Breathe Deeply: fitting a 52mm Throttle body

Words and pictures: Rob Bell


Did you know?

The thermoplastic moulded throttle body, in conjunction with the inlet manifold was a Rover Power train first, designed to incorporate low weight and ease of manufacture. The throttle body itself was manufactured, up until 2001, by SU Automotive. If that name sounds familiar, there is good reason: yes, it is THAT same SU (Skinner's Union) company who manufactured the SU carburettors found practically all MG's of yore. So the MGF shares a significant piece of heritage with classic MG's that hitherto I myself was unaware of until I started researching this article. Unfortunately the passing of SU in 2001 means that the MGF is the last MG to breath through an SU "throttle".
However, not all is doom and gloom for SU. Tthe name and trademarks have now all been transferred to Burlen Fuel Systems of Salisbury, UK, so the name lives on. Moreover, Burlen now manufacture their own version of that original plastic throttle body - this time in alloy - in both 48 and 52mm varieties. It makes a useful alternative to the more common Dell'orto versions.

Getting as much air, and therefore oxygen, into the engine's combustion chambers, is one of the primary objectives for anyone attempting to extract more power from their engine. Of course, the process of engine tuning is a good deal more complex than this (one has to consider exhaust gas extraction, not to mention cylinder head porting, for example), but using this simple rule, one can obtain surprisingly good results, even from engines as modern as MG Rover's 16-valve K-series.

There are two options for getting more air into an engine after upgrading the air filter: a larger-diameter throttle body, or replace the entire intake system with a new, multiple throttle body set up. For this article, we are going to concentrate on the former option - a modification akin to fitting a larger choke SU/Weber carburettors on more 'classic' MGs.

The easiest way to upgrade your throttle body is to replace it with the item used on the MGF Trophy 160 (which is also specified as standard equipment on the MG ZR160, MG TF135 and TF160). Whereas all earlier 1.8 litre K-series engines made do with a 48mm throttle (readily identifiable, as they were frequently made from black plastic until recently replaced by alloy items after the 2000/1 model year), the MGF Trophy 160 used a 52mm throttle body. A 4mm increase of the throttle butterfly's diameter may not sound like much, but remember that the area is calculated as πr2 - so increasing the diameter from 48 to 52mm is equivalent to increasing the surface area of the throttle by a whopping 17%.

A larger intake should mean more power - assuming that the engine has the capacity to use it to make more bang from the fuel (the reason why World Rally Championship cars run with air inlet restrictors - to limit power output from their turbocharged engines), so it is no surprise that this swap has become so popular amongst MG enthusiasts whose cars are equipped with K-series engines. The photo sequence shows you how to perform this swap yourself.

Performing the throttle Swap

First Job is to remove the inspection cover - which is covered in more detail here - but below is a brief description of what's involved.

First remove the engine inspection cover. To do this, you must unclip the hood around the rear parcel shelf, and fold up the rear of the hood (taking care when folding the plastic rear screen).
Next, tug and remove both the carpet and sound deadening material. The inspection cover is now exposed. This is held into place by a number of 10mm bolts. Take care when removing those located under the ‘T-bar’ trim, as there is a sharp edge to catch out the unwary.
If you have a late MGF with rear speakers, access to these bolts can be tricky due to tight access under the speaker box. An open or ratchet ring-spanner are useful for these.
Once undone, tilt and remove the inspection cover.

Looking through the inspection hatch above the engine, identify the throttle body and the throttle position sensor. Remove throttle position sensor loom plug, and carefully put this to one side. Now disconnect the rubber hoses leading to the throttle body: these are the oil breather hose (lower) and the idle air control valve hose (upper).



Now undo the four 8mm retaining bolts that hold the throttle body to the inlet plenum. These are best accessed from the boot inspection panel (the grille is readily pulled clear, pinned in place in three locations along the forward edge). A ratchet ring spanner is perfect for this job – and almost essential for the furthest bolt that is effectively hidden under the rear deck panel. Withdraw the throttle body upward and backwards, as illustrated. Note the path of the throttle cable as it passes over and around the throttle cam on the throttle body - in this case, indicated by the red arrows. The cable is retained in place by a 'T' piece - its location on the cam is here indicated by the white arrow.


Now remove the throttle cable from the butterfly valve cam. Do this by removing the outer cable retainer (slots out) to release the tension on the inner cable. Now the ‘T’ piece at the end of the cable can be withdrawn from the throttle cam. Compare and contrast: the original black plastic ‘SU’ 48mm throttle body (right) next to the replacement ‘Dellorto’ 52mm throttle body (left).
A sensible step at this stage is to remove the throttle position sensor from the old throttle body and replace it for the one on the new throttle body (it is held on by just a couple of small bolts). Doing this avoids the need to visit a specialist with Rover's Testbook to reset MEMS calibration for the new sensor.
New 52mm throttle body, just before installing - a process that is, I promise (!), a simple matter of a reverse of the removal instructions!
Notice the Dell'Orto insignia located adjacent to the throttle position sensor - a departure from the SU it superseded.

Now turn the ignition on (position II), and stamp on the throttle pedal five times in 20 seconds, and switch off. This resets the baseline throttle position sensor reading in the engine management system.

You are now ready to go!


Comparison of 52mm versus 48mm throttle body on an otherwise standard 1.8MPi engine - not a great deal of difference, although there are modest torque gains here and there. Red line = 48mm throttle body; green line 52mm.

Does the conversion work? We decided to find out at Dave Walker's rolling road at Emerald.

We compared maximum power and torque of the standard 48mm throttle body installed in my 1.8i MGF against it equipped with a 52mm throttle body. The absolute peak power gain appeared to be somewhat disappointing: we saw an all-but negligible 1 bhp difference between the two - very disappointing for a 100 quid investment in the new alloy throttle body casting.

But that's not the whole story. Whilst the torque does not appear to have increased much with the 52mm throttle body, there is an improvement to the tune of 3-4 lb.ft over much of the lower rev-range. Out on the road, this modest increase in torque, allied with a much sharper throttle response (which is largely thanks to the increase in cross-sectional area of the 52mm choke and throttle butterfly; any small throttle movement would result in a proportionally much larger opening through which air can flow with the 52mm than compared to the 48mm throttle).

The result out on the road was therefore much more impressive than the absolute gains would suggest; through the gears in town, the car seemed hugely more energetic, lunging forward with much more vigour with small throttle openings. One suspects that one would see much more of a power gain if used in conjunction with other engine modifications - but that's a test for another day!

Is it worth the investment? Well, I had a big smile on my face that couldn't be quantified by measures of bhp, so yes, if you feel that you can justify £100+ on an upgrade that is worth a bhp, an improvement in 'drive-ability' and a lot of smiles, then go for it. Just don't expect too much.