Tuning the C20LET - Breathing/Throttle body

Upper part lip

Attention to detail is a concept where Vauxhall pays only lip service: How else can we explain this botch?

The airpipe here goes around this huge lip. Plenty of scope for optimising with the Dremel. Taper and round off, it won't hurt. The factory left it for us to finish it off, apparently...

pic by Dave

90degree turn

This is a 90degree turn, Vaux-style. The whole of the inlet charge has to make this turn on it's way to the throttle. Torture.

Some think that the rough casting is intentional, to give the mixture a chance to swirl and homogenise. Sorry to break the news, but there's no mix as yet (the injectors are way down the road) and the airspeed under full boost can be a few hundred mph - so much for the 'intentional' bit. They just bolted the castings as they were...but there's a fix:

After some work with the Dremel it looks like it's gonna flow a bit better...

Here's a better one, made by a Swedish enthusiast:

Here's another one made by a German tuner. Slightly more restrictive, but with provision for the plastic 'turbo' cover to fit. It also allows the dump valve to be fitted right next to the throttle body.

This one is DIY (AjRat from MigWeb) but is should flow better than the stock item. It looks funny being 3" wide with the 2" steel pipe sticking before it, but a larger diameter is beneficial when it comes to reducing the airflow restriction at such a sharp bend.

Throttle Plate

There's two of them actually, there's a small one underneath for the very small throttle opening angles.

This is important: you may be losing power at full throttle because you ain't got full throttle! You'd expect it to be at 90degrees when fully open, wouldn't you? I know I did. It was, however, 65-70degrees (no more)

Free power? You decide. Here's how to check:

Take off the 'turbo' cover and then unscrew the four nuts that hold the upper part of the throttle body. No need to disconnect anything, just push it on the side so you can see the throttle plate. Then get someone to press the throttle full open (make sure there's no carpet between the throttle 'stop' and the pedal). The throttle then should be wide open as above. If it isn't, then you need to adjust the cable. Easy, just remove the clip and pull the cable a couple of notches. While you're at it, spray the throttle linkages with WD40, it doesn't hurt. Don't be too hasty to put back the 'turbo' cover. If the car doesn't idle afterwards (but sticks at 2-3K rpm) it means that you pulled the throttle cable too far back. No problem, just pull it back a notch.

I doubt that your mechanic does this check as part of the service (despite what they charge for it)

More potential for improvements! Caveman from Belgium discovered the following wedge in his LET throttle body:

How anyone's expecting to get full throttle with this wedge suffocating the engine, is beyond me. It's meant to smooth out the transition at low throttle openings, at the expense of full-throttle performance. Unacceptable...

For the benefit of others who might discover such an abomination in their manifold, here's how he suggests that you can fix it:

First drill out the top of the screws because they have been stamped. Once drilled, you can screw them out.

With this done there are two holes to fill up.

Then take 2 mushroom shaped magnesium bolts (rivets I think he means) and fill the holes up. The valve thickness is 2 mm. So leave 0.5 mm on the "bolts" and flaten them with a hammer. You can also fill them up with some liquid metal. But does it hold?

Dunno mate, you tell us in a month's time!

It looks like it's gonna hold though!...

The throttlebody gasket could do with some trimming in some cases, too.

Dave found his gasket was protruding by a couple of millimetres. It's not the kind of mod that will force you to wear a g-suit, but it helps...

 

 

 

 

On to the Dump Valve...