posted October 01, 2000 11:30 PM                

But if increasing the pressure of the gasses at the turbo inlet (exhaust manifold) will produce more work, then why is everyone boring out their exhaust manifolds? Accordingly to your post, would this not lower the pressure differential and therefore decrease the heat and energy transferred to the turbine wheel? Does this mean boring is counter-productive and we should only be polishing it to smooth out flow?

ANSWER:

Well, that depends . . .if the exhaust manifold is not flow-limited, then yes, increasing the diameter of the manifold runners is counter-productive.

If the manifold is flow limited, then increasing the diameter of the manifold runners may do you some (or a lot) of good. The pressure may be a little lower, but now you've got more gas available at the inlet to expand and do work with.

"But you didn't say anything about the amount of gas in your other post"

Yeah, I know. It's tough to summarize a mildly complex topic and hit all the points.

Part of the problem is that pressures in the system are all very time dependant. That gas coming out of the turbo outlet isn't a steady source, like a hose, it's a pulse. The pressure at any point in the system varies with time, and it's difficult to "nail down" system performance in absolute terms.

After copping out like that though, let's examine a flow limited manifold runner. The pressure in it is higher than in a runner that is capable of flowing exactly the same amount of gas produced, but it won't be able to sustain that pressure as it gets bled off through the turbine, and not enough gas can flow up to the inlet to replace it. Pressure will drop to the sustainable point - lower than the ideal case. Enlarge the ideal case though, and you've gained nothing - unless you can increase the production of gas to the point where you're back at ideal again.

QUESTION:

What about people who wrap up there downpipes to keep heat in and the exhaust gases hotter after the turbo? The hotter gases travel faster right, and that decreases pressure (good)? Or, the higher retained heat keeps the gases hot and the pressure stays higher and that would lower the pressure differential (bad)?

ANSWER:

Welcome to the real world.

Both are legit ways of examining the problem, and either scenario is possible, depending of the flow capacity of the exhaust system. If it is capable of flowing the exhaust, then the retained heat will service to increase velocity, lower the pressure seen at the outlet, increase the pressure differential, and increase power. If, however, the system cannot flow the gas produced, then the retained heat only serves to increase the pressure seen by the outlet, decrease the pressure differential, and reduce power.

Theory is a wonderful thing. It lets you gain an understanding of what's going on, allows you to ask meaningful questions, and leads you to try things in an intelligent manner. Ultimately though, when you start talking about specific applications, there is no substitute for trying it out and testing it.

QUESTION:

Will boring out my exhaust manifold make more power?

ANSWER:

Well, calculate how much exhaust gas you should be producing at redline at your desired boost level, and then put your manifold on a flow bench. Are you flowing enough? What's the current pressure at the turbine inlet? What was it when you tried a bored-out manifold? (Here's the kicker) What happened when you put the changed engine on the dyno?

Crew Chiefs in F1, Indy car, NASCAR - all these guys have forgotten more theory that I'll ever know, and they still end up trying stuff out on the dyno. But trying stuff without knowing the theory is just blind guessing, putting monkeys on the typewritters and hoping for Shakespeare.

Hope this helps.