trophy_TB_200.JPG (7658 bytes) Throttle Body

Words and Pictures: Rob Bell

What is a throttle body How do throttle bodies work Idle control
Did you know...? Sticky throttle problem Alternative throttle bodies
  Fitting a 52mm throttle body  

What is a throttle body?

TB_Trophy_200.JPG (12240 bytes)
Aluminium 52mm throttle body shown in centre of picture above.

The throttle body is to modern engines what carburettors were to engines of 30 years ago. They are devices that regulate the quanity of air being allowed to enter the engine, and thus, indirectly, alter the fueling. The throttle is modulated by, you guessed it, the throttle pedal - the further you depress the throttle pedal, the wider open the throttle butterfly, the more air that enters the engine, the more fuel that is burnt, and the more power the engine produces. More power, more acceleration.

Where can the throttle body be found? This is shown in the picture, right: if you open the boot of your MGF/TF, and peer into the engine through the inspection grille, you'll see the inlet plenum. To the left of this, you see the throttle body itself (the silver item in the centre of the picture), which is also attached to the air filter.

There are three types of standard throttle bodies available for the MGF/TF. The first is a black plastic item, with an internal choke measuring 48mm. The second (fitted on MY2000 onward MGFs) is an alloy throttle body with the same 48mm internal choke. The final variety of throttle body is the 52mm internal diameter choke alloy item found fitted as standard to the MGF Trophy 160 SE, the MGTF 135 and MGTF 160.

How do throttle bodies work?

TB_48mm_aspects.JPG (26252 bytes)

Throttle bodies are essentially very simple devices, yet manufactured to within extremely tight tolerances.

TB_48mm_opening.JPG (12402 bytes)

As mentioned above, the throttle body is, in some respects, similar to a carburettor, in that it basically consists of a length of pipe in which a flap, hinged through its centre, acts as a 'butterfly' valve to regulate the quantity of air passing though it. Unlike a carburretor, however, the throttle body does not regulate any fuel flow - this function being performed by the fuel injectors in the plenum. The more the butterfly valve is rotated, the larger the opening of the throttle (illustrated opposite, right). The narrowest point in this pipe is the choke - to which the butterfly valve's diameter  is matched.

On a standard throttle body, the diameter of the butterfly valve is 48mm. On the "Trophy" throttle body (so-called, because it was first found fitted to the MGF Trophy 160 SE), the butterfly diameter is 52mm. A 4mm increase in diameter may not sound like much, but because the cross sectional area of the choke is calculated by the equation, pr2 (where p is Pi = 3.142, and r is the radius of the circular butterfly valve), the increase in diameter of the choke from 48 to 52mm results in a 17% increase in area. Result: much more air can flow into the engine (because the resistance to flow is inversely proportional to r4).

TB_potentometer_200.JPG (8933 bytes)

The throttle butterfly is operated by the throttle cable, whose linear motion is converted into the necessary rotary action by the throttle cam on the side of the throttle body (shown in the pictures near the top of the page). On the other end of the butterfly axle is a throttle position sensor (TPS). A TPS is a small potentiometer  (pictured opposite, right, removed from the throttlebody) performs an essential function, as it provides the engine management system information regarding the position of the throttle, and therefore engine load - vital for the fuelling and ignition mapping for optimum performance and efficiency. TPS sensors are normally used on performance engines where airflow sensors may be confused by pressure pulses in the inlet tract. Airflow is not measured, but calculated based upon engine speed and throttle position. If the engine is significantly modified, the airflow characteristics may change beyond the limits of the MEMS fuelling/ignition map necessitating re-mapping (which usually means replacement of MEMS with an aftermarket programmable ECU).

Other throttle body functions...

You may have noticed that there are two other pipes leading into the throttle body, that appear to have little to do with the basic breathing functions of the throttle. The lower of these two pipes is for the oil breather system, that allows fumes from the engine to be recirculated and burnt in the combustion chambers. The upper pipe is the idle control air inlet from the idle air control valve (IACV).

Idle control and IACV

A fully warmed engine at idle has fairly constant airflow requirements and therefore the ignition advance and the idle speed can be set at a constant rate. If any of the environmental conditions vary (engine temperature, air density etc.), then the required airflow, ignition advance and fuelling may need to be altered in order to compensate and maintain an even idle speed.

In carburettor-based systems, to compensate for a cold engine, the choke is operated (which opens the throttle butterfly independently of throttle pedal position) thus raising the idle speed to prevent stalling.

The MEMS system used on the MGF and TF, uses an idle control system whilst the engine is idling: an idle air control valve (IACV) allows the air to the engine to be metered independently of the throttle butterfly. If the engine speed falls below an acceptable limit, more air is bled into the engine. If the RPM goes beyond an upper limit then less air is allowed in. Together with fuelling and ignition variation this system maintains a steady idle with acceptable emissions in all conditions whether the engine is hot or cold. Note that MoT requires that the warm idle is within about +/-500rpm of the manufacturer's specification, so this is a system that needs to be well maintained.

Did you know...?


su.gif (2545 bytes)

Burlen Fuel Systems

The plastic throttle body fitted to all MGFs unil 2001 were manufactured by S.U. Automotive Ltd., Birmingham, UK. Yes, that S.U. (find out more about the history of Skinner's Union, click here). So, like all its MG predecessors, the MGF breathed through an S.U.! A bit of historical continuity that I for one was previously unaware of...

This also means that the MGF is the LAST MG to breath through an S.U. throttle... frown.gif (374 bytes)

Unfortunately, the famous S.U. company finally ceased trading in November 2001, which co-incidently, is approximately the time that the plastic throttle body was superceeded by the aluminium type throttle body (manufactured by Dellorto) that is currently employed on late model MGFs and TFs. ALL S.U. trademarks were then transfered to Burlen Fuel systems of Salisbury.

Footnote: Dana Automotive and Fluid systems now trade from the site that was SU automotive. All S.U. trademarks now belong to Burlen Fuel Systems, Salisbury, UK.

* The plastic throttle body (plus plenum and fuel rail) used on the K-series (and therefore the MGF) was the first of its type to be put into mass-production for a road car in the world. The thermoplastic used in the manufacture of the plastic throttle body were supplied by DuPont using DuPont Minlon engineering thermoplastic resin.

The "Sticky Throttle" problem...

The "sticky throttle" problem is said to occur when the engine is hot, and the idle speed fails to return to it's 850rpm idle speed - staying in the 1500-2000rpm range. Very annoying. There are a number of potential causes, including:

Cause Solution
1. Sticking throttle cable This is a rare cause, but can be caused by lack of lubrication of the inner cable, or by a kink (too sharp a turn) in the cable run. Inspect cable for damage, straighten as much as possible, and re-lubricate as necessary.
2. Sticking butterfly valve Dirt in the throttle body: clean with a proprietary carb cleaner, such as the valvoline product mentioned here.
3. Sticking butterfly valve Distorted throttle body: can be caused by heat in the engine bay, but more frequently is due to injudicous over-tightening of cheap jubilee retaining clips on the hose to the airfilter. Best avoid using jubilee clips where possible, as they assert their clamping force notoriously unevenly.
4. Problem with the IACV The IACV and associated pipework may need cleaning, or the stepper motor attending to.
5. MEMS keeping engine running in cold, fast idle mode inapproapriately Easily diagnosed if the engine appears to run hot in traffic. Problem caused by MEMS being fooled into thinking engine is cold when it is actually warm - usually this is because a fault with the temperature sensor, its contact with the loom plug, or a broken wire in the temperature sensor loom. This problem is commonly solved by simple replacement. See here for more details on how to do this.
6. Throttle position sensor returning false reading Rare cause of this problem- but easy to rectify if present: to reset the TPS, switch ignition on, and depress throttle fully five times in 20 seconds, and switch off for 10 seconds. This resets the TPS.
7. Broken wire to TPS It is not unheard of for the wires to the TPS to suffer an internal break, thus causing an intermittent signal. Needs to be diagnosed with the use of a circuit tester - and if a problem found, the offending section of wire removed and replaced. (The TPS may itself be found to be faulty, and need replacing)

Alternative throttle body options

If you want more power from your engine, you might reasonable assume that fitting a larger diameter throttle body is a 'good idea'. Who am I to argue? There are two options available to you. If you have a 48mm throttle body, then you can consider upgrading to a Trophy 52mm throttle body (more here). But if you fancy something a little more, well, then there is the GM throttle body found fitted to 2 litre Vauxhall/Opel models.

TB_GM_aspects.JPG (35243 bytes)

Click here for comparative dimentions...

This may sounds like quite a radical option, however, there are some benefits, as Dieter and others have found. Firstly, the throttle diameter is larger than the largest throttle found fitted to any K-series engine. At 58mm, its enormous!!! (A scarely believable 46% increase in area over the standard 48mm throttle). And secondly, it bolts straight onto the MG's standard plastic or alloy plenum, once two of the mounting holes are enlarged slightly.

The most tricky part of the operation is the inversion of the TPS to accomodate the fact that the GM throttle butterfly opens in the opposite direction to the one used on the MG (D'oh!). This takes a little work fabricating one's own TPS mounting plate (you can see Andy Phillip's prototype pictured right).

The final finishing touches include reattaching the oil breather and IACV hoses - unfortunately, the GM throttle body has slightly different diameters than those found on the MG's throttle body - but this is not an insurmountable task.

For more information regarding the installation of this throttle body to a standard K-series plenum, see Dieter's website for more information.

But if you think that this is wild, just have a look at Jenvey's web site - junk that standard restrictive plenum and throttle body, and replace them with FOUR direct to head throttles!!! But before engaging that route, it is perhaps advisable to attend to the gas-flow properties of the cylinder head!!!

Installing a 52mm throttle body...

Steve Taylor recently highlighted the method for installing a 52mm 'Trophy' throttle body in the FWD MGZR. Click here to read Steve Taylor's instructions on Steve Child's website. The procedure for fitting this throttle body to an MGF is similar, but ultimately a little trickier because of lack of access. To get to the hidden 8mm retaining bolt under the rear deck panel, use a flat 8mm rachet spanner - otherwise follow what Steve has to say on the matter. MG World will shortly be publishing the full MGF method...


Related interesting stuff...

The following from which contains the principles of fuel injection and engine management design - very insightful!

Air temperature

When air temperatures are high, the density of the air being inducted falls off, thereby lessening the volume of Oxygen available for combustion, if the fuel that is injected remains constant then the mixture will become too rich. To compensate for this the EMS applies a correction to the base map according to a predetermined correction profile. As the air temperature rises so air density will continue to fall and hence the fuelling will be reduced. Information about air temperature is relayed to the EMS by an air temperature sensor. To an extent airflow meters can compensate for lower density air since depending on their type they may show less volume of air inducted and this will cause the EMS to adjust the fuelling accordingly.

Acceleration fuelling

When the throttle is opened suddenly there is generally a weakening affect on the induction since air is lighter than fuel and is drawn in more rapidly. Weakening on throttle opening transients is also caused by the fact that the fuel has already been injected and the inlet valve is open before changes in the inlet manifold can take place due to a throttle change. This is only a transitory affect but it can cause the engine to stumble or stutter on initial acceleration. To counteract this tendency the EMS can keep track of sudden changes in throttle position or load and add a percentage of extra fuel when this happens. The extra fuel is only added for a short period and is then decayed over another short period; this is normally a number of engine revolutions rather than a period of time. This is known as ‘accelerator clamp’.