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mr2greasemonkey · 05-18-2007, 01:51 AM

Compression is the result of the cylinder's ability to seal the chamber and compress the mixture. A good quality compression test is a necessity when you are doing a compression test. When purchaseing any test equipement that relies on a gauge I suggest one with a big face so you can see small changes.

When performing a compression test make sure that the ignition and fuel system are defeated. I like to use a remote starter when the starter is easily accessed. That way the key does not need to be on and the electronics are not powered. If you plan on just cranking the car you can pull the relays or main fuses.

Testing is best done with the engine warm, near operating temp, to get the best results. General rule of thumb is 5 complete cranks. You should see 150-175 on most engines. In peformance application the range will change. Around 130 is the breaking point for decent compression. Once it drops below that it starts affecting the the cylinder's ability to produce power without some sort of induction.

A wet test is where a little oil is added to the cylinder to see if the piston rings are adequately sealing the chamber. You can do a regular test and then a wet test to compare the difference. I find a little plastic syringe with a vacuum hose on the end of it works pretty good. You can see how much oil you are adding and it doesnt make a mess.

The first compression stroke on the guage should be a quick solid jump followed by shorter jumps until you stop cranking. You may notice a little drop when the guage stops, especially with a big gauge, but what you are looking for is peak numbers. Rule of thumb is 15 percent variation between all cylinders.

Keep in mind compression happens rather quickly. You are only checking to see if there is enough compression for the engine to run mechanically. A leak down test is the best way to determine the condition of the parts exposed in the chamber. If you have access to a borescope, handy little self powered inspection camera, you can take a look inside the chamber.

more to come....

05-18-2007, 01:51 AM	#5 (permalink)
mr2greasemonkey Once again....yups Join Date: Sep 2005 Location: Portland, OR Posts: 1,440 Thanks: 4 Thanked 54 Times in 39 Posts iTrader Rating: (0/0% )	Compression Compression is the result of the cylinder's ability to seal the chamber and compress the mixture. A good quality compression test is a necessity when you are doing a compression test. When purchaseing any test equipement that relies on a gauge I suggest one with a big face so you can see small changes. When performing a compression test make sure that the ignition and fuel system are defeated. I like to use a remote starter when the starter is easily accessed. That way the key does not need to be on and the electronics are not powered. If you plan on just cranking the car you can pull the relays or main fuses. Testing is best done with the engine warm, near operating temp, to get the best results. General rule of thumb is 5 complete cranks. You should see 150-175 on most engines. In peformance application the range will change. Around 130 is the breaking point for decent compression. Once it drops below that it starts affecting the the cylinder's ability to produce power without some sort of induction. A wet test is where a little oil is added to the cylinder to see if the piston rings are adequately sealing the chamber. You can do a regular test and then a wet test to compare the difference. I find a little plastic syringe with a vacuum hose on the end of it works pretty good. You can see how much oil you are adding and it doesnt make a mess. The first compression stroke on the guage should be a quick solid jump followed by shorter jumps until you stop cranking. You may notice a little drop when the guage stops, especially with a big gauge, but what you are looking for is peak numbers. Rule of thumb is 15 percent variation between all cylinders. Keep in mind compression happens rather quickly. You are only checking to see if there is enough compression for the engine to run mechanically. A leak down test is the best way to determine the condition of the parts exposed in the chamber. If you have access to a borescope, handy little self powered inspection camera, you can take a look inside the chamber. more to come....