Is there a rule of thumb as to what size pump to fit on an engine build ?
It will be going on a 2.1 forged cdb .

 

11mm.

 

Modified 10mm no good?

 

Are you going avcs and external oil cooler?

 

12 is pointless unless you are on dual Avcs and oil cooler, as banny says with or without Avcs and oil cooler, If neither I suspect a modded 10 will suffice

 

Engine builder should spec it, bearing clearances play a big part, avc's or not, squirt jets or not, oil cooler etc etc

 

Single avcs

 

i've added an (mocal) oil cooler after the engine got forged/build, first i hear that this impacts the oil pump. do i need to upgrade it?
been running with it for over a year now, so far i've got no problems (yet??). i have a genuine subaru oil pump.

 

Is there much difference between a 10mm modified or a 11mm oil pump ?

 

Comes down to running healthy oil pressure, if you're going avcs my gut feeling would be to go 11mm as avcs works off an oil feed. Ultimately as Paul said should go with the recommendation of the builder

 

I was sent a spec sheet of my build and a 10mm modified pump was mentioned.
I was expecting bigger.

 

I have this conversation all the time, the pumps job is to supply the correct volume of oil and the correct pressure. Don't be fooled into thinking the larger pump is better, too big and can cause problems, the relief valve can stick open as it works too much, and the oil can cavitate as the pump tries to force more than required. Ultimately, as has already been said, it's down to the engine builder to spec it, assuming they know what they are doing. Thats not always the case though.

 

Fingers crossed I hope they do.