Technical: The 2.5 Duratec I4

There is no shortage of myths and rubbish written about the 2.5 litre 4 cylinder Duratec, here are some facts.

Whenever something goes wrong with an NC engine someone always says ‘fit a 2.5’ even in the UK where they are rare and expensive compared to the US, where they have read about how cheap they are.

There is no doubt it is The Daddy. The widest bore* and the most CC takes care of that. In standard form (with its original large bore inlet manifold) it’s got a very flat torque curve, some would say dull, lazy and even diesel like in its delivery, yet in a light car like an MX-5 it’s not a bad move and certainly a relaxing drive whilst being quick when it wants to be. You have to remember it’s come from a van and heavy large family car so won’t struggle to power a much lighter car along briskly and with ease.

Look at the torque spread there, it’s almost flat from 2500 to 5000. For the record a standard 2.0 VVT makes 190 Nm @ 6700 rpm.

*You may wonder why I noted the bore size, well the 2.3 has the same bore as a 2.0, but longer stroke making it a more ‘torquey’ engine of the two, but with less of a revvy nature although It can be massively improved with the right mods. The 2.0 is known as an over square engine whereby the bore diameter measures more than the stroke so will rev safely to a greater RPM. A good example of an oversquare engine which really does rev well is of course any F1 engine from the last 20yrs or so. Thinking of overboring a 2.3 block to make a 2.5? Or even massively over boring a 2.5? Forget it, the bore walls aren’t thick enough for that or re-sleeving either and boring a 2.5 to more than Mazdas recommended will lead to a weak engine and the problem of where to get a head gasket from. Be very wary of anyone who claims to be doing this.

The 2.5 engine does not have:

Bigger valves

A higher flowing head

A forged crank

Ductile steel liners

It has:

Cast iron liners and crank

+10bhp and at 700 less rpm

+30ftlbs of torque at 1000 less rpm

The Mazda version has forged steel rods, the Ford one doesn’t.

Balance shafts

Both the 2.3 and 2.5 have a balance shaft assembly hanging underneath, it’s usually removed when fitting the engine in another car due to its size, weight and it saps some of the power from the engine to drive it. These are fitted to counteract second order forces and vibrations which naturally occur in an I4 engine. The bigger the engine gets, the greater the vibration. Removing them won’t damage anything, re-balancing the engine will not get rid of the vibration, nor will a crank damper.

“The ports are bigger and flow much more” is often bandied about on the internet. ‘Bigger’ is a very general term when it comes to ports, you need to be more specific. We look at that in more detail in another blog, but I can tell you that only the inlet port entry is larger on a 2.5, the rest is just the same as the 2.0 and 2.3 VVT heads and that includes the governing parts of a port (the choke points) which are the valve throats and the ‘Min CSA’ which is the minimum cross sectional area of the port.

In my opinion, the best way to get the most out of a 2.5 for the road/occasional track on a reasonable budget would be a set of higher CR pistons, cams and a remap on 99 Ron fuel. 200+bhp and a shed load of torque won’t be a problem with those applied to it.