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MG MGF Technical - 56mm Throttle Body


I was wondering if anyone has fitted a 56mm throttle body to their mgf? as i have noticed there are quite a few for sale on ebay. Also if anyone could please answer the following questions:

1. would there be any performance increase from the above item?

2. is it easy to fit?




56mm sounds huge. I have the 52mm body which is quite common now. I can't say it has improved performance, but the pickup, especially at low revs is vastly improved. The 52mm body is a direct replacement, so it was very easy to swop over,then reset the MEMS with 5 dabs of the accelerator. So far I havn't found it necessary to make any other adjustments.


Thanx for the advice chris,will research on the 52mm throttle body. however i would still like a few more peoples opinions/advice regarding the 56mm throttle body.


We have a 56mm throttle body on our VVC engined midget. In a word, great! Especially as it bolted straight on with nothing more than a minor matching of the plenum with the die grinder.

Paul Walbran

Hi Matt,

A performance increase from enlarging the throttle body only comes if the original size was too small, i.e. restricting the engine. Largely, only when you've done extensive modification to the rest of the breathing of the engine, i.e. porting, higher lift cams etc will the standard TB actually need upgrading. Many who have fitted larger TBs have done so purely because they are alloy units and therefore not prone to the warping that can occur with the standard plastic item.

A larger TB does as Chris says change the 'feel' of the engine, but the reason for that is quite simple - a larger TB flows more air for a given angle of throttle. The 52mm diameter TB flows 17% more than a 48mm, and the 56mm 36%. Rolling road tests confirm that outright performance on standard engines is unchanged. I've seen some pretty outrageous claims being made by some eBay sellers, but let's face it - Rover only fitted the 52mm TB to the 160bhp engines because their extra breathing warranted it (just), on the stock MPi & VVC engines the 48mm item is perfectly adequate.

In terms if fitting, you'd probably be better off going for a 52mm item, it's a simple bolt-on whereas the 56mm doesn't have the crankcase/oil breather connection so requires a separate filter to be added, which will require the cutting or re-routing of the breather pipe. This isn't much work but could be an extra headache if you wanted to return the car to standard for selling on. Have a look at Rob's page on the subject -

Personally, I'd be inclined to put the money towards an induction kit like the K&N, where you'll not only get a definite power increase but a nice induction roar too ;o)
Mike Hankin

Dave andrews has reserched this and found that going larger than 52 actually causes a decresse in performance.

The increse in feel comes not from the fact that the throttle plate flows more for a given position, but that on transient from closed to WOT the vacume in the manifold clapses faster - you wouldn't notice the diffrence from 35% open to 30% open - if you could then you could make the car feel faster by shortening the throttle cable ;-)
Will Munns

I didn't see a loss of power with the 52mm throttle body, but then I didn't see a big gain either - barely a horse power:

A 56mm GM throttlebody (I assume that this is what's being discussed?) is fine, but sadly, you probably won't get any performance advantage from it (as Mike rightly says) based on our experience with the 52mm throttle body.
Rob Bell

This is the same question as is often asked on the MGB board (I have been an MGB owner for many years, but am new to the F). Lots of people think that adding bigger carbs to the MGB will give more BHP. Usually the opposite is true. The size of the carb or throttle body is carefully matched to the breathing characteristics of the cylinder head. If the cylinder head is as standard, the bigger air inlet will slow down the incoming air and power will be reduced. Only when the engine is able to use more air (due to gas flowing and camshaft changes) is an upgrade worthwhile.

Mike Howlett

b's are different to f's though, because the carbs need decent airflow to atomize the fuel, where as in the f the equivilent mistake would be to stick in high flow injectors...
Will Munns

The 56mm TB's advertised on ebay are mostly from the 2.0 T-Series engines I believe. The main problem with fitting the 56mm to the 1.8 K-Series is getting the car to idle smoothly, if at all.

This thread was discussed between 22/05/2006 and 26/05/2006

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