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Here is some much sought after info that i found with a little bit of searching

Sure the forum as a whole would agree to sticky this

Enjoy

The H22A vs H22A4... Here we go:

The H22A is specifically a Japanese Domestic Market (JDM) engine.

Any H22A made and sold in Japan will bear the H22A designation on the block, and only be differentiated by the serial number. This includes engines that were used in every JAPANESE model of Prelude.

The H22A4 is SPECIFICALLY the engine used in the 1997-2001 Honda Preludes sold in North America.

The H22A4 has TWO variants: The base model block and the Type-SH block. The type SH block is machined to allow the proper fitting of the ATTS unit (Active Torque Transfer System) and the transmission. For swap purposes the H22A4[SH] is useless for anyone with a "normal" or non-SH block.

Basic Differences:

1) The H22A series of engines have higher compression ratios: 10.6:1 and 11:1. While the H22A4 series has a 10:1 compression ratio. All engines require 91 or better octane gas, and this is not negotiable if you have an H22A. Significant usage of non-premium fuel will eventually ruin your rings, no matter how much the ECU retards the timing.

2) The early H22A series engines have CLOSED DECK design, while the H22A4 engines have an OPEN DECK design. The closed deck means that aluminum is cast around the tops of the cylinders with coolant port openings, leaving block-to-cylinder bore contact at the TOP of the block. The H22A4 does not have this. Instead, the cylinder bores are anchored only at the bottom of the block, and the head gasket / cylinder head keep them anchored on the top. Mechanically, the closed deck design is stronger. However, the open deck design allows for superior cooling of the top part of the cylinder bores, thus reducing NOx emissions. Later H22A series blocks are exactly the same as the H22A4 blocks.

3) Once again, early H22A series engines are different on the timing side of the engine, since the later model H22A and H22A4 engines are OBD2. This means that the oil pump housing must accommodate different sensors that the old OBD1 H22A's don't require.

4) The crank harmonic balancer bolt is smaller on the older H22A's.

5) The intake manifold is an older design on the older H22A's. ...and on the 11:1 version of the H22A (for the 5 Gen Type-S Prelude) the IM is ported and polished at the factory.

6) The Thermostat housing is different on the older H22A's.

7) The distributors are different on the older H22A's.

Most of the older H22A series engines used type A pistons, whereas the H22A4 pistons are the smaller tolerance type B pistons.

9) The older H22A series engines (lets just call them 4th Gen Prelude engines [92-96]) have different oil pans. (The H22A4 and later model H22A units are aluminum and have a transmission stiffener.)

Basically, it's a matter of the bolt on's and the electronic controls... but only when you are comparing a 4th Gen Prelude H22A JDM to a USDM H22A4.