valve size for turbo engine

[Tru68vw]

I just picked up some new parts for my next engine build 78x92, it will start its life as a naturally aspirated but it will be getting a twin turbo upgrade in the near future. The set of heads I have currently is a set of stock heads that have been extensively reworked (welded, ported, polished) but they only have 40x35.5 valves. My question is will they flow enough, and if not what should I upgrade to? I have heard both sides of the argument, "you want the biggest valves possible for more air flow" and "valve size doesn't really matter under boost, bigger valves is just extra weight in the value train". I want to know some opinions from people who have done it and what you think. I will also take any other advice you have to give. I'm hoping for 250ish HP (with the turbo) but for it to still be relatively reliable. Any advice is appreciated!

[Bruce.m]

It depends…

If you are aiming for 5psi boost without an intercooler typical big house turnkey spec) then maybe not.

As a rule of thumb, take the non turbo estimate BHP & multiply by 1+ boost in bar. Ball park figure

[Buggsy]

That size valves should be fine. 250 HP is a tall order, but very do able. Pay particular attention to spark advance curve, camshaft selection, fuel availability to the engine, and compression ratio. You'll also need dual valve springs.

[Clonebug]

I don't think you'll have any trouble with that valve size. Just add boost to get your hp target.
Will this be carb or FI??

You'll have to work up to the 250 hp level and it will take ignition control and intake temp control.
According to my fav calculators it will take about 15 lbs of boost to get 250 hp from your engine.
Your boosted intake temps will be around 224 degrees at an ambient intake temp of 100 degrees which isn't uncommon.

I'd work on a W/I system at the same time and that will get your IAT's back down to ambient with the right nozzle size....of course I'm assuming you will be blow through since you are going twin turbo.

[slayer61]

Listen to clonebug. He knows of what he says.