Shifting with a Turbo
 

My MX-6 is a 2.2L Turbo. I have been working on my hypermiling technique, and have been taking it very easy. I have been shifting at low rpms and using fifth gear in city driving. One thing I have noticed as i am never really boosting. Doesn't a turbo aid in economy? Do i need to shift at higher rpms in order to get the benefit of the turbo? Not sure how many other eco-drivers here have turbos, but maybe someone with a turbo-diesel has observed ideal shift points?

A turbocharger allows you to downsize your engine while retaining the power of a larger engine. This also allows increased engine loads while cruising which is the main benefit of the smaller engine.

In your case, you already have a fair size engine for the size of your car. The turbo is just a power adder to an adequate engine. I'd do as you have been and just keep rpms low and stay out of the boost.

I guess this is what i get for trying to boost economy out of a performance car!

On to the aero-mods...

your turbo is still providing positive pressure, even though your manifold pressure isn't going positive.

This is commonly misunderstood about turbos. You're "boosting" even when you're not in positive pressure ranges.

Because it's already got a vacuum?

I'll try to answer this, but I'm not entirely sure that I'm understanding what you're asking here.

The idea with a turbo is that even when the manifold is under vacuum, the exact same circumstances (normally) without a turbo would put the manifold under higher vacuum.

Thusly, adding a turbo to ANY engine will decrease the vacuum necessary to induce fresh air to the pistons, because as long as the engine is under some load, the turbo is starting to spool.

To quantify this effect, remove the intake piping from the turbo and pipe it into a cap with a pressure gauge, and pre-load the engine at part throttle w/ the brakes engaged. You'll see positive boost pressure on the turbo's cold side outlet.

The reason most people don't recognize this as boost is b/c the boost gauge is still in vacuum. The reality is that the engine is just requiring more CFM than the turbo can produce under that specific load at that specific RPM, but the turbine is still moving more air than the engine could on it's own, so the result is something like:
(Engine under partial load @ -14in/hg + 2psi = -9.92 in/hg.)
Versus engine under partial load without turbo @ 14in/hg + 0 PSI = 14in/hg)

(This is the engine with the turbo)
This is the engine without the turbo.

Christ, you seem to have some great knowledge on turbos. What differences might i see if i adjusted my boost higher or lower?

I know how they work, but I don't know everything about them by a long shot.

If I had to answer, I'd tell you that adjusting your boost level only compensates for max boost conditions, which you're never hitting anyway if you're eco-driving, unless your turbo engine is properly sized for your vehicle (obviously, it's not, else you wouldn't have been asking about boosting to begin with.)

This would mean that even if you adjust your max boost to something like 2-3 PSI, you're not actually going to hit that at all anyway, so it won't really affect anything, except the (hopefully) rare occasion where you floor it for fun.

Turbos are evil little things for mpgs. See, I just cant help but get into it at least once or twice a tank.... so I'll never actually get the best mileage I could possibly get out of my car.

I have been being pretty good about it. Trying to keep it nice and low. Maybe i will allow myself a "turbo tank" every once and a while...