Is my compression acceptable?

[BrianL1987]

I don't know much about the history of my motor except, that the numbers match & seems to run really well. A quick check of my compression with a cheap (auto parts store tester) revealed: 130 x all 6 cyls. (Actually 1 cyl was about 128). The odometer reads 109,k I'm assuming the motor was rebuilt at some point but don't really know. Are these compression readings acceptable for a stock motor?

[ajmcforester]

According to the 72 factory manual you should be around 171-185 PSI at 300-400 RPM with the SU-carb setup; so I say you are very low. A rule some old Z guys gave me "plan to replace the rings every 120,000 if you change the oil and do all the proper maintenance. You are more than likely won't need to re-hone the block." I was also told that compression tests under 100PSI means re-honing the block (as a rule of thumb). Your at 109, and have lost 25-30% of your compression (assuming the compression numbers are the same for a 73)so I think it might be time for some engine work. Please check to see what a 73 compression should be first I don't have a microfiche reader at home so I can't check the 73 manual, and the library is closed now.

[d240zx2]

Readings can be misleading if you don't perform the test properly. What procedure did you follow when doing the test?

[ajmcforester]

Readings can be misleading if you don't perform the test properly. What procedure did you follow when doing the test?

Great point if you didn't follow the manual the nubers I gave you might not work as a compairison.

Also tell us more about the engine, any mods?

Did you do a leak down yet?

Any oil loss, smoke at the tail pipe, gas smell from the oil?

What weight oil?

Edited April 18, 2010 by ajmcforester
Another point

[BrianL1987]

I'm going to feel pretty stupid if I performed the test wrong.

The method I used was: Remove 1 spark plug, thread in the compression gauge hose. I then turned over the motor about (4 sec.) *noted the result. Then I repeated these steps for remaining cylinders.

The is no noticeable oil loss or, smell of gas in oil. Also, no noticeable smoke from tailpipe.

I have been running 20/50 oil. The carbs are original SU type. Motor completely stock.

I dont know what a "leak-down" is so, I cant answer that one. If my numbers are truely low, maybe I can get away with having top-end re-done.

[ajmcforester]

Great information, my engine tested good on all tests and I run 10W-40 with no oil loss. However my engine has indications that it has new bearings, and rings, I know the cam has new bearings. I have used on other cars heavier oil when the engine started to louse oil, and isn't a bad thing the since the tolerances in the engine change over time allowing for heavier oils. Have you tried lighter oils and if so did you have any oil loss?

Your test is close to how they state to do it. Hear is how it is stated to do it in the 72 factory manual.

With the engine warm, all spark plugs removed and throttle and choke open.(I don't think you need the choke open with SU carbs the text cover 3 different setups) No cylinder should be less than 80% of the highest cylinder. Excessive variation between cylinders, accompanied by low speed missing of the cylinders or cylinders which are low, usually indicates a valve not properly seating or a broken piston ring. Low pressure, even though uniform, may indicate worn rings. This may be accompanied by excessive oil consumption.

The only thing different was they pull all the plugs which would allow the engine to spine more freely. Note they state even pressure like you are getting indicates worn rings. Also it may(it don't mean you will) have excessive oil consumption, which you might with a 10 weight oil. I might try pull all the plugs and do one or two cylinders to see what number you get, since they all tested about the same the first time.

Lastly a leak-down test is a pressure test were you hook up a pressure gauge and see how much pressure is lost over time. This is how I was told to do it with a pressure gauge connected the the spark plug hole at top dead center or close to it to make sure the valves are completely closed, bring the pressure in the cylinder to 100 psi and see how much is lost over 1 hour and that is your % loss. This is not in the Nissan manual, to give you some reference numbers mines tested at 94-97%, my first Z was around 90% with the same gauge with about 40,000 miles of use, both engines were in top shape. On a Ford 302 and Chevy 350 80-85% is considered the threshold for a street engine. This is just another test to see how the top end is, a leaky valve or worn rings will cause leaks. Also the test has about a 5% variance depending on engine temperature changes, air temperature change and how long the hose and brand of gauge. What I look for is consistency, and number above 80%.

[Arne]

I agree. Pull all six plugs, not just one at a time. And holding the throttle wide-open is also important, the numbers will be considerably lower with the throttles closed. I'd re-test, and I suspect your numbers will be higher next time.

[BrianL1987]

Thanks for your responses. I'll try again tomorrow with all 6 plus removed and get back to you. (Arne) I'm not sure I understand holding the throttle open since the motor is just turning over & not actually running.

[Arne]

If the throttle butterflys are closed, they impede the flow of air into the chambers, thereby giving lower compression numbers. Trust me on this, hold or block the throttle wide-open while testing. For the most accurate numbers, I've even known people to remove the SU pistons during the test as well. You want the engine to be able to suck in all the air it can take.

[BrianL1987]

AAAhhhh. I got-it now. Thanks

[ajmcforester]

Arne is right you need to give the engine the least amount intake resistance as reasonably possible. If you test low again try taking the top of the carb off and take out the piston and see if that helps. You need the throttle fully open otherwise your numbers will always be low. Make sure the battery is good and fully charged so you get the proper RPM

[tlorber]

Also, I'm assuming that the "engine warm" requirement has to do with getting oil in the cylinders (a "wet" condition). This will yield a different result from a dry one, where the oil pump has not gotten oil to the rings to help seal. I've heard of people squirting light oil into the plug holes to simulate this prior to the compression test.