Originally Posted by HockeyNomad

Thge heads are all interchangeable on all the zetec engines, the problem comes from the chamber size, it is different in each head, and can throw the compression ratios out,

Second hand focus RS gear boxes seem to be changing hands for circa £500 recently and they come with a Qauiffe ATD diff in them, and also have the benifit or shot peened cogs and revised gear ratios,

Be careful with getting a gearbox from an older zetec as they may have different mounts, for fitment to and escort or mondeo etc, and could need some custom work to get them fitted,

As for the flywheel, im not sure if they will match up or not (i think its smaller), but its a good idea to get the RS flywheel as it will allow you to use the uprated clutches available,

The inlet manifolds are the same on the 1.8, 2.0 and RS engine, all that changes is the throttle body, if youve ever looked at the plastic throttle body on a 1.8 engine then you will see its defo worth upgrading, its the same physical sice as the other ones but the throttle plate is tiny,

the rs exhaust manifold will fit without problem, there are many tubular exhaust manifolds available, from the likes of blueflame, Nortech and Primary Designs,

but i will say it again, it will be easier and simpler to just go for a full rs conversion, and most likely easier on your wallet too,

the standard focus RS engine can see 300 bhp without much work and more than that with a simple turbo upgrade, it has the benifit or forged mahle pistons as standard, the cams are proven to be good right up to nearly 400 bhp,

but if you really are determined to boost the 1.8 go for it, be a good little project,

another thing you could look at is the jackson racing supercharger upgrade if you want boost, might give you enough of a boost to keep you happy,
have aa search on the us ebay should show you what is available and prices etc, they are designed for the 2.0 zetec, but im sure could be used on the 1.8 without much work,