Whats a typical delta P across the hot side of the turbo?
Corky's book talks about the ratio of turbine inlet pressure to intake manifold pressure, and seems to indicate that 1.0-2.0 is the typical range, depending on various factors. Lower is better of course, the lower the backpressure the more hp you'll make, but I expect that the lower ratios will exhibit more laginess.
If you are familiar with the pressure ratio across the compressor side, you'll have no problem with the same concept on the exhaust side. A prime driver for the turbine is P2/P1, inlet pressure/outlet pressure, on an absolute scale. What is typical, I can't say. Sure would like to see some turbine performance curves, but those are like finding hens teeth it appears. The new Garrett catalog has some turbine curves though, for the GT-40 the curves start at P2/P1 = 1.2 and go up to about 3.0. Remember this is turbine P2/P1, not turbine P2/boost.
Whats a typical exhaust backpressure? (after turbo)
I dunno, I would expect it to be smallish, something in the 3-4 psi range is what I would guess. Depends on mufflers (or lack thereof), cat converter or not, number of bends in and length of the tail pipe, downpipe size, elbow design (ie TH style vs. stock), and integral or external wastegate.
Suppose your setup required a pressure ratio across the turbine of 3.0 to drive the compressor side. You have 5 psig right at the turbine outlet with a stock exhaust system. The turbine inlet pressure would be = 3.0 x (5 + 14.7) - 14.7 = 44.4 psig.
Now replace the downpipe with a THDP, get rid of the cat, single 3" exhaust, etc... and this reduces the turbine outlet pressure to 2 psig. What is the new turbine inlet pressure? = 3.0 x (2+14.7) - 14.7 = 35.4 psig.
So reducing the turbine outlet pressure by 3 psi made a 9 psi difference in turbine inlet pressure (which is the backpressure the cylinders actually see). I would expect a nice power gain from that reduction in backpressure on the engine.... ah, the magic of the pressure ratio... shows the importance of a good high flow exhaust system to a turbo car.
One of these days I'm going to get around to measuring the backpressure on mine. I have a theory that the guys that seem to need dual deltagates or some mondo big flow single wastegate really have a high backpressure problem that they don't know about. Instead of more wastegate they really need a bigger turbine side. If they've got so much excess exhaust that they need a lot of wastegate to get rid of it, they'll be better off with a bigger turbine and letting it go out that way instead. It'll result in a lower backpressure and mo' power.
John
