Tech:Engine/A Series/Heads

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. 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 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 revheads.

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. It had a reatively wide angle between valves at 12 degree but narrower than a 16V, but the biggest difference is that it has 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, 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. 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 sujest it is adventagous onlu up to a 800cc engine.

Parts for the 20-valve head are more expensive than the 16-valve head (eg. about 20% more for Toda camshafts). Unlike the 16v heads the 20V uses bucket over shim design unless the 16V engine

more to be added shortly