Hey, how's everyone doing? I just build my 12a BP and had it running for a little bit but it turns out it's got pretty bad compression, about 60 psi in each chamber. All the parts are new except for oil seals. I think that the problems might be in the side seals cause even though they are new, i still had to grind them down to the right size to fit. Does anyone know anything else that can give me bad compression? Also does anyone know if it is a good idea to check the compression before putting the engine in the car, by hooking up the starter to the flywheel and cranking it, with all the oil already in the engine? I would really appreciate any responses.

 

Maybe you get a bad compression because of the BP? Less comression coz of the increased overlap.

 

I think you have to let the engine bed in for a bit before you get good compression ?

 

Thats what I have heard. You need to break in the engine properly before full compression will be reached. Also, is it possible that its lower than "normal" because its a BP as Aico said?

~T.J.

 

Everyone is kinda right..


1st A BP will have lower compression because of over lap
2nd The engine needs to be broken in before compression will be good.. And 60PSI is about right for an engine that just started up..


But you should get ~90 after broken in.


-Zach

 

I have had the engine running for more than an hour all together but also it wasn't starting from a key, only by pull starting and when it is in the advanced setting. I'm really lost now.

 

Breakin asumming new seals takes at least 1K miles


And for bearings ~3K miles. But the bearings will not affect compression.


I would suggest running it for a while before tearing it down..


More important then overall compression is the difference between blips in the chamber. Also the difference between the rotors..


Hope that helps a little.




-Zach

 

I got bridgeport and even before its broke-in, it built more then enough compression to start, you got other problems. Any rotary I rebuilt will start right off, unless something else is wrong.--WITH OUT DOUBT.

 

I have heard of good BP engines not wanting to start new.. I think it has to do with porting and apex seal groves needing to wear in a little. It could also have to do with less then perfect housings..


But after running it a little while it should build more compression.



-Zach

 

All new side seals are oversized and have to be ground to fit. If you held the tight side of the mazda spec, it is not the cause of your problem. While it is not too critical to grind it down perfect, the mazda spec on the gap between the side and corner seal is quite liberal but still shouldn't affect it that much. Did you check all the housings and clearances prior to slapping it together?

 

why are all new side seals too big? wouldnt the manufacturer fix a problem as significant as that?

 

it is not a problem, it is done that way on purpose so you can have better tolorances. Perfect fit for each engine.


-Zach

 

I don't want to argue, but I have to disagree. After assembly, and you turn it over and over by hand (with oil in the chambers ofcourse) and it builds more then enough compression with the carbon seals(even though the break-in period will bring compression up even more). If you are fairly used to what a good compression rotary should feel like when turning it over by hand, you will know its got relatively good compression in the chambers.

The housings would have to be unbelievably shotty to not start it. I mean crom flaking of massive amounts and even then, you find old engines with horrible flaking running really good, even though compression isn't like new obviously. And obviously with a bridgeport or any high rpm engine should be using excellent condition rotor housings since carbon doesn't age them at all and will build more HP then crap housings. Hope your apex seals, springs, etc are all in good shape and move freely when you installed them on the rotors? But the point to me is, unless there is some other issue, it has to be related to something mis-adjusted/not working, or bad engine assembly?

I installed one engine that took pulling it like you mentioned above to get it started or starter-fluid, and it turned out the leading ignitor was bad and after changing that, it started on the first try all the time(if you pumped the gas when cold though before turning the key).

 

I don't disagree with you at all, just saying there could be other possibilities.. As for the other problems with starting i bet that is the case.. even with 60PSI the car should start.. Althought that is on the low side.


-Zach