Mann

I have been working on Rotaries for about 7 years now and have only needed to build about 4 motors. I wouldn't call myself an expert but I do know alot about, and love these motors.
However, It seems that in the rotary Community there is a big misuse of compression test and the NEED for them. I see all these post where people are asking where can I get a compression test, my compression is 120, how much compression should I have after a month, etc, etc. Frankly, I'm sick of hearing about compression! Compression is as important in a Rotary as it is in a piston motor. You either have it or you don't. You don't hear guys with pistons talking about how much compression they had on the #1 piston at the drag strip. So why is the first thing some1 wants to ask me is how much compression do you have? Who cares?
I am not trying to take away the fact that compression makes a difference, just that it is not so important. A motor that has 105 compared to 115 is not going to run as good, but the average driver couldn't tell anyway.
I have never done a compression test on any of the motors I have rebuilt, never had to! They started - they idled - they reved - they worked, that was it. Breaking-in a engine is the most important factor when it comes to Compression, gradually the compression builds and good things happen( it starts easier, pulls harder).
By the way, Mazdatrix, Racingbeat, and all the mechanics that I know hardly ever uses a compression tester! Mazdatrix takes the trailing plug out, has some crank the motor, and puts their thumb on the hole---if the compression blows your thumb off the housing then its good. Others say, if it starts easily then the compressions good. I believe that if you are a good Rotary mechanic, you won't need a compression # to tell if you need a motor. I think it only good for bragging. " i got 125 psi on both rotors". WOW

Like I said, I am not an expert and this is just my "two cents". I would like to hear from others ( experienced and not) what there outlook on compression is! How important is it/ is it getting carried away/ what horsepower differences can you expect from diff compressions/ how u check compression/ etc etc. please share your knowledge on this subject. I am really interested in what others think.

Thanks alot for reading.
Mann

j9fd3s

iTrader: (3)

that and a compression test tells you nothing about the rotor oil seals, or water seals.

mike

now

iTrader: (1)

well its nice to have a mazda compression tester on
hand to check the condition of a motor, you can clearly
see if there is a problem with a side seal for example,
there are lots of rx7 shops that say that compression
tests are useless, but that is because most of them
don't and will not spend the money for the correct
compression tester.
I have found that the printed out compression test
is great tool to find the condition of a motor , i will agree
that a motor can have great compression one day and
blow up the next, that just the way a rotary is.

but when you start to deal with more than 2 rotors
you will find it harder and harder to tell if one face
has lost compression, a 3 rotor with only one bad face
(due to a stuck side seal) will run so smooth you might
never know that you are missing compression on one face.

matt

Evil Aviator

That's pretty much the method in my neck of the woods. I'm not even a mechanic, and I can tell you that my 1Gen's engine has just about had it just by the way it drives, so there is no need to go through the trouble of a compression check, lol. The more official compresson check is mostly for people who are considering buying a used car, and want to know about how long it will be until a rebuild.

EpitrochoidMan

I agree, it's not bragging rights. HOWEVER, compression tests can be invaluable for determining the condition of an engine. Sure, you don't need to do a compression test on a bad engine to find out it's bad. But like now said, compression tests can tell you important things.

j9fd3s

iTrader: (3)

not really it can only tell you if it works today, it tells you almost nothing about the condition of the housings

mike

Mark'sMazda

I bought a cheapy compresion tester from sears. It's nice to have around when looking at used cars, J-spec engines, and so on.

From what I have seen/heard you shouldn't worry too much about what psi the motor pushes, as their are many, many varibles. What you should look for is even jumps. make sure all faces are equal, and both rotors are the same.

mazdized

It is a some what effective way to tell if an end piece has cracked or the housing is slightly wraped from high temp. How ever most people don't realize a compression test will show how well it is sealing during compression, but it will not show how well the motor seals during the combustion area of the rotor housings which is most important.

KevinK2

my used 93FD started life with me at about 6.8 compression, dropping to 6 then 5's in a few years of driver ed track use. idle vac was also dropping, but it still ran strong. Engine blew in the 5's ...

low idle vacuum could be for various reasons, but properly tested compression in 5's for FD means end of the road is near, imho.

t-von

iTrader: (8)

I'm glad someone has started a thread about this subject. Here sometime next week, I will have a large write up on this subject. Stay tuned!

j9fd3s

iTrader: (3)

yes the dr only need compression in the combustion cycle!

mazdized

Oh I need sucky sucky in the intake too.

j9fd3s

iTrader: (3)

speaking of that the patient is ready

mike

mazdized

OH is the patient clean and shaved for operationnnn?

j9fd3s

iTrader: (3)

yes dr, the patent is ready, heavy internal bleeding

mike

mperformance

I have been doing compression testing on all my engines. It helps me determine many variables that otherwise I would take for granted, specially during break in.

I used compression testing to determine the condition of used engines, freshly rebuilt engines, and engines that have been abused at the track. It is a nice way of knowing a bit about it's condition.

I specially know that a side seal clearance of more than .002" will render a very low compression reading usually lower than 80psi with good housings and new apex seals. That is explained in most mazda rotary engine manuals and is dead on. Marginal compression is 85psi and anything lower than a 75psi will be almost impossible to start without unusual aids.

thats from my personal experience.

seveninphilly

iTrader: (7)

this is a very interesting topic to me as well.

How much do the other seals (oil, coolant) in the engine affect the compression numbers?

How much do contaminants (coolant, flooded, excess oil) in the chambers affect numbers?

As in what numbers might one expect from failing seals other than apex? Assuming the engine will run/not run.

Project84

I agree. It can only tell you of the apex and side seal condition at the time you do the test. So many people on this forum fear the apex seals that any time something happens to the car like a backfire, flat tire, low fuel light, light bulb burns out, or a CD skips, they come running to the forum asking if they accidently blew an apex seal. Recommending a compression test will give a yes or no answer to that one question. Yes a seal is blown, or no it is not. Comp test doesn't promise that they engine will last for a given number of months or miles. Not for a rotary or any other engine.

12abridgeport

I do agree that compression testers are not that important on rotaries, however, i believe they could be used to determine how much power a motor has, there is going to be a big difference between 90 psi and 125psi.

Just for kicks i've actually put together a motor with only 2 apex seals on the rotor and tested the compression, it came out to be 85 psi. That shows that the compression testers only show you the highest sealing rotor side in a chamber, so it's possible to have one side of the rotor having like 80 psi and the other two around 100 psi and the tester would still show just 100psi.

Dcrasta

without modifying a boinger compression guage or using a rotary compression guage your info is not valid, so no wonder people find the data useless. If you use the proper tools, then the information would be useful. I think compression test is a good way of telling how well the engine will be able to suck, squish, bang, and blow. if its not sucking strong, or squishing well, how is it going to be able to bang and blow? but hey common sense isnt that common

weaklink

I find the opposite to be true. Most people on the forum are in denial when the engine goes. Despite the hard starting, low vac, smoke, shaking, etc. they ask about #1 the map sensor, plugs, ignition amps, too rich, too lean, vac hoses, fuel injector cleaner, MMO, premix. Though it may be easy for you rotary veterans to simply see the truth, it's not so easy for our forum brothers who don't want to plunk out the cash for a rebuild, especially since they just spent it all on upgrades to pop the motor. The compression test, done correctly, is a good tool for you to break the news to them. It removes you from the equation. Nobody likes the bearer of bad news. Blame it on the compression tester!

mperformance

I agree...sometimes it's bad news but a bad compression reading is all it takes to get convinced that the engine is crap!