Hey guys, It seems that time and time again I see people (including myself) posting up questions looking for difference between the OBD-1 Pr4 head and OBD-2 P75 head. I have a couple of each laying around, and decided to take a quick looksy.

As we all know, the b18a/b/b20 heads can be switched out with no modifications necessary. This is with the exception of the p8r head, but Im not including that.

On with what I found,

First thing I looked at was inside the valvetrain casting in the head. Overall everything is the same. The only difference I can see is that the p75 head has a 4th oil return to the pan, while the pr4 has more of a vent hole.

Pr4 on the right, p75 left


Picture of the p75 oil return


Pr4 oil return Vent


The next thing I did was take a quick measure of the intake and exhaust ports to look for any noticeable differences. It has been brought up before that the p75 has bigger ports that flow better, but I failed to see nythign different. I measured quite a few different surfaces, and everything was identical between the two. Im no professional, but if there is a difference I didnt see it.

Intake Port


Exhaust Ports


Next up, I took a look at the head surface of the two heads. This is where I found the most difference between the two. As can be seen from the pictures, the p75 has much larger coolant ports than the pr4. However the pr4 has on additional port on the return side of the cooling system, the p75 does not have this. The chambers in both appear to be identical.

Pr4 left, p75 right


Pr4 head


P75 Head


P75 Center Cooling Jacket


Pr4 Center Cooling Jacket



The last thing I looked over was the cams. Once again, there have been claims of slightly bigger cams in the p75. From what I found, the lift on both appears to be the same. I had no good way to easily test duration, but if there is a difference it is minor

Pr4 on the right


Overall, an easier to give to someone is that the heads are identical. If your really worried about cooling, the p75 appears to be a bit better, but deck rigidity may be sacrificed a very small amount. The ports also appear to be identical. When it comes down to overall performance, they should be the same.

Hope this helps answer someone questions.

On the p75 head it's the intake manifold that gives it a better flow. Overall the head is the same. But good write up.

What is the major difference between the OBD-1 B18b1 and OBD-2 Intake manifolds? I dont have them both in front of me to do any measuring on.

nice write up. from what i know, the major difference between the a1 and b2 resides in the ecu. that besides what mp82 stated concerning the better flow on the p75 intake mani.

basically, yes. now, if you have the b2 ecu you will feel a difference although a slight one. the b2 ecu ignition and fuel maps are slighly more aggressive than the a1.

i don't think the ecu is gonna make the car run way rich. it's a slightly more aggressive ignition and fuel tables, so it's not so big of a deal. do you have an aftermarket fuel pressure regulator? which psi is set? also, have you checked your o2 sensor

o2 sensor is narrowband, right?

what mods do you have done in your engine?

strange... imo you're not supposed to be way rich...

yep, especially if you're running a dual point fuel pump... wait, why a dual point?

oh. so is it a daily driver?

hey gk, do you know the heat range of the plugs you're using in your b18?

The intake ports of the P75 and PR4 are different. The do flow about the same though. The attached photos show molds of the PR4 (left) and P75 (right).

nice you made a mold out of it...

far as i know. the ls head 96 up is were all the things mentioned become different. from the size of the ports. to the change of the intake manifold. all for better flow. to even the maps and stuff in the ecu. all started at the obd-2 era. from 92 up obd-1 pr4 p75. identical. 90-91 obd-0 terrible air flow. sorry for nothing waist of a good b series motor compared to the 96 up.

where the difference's in the power come
90-91 near 130 hp obd-0
92-95 near 140 hp obd-1
96-01 near 146 hp obd-2 a/b
all in electronics. ignition timing. and a lil more.

If I use a bare pr4 head, w/ just valves/springs/and retainers can I swap every thing else (cams, IM, valve cover, etc) from my p75 head (minus the toasted exhaust valve, from my '01 crv) and bolt it up to my B20Z block using a '01 head gasket set?
Is there other modifications required?

yes you can.

Alright, I was told the B18b1 head and Intake manifold flows better then the B18a1 head and B18a1 intake manifold, I was also told that the B20z cams are more agressive then the rest of the (stock) non vtec cams?

More than likely, they're not. Also... there's no way to really tell B20Z cams from B20B, B18B, or B18A cams.

It will fit, but you open yourself up to the risk of a coolant flow problem and potential leak.

You have a p75 head on the right stock, and a pr4 is on the left. Notice the coolant port on the pr4 head at the bottom that the p75 does not have. The p8r headgasket does not accommodate this port which could lead to problems.

Old but a nice right up