Have been having compression problems...

@ 900 rpms used to be about -425mmHg now it is about 100mm less

shows around 11"Hg at 800rpms...even lower with the a/c on.

then it will get closer to -400mmHg....?

how does that happen? side seal? tested the compression and mazda sez I have low compression on the front rotor. Is -389mmHg around 1000to 1100 rpms low compression?

TIA

 

mine is anywhere from 380-420mmhg between 900-1100rpm. It seems that a couple that i have seen, if the rpm drops from 900 to 800 the vac will drop by half almost. So the AC causing more stress on the engine might do that.

 

thanks...

a/c kills my idle

 

Vaccuum readings taken with the A/C load are completly erronious. Go by your vaccuum without any load on the motor. IE, base idle.

 

a/c kills my idle too.....so I run my running lights to kick up the idle a bit.

 

You're idle vaccuum doesn't really necessarily tell you what your compression is... it just should be within spec.

 

Vacuum readings have more to do with engine load and idle speed than engine compression.

When the AC is on the Idle Speed Control opens up to keep the car idling, this lets more air into the intake manifold, this lowers the vacuum reading.

If the idle is higher then you get more vacuum.

 

Well I agree vacuum at idle is not a good way to compare one engine to another or even tell you anything directly about the state of compression for your engine, but I think it is always to keep an eye on it.

I have always watched my vacuum at idle and have seen small changes over time. When I first got the car it idled at 950 rpms at -18in/hg (pre-PowerFc so I dont know exactly what the converts to mmHG but I would guess -465mm/Hg). Then I tuned my idle to 900 and it dropped down to -450mm/Hg. Then this summer my idle started dropping as the weather got warmer until last week it was around 650rpms on startup which would put my vacuum at -400mm/Hg. I am not sure why it kept dropping but I finally couldnt take it anymore so I re-adjusted everything and got it back to about 975rpms and now it is only -445mm/Hg. 975 is higher than 950 so I would expect the vacuum to be higher but it is not, so I think I have lost a bit of compression (by-product of my many mods, general lazyness, and self tuning learning curve). Of course it could also be one of the many things I have capped with a rubber cap leaking a bit over time. Either case something is not exactly right.

SO I guess what I am saying is that you can keep an eye on your vacuum over time to tell you if something is going on with your engine but it certainly isnt a conclusive measure of anything. There are too many variables.

Shawn
www.3rdgenrx7.com

 

Compression test revealed low compression on the front rotor..

(gettin' good and rebuilding the rotary engines)