Tuning the C20LET - Breathing/Intake manifold

 

Smooth and Flat

This is the view from the cylinder head. An injector hole can be seen at the top. Note inside the rough surface and the casting flaws on the left. Nothing the Dremel can't fix.

No need to spit polish the whole thing, as we do want some 'roughness' at this stage, to keep the fuel atomised. But this is a turbo engine and the air is already fast&furious.

Nut torquing

Be careful with the intake manifold nuts. They have to be fitted properly, just like the cylinder head bolts. (Chris-cross, from the inside outwards). Tighten them finger-tight, then progressively half a turn each, until you do them with the torque wrench. When removing them, follow the opposite routine (Chris-cross, from the outside inwards). This is the case with all such sealing surfaces, (the exhaust manifold for example).

The nuts themselves are copper, and they are meant to deform as they move down the studs. Vaux recommends that they are changed every time. I don't think it's a good idea to reuse them, as there is the risk of some getting loose later on. That would be risking an intake leak - potentially damaging the turbo. ANY air leak can be damaging for the turbo, as it upsets the pressure balance around the bearing and this affects the oil flow (yet another fact so many 'tuners' ignore!)

 

Gasket fit

Apparently the stock intake gaskets may protrude into the intake ports! Another chance for Vauxhall to show off their obsessive attention to the most minute details! It's only a millimetre or two (maybe three) but it shouldn't be there. Fit the gasket temporarily (as above) and then draw with a pen the upper edge of the intake runner. Then use a Stanley knife to cut off the excess. Incidentally, the upper part of the port carries the majority of the flow and it's the one most sensitive to obstructions. And that's the one they managed to get wrong!

 

One-way valve

There are two breather hoses ending at the rear left of the cam breather cover. The thick one goes to the turbo intake. The other one goes under the throttle body. Somewhere in the middle it's supposed to have a one-way valve. This ensures that the camcover is vented when there is vacuum in the intake manifold, but is not pressurised when the manifold is under boost. This is important.

These little valves are known to perish, and some older cars may miss them altogether. I've seen this pipe split and dangling happily on some older cars. There's an alternative to using an OEM valve, you can get one from a Pet-Shop, they're used for fish-tanks. Just blow through it: it should flow one way only. Make sure that the arrow should point towards the intake manifold (the camcover should NOT be pressurised).

This pet-store valve may not live too long in the harsh petrol/oil/pressurised environment of the engine, but neither do Vaux valves.

Home-made manifolds

Some people think that the stock LET inlet manifold is rubbish, and they have a go at making one themselves. Then they sit back proudly, admiring their creation (click for larger photo from Swedish Racers):

A cylindrical 'surge tank' is certainly easy to fabricate. It may even fit where the stock one won't, like some kit cars (fair enough then!)

But that doesn't make it any good - cylinder #4 will not get the same airflow as #1. If the airflow is different among the cylinders, the fuelling will be all over the place, won't it? So the ignition needs will be different for each cylinder. But even if these are all smoothed out (and no cylinder detonates), the different airflow will result in each cylinder producing different amounts of power, so the delicate balance of the 4-cylinder design goes out of the window: some cylinders will be working against each other.

This guy thinks that because this design works fine on a fuel rail, it will also work for the inlet manifold. Well, I've got news for you sunshine: liquids are not compressible, but air is. There will be shockwaves in that cylinder playing games with your airflow. At least it looks trick, I give him that!

This manifold suffers from the same 'unequal feed' syndrome. The injectors will supply the same amount of fuel in each cylinder, but even a variance of 10% in air under boost (conservative estimate) will lead to one cylinder having AFR 12:1 and another one almost 13.5:1. That's paying money and fabricating something that performs worse than stock.

This is a more decent attempt: The volume of the surge tank will equalise the pulses and airflow to some extent. Still not ideal, but perhaps tolerable.

This one is fitted in a Manta: Looks good, but they could have made the tank a bit bigger, avoiding the sharp edges as well. They have to be smooth inside, if standing waves are to be minimised.

 

The inlet manifold below has a better chance to work properly. I don't know if they've done any flowbench tests on it, but it does look like someone has spent some thought before he started welding:

The thermal wrapping is a nice touch. We spend so much effort to lower the air temps, that it's a bit silly to let the manifold get hot from the radiated heat and preheat the air just after the intercooler. Not fair.

Below is a high-flow inlet manifold for the Skyline GTS (six cylinder, single turbo). It shows how it should be done. Note the shape of the 'rail', the aim is to have identical flow in the runners, under pressure.

 

Here is another decent high-flow intake manifold for a Skyline. Note the shape of the plenum, and the 'trumpets' inside

 

This custom-made LET manifold is even more extreme. It's clearly made to handle boost pressures of several bars. Note the heavy-duty linkage, butterflies and the throttle position sensor at the end. During those very long nights in Finland, some people choose not to hibernate...

 

Here's how to fabricate your own intake manifold (local copy here)

 

On to Cylinder Head...