Definitely an excellent result Andrew, and very helpful evidence that further supports the theory Monty is hiding a restriction.
For comparison with your Webers, Monty's runners are 45mm OD, so 42mm ID when the wall thickness is taken into account, and it's well understood 42mm runners will easily support 230hp.
![[Linked Image]](https://i.ibb.co/x8GXT62J/Fusion-Fabrications-Inlet-Runner-Size.jpg)
The interesting fact here is a Duratec head is known to flow vastly more air than a Zetec head right out of the box, it features highly optimised port angles and thinner 5.5 mm valve stems. Monty's Duratec head is also far from standard, Zetec intake valves are 32 mm and the exhaust valves are 28mm, Duratec intake valves are 35 mm and the exhaust valves are 30mm. However, I've gone 1mm bigger still so 36mm & 31mm meaning I've ended up with a massively bigger valve area than a standard Zetec.
The port holes on a Duratec exhaust manifold gasket are 44.5mm but Monty's exhaust primaries are 42mm ID, so it was pointless taking the ports out any larger than 42mm. While heavily worked by A.L Developments in the bowl areas to work with the 1mm larger valves, and with nicely reshaped throats, when the head arrived I was a bit surprised to see the exhaust ports were left at the standard 38mm diameter.
Alan claimed this was intentional for torque, and while I understood and accepted the theory, I also felt we could take the exhaust ports out to 40mm without any negative effects, so that's what we did. My argument was bringing the ports out to 40mm should still give good gas velocity, and a 2mm anti-reversion lip would also still be sufficient to counter the effects of the considerable valve overlap produced by the Newman Phase 3 cams. Subsequent testing has proved I was right as Monty has a nice stable idle, and wonderful low end torque too.
Without any question Monty's big valve fully ported head running Newman Phase 3 cams and 160 lb beehive valve springs will support 270hp, so that just leaves the inlet manifold and exhaust system to hold the engine back. Monty's 42mm exhaust primaries should support 250hp if I paired them with a set of 45mm ITBs, but even on the 42mm inlet runners the whole system should definitely not be producing any form of restriction up to 230hp minimum!
Last week I had a meeting with James at M2R Motorsport to discuss the missing power, don't get me wrong I'm very happy with the results but it's quite clear Monty still has a lot more to give. James who fitted the cams and head was genuinely surprised and disappointed we only made 182hp, his comment being
"I was genuinely expecting close to 230hp". I suggested the FF long runner inlet was the likely restriction, but James countered this theory by saying his ST150 with the same cams and a similarly ported head but on standard vales, gave 213hp.
James went on to explain this was achieved on the stock plastic Ford factory inlet with the swirl flaps still in place but held open, and the truth is while it is proven to deliver more torque, the FF inlet isn't actually massively different to the factory inlet. James has also seen many Duratecs making 200hp plus on the FF inlet, and Matt at Fusion Fabrications himself claims his inlet has been proven to 230hp. There's definitely something else at play with Monty, even on a standard head, ST150s running Newman Phase 2 cams and an FF inlet will typically make 190hp. This was Phase 2 Monty spec, but in this guise he only made 172hp so at least 10% down on our expectations even back then.
Clearly we need to find the restriction that's been hindering Monty's true potential, and there's now strong evidence to suggest it's not actually all about the FF inlet as we've long assumed! If we now take Monty's Stage 3 result of 182hp and add 10% we get 200hp, but it seems highly likely there's more to it than that. Comparing Monty's Stage 2 & 3 graphs reveals the same flow restriction continues to hold Monty's Duratec back around the 180hp mark, and this even with the new trick cylinder head, bigger vales, and the Newman Phase 3 cams.
Having closely studied all the evidence, James and I both concluded it now looks like it would be a mistake to put all the blame at the door of the FF inlet. The way I see it, this is actually really good news as it suggests when the restriction is found and removed, I'll be left with the same super flexible Stage 3 Monty under 180hp. However, over 6,500rpm he should continue piling on power all the way out to 8,250rpm. If we follow the direction of travel on Monty's Stage 3 dyno graph everything points to a peak figure of 213hp, although it's entirely possible the restriction could be starting to hold the engine back far earlier around 5,000rpm, in which case we'd likely make 220hp plus.
With all this in mind, my suspicions are now swinging towards the rest of the exhaust system, it's certainly big enough and the primaries are equal length too, and don't get me wrong Matt at Fusion Fabrications certainly built me a lovely full system from cylinder head to tail pipe.
![[Linked Image]](https://i.ibb.co/7Xjz7JT/FB-IMG-1675526311267.jpg)
![[Linked Image]](https://i.ibb.co/zmp0fQF/IMG-20230213-WA0005.jpg)
![[Linked Image]](https://i.ibb.co/MS8BmTT/IMG-20230213-WA0007.jpg)
![[Linked Image]](https://i.ibb.co/hXWdf1B/IMG-20230213-WA0008.jpg)
However, I'm now starting to wonder if the internal honeycomb structure has collapsed inside the so-called 'Spots Cat'

![[Linked Image]](https://i.ibb.co/nCpr2s8/IMG-20230213-WA0006.jpg)
Another significant piece of evidence is I'm logging 100 kPa all the way to 8,250 rpm, seeing atmosphere makes the intake even harder to blame as it virtually rules out an upstream intake restriction. If the FF inlet runners, plenum size, throttle body, or air filter were choking the engine, you'd expect manifold pressure to fall away as airflow demand increased.
For example:
3,000 rpm: 100 kPa
5,500 rpm: 98 kPa
7,500 rpm: 94 kPa
8,250 rpm: 90–92 kPa
The above would indicate the engine is trying to draw more air than the intake system can supply.
Instead, I'm seeing numbers that are typically:
3,000 rpm: 100 kPa
5,000 rpm: 101 kPa
6,500 rpm: 103 kPa
8,250 rpm: 101 kPa
That's telling us the plenum is staying essentially at atmospheric pressure.
However, a partially collapsed substrate in the sports cat would behave like this:
* Idle perfectly
* Cruise perfectly
* Feel strong in the mid-range
* Only become restrictive once exhaust mass flow gets very high
Which is exactly what we're seeing here.
![[Linked Image]](https://i.ibb.co/Z6rTgx2F/IMG-20260620-WA0012-1.jpg)
While Monty's above horsepower line clearly goes flat from 5,500rpm to 7,900rpm, the below Caterham 420 with presumably no exhaust restriction goes from a Monty matching 180hp at 5,500rpm, to 233hp at 7,800rpm!
![[Linked Image]](https://i.ibb.co/tpyTSwXL/Caterham-420-on-ITBs.jpg)
The evidence pointing to an exhaust restriction above 5,500rpm is building strongly chaps

Remove the restriction, and Monty really should do this......
![[Linked Image]](https://i.ibb.co/TQF0C1n/IMG-20260620-WA0014.jpg)
Finally, when I recently had Monty on the dyno we did a second run advancing the ignition timing by 2°. In doing so Monty only gained 2 hp, and that's exactly what I'd expect to see if the engine had reached a physical airflow
or gas-exchange limit rather than simply needing more ignition advance. It also reinforces the idea that the missing horsepower is likely hidden in the engine's breathing and as we're recording atmospheric pressure it's highly unlikely to be a restriction on the inlet side, it's also not the calibration as we're pinning the optimal 13.0:1 AFR under full load & wide open throttle.
So despite Monty having a rather loud exhaust, noise is not always proof there's no restriction, catalysts are a classic example of this as a partially blocked catalyst often produces:
* Normal idle
* Crisp throttle response
* Excellent mid-range torque
* A very loud exhaust
* Disappointing top-end power
That's because the engine isn't trying to move huge quantities of exhaust gas until the upper rev range, below 5,000rpm the catalyst may cope perfectly well. However, above this and increasingly all the way to 8,250 rpm exhaust mass flow goes up dramatically, it's only then the restriction becomes significant enough to limit power.