Is it a good idea to have the cast liners removed from the exhaust ports? Will this improve or mess up flow? Also while its in the pro shop, besides port& polishing and valve job,any other mods I should have them perform?
Regarding the linings; I have heard from several reliable sources that it is not worth the effort and is a good way to ruin the head. If you need the extra flow look for the earlier head, what is it? the N42? Most important is to match the intake and exhaust manifold ports. They are not well matched from the factory and can be over 3/16" overlapped which will greatly decrease flow
The steel liners are only an emissions device.They're only purpose is to keep exhaust journals hot enough to burn off excess hydrocarbons.
Steve is correct,the earlier head is a better bet(N42-larger square exhaust ports with no liners), but if the options not open to start over with different head, a reputable head man should have no problem removing liners.
Don Potter of DLPE built my head. It is an N47 that has been P&P. He recommended leaving in the exhaust liners as they act as an "anti-reversionary" device. Don't ask. It is however a good thing. Anyway, my car tops out at just under 150mph with a 3.9:1 differential. The N42 according to Potter is a better full race head but holds no advantage on the street.
Yes, I too have heard this. In actuality, superheating the gasses expands them even more as they are coming out of the head. Going from a high velocity small opening and superheating the gasses forces them into the header and actually aids the flow of the exhaust. The anti-reversionary feature implies that as the pressure drops coming out of the liner area it decreases the backpressure at the valve and the vortex created by the gasses spilling out into the header increases the flow.
I just made all of this up. So am I full of ****? LOL