Throttlebody Modification
Last Updated: 30-12-02

Figure 1	Figure 2

The throttlebody is what allows air into the engine. When the accelerator pedal is pressed it tugs on a lever which opens the throttlebutterfly (a circular flap) which allows air into the inlet manifold which then mixes with fuel in the cylinder.
The amount of air allowed in throught the throttlebody has a very significant affect on the running and poweroutput of the engine.
A turbo car produces more power by compressing the air so it enters at a higher pressure. Although it might have the same throttlebody, more air enters so there is a more affective combustion and hence more power.
A set of carbs or throttlebodys also increases power and torque. These dont compress air -it is still 1bar (atmospheric pressure) like a standard car, but they allow more air in. How? Because the throttle body is the bottleneck in the system. In the standard XE engine one throttlebody feeds 4 cylinders, if you replace this with twin carbs or 4 throttlebodies then each cylinder has its own throttlebutterfly and thus each cylinder gets a lot more air.
Turbos, superchargers, carbs and throttlebodies cost a lot of money, so while you are saving up there is a mod which you can do to the standard throttlebody to increase the airflow past the butterfly.
This has become known as the throttlebody mod.

In Vauxhalls wisdom they decided to add a segment of metal (same shape as a segment from an orange) to the throttleflap in the XE throttlebody. I believe later LET engines didnt have this. The segment protrudes a long way from the flap. This means that it disrupts airflow, it looks especially bad at quarter throttle, where the air has to flow right round the segment. If the segment wasnt there then it would have a much quicker and easier path - air could quite simply slip over the top of the throttlebutterly...

No prizes for guessing what's going to happen to the metal segment hahahaha!

Note: You will need some chemical metal, such as metalix, a screwdriver, and a chistel and hammer, and maybe a hacksaw with a metal blade. It is also a good opportunity to clean the throttlebody up with some carb cleaner. - watch your eyes with the spray carb cleaner!

Method:

1) Disconnect the battery for safety and pop the bonnet. Remove the 4 screws which hold the SFI box down.

2) Before yanking the SFI box away, undo the large clip from the Air mass meter and the hose at the passenger side of the SFI box. Withdraw the SFI box.

Check to make sure the rubber sealing ring is in the groove on the bottom, if not it might be stuck to the throttlebody - rescue it.

3) The throttlebody is now exposed. Remove the accelerator cable - it has a circular piece of metal holding it in, you need to push the cable towards the throttlebody, and slip the end of the cable through the gap in the side of the arm that operates the butterfly.

4) Pull off the large breather hose and the smaller one. There is a tiny hose to the fuel pressure regulator and another next to it which you might need some very fine needlenose pliers to remove.

5) Undo the electrical connection on the drivers side of the throttlebody and the 4 bolts holding the throttlebody down onto the inlet manifold. Withdraw the throttlebody and check to make sure the thin gasket under it is in good condition. This can be reused if it is ok.

6) Cover the opening to the inlet manifold with a clean cloth. Dont worry if there is a trace of oil in the manifold, oil can enter from a breather hose.

7) Rest the throttlebody on a workbench. The first job is to unscrew the throttlebutterly plate from the body, so it can be removed in order to safely remove the metal segment. This can be hard work! The screws have a fine thread and screw directly into the spindle.

8) Once removed the throttleplate can be clamped in a vice. This is a good idea to avoid it bending - yep, its time for some brute force, but no ignorance will be required.

9) At first it looks like the segment is held on by 2 screws. However, after doing this mod on two different throttlebodies I can assure you there is a bit more involved than a small Phillips screwdriver.
By all means try and unscrew it, but I think they are stamped or spot welded or something. In the end I got a saw and sawed off the heads of the screws. In between sawing, a hammer and chisel can be used for some extra persuasion.

10) Finally when the orange segment gives up you are left with a nice thin plate. Tidy up any rough edges with a file(I rounded off the edges of the plate with a file for smoother airflow round the plate.)

11) Next the holes need to be filled with chemical metal. If this isnt done it would be like having the throttle slightly open at idle - I can imagine things would go a bit mad. Both times I did the mod I used Isopon Metalix - great stuff. Just like their other products you mix it with a little hardner, work it into place, let it dry and then it is easily sanded off for a nice smooth finish.
Metalix contains aluminium and a special resin so can withstand the heat, but make sure you get a nice smooth finish so that none can crumble and fall into the engine.

12) Before refitting the plate to the throttlebody file off and rough edges to make sure there is no chance of bits of metal falling off. Swarf in the engine would be a disaster.
Refit the plate, making sure you get it the same way up. If the screws become excessively hard to screw back in then swap them round - you may have them the wrong way round.

13) Manually check the throttle opens by operating the level. If its a little sticky - no worries, you wont notice under foot!

14) Refit is the reverse of removal.

Note: Figure 1 shows the metal segment - highlighted in red.
Figure 2 shows the throttlebody in all its glory.

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