On a stock engine that produces let's say 17 inHg at idle, what will the vacuum be after the street port?

What will be the effect on cranking compression? 5%, 10% drop?

 

mine seems lower than most at about 105 110 on used housings and sideplates. Normaly it seems like it is more around 120 but who knows. There are so many different factors that can be affecting things. Oh and my vac is only 13 at a grand or so, Definitely low but the compression is good hmmm.....

 

Are your numbers taken when cold? I realize now I left out 'compression taken when cold?'.

Mine is an FC-TII and producing ~78 psi cold. But from the sound of the pulses, doesn't seem it's cranking over 200 RPM. Spec when warm is ~85 psi @ 250 RPM.(Recently, I took a reading w/ the upper manifold off and I swear I saw ~105 psi being hit. Now I think I was hallucinating!!)

Is the stock compression spec @ op-temp on the FD > 120 psi? Or is 120 what others are seeing w/ the aggressive street port when cold? Regardless, if op-temp spec is > 120, it's amazing when compared to the spec of the FC-TII w/ just 1/2 point lower compression ratio. I wonder if the cranking RPM is considerably higher on the FD?

 

That was warmed up. If you think about it technicaly the compression shouldnt change no matter what port it is, cuz the compression is not even near the intake or exhaust?

 

I'm not to sure about that. Isn't the purpose of porting to effectively increase the time the port is exposed? Another words allowing the rotor to sweep pass BTC longer? I'm just curious. If the answer is yes then the compression will be lower along w/ the vacuum just like a piston engine w/an aggresive cam.

 

yeah compression stays the same check out this
https://www.rx7club.com/showthread.p...ession+numbers

 

Ok fair enough. I'll accept compression READING change is negligable. I realize now the thread's title should have read 'compression reading'. Yes, static compression doesn't change but the READING should change w/porting work. Oh, in my post above, BTC should have read BDC.