they mean? Again, opinions vary, but we can set some guidelines which will be of use. First, cylinders with less than 100 psi compression (dry) are probably not firing efficiently and, if the compression is much below 100 psi, may not be firing at all. Secondly, most MG engines in good condition will read about 150-160 psi maximum. If you have more than this, you probably have modified, high compression pistons (assuming the standard camshaft is still being used) or the cylinder head has been milled significantly. The normal range, on an MG, is between 120 psi and 150 psi for an engine that is operating well. A newly rebuilt engine will be slightly higher than 150 psi after it is broken in and may be somewhat lower than 120 as the rings and bores are mating themselves together. The biggest concern is consistency of the readings. The closer the cylinders are to reading exactly the same the better. This means that, all other factors being equal, each cylinder is operating exactly the same when it fires. This will give the best balanced engine from a smoothness standpoint. If all the same is the best, what is the acceptable and the unacceptable?

There are a couple of schools of thought on this. The first one says that all of the cylinders should be within 10% of each other. This sounds good, but they do not tell us what their reference cylinder/reading is, nor do they tell us whether it is a plus or minus situation. If we have readings of 150-145-135-140, what is our reference? If it is the high cylinder, then number three is exactly 10% below number one and our test shows the engine being "good". If, however, number three, our lowest reading, is our standard, then number 1 is out of limits as it is more than 10% above number three. If we use number four cylinder as our base, both the number one and three cylinders are good because they are within, plus or minus, 10% of number 4. If I were to use this system, I would use the highest cylinder pressure and want all the other cylinders to be no more than 10% lower than my highest reading. There is, however, another theory, one easier to remember and use.

The second theory simply says that all of the cylinders should fall into a certain range. Roger Parker, a well know English enthusiast, wrote me that he used a range of 20 psi. Thus, all of the readings should be within 20 psi of each other. I have used this procedure for several years and find that it works well with the average road car. Some other sources feel that the range should be 10 psi. This will also work and gives the added benefit that you will be ensuring that your engine is always developing the maximum efficiency that it is capable of. But, it will also lead to more frequent engine rebuilds, often of engines which are fully capable of being tuned for good driving performance and capable of meeting emissions standards. I would use the 10 psi difference standard on race engines and use the 20 psi standard on street engines.

What about low readings? If we do a test and find 150-145-25-140, what does it mean. This is when you must perform a wet compression test to gather additional information. If the wet test gives us 160-155-25-155, we can assume that the problem with number three cylinder is not related to worn rings. This would leave us to investigate a bad valve, a hole in the piston or broken piston rings. If our dry test showed 150-145-120-125 and our wet test showed 165-155-150-155, we would assume that we had a ring related problem because of the significant increase in compression on cylinders three and four with the wet check. We might also theorize that we had excessive ring and cylinder bore wear due to an excessively rich mixture with the rear carb causing the oil to be washed away from the cylinder wall. This would cause us to examine the rear carb before re-installing it after the engine was rebuilt.

The last area which we may examine during out test is when you have symptoms of a head gasket leak. This will normally show up on a compression test when you have two low cylinders next to each other. Such a reading might be 150-80-85-145 dry and 165-85-90-160 wet. You do not have a significant increase in the dry vs. wet readings and the cylinders are adjacent. This will normally indicate a bad head gasket seal between the two cylinders and that, when compressed, part of the mixture is leaking into the adjacent cylinder.

A compression test requires relatively inexpensive equipment to perform and takes little time. The readings obtained should be noted and kept for future reference. That way, you can catch problems as they begin to develop and, often, avoid an expensive repair. It is a simple procedure which any home hobbyist can perform in an afternoon. A good time to do it is when tuning your engine as it will help ensure that you are obtaining the maximum state of tune your engine is capable of. It will also tell you when you have problems that simple tuning will not correct. Replacing a spark plug on a misfiring cylinder will not correct the problem if the root cause is low compression. Performing an annual compression check is a good way to ensure many miles of trouble free driving enjoyment.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright © 2001 by Les Bengtson.

Technical Information for Gunsmiths and for Classic Car Enthusiasts