> It is really hard to get a good handle on the compression
> of a rotary engine as things happen too fast. The usual
> way is to take the Shrader valve (tire valve) out of an
> ordinary compression tester and watch it as the engine turns
> over. You should get at least three pulses in a row all "about" the
> same value. "About" is the problem. At 250 RPM the compression
> should be 85 psi plus or minus 10 psi chamber to chamber.
> It also varies with cranking RPM and altitude of the shop.
> At 10,000 feet it is down to 57 psi. At sea level and 300
> RPM it is up to 95 psi or so. At 200 RPM it is 73 psi or so.
>
> Mazda has a special device that shows the pressure in three windows.
> Special tool 49 FO18 9A0 is it. I have never seen one.
> A very very expensive tool. I have never seen or heard of
> a rotary engine builder or re-builder that had one. They should have
> them as that is a major issue with after market apex seals. Stock seals
> have a slight crown. You can see it when you put them belly to
> belly. The after market ferrous apex seals I have seen so far have
> no crown. This means you lose compression and power.
>
> This is another area where a PICAXE will help greatly.
> You can use one of the analog to digital converters to
> measure the compression with a Free Scale pressure sensor.
> Say ten times a second. Send the data to a PC which is easy to do
> with a PICAXE through the serial or USB port and you have an
> under $75 rotary engine compression tester. Not counting the cost
> of the PC but you already have one of those or you would not
> be reading this.
>
> Paul Lamar ...No rotor no motor.


When checking the compression on a Lycasaurus, A and P mechanics always

(or

should) do a differential compression check using two pressure gages every
100 hr and at annual inspections.  Using a warm engine, each cylinder is
checked individually with compressed air through a regulator set to 80 psi
at TDC while the engine is immobilized.  The greater the leak rate the

lower

the pressure reading on the engine side of the regulator.  A typical

reading

may be 78/80.  If the reading is too low, a little oil can be squirted

into

the spark plug hole.  If this method is applicable to rotary engines, a

big

advantage is that the low compression rotor can easily be determined.

Norm DeBoer

That is a good idea Norm. In fact all three chambers on each rotor
can be checked. Lock the e-shaft at Top Dead Center  for each chamber
and pressurize each chamber in turn. Note the rate of pressure leak down.
Six tests in total. Not only suspect rotors but individual leaking
apex seals can be determined.

Paul Lamar ...No rotor no motor.

Yes that is exactly right!  Some one much smarter than me came up with this
method of inspection long before I got my A & P.  Each measurement is
recorded in the engine log book and when the readings start to go south, it
is an advance warning of impending problems.

I did a Google before I posted the last and found some interesting info from
operators using reciprosaurus engines:

http://www.napanet.net/~arbeau/swift/o3002.htm

There are others...

Developing a Standard Operating Procedure (SOP) for this method of
compression check for the Rotary Engine may be more acceptance criteria for
some of the unconvinced...

Norm

Norm I am sure you have heard of the trick of taking the rocker arm
off and smacking the end of the exhaust valve stem with a hammer
to seat it better :)