Heads

[dwg86]

I have read before that the 7120/0630 heads flow the best. Now I have been reading that the 0331 heads flow better although the exhaust ports are smaller. Does anyone know or have flow numbers to show how the different castings flow?

[StroktWJ]

This was posted on another site.

Cylinder head flow figures (cfm) at 25inH2O pressure drop are:

Non-HO head #2686

Valve lift (in)..... 0.2 ... 0.3 ... 0.4 ... 0.5 ... 0.6
Intake flow.... 122.0 168.0 186.0 189.0 192.0
Exhaust flow....88.0 114.0 130.0 134.0 138.0

HO head #7120 & #0630

Valve lift (in)... 0.1 ... 0.2 ... 0.3 ... 0.4 ... 0.5 ... 0.6
Intake flow.... 66.0 128.0 179.0 206.0 209.0 209.0
Exhaust flow. 55.0 100.0 120.0 136.0 141.0 141.2

HO head #0331

Valve lift (in)..... 0.2 ... 0.3 ... 0.4 ... 0.5 ... 0.6
Intake flow.... 114.0 165.0 194.0 199.0 205.0
Exhaust flow....94.0 117.0 126.0 130.0 133.0

[Muad'Dib]

I think alot of this has to do with what year range the head is going into also.

7120 head is the sweet spot for any build for a 87-96.

The later year heads would be better suited for 97-01.

Just my opinions of course... Ease of the swap, how much machine work will have to be done etc...

[RAPTORFAN85]

The 0331 head doesn't flow more CFM's then the 0630 or 7120. The ports are smaller because they filled in the dead spot on the floor of the runners. The smaller exhaust ports increase the exhaust gases velocity, Building more torque. The drawback is a couple of top end HP, but thats not what most of us are looking for.

[SIXPAK]

Exhaust port was altered to help the twin cats work properly.

[1bolt]

The intakes of the 0331 flow more CFM and are physically bigger and slightly higher than previous HO heads. The exhausts are smaller and higher, the exhaust port floor is a dead spot on earlier HO heads (see the sticky thread at the top of this forum, for velocity maps), the 0331 fixed this but they flow less CFM, due to being smaller. The smaller port creates better velocity into the iron exhaust manifolds, which are well matched to the port in size, and have a reversion step built in, Most importantly the iron manifold has considerably longer primaries than previous HO headers (believe it or not they are 3-4 inches longer on average) and they meet the collector pipe more smoothly than most of the primaries on the stock tubular headers do (many of them 90* into the collector it's even worse on the renix header). The 0331 iron exhaust is somewhere around 9-10 inches on average per primary, versus about 5.5-6.5 for the HO headers... The 0331 intake runner length is 11.5 versus the early intakes 7.5 to 8.5

Design wise those are all power UPGRADES over earlier HO's... But using "conventional wisdom" or looking at them intuitively they might look like downgrades. So the 0331 in addition to cracking is often said to be the worst performing HO head. When it is probably the best performing.

The 0331 system was ostensibly designed to counteract losses from emissions "upgrades"... (and from what I see: make the engine more optimally tuned for what Jeeps are used for most).... low and mid range torque. However I believe they more than hit their mark. I believe the longer primaries help scavenging so much that they even increased horse power a small amount while bumping torque significantly.

The correct way to describe it is they moved the RPM of peak torque lower (longer pipes do this) while improving torque on either side of the peak (enhancing intake port CFM and better exhaust scanvenging will do that)

We've also found out that Rick Mudge (see the sticky thread again) who worked the heads on Garth Hill's 8 second 4.0 also was involved in the 0331 head for Mopar (and presumably the intake and exhaust as well). It stands to reason that someone who's ported an 8 second 4.0 head knows what he's doing.

Engine Analyzer Pro predicts that the 0331 (as a system) will make more torque AND more horsepower than an earlier HO (system) using the same cam. That is only changing the head, intake and exhaust. I have not done a comparison using the newer cam. I think the later cam and ECU are a far bigger deal than the fact that they put two cats right at the end of the header... Two cats are usually better than a single big cat (less back pressure, because the cavity that contains the catalyst is much smaller)... look at where the cats are on a Mustang yep... same place, and there's four of them.

[SilverXJ]

I think the later cam and ECU are a far bigger deal than the fact that they put two cats right at the end of the header... Two cats are usually better than a single big cat (less back pressure, because the cavity that contains the catalyst is much smaller)... look at where the cats are on a Mustang yep... same place, and there's four of them.

Actually, those with the two precats also have the large cat as well. The smaller cats are there to clean up emissions when it isn't up to temp yet as they will heat up faster.

[1bolt]

Got me there I've never actually been under a 2000+ so I was assuming.

[RAPTORFAN85]

There is a third cat farther back but it has no O2 sensor so theoretically it could be removed without affecting how it runs.