cylinder head FAQs

[amcinstaller]

a question was asked recently on another forum i frequent, asking the main differences between different years, just physical for now, no flow numbers involved. so i jumped over here, whipped out the cylinder head FAQs, but there was only casting numbers for the various years. seems to me that common knowledge is that the 7120 head is the "best", but is this true? maybe can we add the various designs or "features" that each head has, like the later ones having mounting bosses for the coil pak?

[1bolt]

Renix heads (can't remember casting number) have the lower ports and presumably less flow (don't recall actual flow numbers though)
7120 (91 to 95 I believe) this head somehow got the reputation as the "best" head... I can only guess because it's popular for the 4.2 head swap because some misinformed early swappers claimed the later 0630 head didn't have a water temp sender boss.
0630, 96-99 (98 on a ZJ I believe) this head is supposed to be identical to the 7120, I haven't flowed my 7120 yet but I believe it will flow the same. This head supposedly doesn't have a water temp sender location, but it does in every one I've ever seen... there's a small bolt in it depending on what model Jeep its in.
0331 99+ (98+ on ZJ?) this head is prone to cracking in its first few years of casting. It has Ports that are higher on the head and the intakes flow better than the intakes of a 0630 (and I suspect the 7120 as well but I will confirm that at some point).
The exhausts on the 0331 should be better for torque (perhaps worse for high RPM horse power, perhaps no difference), they are smaller, shapped much better, and the floor has been raised a lot while the "roof" was raised slightly making for a higher velocity port, more exhaust "scavenging" and thus better low end torque. How much is anyones guess as no ones done a dyno shootout of Jeep heads (unless HESCO has but they would never make the sheets public).

I feel the need to point out that the last part about the 0331 will be controversial, because it flies in the face of every assumption about 4.0 heads on the net. It's a crack prone head so it really doesn't matter most people aren't going to be brave enough to take a chance for a few extra foot pounds, if it means possibly having to do it all over again.

The exhaust port floor on previous heads was extremely lazy, the area that Jeep filled up on the 0331 (making the port smaller) was this lazy area, and actually makes the velocities higher in the port, while flowing slightly less volume. A good pocket porting and valve job would (In my opinion) make the 0331 flow significantly better than any previous 4.0 exhaust. I believe there's a WHOLE LOT left on the table because this port is smaller and has more meat to work with in terms of shaping.

[seanyb505]

0630,... This head supposedly doesn't have a water temp sender location, but it does in every one I've ever seen... there's a small bolt in it depending on what model Jeep its in.

My 97 XJ has this filled with some metal. I can see where the hole is, but its not a bolt filling it.

[gradon]

My 1/96 0630 still has the sensor.

[dwg86]

It would be nice to see some flow numbers...anybody??? What years are prone to crack, and what did Chrysler do to fix it? The engine I bought to build my stoker was a 2000. The engine was removed because of a cracked head. So far my 2003 Wrangler head is still good(95,000 Miles)... knock on wood

[amcinstaller]

i picked up a 93 engine for my stroker build, based on the 7120 head beliefs. thats all ive really read, that the 7120 was the best, maybe on Dinos site i saw that one. i plan to as well switch to the newer intake manifold, will i encounter any troubles if i just get the PS pump to match the intake?

[RAPTORFAN85]

I have a 0331 casting head, but it is from Alabama Cylinder Heads so it is reinforced in the areas that normally crack on stock heads. I am going to send it out in about a month or so to be flow tested and have a full port and polish done so at that point I can give you some hard numbers on flow both before and after the port and polish. It should be very interesting as I believe that once the P&P is done it will probably flow better than any of the other heads. It also won't crack because of the strengthening done to it.

[Muad'Dib]

Renix heads (can't remember casting number) have the lower ports and presumably less flow (don't recall actual flow numbers though)
7120 (91 to 95 I believe) this head somehow got the reputation as the "best" head... I can only guess because it's popular for the 4.2 head swap because some misinformed early swappers claimed the later 0630 head didn't have a water temp sender boss.
0630, 96-99 (98 on a ZJ I believe) this head is supposed to be identical to the 7120, I haven't flowed my 7120 yet but I believe it will flow the same. This head supposedly doesn't have a water temp sender location, but it does in every one I've ever seen... there's a small bolt in it depending on what model Jeep its in.
0331 99+ (98+ on ZJ?) this head is prone to cracking in its first few years of casting. It has Ports that are higher on the head and the intakes flow better than the intakes of a 0630 (and I suspect the 7120 as well but I will confirm that at some point).
The exhausts on the 0331 should be better for torque (perhaps worse for high RPM horse power, perhaps no difference), they are smaller, shapped much better, and the floor has been raised a lot while the "roof" was raised slightly making for a higher velocity port, more exhaust "scavenging" and thus better low end torque. How much is anyones guess as no ones done a dyno shootout of Jeep heads (unless HESCO has but they would never make the sheets public).

I feel the need to point out that the last part about the 0331 will be controversial, because it flies in the face of every assumption about 4.0 heads on the net. It's a crack prone head so it really doesn't matter most people aren't going to be brave enough to take a chance for a few extra foot pounds, if it means possibly having to do it all over again.

The exhaust port floor on previous heads was extremely lazy, the area that Jeep filled up on the 0331 (making the port smaller) was this lazy area, and actually makes the velocities higher in the port, while flowing slightly less volume. A good pocket porting and valve job would (In my opinion) make the 0331 flow significantly better than any previous 4.0 exhaust. I believe there's a WHOLE LOT left on the table because this port is smaller and has more meat to work with in terms of shaping.

Thanks very much for this, im going to add it to the FAQ.

[SilverXJ]

0630, 96-99 (98 on a ZJ I believe) this head is supposed to be identical to the 7120, I haven't flowed my 7120 yet but I believe it will flow the same. This head supposedly doesn't have a water temp sender location, but it does in every one I've ever seen... there's a small bolt in it depending on what model Jeep its in.

Both 0630 heads that have been through my hands did not have the sender port. There was a flat spot that could be drilled and tapped, but no port. However, I have also heard of some 0630s having the port filled. Perhaps earlier runs on of the 0630? Both of my 0630s were off of 98s.

0331 99+ (98+ on ZJ?) this head is prone to cracking in its first few years of casting. It has Ports that are higher on the head and the intakes flow better than the intakes of a 0630 (and I suspect the 7120 as well but I will confirm that at some point).

The 0331 head was never installed on a ZJ, which stopped production in 98. However, the 99-04 WJs have them. Also, from what I have heard from Jeep parts departments and mechanics is that they fixed the crack issue after 2003.

[John]

The info I have also indicates that 2002-2003 saw a change to remedy the head cracking. They were looking at a class action suit and quietly addressed the problem.
John

[Mgardiner1]

Correct me if i'm wrong, but i thought the 0630 heads were the start of smaller or reshaped exhaust ports to improve emissions and speed up engine warming cycle, thus slightly downgrading flow.

[SilverXJ]

Correct me if i'm wrong, but i thought the 0630 heads were the start of smaller or reshaped exhaust ports to improve emissions and speed up engine warming cycle, thus slightly downgrading flow.

That was the 0331 Head.

[1bolt]

Personally I think the main reason they made the exhaust ports smaller was to improve torque because they lost some of it with the pre-cats and emissions oriented ECU programming.

Also because the previous exhaust ports are terrible... They flow only slightly more CFM even though they are almost 25% larger. The smaller ports sacrifice HP that most Jeeps will never rev to (in other words HP near the rev limiter) for lower RPM torque that we all use every day.

I wish I had a chassis dyno, I'd love to run a stock 98 or earlier HO 4.0 against a stock 99+... then lop the pre cats off and see how much more torque its making than earlier 4.0's.

I bet it's making 15 more foot pounds, if the pre cats were deleted. based off the 99+ manifold and better intake and exhaust port, probably the same or a tiny bit more peak HP due to the inertial ramming of the 99+ intake.

Anyway Alex has already flowed them, and so have I, I think we both found that the 0331 flows a bit better on the intakes than the 0630 and 7120, and the exhausts flows around 10 CFM lower (smaller area but faster velocity makes up for most of the reduction in size) I am just waiting on flowing the 7120 before I do a writeup of my results.

Unfortunately a bad economy and slow work have caused me to spend just about every waking hour trying to make ends meet lately so I haven't had time for cool shit like flowing Jeep heads.

[Muad'Dib]

Anyway Alex has already flowed them, and so have I, I think we both found that the 0331 flows a bit better on the intakes than the 0630 and 7120, and the exhausts flows around 10 CFM lower (smaller area but faster velocity makes up for most of the reduction in size) I am just waiting on flowing the 7120 before I do a writeup of my results.

I cant wait for that... It will be some valuable information!

[the88thpianoman]

It seems I was directed to this forum at the perfect time, just as this thread was popping up. I'm planning a very simple and low budget 4.5L stroker build for my 2003 WJ, and the cylinder head is still my biggest question mark. Obviously my stock one is the 0331, but can anyone confirm if Jeep had already addressed the cracking issue by the manufacture date of my Jeep (08/2002)? If my 0331 isn't subject to cracks, then it looks like a port and polish on it is all I'd need to do to meet the needs of my stroker build. I appreciate anyone's input. From what I've read so far, it looks like there are a lot of VERY knowledgeable people on this site