I recently ran compression on a friends 13btt and got 40 across, we are going to rebuild the motor but what are other possible reasons why compression would be so low?

 

low cranking rpm when doing test..throttle not wide open..
many factors.

 

I'm wondering more about internal problems, like bad rotors,? Housings? Can those cause low comp?

 

Mainly it's the Apex seal(s). Those are what create the compression between the housing and rotor. It's like piston rings sealing the gap between the cylinder wall and piston while it's moving up and down. Pretty unlikely that it's the rotors or housings unless they're completely trashed for some odd reason.

 

Good way to put it, gonna rebuild my first 13b in a couple weeks *jizz