I did a compression test this evening and both rotors are giving even burst to ~88 pounds.

I know this is getting low and I'm curious what the consensus is regarding when a re-build is due. Less than 85 pounds I'm thinking?

Both of my rotors give three equal pressure burst so it's not showing an obvious bad apex seal.

I used a standard piston compression tester and held the valve open and watched it bounce. What are your thoughts regarding the accuracy on that test compared to a rotor tester?

Last question.. can anything affect compression other than rotor, apex seals and housing quality. i.e. could a vacuum leak or something else affect the psi?

Pretty sure I'm not gonna like the answer but feel free to deny or confirm I need a rebuild.

 

I think Mazda's lowest acceptable comp. # is 75psi. I don't let my motors get even that low though. My worry is that the apex seals could be getting so worn/thin that they have a greater chance of breaking and taking out housings. I could be completely wrong, but better safe than sorry.

 

make sure that the engine is warmed up fully, charge the battery or maybe even keep a charger on it during the test and be sure to hold the throttle wide open during the test.

~80psi is where you should definitely think about cracking her open.

side and corner seals also affect compression, side seal gap can be excessive or a corner seal stuck or with a weak spring but all compression being even i seriously doubt that to be the case as all the side seal gaps would have to be excessive.

 

It's low but seems even. If it still hot starts fine keep on keeping on. Could last another couple years as is.

 

Thanks for the replies. I did warm the car up prior to testing it and throttle was wide open. Putting a charger on the battery while testing sounds like a good move to make it turn over a bit stronger. I may give that a shot and see if it creeps up a bit.

 

That's really a lot if you are holding the bleed valve open, my rebuilt engine bounces around 65 if I recall correctly when holding the valve open. I think you are supposed to not hold the valve open to see the pressure build up. I also got higher numbers with it hooked with jumper cables to my other car. I ran my old engine with around 80 peak pressure for a couple years and a lot of boost till it finally blew.

 

the ideal way to setup a compression tester is leave the gauge pressure relief alone during the test and remove the schraeder valve from the hose prior to installation. the pressure will bleed off quick enough that way, you are just looking for peak bounces anyways.

 

Find a shop with a Mazda compression tester and have them do it if you want to be more confident.

The piston compression gets iffy and i never liked it. Also, you cant factor in a weak starter using the piston tester.

 

"It's low but seems even. If it still hot starts fine keep on keeping on. Could last another couple years as is."

i agree.

it is easy to get a finer look at a compression test w just your normal piston compression test instrument.
remove the schrader valve.... do all the usual things... warm engine, open throttle, charged battery and remove the other lower sparkplug...

then crank the engine w a video camera on the gauge.

play it back in slo mo and you will clearly determine if you have even pulses/rotor faces.

do keep in mind that compression checks show compression at, say, 250 rpm. they are great for determining if you have a broken apex seal (two adjoining low pulses) or a side seal problem, one low pulse, or just generally a worn/carboned up motor.

if the motor will hot start... it may often drive well for a couple of years as since the rpm increases the apex seals due to centrifical force tend to seal better.

i remember the 82 Runoffs... Chris Dembs was on the pole and his RX2 wouldn't hot start on the grid. they poured some oil down the Weber carb, it started and he blew off the field.

so there can be situations where you generated lowish but even comp numbers but by adding a fuel toggle switch you can get your rotary started and from that point you have no problems... perhaps for a few years.

just sayin'.

howard

 

Thanks again all. I'll be staying pat with it for a while based on the feedback.

Howard, I've read several of your posts and love the 'flavor' of your posts.

You always have a story based on some experience. Cool stuff!