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Old 20-01-2019, 12:42 PM   #21
Luke Plaizier
Lukeyson
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Join Date: Mar 2005
Location: Maitland, NSW
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Default Re: WQ Fiesta XR4/ST150 with Mazda SP25 engine

Summary of interesting findings:

2.5L stock motor is good for 430whp at around 18lbs. Confirms comments earlier and elsewhere.

To accommodate the increase deck height in the MX5, they've had to fabricate slightly lowered engine mounts for the Manifolds to all fit. So not sure if this is also a thing for the Fiesta.

They talk about swapping sumps but that's only to accommodate the RWD conversion. I expect no sump swap in the Fiesta. Also, the sump swap means that the Balance Shaft has to be removed. But I'd just as soon keep it on the 2.5.

Just about everything swaps across - there's talk of needing to move the PCV valve over from the 2 Litre. Some talk of having to change the loom plug over form Mazda to Ford for the intake VVT controller (and yes, I'd incorrectly stated that the Exhaust cam was phased in an earlier post- my bad).

Crank pulley would need to be swapped - it has a different number of teeth between Ford and Mazda.

All the Ford ignition gear should fit into the Mazda head.

The 2.5 head has much bigger exhaust ports and flows significantly better than the 2.0 head - even better than some of the special 2.0 Race-Heads that some companies were selling aftermarket apparently. So the big thing will be the jump in port size when bolting up the 2.0 headers. If there's any way that the 2.5 headers can be used, I'll give it a go. Otherwise the suggestion is to die-grind the inside of the 2.0 headers to better match the 2.5 head. But the 2.0 headers will then be a restriction going forward. The 2.0 headers need a gasket, the 2.5 headers are apparently grooved O-Rings. You could use a 2.0 Gasket and get within 1mm of the 2.5 ports. Keep in mind that this is all MX5 specific so I'll have to pay attention to differences if tried on the Fiesta.

The factory ECU on the MX5 was fine running the 2.5 swap without too many mods, but as the mods stack up the ECU might start to struggle. The factory map can start, run and putter-around the 2.5 fine, but don't do a power run. The 2.5 has slightly bigger injectors, keep those.

There are lots of individual posts about certain sensors needing to be swapped from the Ford into the Mazda block. I can't recall where the crank angle sensor is - whether that is what's on the pulley or not.

The 2.5 swap looks to be what everyone is recommending in MX5 land - if you have a high mileage NC MX5, do the 2.5 swap first, and than start your mods. The NC MX5 is also down close to 1100kg and in itself looks compelling - but the things are still around $12k, so some 3 x the price of a similar aged XR4/ST150, which takes it off the table for me in the short term as a cheap shed-friendly nugget build.


Lukeyson
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