3.0 litre heads on 2.5V6

[Fylnn]

If you put 3.0 litre heads onto a 2.5 V6 engine do the larger valves foul the pistons?  I am running a PRC GTV6 rally car and to keep within the intent of the rules and keep it classic etc.  so to answer the obvious question, yes I want to stick with the 2.5 capacity.  But I can play with valve sizes and the like.  I am looking to open up the inlet runners in a quest for more power.  It is running a Haltech with later style Commodore injectors so it is easily tunable for any power gain with more airflow.  I have a spare 2.5 V6 out of a 75 which is donating the inlet runners for machining out.  But I got to thinking maybe I should take the heads off the spare engine and port them out properly and rebuild them as well.  But also go from the standard 41mm to the 44mm out of the 3.0 litre engine.  But I have no need or desire at this time to touch the block as it is working just fine.  But then the concern is that the cutouts in the pistons, especially if the heads get skimmed, might not allow accommodate the larger valve size. 

[Duk]

If you haven't done so all ready, read this: http://www.motozane.com/oldtunnelV6Porting.html
And then read it again.

My suggestion would be to work the 2.5 heads and not worry about bigger valves.

[Fylnn]

Yes pretty interesting stuff.  I am told opening up the inlet runners and plenum outlets is worth about 20HP.  Doing this can't hurt either.  I have 133HP at the rear wheels, closer to the magic 160 at the rear wheels the better.

[Duk]

No bigger than 85% of valve diameter for the runners. But you'll soon realise that the ports are a bit too small and not as consistant as they should have been in their placement (core shift during casting).
My 3 litre ports were pretty ugly, especially around the pocket and the short side radius. But the runners were terrible.

[Fylnn]

Looking carefully there is a lot of tidying up that can be done to correct the original manufacturing issues.  I have a spare 2.5 engine out of a 75, i am about to pull off the heads and start looking at a proper head job.  My next assumption is that the 2.5 heads are interchangeable between a 75 and an 83 GTV6 engine.  That could soon develop into an engine rebuild on the spare engine, but first things first.

[Fylnn]

Does anyone know if 2.5 Alfa 75 heads fit onto a GTV6 block?

[Duk]

They're the same engine.

[Fylnn]

thanks again