Does overlap affect compression test results?
 

Hey guys, I was wondering how bad overlap messes with a compression reading? I saw on another post that Mazda min compression suggestion is 85psi which is just a hair less than what mine is however my car dynoed well and ran awesome.

Could it be do to porting that has significat overlap? Would that cause the compression to read low? I'm thinking it would

STEPHEN

YES, the compression specs are for stock engine. Steve X dealer tech

It shouldn't matter that much.
The dynamics of a compression test is a lot different than the engine running under combustion under load.


-Ted

it doesn't matter, but if you don't hold the throttle full open, it will affect your numbers

What are the suggested numbers in psi that a large street port engine should make?

STEPHEN

If it's like the Mazda tester...
Anything over 100psi is great.
Anything from 80psi to 100psi is good.
Below 80psi is looking at a rebuild soon.
Below 60psi (Mazda spec) is a bad motor.


-Ted

the port job does not affect compression

you should be looking for 130psi on new rotor housings, 115-125psi on used rotor housings

if the number is lower but above 90psi, you are still breaking in your apexseal/rotorhousing wear