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Compression Test

Old 03-Oct-2005, 10:29 PM
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Compression Test

Hey guys,

I'm thinking of buyign a b16a2 JDM engine. When some of you guys say have a compression test checked on teh engine, what is it exactly? Is it actually the compression ratio of the engine? Does anyone have a pic of one performed on an engine. What kind of numbers should I be looking for on a used JDM engine?

Thanks in advanced.
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Old 04-Oct-2005, 12:31 AM
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A compression test gives a basic idea of the health of an engine. It involves cranking the engine over and using a compression testing tool to measure the PSI in each cylinder.

Each engine has a different spec as to what a good result is. For a JDM b16a second gen. test results should be 200+. Rule of thumb is all cylinders should be with in 20% of each other.

Aside from the compression test, also check the oil in the engine. Ask the shop to open the valve cover and inspect the head, as well as drop the oilpan and inspect the oil pickup and crank for scoring.

This is not a b series, but its a miata engine we are testing in the pic.



Specific things also to look for on a b series is leaks around the vtec solenoid and leaks from cylinder #1 where the head mates to the block.

Be careful when buying and make sure the shop gives you a written warranty.

Hope this helps!
Eric
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Old 04-Oct-2005, 09:21 AM
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Cool thx,
Is this also a leakdown test?

I noticed you're one of the guys that everyone talks about at Teknotics. I've heard alot of good things about you guys. In fact I'm hoping to buy an engine from you guys somtime in the upcoming year. Hope to talk to you guys soon about it.

Thanks for the help so far!
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Old 04-Oct-2005, 09:23 AM
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Also another question,

How do you you know that the PSI should be 200+? Is there some calculation that is used?
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Old 04-Oct-2005, 09:42 AM
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its not so much what the number are but how consistent they are. Unless you get like 130 on all cylinders, then i would be concerned.
But a few things, make sure this is performed on a COLD engine. Also make sure that the throttle body is held wide open.
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Old 04-Oct-2005, 01:48 PM
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Originally posted by tranh2
Also another question,

How do you you know that the PSI should be 200+? Is there some calculation that is used?
Nope. If you look at the guage, it reads 0-300 PSI.

This type of test on a engine out of the car is called a "dry test" (vs. "wet test" when in the car). There is oil in the engine, however you are manually cranking the engine by putting power to the starter and turning the flywheel.

As Zeeman said, just as important as the reading is the equal result across all cylinders in determining the heatlh of the engine.
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Old 04-Oct-2005, 02:01 PM
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Compression tests should be done on a WARM engine, but cold is fine if its the only option you have, the numbers will just be off a bit in the extreme cases. Doing a leakdown test is better to do on a cold engine
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Old 04-Oct-2005, 05:14 PM
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is there a specific size tester that i need (10mm, 12mm, 14mm) or will the standard size fit?
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Old 04-Oct-2005, 06:49 PM
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In or out of the car makes no difference if it's wet or dry.

A dry test is when there is no oil put into the cylinder itself by you manually.
A wet test is when you do put a drop or 2 of oil into the cylinder to check if the compression PSI rises or stays the same.

Generally if PSI rises on a wet test there may be a bit leakage past the rings that the oil in the cylinder helped to seal.
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Old 04-Oct-2005, 07:48 PM
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Originally posted by sohcnotec
In or out of the car makes no difference if it's wet or dry.

A dry test is when there is no oil put into the cylinder itself by you manually.
A wet test is when you do put a drop or 2 of oil into the cylinder to check if the compression PSI rises or stays the same.

Generally if PSI rises on a wet test there may be a bit leakage past the rings that the oil in the cylinder helped to seal.
Shouldnt PSI decrease if air is leaking out of the cylinder past the rings? Or is it the other way around and air is leaking into the cylinder past the rings?
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Old 04-Oct-2005, 08:25 PM
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I only do a wet test when I get low readings in one or all of the cylinders.
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Old 04-Oct-2005, 09:19 PM
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Originally posted by tranh2


Shouldnt PSI decrease if air is leaking out of the cylinder past the rings? Or is it the other way around and air is leaking into the cylinder past the rings?

It will be lower if air is leaking out of the cyl past the rings. But it MAY increase when wet cuz the oil helps to seal between the rings and the cylinder, therefore less air is now bypassing the rings. No change in readings could mean a bent/burnt/damaged/misadjusted in/exh valve or even the valve seat itself.
Makes sense now?

I agree w/Starboy, you only really need to do a wet test if a cyl may be lower than the others. It helps to determine if you're losing pressure past the rings or the valves.
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