boost bypass

[dbuzz77]

anyone know if you can shut the boost off altogether for highway cruising for better gas mileage? (legacy turbo)

[BAC5.2]

You can't.

When you are cruising on the highway, you shouldn't be on-boost anyways unless you try to accelerate.

[sammydafish]

even if you were on boost (going up an incline at a steady speed) you would actauly get better economy since the compression is higher.

[dbuzz77]

I guess what I am saying is that I think boost comes on at a lower rpm in these cars, so it is probably asking for more fuel(WRX's get in the 30's , i get high 20's). I thought of opening up the boost control solenoid, but I don't think it is rated for continuous use.

[BAC5.2]

WRX's don't get in the 30's, they get mid 20's.

Opening the boost control solenoid would revert the turbo to wastegate boost.

Junior - I don't think that's true.

On boost, you do have higher compression, but to fend off det. fuel is added and timing modified.

High 20's in MPG is pretty damn good. My Forester doesn't even get high 20's on the highway.

[scuzzy]

I'm lucky to get IN the 20's in the city. I get about 25-26 on the highway.

[rallysam]

I'll say what I always say to this question: Who cares what the turbo is doing, you control the intake manifold pressure with your right foot. Lift it a hair and the throttle cuts the pressure just like you had no turbo.

[Project_Legacy]

true. just get a boost gauge, and keep the throttle under boost. i drive for hours in a day, thru the city. i believe i get pretty good mileage from it, by most of the time staying off boost.

[dbuzz77]

you guys have to know I am in Canada and WRX's do get in the 30's here. I usually get 25-28 but I thnik I need a new o2 sensor. Thanks anyway.

[BAC5.2]

30 km/L maybe. Not miles per gallon. And it would help if you had Canada as your location.

If you want better gas mileage, keep a handle on your right foot.

[vrg3]

Well, using your right foot to control boost is different from limiting the turbo's boost. When my intake manifold is at 0 psi, intake air is at a higher temperature than it would be if there were no turbo and I was at WOT.

When the boost control solenoid is active, it is bleeding pressure away from the wastegate actuator. If you disconnected the solenoid (but pacified the ECU) then you would have wastegate-level boost.

It would be interesting to see if you could actually achieve better mileage without boost. Both Junior and BAC5.2 are correct, so it depends on the engine management and on the actual boost level. My suspicion is that on an otherwise stock Legacy Turbo you'd get the best fuel economy with wastegate-level boost.

You can eliminate boost semi-permanently by disconnecting the wastegate actuator from the wastegate flapper rod and then using baling wire to hold the wastegate in the fully-open position.

You could alternatively set up some kind of system to supply pressurized air to the wastegate actuator at all times. If you used a little pump with good seals you wouldn't have to run it very much because the actuator is sealed.

[rallysam]

Well, using your right foot to control boost is different from limiting the turbo's boost. When my intake manifold is at 0 psi, intake air is at a higher temperature than it would be if there were no turbo and I was at WOT.

shhhhhh! I'm simplifying! I'm waving my hands and shouting "to first order!"

I added about 4 mpg by keeping manifold pressure below -10 in and keeping my AFR stoich. But, it was like driving a 3 cylinder Justy

[vrg3]

But if you're approximating, we all make the same fuel mileage anyway.

[rallysam]

The physics god spoke to me this morning and said to approximate to 1st order, not to 0th order or 2nd order.