Best flowing heads?

Free-flowing heads, in order:

#7120 (1991-1995 OBD-I)
#0630 (1996-1999/2000, OBD-II w/o COP/DIS ignition)
Third place is a tie - the #0331 (COP/DIS OBD-II, 2000/2001-2007) flows better on the intake side of things, but is restricted on the exhaust. The #2686 (1987-1990) flows better on the exhaust, but has restricted intake due to lower ports and a sharper turn into the cylinder.

Also note that the exhaust ports on the #0331 head are smaller (meaning you'll have some trouble using the later header on the earlier head,) so you may want to just use an aftermarket header or an OEM that fits the earlier head. The 1999 intake should swap without too much trouble, fitting up to the aftermarket header or the earlier OEM unit.

I'm still trying to collect heads so I can flow one cylinder from each on the same bench to get a better benchmark (relative flow from one head to the other, I/E bias, ... and some porting experiments) but I've not yet gotten all of the heads I need.

5-90 do you have any flow data on the 0630 or 0331 heads after mild porting? I was just wondering what the improvment was if any.

ColoradoRaptor said:

5-90 do you have any flow data on the 0630 or 0331 heads after mild porting? I was just wondering what the improvment was if any.

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Not yet. There is always some room for improvement on a rough-cast production head, but that's why I'd like to collect heads - find out how much room for improvement there is.

5-90 said:

Free-flowing heads, in order:

#7120 (1991-1995 OBD-I)
#0630 (1996-1999/2000, OBD-II w/o COP/DIS ignition)

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what kind of difference are we talking about here

I know a inline ford 300 can really scream with a port and polished head. Any engine will benifit with better cylinder filling.

ehall said:

what kind of difference are we talking about here

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As I recall, it's really not that much on the intake side (the #7120 is slightly better, but not so much that the #0630 can't catch up.)

It's on the exhaust side - OBD-II vehicles tend to have slightly undersized exhaust ports, which keeps the exhaust gas hot and shortens "catalyst light-off time." That's where the #0331 really sucks (apart from the cracking issue for 2000-2001 heads,) it's the reduced ports. The intake side is comparable to the #0630, but the exhaust cripples the head.

And, it's not just "cylinder filling" that improves the engine - it's also "cylinder emptying". Increasing exhaust flow (within bias limits - I think it's usually taken as a 65-70% bias in favour of the intake side) helps to reduce pumping losses, which can be classified as "parasitic drag" akin to the slight drag of the bearings, the greater drag of the valvetrain and rings, ... That's what makes rollerised valvetrains so popular, but rollerised valvetrains is still a complement to other mods, and not strictly a power adder in itself (it will free up some power lost to parasitic drag, but there are better ways to free it up...)

so what kind of difference are we talking about here