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Originally posted by shadowmaker
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The rate of gas cooling between the manifold and turbo inlet will be determined by the velocity of the exhaust gas, the difference in temperature between the exhaust pipe and the atmosphere and the rate of heat transfer between the inside and outside of the pipe. My 200 deg example was only a guess as to what cooling might take place in the system - a couple of probes will soon provide the required information for whatever size system you are considering.
As far as the gas velocity is concerned - the larger the pipe, the lower the velocity and the more time the gas has to cool down between the manifold and turbo but has the benefit of a lower pressure drop than with a smaller diameter pipe.
The rate of heat transfer across a stainless steel pipe is much less than that for a mild steel pipe so the use of a stainless steel system will give higher gas temperatures than the same system in mild steel.
Also keep in mind, part of the energy transferred by the turbo to compress the intake air comes from expansion of the exhaust gas as it passes through the turbo and this expansion reduces the gas temperature if the turbo casing stays at a relatively constant temperature. The exit temperature of the exhaust gas leaving the turbo will then be much less than the entry temperature. You can increase turbo efficiency by reducing back pressure in the system after the turbo discharge to take advantage of this expansion.
We are into a complex area of fluid flow and thermodynamics - I have tried to keep the essential elements simple to give an appreciation of the areas that are worthy of experimentation.
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