Originally Posted by pgfpro (Post 152837)

I'm sorry but I'm not understanding your question???

Turbo manufacturers will tell you that there are two things that turn the turbine wheel. Heat and Exhaust flow(temperature and pressure)

I think you agree with the exhaust flow part? On the heat side this is why they use advance materials to make the turbine wheel and when you use header wrap you actually help the turbo spool.

On rear mount systems that I have built and use from a major manufacturer you loose a lot of this energy and usually have to up the size of the turbo hot-side itself because a loss of the turbo's turbine efficiency.

Originally Posted by theholycow (Post 152849)

I agree with the exhaust flow part, correct.

On the heat issue, I'm ready to accept that it does what you say it does, but I'd have to understand how it does that.

As I understand, heat cannot itself turn a turbine; it can expand a previously cool gas, which in something like an exhaust system would increase the gas's velocity and pressure, which would certainly turn a turbine. However, in an exhaust system, the exhaust gas is hottest when it comes out of the cylinder*, so it's not going to pressurize any more than it already has between the piston and the turbo.

*: And maybe again when it comes out of the catalytic converter, but that's not relevant here.

Originally Posted by shatto (Post 152842)

Interesting is about heat and gas flow. I once read a dissertation on high performance exhaust systems and the end result was that too big wasn't good because of heat loss. Seems the hotter the exhaust at the tip, the better for scavenging. Something like that.
On the other hand, if keeping the heat in was so important one would think Lockheed would have placed the turbochargers on the engines, not 10 feet away on the booms of the P-38. Or used two stage mechanical superchargers like the other inline engines used..

Is it not true that the reason we need exotic high temperature materials for the impeller because they want the turbo as close to the exhaust valves as possible......to overcome....."turbo-lag?" Is that lag not the reason racers who need instant power do not use turbochargers but default to the good old Roots, in drag racers?

Originally Posted by Erik (Post 152844)

Keeping the exhaust temp high (with header wrap) will keep the velocity high- b/c the exhaust gas will contract and slow down as it cools. More heat loss between exhaust valves and turbo will mean less energy to spool the turbo.

I think that there are two perspectives going on here about the whole heat vs. velocity argument about what spins a turbo. The heat of combustion is what provides the energy that causes the gasses to expand. The expanding gasses move to an area of lower pressure and as a result there is a pressure gradient. The movement of gasses toward the atmosphere (their mass and velocity) provide the force that spins the turbo.

So, the heat of combustion initially provided the energy that gave the exhaust gasses the force (a function of their velocity and mass) that that they impart on the turbo impeller.

So both camps are technically correct- they are just looking at it from different viewpoints.

Just like wind energy from a sea breeze is technically a byproduct of solar energy because the sea breeze is a result of differential heating of the earth's surface by the sun.

Originally Posted by pgfpro (Post 152878)

The turbine performance increases as the pressure drop between the inlet and outlet increases, i.e. when more exhaust gas is dammed upstream of the turbine as a result of a higher engine speed, or in the case of an exhaust gas temperature rise due to higher exhaust gas energy.

Originally Posted by theholycow (Post 152895)

Ok, I can imagine that heat is conducted out of the exhaust gas through the turbo, so it's cooler and denser on the output side, which could conceivably result in higher pressure on the input side than the output side...and the more dense exhaust gas now requires more energy behind it to get it through the rest of the exhaust system (unless it's a tailpipe turbo).

So far, that gets us to where it is using exhaust energy to shove more air (and therefore fuel) into the engine, reducing pumping loss on the intake but increasing it on the exhaust. At most, if it was a 100% efficient machine, it would seem to come out even.

Erik's explanation makes more sense to me, though.

Originally Posted by theholycow (Post 152903)

I wish I had that level of skill and experience.