Quote:
Originally Posted by Zsteve
Ive had a couple of turbo cars and lived in the forums for those cars and I never heard about heat being what caused more power. It was always air flow in and out causing the turbines to spin. The wrapping of things was to keep the heat away from the engine, thus the intakes so cooler air went into the intake mannifold. Heat is actually bad in turbo cars as when you get too much heat from the turbo and the engine temp gets so hot that at some point you will not net any gains from a bigger turbo cuz the heat is causing so much heat soak and reducing power. Just what Ive learned from having a couple of turbo cars but I could be wrong.
Just a side not too, would having a diverter valve that dumps the excess air back into the intake instead of venting it help with the lag of having the turbos far away?
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Issues are being conflated here.
First off, heat is the only source of power in an engine. That's why we burn gas under pressure--to create heat. More heat, or more accurately, more delta-Temp, always equals more power. This is consistent with your observation that, given a combustion temperature, a colder intake temperature will result in more power, because there is a greater delta-T. Likewise, given an intake temperature, hotter combustion temperature will create more power, because, again, there is a greater delta-T.
If we had the materials to do it, we would run engines absolutely as hot as we could make them, and by that I mean create the highest combustion temperatures possible. However, heat soak into ancillary parts of the engine, wear, cooling required to avoid materials failure (including oil breakdown, melting metal, etc), and other considerations require that we have to balance heat production with reliability. Thus, we have to limit heat generation for practical reasons.
As for turbos, certainly air flowing through the turbo makes them run and, by implication, more airflow through a turbine will spin it faster. That's not wrong, but it's missing an important point. Hot air is more energetic than cool air. When a molecule of extremely hot air hits a turbine blade it imparts more energy on it than the same molecule of cool air. Thus, the equation PV = nRT informs us that higher T => higher P, and where P (pressure) is a corollary for force applied to the surface of a turbine blade, higher T = more force applied to the turbine blades. Thus, a turbo will perform better if hotter air is running through it. However, for the same reasons as mentioned with engines (namely materials considerations and inability to restrain the heat to just the turbine area), heat has to be managed. If we could run turbos hotter without reliability issues and heat soak, we would. Hotter air through the turbo, where that heat doesn't negatively affect other parts of the system like intake temp, means more power.
Again, just look at the designs of cars. Productions cars, supercars, and race cars all try to put turbos as close to the engine as possible so the air flowing through them is as hot (and thus energetic) as possible.
A remote system, while easier to install and probably perfectly functional, will not outperform a properly designed turbo system where the turbo sits near the engine. Too much heat (i.e. energy) is lost in transit to the remote location. You may think that is crazy, but consider this example. At idle, the average EGT on my STi at the headers was 850 degrees F, but by the time it left the exhaust pipe, maybe a second later, it was not hot enough to burn my skin. Heat, and thus energy, is convected away from the exhaust system rapidly as it follows the exhaust path, and the expansion of the exhaust stream in the lower pressure of the exhaust tract also rapidly cools it (back to PV = nRT). Thus, a turbo that sits where the muffler is will have access to far less energy from the exhaust stream than one mounted near the engine and will not be as efficient. Since turbos fundamentally work by recapturing energy in the form of heat, it makes sense to put them as close to that source as possible.
But to reiterate, I'm not saying that a remote system won't work, or that you can't make power with one, I'm just saying that those who would claim they are as efficient, or work as well as a properly designed system, are just trying to sell you something.