Just having a look under my car the other day to fix a leaking gasket i was doing some measuring and found the following..

Exhaust port diameter: 37.5mm
Stock header port 39.5mm
Gasket diameter 46mm OEM EJ gasket (3 bolt dual port)

Can anyone explain why Subaru made OVERSIZED gaskets?
This does not seem beneficial to flow to me? maybe they made it oversized to keep the gasket away from the heat to stop it deteriorating prematurely..?

I tried looking for a gasket that was 40mm? but couldnt find one..

Reason being is that i see a lot of people open up their ports on the headers to match the gasket size.. so called port matching..
Even Grimspeed port out their headers to match the OEM 46mm gasket??!!

By my reasoning if the head and the exhaust have the same diameter roughly then wouldnt it be better just to get a 40mm gasket and be done with it!! surely this would be better for flow than the factory mismatch that leaves carbon rings everywhere

Can anyone comment on this ?
Is it worth making my own gaskets or just go with the OEM..?

Makes it easier to port heads using gasket lol

Believe it or not thats the first thing that came into my mind..

I think it has something to do with an ANTI reversion STEP. to help scavenge exhaust gasses...

I wonder those guys that are porting their exhaust manifolds to 46mm ie the size of the gasket are they going backwards with regards to flow..

Adn if so why does grimspeed offer 46mm PnP headers and cross pipe and claim 10-15HP in mid low down power..

quote..

Our Subaru boxer engines kill our spool up time, most notably because of the very long pre-turbo exhaust tract(exhaust manifold, up pipe). To directly combat this loss of spool up we will port and polish all 9 inlets/outlets of your 3 piece exhaust manifold. We enlarge each port to match up with the stock subaru gaskets, porting 6-7mm of material at the entrance/outlet of each port and then tapering down into the port. This takes out the large steps in flow and smoothes out the port to port connections. Every piece is perfect matched to eachother for optimal flow.

As far as gains, 3rd party unrelated Subaru owners have seen ~15 whp/wtq gains, most notably in the low and mid range. You should also expect to see quicker spool up time.