Made a big boo-boo due to inexperience.

Bought a head the seller thought was a B18B high compression head. It was ported and polished and rebuilt. The top of the head was flat with no signs of FI ports being filled. I was quite excited because it looked like a great upgrade...

My car before upgrades was this:

B18D with 0.040" pistons (compression of 155 psi for all 4 cylinders)
Standard head with no modifications and standard head gasket
K cam
original exhaust and manifold

Recent Build:

Comparing the old head we realized the new head had a much bigger combustion chamber and a larger exhaust valve. We were too into the build and likely should have stopped but installed it anyways. Turns out it's a 74 F head unique to the Canadian and Euro market, with the 92mm exhaust chambers and 44mm exhaust valves. The FI ports were non-existent. Probably shouldn't have installed it but did anyways.

What I have now:

Same B18D block with 0.040" pistons (120 psi across all 4 (low compression head)
B20F head with 92mm combustion chamber and 44mm valves
K cam
0.030mm head gasket
123GT siamese manifold
2" exhaust

Thoughts:

The car runs pretty well surprisingly. I find it spins much easier and I can rev it a lot better--probably the exhaust improvements. Just not a lot of torque. I'm not upset but more frustrated that I didn't understand the differences in the two heads. I realize the diameter of the pistons and the combustion chambers don't match but I'm likely not going to bore out my block to 2.0 right now.

Should I take my old head and shave it to B18B spec? Upgrade it?

or

Take the B20F off, shave it to B20E specs. Live with the overlap.

Well, you could bore it out to B20 specs to make best use of this head and your other mods. These late heads are rare and desirable to some due to the improved exhaust ports. Those on the earlier ones are bad and can't be improved unless you go to welding up the port floor and reshaping. Depending on what you paid for it you might make a decent profit. If you then bought a good B18 head and did some basic work on it you might not be too much out of pocket. I have used a B20 head on a basic B18A engine and it ran smooth and sweet. Of course it was low compression but would cruise all day at 70 without overdrive. That was in an Amazon.
Easy way to ID the heads is to look at the top of it. If the cast in Volvo name is outside of the rocker cover just above the exhaust and in the centre it is B18. If it's inside below the rocker gear it is B20.

Good advice. It might be better for me to find someone who has a freshly milled B18 head and who is looking for this kind of head.

I can answer your question by way of direct comparison to what I did. I built a B18 with .040" over pistons and "K" cam. At the time I was the mechanic at an "indy" Volvo repair shop and went through a pile of cylinder heads looking for a good candidate to rebuild. Fuel injected B20's were more recent then and not likely to be in the pile but I figured I'd at least find a good B20 carbed head with 42mm intakes instead of the 40mm B18 valves. What I found totally amazed me--an obviously carbed head (with the locating rings) and the 44mm intakes like an "E" or "F" head would have. I consulted Greenbooks for compression specs and found specs for deck height. I don't recall the height of my head as found but do remember calculating it had a ratio something in the 8.5:1 range. I milled the head .125" to bring the CR up into the 10+:1 range. I left the intake ports alone but port matched the exhausts and cleaned them up a bit. Without a doubt it all worked. I used the 1968 SU's - I think DX needles worked OK. I had helped a local longtime Foreign Car parts house that had a big box of loose needles and I had an SU book with applications. In exchange for organizing their needles I could set aside those that might work for me. After putting many 10's of thousands of miles on it the car (1968 144S) got rear ended. I replaced it with a 1969 144S w/B20 and after I was satisfied the bottom end was good-transferred over the head and cam. While my B20 was still stock I received a "speeder" on a local highway at 6am on a Sunday at 103 mph (local police-pleaded not guilty-knocked down to 65 mph). A Road & Track road test in 1969 listed top end as 103 mph. Later on with the bigger head I did an RPM calculated 115 mph at Watkins Glen. -- Dave

Seems like shaving the head and premium gas will help performance.

Shave the head and only use high test...Make sure your distributor is working correctly on the advancement.

Thanks!