There are three different 3S-GTE heads. These correlate to the three models, ST165, ST185 and ST205. I haven't got references on the ST165 head (no I won't pull mine off to get photos!) so I'll concentrate on the changes made between the ST185 and ST205. I believe the ST165 head is very similar to the ST185.
The shape of the intake port was the main change made with the revision in late 1993. Previously the 3S-GTE had TVIS (Toyota Variable Induction System) which had two intake runners per cylinder (but one large intake port in the head). One of these runners had a butterfly which was closed at low rpm (up to around 4300) to increase the flow velocity down the other runner. This improved mixing and hence low speed torque. I have termed this the "large port" head, just as the 4A-GE has large and small port variations.
However this TVIS system requires a large sized port in the head. Although not visible in this photo the large port tapers quite dramatically. Some people have found that after removing the TVIS and fitting individual EFI throttle bodies, they have had problems getting aftermarket ECUs set up due to insufficient velocity causing poor mixing, something to keep in mind.
The photo on the left clearly shows the rectangular nature of these "large" ports. The photo on the right is the non-TVIS ST205 head. You can just see that the ports are smaller and more oval.
The exhaust ports are significantly changed on the newer engine. On the ST185 each pair is symmetrical and evenly spaced. The ports are quite narrow, almost round, as seen below on the left. Having only one nut at the extremes of the manifold means the nuts are prone to backing off and the manifold warping.
The ST205 ports in comparison (photo on right) are longer (by about 10mm), but are a lower, more oval shape. Additionally the cambelt end port exits the head on quite an abrupt angle, making it longer still to maintain the same throat area. The ST205 has a different flange bolt pattern, with an additional nut each end which hopefully cures the warpage problem the older manifolds had.
The ST185 and ST205 combustion chambers appear virtually identical. The valves are the same diameter but I'm unsure if the ST185 intake valves are stainless with waisted stems (as they are on the ST205).
The photos show the water jackets vary slightly between the ST185 (left) and ST205 (right). The heads are interchangeable however. Just note that aftermarket Japanese gaskets are probably intended for the later engine by now.
In terms of modifications that can be done, the combustion chamber has plateaus where the flat surface of the head extends into the chamber. These have quite sharp edges which will act as detonation initiation points. Also the whole chamber is a rough cast finish. I've had my ST205 head blended to help prevent detonation as shown in the photo below. The ledges were blended in and the cast finish blended smooth.
The head designs are both good, yet the ST205 one is definitely the best choice for modification. The lack of TVIS and dual runners give cleaner flow, which in combination with the smaller ports (without the taper) will give better overall flow. The way the ST185 port tapers down is not ideal for good flow.
The ST205 head is very well finished stock. The gains to be had porting and polishing are not huge, and many recommend for anything less than 400bhp you don't touch it. All I had done was a lick around the rear of the valve seats where the casting suffers a bit from shrinkage.
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