As others have said if the exhaust manifold is getting red hot then it's usually valve timing (I had this happen with a Ford Capri 2.0 pinto after engine change - low on power and manifold glowing red! - was only one tooth out)
However , whilst it was low on power it would drive much faster than 40mph.
Could I suggest another potential issue - has the new engine got a faulty wasted spark coil pack , this would only give you real power on two cylinders (hence 40mph) and would throw unburnt fuel towards the catalyst (this would get rahter hot!) - Just a thought.
New coil pack cheap (worth changing anyway) but if this has happened the catalyst my be cream crackered now (it can melt the core!) but these aren't very expensive these days
Edited by Big John on 27/02/2016 at 10:19
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