car is down for this winter and i all ready started on projects
car is running fine no problems

i was thinking about having the engine rebuilt as i was bit abusive this year

clean up the internals and change all the seals

so i did the compresion test i think its 119-120pis F&R just wanna make sure i did this right

here are the videos

http://www.youtube.com/watch?v=LBP2697bo6o

http://www.youtube.com/watch?v=k-JG3apCzzg

 

from what i understand anything over 100psi is good and 120psi would be really good

now i did the compression test with UIM off

 

Now convert to 250rpm cranking speed and you'll have your result. Oh that's right you need a rotary compression tester to convert.

 

ok so what would be good rotary compression tester....most tests i have seen where done by regular tester like i used

 

TR-01 Rotary Engine Compression Tester -- but its sold out

 

your compression looks fine man

 

You don't need a rotary specific gauge. If you have a regular compression gauge, what you need to do is hold the pressure release button on the gauge while the needle bounces and you just read it at it's highest point during each compression. Also make sure that whoever is cranking the engine that they hold it at WOT, otherwise your readings will be low.

 

so what number should i see when i hold the pressure release button on each compression

 

The regular compression numbers. Just search for the DIY's or look in the FAQ's there's plenty of guides that show you how step-by-step

 

An explanation of compression numbers should be on the forums somewhere. 120 psi is excellent for a used motor.

 

You need to hold the check valve so it releases the air to do a proper compression test. It needs to pulse or the reading is useless.

thewird

 

so you can use a regular psi gauge to test the compression?

 

I saw the needle jump up to 90psi when you had the release button pushed.

 

yea it jumped to 90psi three times

but it's not a rotary specific tester so numbers will come out low

just to compare

i did a compression test with same tester on RX8 with 40k and also got 90psi on all faces f&R

i am saving up for rotary compression tester to get actual numbers

 

When you do a compression test with a regular piston tester and you hold the pressure release button, it's only going to bounce 30-40 psi because it's bouncing per each face of the rotor. At the end you essentially add up the 30-40 psi bounces for all 3 faces which equals the total compression.

The reason that in the video it looks like it's bouncing to 90, is because the needle was all the way up to 120 when he released the pressure as the engine kept cranking. If you notice, the needle doesn't bounce from 0-90, it bounces between 60-90, which is 30psi per face.

The reason piston engine testers are not incredibly accurate, is because #1, it's difficult to tell exactly what psi each face is bouncing up to when you hold the pressure release button, and #2, if you don't hold the pressure release button, you're allowing extra pressure to build up in the gauge because it's almost impossible to immediately stop the engine after 3 turns, so it would give you a higher false reading that what you actually have.

From the videos it seems like the compression is ok to good, but not great, each bounce seems to be over 30 which should net you over 100 or close to it for all 3. A really good engine should be bouncing close to 40 if I'm correct.

 

thanks for the info that helps...

i have seen what i did wrong

but i got confused after i did compression test on the rx8 with that same tester

i hold the pressure release button from the start and and it was bounced from
0-90 and continued bouncing from 85-90 i think this was do to higher rpm

 

I have never read anything about adding the numbers of each rotor face to get a compression reading. I've tested a with a normal compression tester and gotten 105-110 on each pulse while holding the button. I don't think the method of adding the 3 numbers together is correct.

 

what brand tester did you use

 

I don't remember the brand but it was the nicest one at Advance auto parts. It was probably less than $50.

 

There's nothing wrong with using a piston tester and it can give you accurate results for a MAXIMUM pressure generated by the three rotor faces.

Where it lacks is in differentiating between the three faces. However, when you hold the release in, you can tell if one face is generating more pressure than another in a relative sense.

The reading when holding the release in will vary from gauge to gauge and between methods of holding in the button because of how fast the air can flow out in each case.

I can see how you might think this, but it is not true and the number of turns has nothing to do with it. Here is why:

Say the actual compression would be 115 117 120. The piston tester reads 120 after a few seconds cranking. You could crank all day and the pressure would stay at 120 because that is the maximum pressure that the best rotor face can generate. There is 120 psi on the other side of the check valve at the gauge.

Just because it generates that pressure over and over, does not raise the pressure at the gauge. Only if the pressure in the combustion chamber somehow became greater than 120 psi would more air be forced into the gauge and cause the pressure to increase.

Maybe you're thinking of it like a pump, which it kind of is... but if a pump can generate up to 2 psi and it fills a balloon up to 2 psi, no more air can be forced into the balloon because the forces on each side of the balloon's entrance are balanced.

The real problem with a piston tester is the fact that you cannot get an accurate individual face reading. This is because when you attach the gauge to the plug hole and have no check valve in the end, you are adding the gauge tube to your system. This increases the total volume and changes your compression ratio. This is why rotary comp testers have the pressure sensor right at the plug hole.

All that aside, if one of your faces is trashed chances are that you won't have one face making perfect compression so a maximum reading has always suited me fine.

 

I don't think it works that way. Regardless the OP isn't doing this compression test correctly. I've personally never done it and held the pressure relief button down. He is suppose to remove the schrader valve to get the true face readings. The schrader valve is a restriction that's why it's removed. You then reinstall the schrader valve to get an accumulative reading (no pressure relief). All experimenting I've done with compression testing, I know for a fact that a 2 rotor wont fire up with pressures in the 60's.


To the OP with a piston tester, you want to see individual bounces of 85 psi or higher to consider that engine some what healthy with a piston tester. I've personally never seen numbers higher than 100psi with a piston tester.


Anyone checking the bounces of the rotor while holding the pressure relief is doing the rotary compression test incorrectly.