52mm TB MHB000261

Job just done!
The result is impressive and it's impossible to say it's only feeling or placebo. Not at all: easy to notice the difference before / after.
The throttle response is absolutely immediate, beefy and massive. Not any delay: it's brutish!
Hence it gives very fast accelerations: I'm chuffed!
The diameter difference doesn't look that huge, only 4mm, but in my math that's 17% more air flow which is far from negligible.

Indeed that doesn't make a very sensible difference in bhp: it's the same engine, bore, stroke, you name it but when accelerating it's stunning and it can glean some precious tenths of second if not a few more. Very useful when overtaking or changing gears while uphill: faster than before going up in the revs.
I'm dreadfully curious to know whether my better half will notice any difference: place your bets!

The job was easy but one mod I've to do: drilling another hole to insert the end of the cable on one side and dig a little groove on the same plastic arm at the other end where the cable is to be inserted.
It wasn't intended to be fitted in a 1.4 indeed. That's why.

In the beginning I was puzzled because the length of the cable wasn't adequate then no direct fit but after a good five o'clock tea, I was clear in my mind.
Just enough time to take the exact measures to avoid a mess and that was it.
Then I asked my better half to give me an eye and a foot to verify that all was fine when pushing on the pedal and hey presto rebuilding the whole stuff including the bar strut and testing on the road the brilliant result. And don't forget to push 5 times on the pedal with the key in 2nd position to reset the TB.
Phew!

Yes I was very lucky to find one in England via Ebay for a very fair price (£35 + shipping + custom taxes).
Initially the engine was restricted to 84 bhp (opening only at half). I'd just cut off a little bit of plastic and then the TB was opening fully.
Before the TB swap, my 103 bhp 214 cabby - same engine as it looks - gave me the impression to be swifter, more lively & efficient than the 25 1.4 hatch.
But now as I've driven both cars on the same twisty and hilly country road, I can ascertain that now the 25 1.4 has unquestionably more torque than the 214 cabby.
And mind that they have got both same air box mod (holes) & same K&N filter, and both cars have approximately the same weight.
The only difference now is the TB: 48mm in the 214 cab, 52mm in the 25 1.4.
To be fair I've to add that the ECU in the 25 had been slightly remapped, essentially improved advance. Despite that before the TB swap, the cab looked slightly better.
Today in the 25 I was driving uphill on some parts of the route easily in 4th gear, when in the cabby the 3rd looks more appropriate & efficient.
The 25 is my better half's car: I'm jealous now!

You're quite right as well. The only example I've got is my KV6 2.5 when she was remapped in July 2021 by a well famed and respected race circuit tuner garage.
As expected they took measures before and after on the bench i.e. at the wheels. Hence I've converted them to match with the factory figures done at the flywheel (converted as well from metric value).
Mind that all my mods (air box, cotton filter, resistors for E85 and full SS MIJ exhaust line) had been done before and that the car was fuelled with full E85 (i.e. 85% ethanol in Summer).
The 75 was given 195 bhp before the remap, that is 20 bhp more than from the factory... And 203 bhp after the remap that gave only 8 bhp more, because they were a bit uneasy with the Jatco auto box. They are obviously used to remap more modern and powerful cars nearly exclusively with manual box! But they did a very good job. Mind they tested the car before because they were very dubious about my E85 home made mods (they are doing the conversion themselves). They told me in the beginning that unless the engine and the car were perfect they will not go further. I was shaking in my boots as you may guess... After a very close exam they found her in very good nick for her age and accepted the deal. Phew!

Hence we may guess that both my not tested K4 engines have already made significant gains from their home made conversion to E85. That's another important parameter indeed.
Anyway the difference between the 2 engines after the 52mm TB swap in the 25 does give the evidence that in the 214 mk2 cab the E85 conversion was not yet enough because she keeps her factory fitted TB (by necessity, I haven't found any straight alternative) when the 25 is obviously enhanced by the 48 to 52 swap so much that now the test on the road has inverted the positions: the 25 from second becomes first.
Supplementary holes at the bottom of the air box to feed with more air, and a tad warmer as well (which is meliorative for ethanol but detrimental for petrol BTW) plus cotton filter less restrictive were likely not enough to take full advantage of the difference. Remained a restriction at the 48mm TB. Now it looks that the flow is coherent and not reduced as before.
At least that's the lesson I'm inclined to learn from the experience when I'm taking all data and parameters into account. But you may be right despite that: a TB swap in the first place without any other previous mod might have lead to another conclusion.

EDIT: but incidentally if a better air flow from the box for the above reasons isn't sufficient with a 48mm TB, maybe the reverse may be true: fitting a 52mm TB without improving the air flow within the box. I'd noticed a big improvement at least in the throttle response in my KV6 the day I drilled the holes at the air box bottom...

Some afterthoughts...
As I'm neither an engineer nor a mechanic but a pragmatic man who does some empirical tests, I'm inclined to think that every stage in a chain are the following and that they should be coherent:

1. the air box opening diameter
2. the filter
3. the TB diameter
4. then the mix taking into account several parameters such as kind of fuel & octane, ECU parameters, injectors, you name it
5. the engine itself witch is even more complicated for a ignoramus as I am
6. the exhaust line

I have nothing more to add regarding the 2 first stages, but the 3rd one: only upgraded in the 25, but stock in the 214 and the 75.
I shall say nothing about the stages 4 & 5 I don't know a jot about. That's engineer business.
The 6th stage resulted in 3 different solutions among my 3 Rovers:

• 214 cab: no mod. I didn't found anything suitable so far.
• 25 1.4: swap of the factory back box for a 1.8 VVC one with a slightly larger diameter (+ 0.5 inch) and twin pipes.
• 75 V6 2.5: swap of the factory line for a full SS derestricted exhaust line.

That may be clearer now... in my mind as well.