Think of it as a chest X-ray for your engine
At some point in the restoration process, the engine will come into question—to rebuild or not to rebuild. An engine rebuild is a lot of work, and to do it right costs a lot of money. You certainly don't want to do it if the engine is internally in good condition. That $3,000-$5,000 could go for a new paint job, new chrome or even some rare number-matching parts you may be missing. Answering the usual diagnostic questions such as "How many miles are on it? How much oil does it use? Is it down on power—" will help with the decision, but a simple 30-minute compression test can answer a lot of questions if you know what to look for.
The key to any internal combustion engine is cylinder pressure. Power and efficiency depend on strong compression and the ability to contain the combustion pressure in the cylinder. Ideally, this intense pressure from the burning air and fuel will not escape past the piston rings or valves, exerting maximum force down on the pistons, rotating the crankshaft. All factory-assembled engines have a cranking compression specification that you can test your engine against to determine what condition its internal parts such as piston rings, valves and head gaskets are in. You can find these cranking compression specifications listed in repair manuals and sometimes in the owner's manual. If your engine has been modified with a different camshaft or cylinder heads, the stock specs are meaningless. Still, a compression test will provide a good indication of the condition of the engine by comparing one cylinder to another.
The test is simple. A compression test gauge (similar in concept to a tire pressure gauge) is screwed into the sparkplug hole. The engine is turned over several times (but not started) with the starter, and the gauge records the pressure in p.s.i. generated by that cylinder. All cylinders are checked, then the numbers are compared to the factory specification and each other. The general rule is that the lowest cylinder should test a minimum of 80 percent of the best cylinder. However, if the test numbers are consistently 10 percent down from factory spec, it's time to start setting aside the budget for a rebuild.
An engine in new condition typically should indicate a compression number of 200 p.s.i. When testing our engine, all cylinders check out at 165 p.s.i. plus or minus 10 p.s.i. What you can deduct from this is that the engine is about at 80 percent of its original capability with pretty even wear in each cylinder. Time for a rebuild! If only one or two cylinders had low readings and all of the others were up around 200, then you could deduce that those cylinders had unique problems such as burned valves or a camshaft going flat. If the sparkplugs were oil fouled you could pin the problem possibly on broken rings. Even more telling—if the two low reading cylinders are next to each other it could be a blown head gasket between the two cylinders.
Although a compression test gauge is not a definitive trouble-shooting tool by itself, it will definitely tell you if an engine is in sound condition or not, and in the hands of an experienced mechanic it will quickly lead you to the source of the trouble. If you are about to purchase a used vehicle and have doubts about the condition of the engine, it's a wise move to purchase a $30 compression gauge at a local parts store and do the test.
The first step is to bring the engine up to operating temperature then remove all sparkplugs so that the starter can spin the engine over at a good rate of speed. Keep the spark plugs in order so that you can inspect them if you get a low cylinder reading.
Next, remove the coil wire out of the distributor. Some electronic ignitions may require disconnecting to prevent damage from cranking the engine without the spark plugs connected. Check the service manual for your vehicle to see if that's the case.
Lock the throttle in the wide-open position. This will allow the engine to suck in enough air to provide an accurate reading on the compression gauge. We used a piece of coat hanger for the job.
A compression gauge like this can be purchased for about $30 in most auto parts stores or rented for a couple of bucks a day. The hose quick-disconnect feature makes it easier to screw into the spark plug hole.
Screw the hose into a cylinder making sure that it is firmly hand tightened so that the O-ring will seal in the cylinder pressure.
Attach the gauge to the hose then have someone crank over the engine while you watch the gauge.
The needle will continue to climb with each successive compression stroke, then stop climbing. Stop cranking the engine over and write down this reading. Release the gauge pressure by pushing the relief valve and move on to the next cylinder.
If a cylinder has a low reading some mechanics squirt a little engine oil into the cylinder and retest. If the piston rings are bad the compression will increase 10 or 15 p.s.i. If the culprit is burned valves it will not.
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