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MrWOT
11-14-2012, 11:56 PM
I'm going to fill this in later when I have time, but here's a few helpful links

http://www.turbodriven.com/performanceturbos/matchbot/index.html
http://www.turbobygarrett.com/turbobygarrett/garrett_nomenclature

MrWOT
12-10-2012, 12:30 AM
OK.

So the first thing you need to know when picking a turbo.

Your power goal.

A turbo is not a "turn it up for MOAR GO!" device, though it may seem like it sometimes.

A turbo is designed to operate in a given range. The turbine will only flow so much through it, and requires a given pressure behind it (in the exhaust manifold) to drive the compressor.

A wastegate bypasses exhaust away from the turbine to keep it at a desired speed, but no matter how large the wastegate, there is always the given drive pressure in the exhaust manifold.

What makes power? Power is getting the exhaust out of the chamber, and packing it with nice cool, well mixed air/fuel. The pressure difference across the head is the key to power. If intake pressure is higher than exhaust pressure, life is good. Exhaust higher than intake will push the mixture back on valve overlap.

Now it is still possible to make decent high rpm power with a small turbo, the 3S and the CT26 are a great example of this. It's the valve overlap. If there is minimal valve overlap, there is minimal time for the exhaust to flow back. So while you might not get all the exhaust out, you still have all those degrees of intake cam to fill the cylinder, minus whatever is left in the chamber.

This is why the stock CT26 responds well to just increasing the boost. But the drive pressure needed to maintain boost at high rpm is very high, and the exhaust manifold pressure is well above intake pressure. This also makes EGT rise with exhaust pressure.

So with smallish cams, a small-medium turbo is a good choice.

With larger cams (you're aiming for power above 5500), you need to pick a turbo with a turbine matched to your rpm goals. For a daily driver/road warrior, err on the smaller side, provided you aren't going to take it to the track for 15 laps. For extended high rpm power, err on the larger side to keep the exhaust flowing.

Certain technologies exist that are the best of both worlds. VGT (variable geometry turbine) turbos are a good example of this. The use vanes in the turbine housing to change how much of the turbine is exposed to the turbine housing. That way it's a/r changes and turbine drive pressure (backpressure) is minimized. So you can use a big cam one of these and still have good low rpm power.

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To be cont.