Mk1s came with a version of the Crossflow — the XR2 running the biggest at 1600cc, and designated 771M. It’s a unique block with no side mounts, while it’s overall length is shorter to fit in the confines of the Fiesta’s engine bay. Therefore, it has its own crank, water pump and clutch system to compact it. Apart from these differences, tuning is pretty much the straightforward Crossflow route — the biggest capacity will instantly give you more power, providing a great base too.The standard carb will release, circa 110 bhp coupled with up to a Kent 234 camshaft. After that 40 DCOEs release a further 15 bhp and provides the way for future performance. The rest is standard Crossflow stuff: bore it out to 1700cc, fit a ported, big-valve head, a purpose-made four-branch manifold, suitable electronic ignition and decent exhaust and you should have power to around 145 bhp without steel components and according to state of tune. That is enough for a lively car since the Fiesta is light — there’s many an example running mid-12-13-second quarter miles with Crossflow power, which certainly isn’t shabby!
Tuning the Crossflow? Start with the 1600 version if you can.