Compression Test

Can anyone enlighten me on what I am looking for ie. what makes a good compression test result??
what should the result be for an inboard V8???

You want similar numbers across all the cylinders(within 5% or so). I don't know what the acceptable limit is on your particular motor, but I'd expect something in the neighborhood of 170psi from each cylinder.

-Philip

turn the motor over about 5 cranks and it should hold 50-55 psi for a few seconds.

50-55? I hope those are dry numbers.


Don't forget to do a leak down test too!

Every gauge will read different. Depending on the compression ratio and gauge used you should have between 110-160 psi. The allowable difference is generally 10%. If you can watch the gauge, crank the engine until it stops increasing pressure. Should take about 8 revolutions but may take more or less.

A leak down test will tell quite a bit, but generally unecessary and very time consuming. A quicker way is to graph a vacuum transducer, but few have the equipment or knowledge of how to do that.

a leak down test is the only way to tell where you are losing compression and why. Time consuming? Unnecessary?

I agree - leak down tests are bulletproof and not really all that much more work than a compression test. You can also tell a lot about the condition of the valve train with a vacuum gage.

How many times have you seen a compression test turn up good yet the CLT test shows a failure? There are times yes; they are far and few between, but the engine will almost always have a slight miss that can easily be detected, especially with burn time. Some will do a running compression test even. Typically most start with a compression test then run a CLT on a specific cylinder to find the exact problem if there is one. I would guess less than half the auto techs out there even own a CLT checker. They are twice the price of a compression tester. I would also venture to say a CLT test takes 4 times as long as a compression test. Remember each cylinder has to be right on TDC or the motor will turn over and all but the one spark plug should be installed.

I guess we both are assuming and should be asking the question of why run a compression test? Is this a used boat being purchased or a boat that is running poorly?

A vacuum guage can tell a little, a transducer and a labscope triggered off of number one can tell you which cylinder and even start to narrow it down to an intake or exhaust problem. The technology has been around for a long time, but few know how to utilize it, let alone have the equipment to make their job easier.

Used boat being purchased

To properly test compression remove all spark bolts, disable ignition system and crank engine with proper batt. voltage and throttle wide open. 10% difference in holes is max allowed. leak down test will let you pinpoint problem, after cylinder is at top dead center fill with 100 psi. and listen at intake,exhaust and oil fill or breather

Hey Clint,
Is that compression test method wet or dry?

Always start dry, then if you want to check rings then redo the test wet. Compression will jump up some wet, but if it is rings there will be a huge increase in compression.

Clint is right about holding the throttle WFO, I just use a throttle stick and often times a remote starter if it is convienent.

You can ground the coil wire if it has one (Spark testers work well), and often WFO is clear flood mode so it cuts fuel.

Here's a word to the wise, make sure the impeller is also checked so you don't end up with an overheated motor and a cracked cylinder wall on your motor-then the compression test won't even matter! J/K-take it to a Mech, pay the service fee to have it checked out and they will know what the compression should be. I paid $40 to have my boat checked before I bought it, well worth it, just make sure you are specific on what you want checked so you don't end up in the situation I am in now...

Great advice in this thread.

Peter - I see now what you're saying on the compression vs leakdown tests. If the compression test shows a very healthy engine, I'd be pretty surprised if a subsequent leakdown would find anything. I also agree on doing the dry test first.