sentences:  Only two Hydras were built, the type never entered production.It appears the Hydra was built as a "backup" design in case the newer sleeve valve engines being designed by Roy Fedden at the same time proved unworkable.Design of the Hydra was led by Frank Owner, who built an experimental V-4 design to test a new twin overhead cam design, a first for Bristol engines which normally used pushrods and rockers.