Question concerning compression
 

Alright, this is quite a common topic, I know, I've searched. I just haven't gotten the exact answer(s) I need.

So a while back, I bought a normal piston compression tester, which looks like this...

[IMG]http://img463.imageshack.us/img463/593/nnn0140ug.th.jpg[/IMG]

Alright cool. Lets test the compression. So I have read that you don't want the total PSI for each face of the rotor. I've read to hold the valve on the side in, to let the needle bounce for each face. (I also read later to just remove it, but I'm not sure if it's the right valve; I read there are sometimes two valves.)

Okay. The compression I got was 35, 35, 35 for the rear rotor. Front rotor, I got between 33-35 for all three faces. All bounces of the needle were even with each other for each rotor. I read somewhere a while back that those were good numbers. Then I also read that I want numbers like 110-110-110 for each rotor. :confused: After reading that the numbers I got show the engine has low compression, I was a little confused because I read different numbers I should get when I test it.

I tested it correctly, unplugged crank- angle sensor, EGI fuse, put the tester in the top spark plug hole( I forget which one is L and which one is T) and floored the accelerator while cranking. This is all following the instructions listed in the Haynes manual.

Anyway, I read that someone else had got similar numbers as I did, all around 35. Then I read more and found out that the low numbers could have been because they didn't completely remove the valve on the side, and instead just held it in, as I did.

I am going to test the compression one more time, but I have a question about the tester itself. Supposedly there could be two valves, but mine only has one shown in the picture above. Is that the valve that I have read about to remove completely? Or is this not a suitable tester, though I'm pretty sure it would be? Well, if it is, how do I remove it? It doesn't just unscrew.

[IMG]http://img268.imageshack.us/img268/1...n0150mx.th.jpg[/IMG]

[IMG]http://img277.imageshack.us/img277/7...n0161xq.th.jpg[/IMG]


Sorry for the low quality images, they're from my phone and such a long post for a simple thing to do, I just want to make sure I get this straight so I can go from there.

Thanks,
Jordan

You want between 30-35 on each face. Sounds like your motor is healthy. If you are using a tester that has no check valve, It will give you 3 readings for the whole rotor instead of the true compression of that face.

Hrrmmm.... can anyone explain why I have been reading that I need around 110 on each face or so? I have also been told this too, and that I have tested it incorrectly. Honestly, I think the engine is good, the "psssh's" it makes while testing are strong, even sounds. But hey, I could be wrong.

i think you want 70 +

Please don't post such mis-information. My tester has the Schrader valve removed and I pull almopst 130 PSI per rotor face. If you have 30-35 per face than your car is not in running order.

The compression on these things is measured by each face (3 faces per rotor) or as a total compression rating of each rotor. So if you have a piston tester with no release valve than it will crank and keep showing you the total rotor. With a release valve you will see it bounce 3 times quickly showing a reading for each face.

SO
With the proper tester you will get 30+ 3 times each. Close the valve and you get a total. If your tester has no valve it will show you 3 maybe slightly different numbers but they would all be measuring in fact total compression on that rotor. The limit for a healthy motor is 85-90 on each rotor with no rotor or face significantly lower than the others. Rotaries are not high compression motors. If your seeing 3 bounces of 30+ OR a reading of 90 or more you are "healthy" Sorry if this doesnt help. If you were told you should have 110 on each face than that is not being tested the correct way. You have tested it correctly yourself and have good compression.

Your not testing it right, and you are yourself posting mis-information. Though you have good compression for the whole rotor, you dont have any idea what each face looks like.

Thanks. I hope your right. But there are still many people out there that think it should be around 35 and many think it's supposed to be around 110.

a friend of mine got one of those testers when we went to test his car and the part that you want to remove is at the tip where you screw it into the hole for you spark plug. Both my car NA and his TII were in the 90psi range all round 35 would be really low, unless I'm missing out on something.

Ummm NO. I got those readings per rotor face. if I left the valve in there I'd have an inaccurate reading. Maybe you should do a bit of research. You need 3 even bounces of 70 PSI or more. If you leave the Schrader valve in, you will get skewed results.

Yeah, I'm not quite sure myself, either. Maybe I could try a different tester. Though reading several other people having gotten similar numbers, could it possibly be a coincidence the numbers are the same or that my testing is wrong?

compression test
 

By holding the pressure release in you're venting a good portion of each compression "stroke". As the previous post remarked, there is a valve on the spark plug hole insert that needs to be removed to give a more accurate reading.

I thought the valve just lets the pressure off so if you leave it in you'll get the max pressure (the needle doesn't bounce) instead of being able to see the pressure on all 3 faces with the needle bouncing.

I don't think there is one on mine... and it's a normal piston tester.

so what...nobody know the correct way to check compression with a normal tester?

someone should just contact a Rotary "Pro" and see what they have to say...

LOL, guys this has been brought up soooo many times. The compression per rotor is between 80 and 120 if your higher than that good for you... The compression per face is only 30-40. You need a tester with a valve that is constantly open to get a TRUE per face reading.

If you can only get it for the rotor, than all you can do is make sure that the reading doesnt change much relative to the face, this would indicate a blown seal on one side.

If your saying that you have 110+ per face than you would have 330+ per rotor, thats ludicrous, youd need alchohol to fuel that. Rotarys are NOT HIGH COMPRESSION. Sorry i know its kinda confusing because there is the good testing method and the good enough testing method, but trust me on this one. If your seeing even bounces of 30+ and a valve closed (retaining pressure) reading of 90+ you are good.

I hate to bust your bubble -You're fooling yourself in saying that you're not providing misinformation - , but rotary compression is not 30psi per face. Stock rotor faces have 9.7:1 ratio in the na s5 9.7*14.7=142psi completely perfect sealed compression.

Multiplying by face makes no sense and is like saying that total compression on a piston engine is the combination of what every piston has. The 6 rotor faces are why many companies/people consider us as 6 cylinders.

The issue here is the testing procedure, the side valve does lose pressure and typically results in number around 30psi as I found in my running car and a friend's car. 30psi air + fuel will not combust very well. Removing the top valve (or plug in side) will result in full pressure readings.

Proper numbers per face are 100-120psi, tested with the top (or bottom) valve removed and not the side.

Its PER face. You should see 3 bounces of about a 70 or higher. Even 70 per face is kinda low. Hence when you see readings saying 0-80-80, that means that you have lost compression on that one face. I would say the best way to know what your seeing is, just take your finger off the tester and let the gauge read. You wont get the 3 bounces, but you should get one reading telling if its above 35. If your that low, look into a rebuild kit.

Ok, i was wrong in saying that the high value is the sum of the faces, your right sar that makes absolutely no sense. The simple fact is the guys compression is fine though. Heres the best page ive found on compression testing procedure, and the method i have always used. http://www.rotaryresurrection.com/ro...lder/comp.html

Sorry for adding to confusion :wallbash:

Okay so 35 sounds low. But say I do remove that valve... it will pretty much show good compression won't it, because I got like 35 on every face for both rotors. And when an engine needs a rebuild, or is blown, usually something goes out like a side seal, apex seal, etc, so if I do have low compression why would all faces show nearly the same number?

Next step... different compression tester I think?

the compression ratio of your rotors has nothing to do with actuall compression. High or low compression turbo motors all have the same compression (120 on a perfect motor).

your a little mistaken
reason why he is prolly getting 30-35psi is cause he was holding the check valve in releasing pressure. hence the REALLY low numbers 30-35 is a dead motor.

if the compression checker has no check valve then what you will see is compression for each face. the check valve holds the pressure it so you see the max pressure and that is it

if you don't have the release valve chacnes are you don't have the valve that holds pressure in. if you don't have the valve that holds pressure in your going to read a lot higher numbers

how about this I was getting 110 in bounces of three. was not holding a check valve down or anything like that.

I think your doing it wrong buddy

wrong buddy your the one missinformation on your side. I get three BOUNCES of 110 psi

incorrect what you need is a tester that HAS NO VALVE. when you ahve the valve and push the button your venting pressure. so therefor your readings are not accurate. you can get readings when you don't have a valve to hold the pressure in there. have you ever seen a true rotary compression tester? there isn't a valve on there to hold down. and they read usually 90-100+psi depending on motor.



no 110+ per face doesn't mean 330+ per rotor.

they don't add up. that's like saying if I have 110psi on one cylinder that means my motor has 440psi on the whole mtor if it is a 4 cylinder and 880psi for the motor if it is an 8 cylinder. just doesn't work that way.

if your valve is still in place when doing the test your doing it wrong. at that point your releasing all your testing pressure. the amount that gets released to atmosphere can vary depending on a fwe things unrelated to the health of the motor. you need to remove the valve all the way off the tester. at that point you can test true compression

then somebody needs to fix his page cause he is confusing and missleading people like you who are t hen trying to mislead other people

^^WTF 5 posts to say the same thing!

Just let it rest for christs sake, someone already said there are different numbers you can get depending on tester and procedure. I already said i was wrong in stating that 30-35 psi was real compression per face. Regardless 30-35 psi per face has been my results on 2 different healthy rx7s. And tell me this... How is the amount of pressure blowing off going to change on the same tester, why would their be any difference its plugged into the same spark plug hole? All your looking for is even bounces and a compression of over 90 for each rotor.

synesthete please do not post in this thread anymore. Your answers are clearly wrong and you don't understand the subject matter.

To clear things up.

When compression checking a motor, the motor should be warm, and the throttle should be fully held down while cranking at 500 RPM.

You check each rotor face. In an ideal world you would have a compression check like this:
120-120-120 or higher on each face.

Each face would show a full compression.

If you have a compression check of any face under 80 the motor needs rebuilding (although myself I consider anything under 90 psi as needing rebuilding as the parts are near failing, but probably have not worn out anything too expensive at that point.

a compression check of 90-30-90 would show a blown side seal

A compression check of 30-30-90 would show a blown apex seal

So your telling me that both my rx7s are blown on all 6 faces, even though they run great, and the posters motor is blown even though he believes it is healthy..... And rotary resurection lies about the numbers you will see. I may be confused on what im actually measuring but at least im not screaming rebuild to someone with a healthy motor.... I concede this argument, no more posts from me.

The key is you are only doing half the test if you are following Kevins write up:

You were holding the valve on an piston compression gauge... not letting the valve out.

So again, it points to you didn't know what you were doing

So to further clarify the simple subject:

Holding the valve in you are looking at the differences between rotor faces, letting the valve out you are looking at actual compression. Holding the valve in and seeing 30 PSI is nothing, and only half the test. That only shows the difference between rotor faces and not the actual compression

Actually i've been saying to do both. And with both compression testers i've used if you dont hold the valve open you get 1 reading only, little or no bounce at all.

I think i lost my credibility when i said the rotors compression was total of the faces, I can understand that, sorry for my ignorance, but as far as i know that was only mis-information i posted. But i apologize for the confusion.

Of course this is why I say use a rotary compression checker or build your own.

Piston checkers are easily mis-understood when doing a check on a rotary motor.

Thats because you got:pokeowned

Okay, so the numbers I got show that I DO NOT have a blown motor, right? They do show that the compression could be low, or it could not be low, because I need to test it without holding the valve in? Am I getting this right? If so, how come when I don't hold the valve in, I just get like 110 and not like 110-110-110? Is that because I don't have the other valve on my tester to let the needle fall back down or because technically, it is for a piston motor?

So what I need to see after knowing that it's not blown, is like 110-115 without holding the valve in? And that's it?.... Good motor?

sorry I will make what I was saying a little more clear. different testers depending on how big their bleeder valve is can vary the amount of the reading.
so you tested a good motor on 35psi doesn't mean that is the best way to do the test.
you want even bounces your right. and your right you want them over 90. but if you are holding the valve open that is releasing pressure. if the pressure gets released it never makes it to the tester. therefor you really don't know what pressure your motor is really making cause there is so much pressure being bled out to the open air that doesn't even get tested

and if anything icemark wouldn't a tester without the valve in there in the first place be the best method?

that valve is just a one way valve. once the motor pushes air into the tester the one way valve holds it in there and doesn't release the pressure till you push the button. most of the engines out there are piston engines and you really are testing only one cylinder, one piston and so forth. this way with that valve in there you take out your spark plug screw in the tester go into your car and crank it then come out and read what it says. makes life easy if you want to do something like this on your own. if that valve wasn't there you would need someone watching the meter as you crank so it would require two people. remember we in america are lazy :D. now since it is a one way valve though and holds that pressure in the tester itself it can only read peaks and that is all. but when you have someone crank on the motor and you hold the valve in not all your pressure is going to the tester. most of it just squirts out the valve and you only read 30 psi. that 30psi means nothing for the health of the motor. only thing it might SEMI be usefull for is making sure yuo have even pulses but even that I would be leary of being that most the pressure just gets released to atmosphere bypassing the tester. best test would be to remove the valve all together and make sure you have even pulses then. this way you should read much higher numbers and be more accurate as the full pressure should be pushing againt the tester. also since the valve isn't in there the pressure will get released again as the rotor moves away from the spark plug on what would be the power cycle of the engine.


icemark I do have a question for you though. as far as the tester goes since I haven't had a chance to really try this yet. when using a tester for a piston motor with the check valve removed vs with it in there how much of a difference should peak pressure be? I know at least with a piston motor using the valve it takes a few cranks to actually get a peak reading. which one would be best to go by for the peak?

Holy crap, we got people that don't understand the concept of "pressure"? :eek:

http://fc3spro.com/TECH/FAQ/compress.html

Sorry for the late reply, but I just got around to reading the PM from the original poster... :)


-Ted

Yes, that is what I use.

Also as Ted reminded me, (and I didn't post before) compression should be checked in the trailing plug holes. Yep the "T"op spark plug holes.