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I'd give that theory a rating of "half-truth" at best. The idea that a hot gas exerts more pressure than a cool one is correct, but that's STATIC pressure. Imagine a gas confined into a specific space like a sealed box or a balloon. That gas is exerting a certain force on the walls of the container. If you heat the gas above its current temperature, the molecules become excited and want to move further apart from each other... this causes the force exerted on the walls of the container to become greater. Hence, the concept of temperature's effect on static pressure.
Now, imagine that you have a PIPE, rather than a confined space, and that the gas is MOVING, rather than just sitting there. Now we're talking about DYNAMIC pressure. Here, velocity and cross section have a far greater effect than temperature. Think about a garden hose, and the water coming out the other end. Turn the faucet on just a bit, not much water comes out. Turn it on more (increase velocity) and you get more pressure at the end of the hose. Squeeze the tip or attach a nozzle (reduce cross section) and the pressure builds even more.
Static pressure and dynamic pressure combine to form the total amount of pressure in a system. In the case of exhaust, yes- the temperature will have an effect on the overall pressure (or back-pressure) present in a system. However, this effect can be easily be overcome by the factors of dynamic pressure. In other words, I'd rather have a hotter, more free-flowing exhaust than a cooler one with lots of kinks, bends, and restrictions.
HOWEVER- your conclusion is correct, to an extent. The 3" turbo to cat downpipe will probably have a noticeable effect, but mostly when the rest of the exhaust is also uncorked. You can have the biggest downpipe in the world, but if you're attaching it to a pinhole sized stock exhaust, all that air coming out of the turbocharger will still get backed up trying to squeeze out the tailpipe. It'll get backed up after your nice big downpipe rather than right at the turbo, and maybe that huge pipe will allow a bit of a "cushion" so that the exhaust can flow freely for a bit before getting stopped up, but backpressure will prevent you from seeing the full gains of your modification. Backpressure in a normally aspirated car is what produces torque (so I've heard), but in a turbocharged car it only increases temperatures and lengthens turbo spool-up time. So, for minimal restriction, go 3" all the way. For a quieter and more well mannered exhaust, go 3" downpipe to cat, and 2.5" cat to tailpipe. The smaller diameter after the cat is less significant, because the exhaust is moving at its fastest right out of the turbo. By the time it gets past the cat, it will have already slowed significantly due to a couple of bends as well as passing through the converter. The 2.5" will be less of an issue because of the decreased velocity (think squeezing the hose with not much water flowing through it... doesn't make much of a difference, does it?) I went the 3" all the way route. It took the meineke guy less than an hour to do all the installation and bending from the downpipe to the cat, then the cat to the rear axle and up and over, then to the flowmaster, and finally to the tailpipe. Be ready and willing to accept both acceleration induced neck strain and a very loud exhaust. Good luck, and I hope this helps.
-Jon
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1LT Jonathan T. Belmont-- '91 Coupe 166K 'Gustaf'
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