We recommend for both the 1.4L & 1.6L Vauxhall, the later triangular ported head (see picture right). The 1600cc cylinder heads with the triangular ports are found on the X16XEL engine from the Astra/Zafira from 1998 to now. In tests we have found that the triangular port shape, which has a smaller entry gives good gas speed and therefore improves throttle response, also the overall shape of the inlet tracks gives even better flow than the early GSI head.
Triangular Port Cylinder Head, Part no 90 400 187
As a second option, you could use the early GSI head (see picture below), this has the larger oval port size with a slight raised arch in the middle. These are very rare and only fitted to a few cars but we would still recommend the triangular port head as the best choice.
Corsa GSI cylinder head, Part no 90 472 726
GM produced a huge variation of cylinder head castings for this engine design, there is even a version that is similar to the GSI head in shape but much smaller, this head produces at least 10bhp less than either of the 2 recommended heads and cannot be modified to overcome this issue.
Due to the variation in GM designs and port sizes, the inlet manifolds that we produce are made with a small port size, which can be modified by the user to suit various 1.4L/1.6L heads. Unfortunately it is impossible to manufacture a manifold for each of the GM head designs.