The book I have for chamber volume has the entries swapped.
I swapped a pi whole, so I've never actually seen a pi combustion chamber.
I had to look it up; the reason to keep the NPI heads for a budget build is the midrange; pi heads/intake combos are peaky around 6k rpm, not great for a DD. Great for racing.
NPI heads breathe better in the midrange, where the pi heads are more peaky; even more peaky than the pi intake swap.
(If you've done the PI intake swap, you will notice a lot less midrange untill ~3k rpms. But it doesn't run out of GO at ~4200, either.)
The extra feel over 3500rpms is offset by the amount of time you actually get to use it; my unmodified Tbird is a bit faster than the very modded Cougar redlight to redlight. (Both cars have dual exhaust, but that only really helps with a high flow cam.)
The PI intake has more power over ~4200, but if the cops see you over 4200, it will not be a fun day.
The 03-04 4v heads are way more expensive, and rarer; no one wants a conti motor, so they're cheap, conti owners just buy new cars.
And I found a unicorn; my heads had xr3e cams in them.
There is a handful of hp available on all this; if you want more than ~20hp, you want an aftermarket cams, and reworked heads.
The cams are the biggest restriction until you hit over 300hp.
Npi stage 2 heads flow better across the board than pi; especially midrange.
If you want a set of racing heads, I'd send your heads to MHS with a pile of money.
Buying a set of trickflows is probably cheaper.
There's a thread nick wrote here somewhere.
BTW, you won't be able to get a tune that eliminates the rear o2 sensors anymore, so I'd plan on highflow cats.
I bought a book awhile back that a Dyno guy wrote, where he goes thru every combo I've ever heard of, with HP and Torque curves.
You should find a used copy; it makes for great bathroom reading.
Building 4.6/5.4l Ford Horsepower on the Dyno:
Thanks for the clarification. I don't know much about "truck" mod motors as far as when they went to PI and such. Got triggered by the "any" 99 up 4.6 as the much more common Panther cars weren't PI till 02.
I've been here a long time, lol.
I personally prefer the Romeo engines, but I wouldn't swap a mustang anything
into one of my cars; they're always beaten to death.
Say what you will about truck motors, but they haven't usually been abused too badly, unless the oil isn't changed regularly.
You can tell just by looking, "No, that intake won't clear the hood."
Gran Marquis are great parts cars; they usually get totalled by rear end damage.
03-04 Mach 1 and Cobra heads are better flowing than 99 Connie heads, they're the "final revision" for the 4v/cylinder 4.6 head. The Aviator got these heads as well.
The heads are slightly better; but you'll never feel the difference unless you buy some serious cams, and even then the real difference on the track is a few hundredths. (same cams, different heads)
The Best Flowing DOHC heads are the 97-98 B heads. A cobra intake and cams really lights them up.
But the IMRCs are a bitch to deal with.
Navi heads will flow more, but IDK what intake is usable without going custom.
You do not want that 03-04 block, however; for an aluminum block, you want a Teksid Mark block.
They have been known to handle 1200HP for more than one season.