Basically there are four different types of head available for the A-series motors. This information is definitely applicable to the 4A variant, but the heads can also be used on the 7A (1.8L block). The heads are as follows:
SOHC - this version was used on the 4A-C version of the 4A. It was available in some Corollas (AE71, AE82) and the Australian-spec Sprinter. It was created because it was cheap, and is not designed for high-performance or even economy. I features a wedge shaped combustion chamber in a non-crossflow design. While it would be possibly to improve the performance of a 4A-C while sticking with the SOHC head, there is no point, as it would be cheaper to swap to a DOHC head.
DOHC (F) - this is the narrow angle head used on the majority of 4A motors. It was available in Corollas from the AE92 through to the AE102R. It was, like the SOHC head, a low performance economy design, however it had 16-valves instead of 8. It can also be modified to provide more power, but also like the SOHC head, would require many parts to be custom made (eg. cams) and it would therefore be cheaper to swap to a DOHC (G) head.
DOHC (G) - the G-head is the most common of the performance heads available for the 4A motor. It had a wider angle between valves than the F-head, creating more theoretical potential for higher power, however it stuck with 4-valves per cylinder. This engine was available in Corollas from the AE82 Twin-Cam through to the AE101. This head has the most aftermarket performance parts available to it, making it the best option for many revheads. It features a shim over bucket design that while under normal use is highly reliable can however provide problems with high lift cam-shafts causing it to "flick a shim". This is where the edge of the cam lobe will push against the side of the shim causing damage. It can be converted to a bucket over shim design using either aftermarket or OEM parts (off a 1GG engine I believe).
DOHC (G 20 valve) - this variant of the G-head first became available in the AE101 Corolla in Japan and only lasted until the AE111. While it was designated as a wide valve angle Head by Toyota (G -series head) the angle between valves it was much narrower than a 16 valve at 12 degrees. However the biggest difference was that it had 5-valves per cylinder versus the 16-valves' 4-valves per cylinder. This is an uncommon setup, used only in a few other engines including the 3.6L V8 in the Ferrari 360 and the 1.8L Audi 4-cylinder (Audi S3). Interestingly there is some debate to be had on whether a four valve head is adventagous over a five valve, the reason being of the larger area shrouded between the intake valves which restricts flow. Some theories suggest it is adventagous only up to a 800cc engine. The two main advantages that the 20 valve engine had over the 16 valve and they were that firstly it had a far superior intake port angle. On a 16 valve engine it had a conventional angled short port, however on a 20 valve engine it had a very long and straight port much like a motorbike head. The other feature that both the 20 valve engines had that none of the other 4A engines has is a form of variable valve timing. It was variable in only the crudest form being in that it was just a two stage step in the intake cam valve opening angle. While the 20 valve already had a bucket over shim design it was fitted with small buckets that are very limiting with maximum valve lift. It is generally regarded that 9.5mm of lift is the maximum without special measures before the cam lobe starts to push a sideways force on the bucket. Interestingly the highest power figure heard of from a naturally aspirated 4A-GE 20-valve is about 220hp, whilst the 16-valve has been known to produce 250hp whilst naturally aspirated. This could be put down to the over shrouded valves combined with the limited valve lift and lastly the fact that the valves can only be upsized 1mm. Parts for the 20-valve head are more expensive than the 16-valve head (eg. about 20% more for Toda camshafts).
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