Added Oil for Compression Test
 

Hey,

just yesterday I felt like rechecking my compression to see if it has dropped any since last year. I believe I have the old starter on my 04 because all the maintenance stickers read "n3z2" and it does take a while to crank sometimes.

Used an Eqqus steel braided tester with check valves removed so that I can see the fluctuations.

Both trailing and both leading read up to just under 90psi. Since I don't know starter RPM or haven't done much calculations I don't really know how to interpret this. Though battery is good (Duralast Gold got in 08).

Here's the part I'm wondering about. After doing these tests, I added about one ounce of engine oil 5-30 into one of the trailing holes. Repeated the test, and results jumped to 105-110. One more time and it went back down to just below 90. This is all psi btw.

What does this mean to you? To me it makes me think that a little bit of oil has filled up a fairly large gap in the compression. Is this difference as large as it I think it is?

It makes sense that the compression would go up significantly, but I think where you're mistaken is in thinking of it as "a little bit of oil". It's a vast amount compared to what would normally be supplied during engine operation. As you'll probably notice when you start the engine again and it behaves like a crop-dusting plane. :)

It's been shown that adding oil via premix can raise horsepower slightly, and that would be due to improved compression. But the amount of oil needed to achieve that is quite rich, and even then is far, far less than that one ounce you squirted in.

Adding oil during a compression test is usually a method of telling whether bad compression in a conventional engine is due to rings or valves. If adding oil raises compression, then the rings were bad. If it doesn't, then it's valves. (Or possibly head gasket.)

I don't think it has any particular meaning in an engine with good compression.

Ken

Adding oil will give you a false higher reading. And for RPM, you should get a proper rotary engine compression tester. Like mine. :) The group buy is still going on!
-John

I see. Thanks much. But I do want to know if this really is a large gap. Anyone good with numbers and measurements?

I know that apex seals are supposed to remain in contact with the housings. What about the corner / side seals. Would these latter seals cause the difference of compression with no/1oz of oil added?

yeah it's gonna go up wthen adding oil it makes a better seal i'd just go with ur first readings!

Right. So at this point, lets say I have the weakest 04 starter in. Does 90psi seem right?

It is not that hard to work out how fast the engine is turning over .
Take a video of your compression tester as you crank over the engine .
Now with a stop watch playback the video at a suitable point hit start on the stopwatch and start counting the pulses.
Hit stop when you get up to 20-30 pulses .
Now from the recorded time and your counted pulses you can calculate RPM .
Every pulse is 1 rpm .
Takes a few goes to get the count right and your brain doing two things at once but I actually managed it .
Now compare against the rpm vs psi chart

The faster you crank, theoretically you should get higher compression numbers. *Just making sure, that's correct, right?* So if you have the slower rpm spins, you may be looking at a lower number than what your compression truly is.

Brettus is a genious!!!!!!!

Brettus. Awesome man thank you. I've read up on almost all the compression threads that's why I didn't want to annoy people with stupid questions, but thanks a lot.

Also why I said, lets say I have the lowest spinning 04 starter. Wanted to give my motor the benefit of doubt because it very well may be the starter or batter just being weak. Maybe I should look for a local 04rx8 guy and see if he'd be willing to devote some time to side by side comparison. I did this with my friend but he has the 06 model.