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dhirocz · 03-08-2018, **(Subject: Boost gauge and hypermiling theory )**

So I noticed when dropping out of the mountains, the boost gauge starts to flutter when you start to lug the engine...I either drop another gear, lift on the fuel pedal, or both, whichever is appropriate.
I noticed I get far better mileage if I run the engine with both minimal boost and keeping the needle from fluttering. It makes almost no blowby, runs quieter, and gets better mileage.
I'm thinking when it starts to flutter, it's pushing more boost into the cylinder than it can ingest at that speed...the piston comes back up, and some of the boost is pushed back out the valve before it closes, causing the flutter in the gauge. That explains why either lowering the boost or downshifting makes the flutter go away.
I can visualize that pneumatic spring action in the cylinder causing the engine to work against itself and cause liner and other issues over time. Is anyone following me on this? Am I on the right track here? I figured the better I can read the gauge, the better off I'll be when trying to save on fuel...

Rawze · 03-08-2018, **(Subject: Boost gauge and hypermiling theory )**

the isx is known for intake cavitation issues on some model trucks. Prostars seem to do it the most.

dhirocz · 03-08-2018, **(Subject: Boost gauge and hypermiling theory )**

So you're saying it's more of a function of intake cavitation? I was thinking it was valve timing and boost vs rpm...which had me wondering if there is more MPG hidden in the cam profile.

Joesixpack · 03-08-2018, **(Subject: Boost gauge and hypermiling theory )**

"Hypermiling". Is that like in Star Trek when they bend space and time to get from A to B? LOL! Some of the mileage claims on here seem as realistic as Star Trek!

The compressor can surge a bit when your at a weird place on the map. Kinda doubt it's some kind of cylinder reversion. Even at 1200 rpm that's 60 hits per second.

snailexpress · 03-08-2018, **(Subject: Boost gauge and hypermiling theory )**

(03-08-2018 )dhirocz Wrote: So you're saying it's more of a function of intake cavitation? I was thinking it was valve timing and boost vs rpm...which had me wondering if there is more MPG hidden in the cam profile.

I don't know the physics of this but sometimes it quit and fuel mileage is up. I can hear air in the intake bouncing back little bit like some resonating cavitation. When it quit the air flows smoothly
It does same on 871 and 2250.
Timing and overhead was checked numerous times by different people. Mandated and deleted engines the same.

Magard · 03-08-2018, **(Subject: Boost gauge and hypermiling theory )**

My needle is violent under hard load. Thought that was just normal. Is it of major concern.

Chamberpains · 03-08-2018, **(Subject: Boost gauge and hypermiling theory )**

(03-08-2018 )Joesixpack Wrote: "Hypermiling". Is that like in Star Trek when they bend space and time to get from A to B? LOL! Some of the mileage claims on here seem as realistic as Star Trek!

Ya, its the well know episode where Rawze bends a single gallon of Diesel to travel the entire universe and still has a drop or 2 left after the endeavor. All the while never gets any where close to light speed.

Hammerhead · 03-09-2018, **(Subject: Boost gauge and hypermiling theory )**

(03-08-2018 )Magard Wrote: My needle is violent under hard load. Thought that was just normal. Is it of major concern.

No this is not normal.
Is it a mechanical or electronic gauge?
If it is an electronic gauge, then you may have a sending unit issue, wiring issue or bad gauge.
If it is a mechanical gauge, then verify the gauge is functioning correctly.
If the gauge is working correctly, then you have an issue in the charge air system that needs to be diagnosed, because the boost pressure should remain steady at a given rpm/work load/throttle position.
Based on what you're describing, it sounds like something on the intake tract, under pressure is opening and once the pressure is relieved slightly closing, and repeating this cycle rapidly and the gauge is showing it. If this is actually happening, you should hear audible air leak pulses from under the hood if you roll your window down. If it is a mechanical gauge, inspect the gauge signal hose closely.

03-09-2018, **(Subject: Boost gauge and hypermiling theory )**

its a small cpl volvo above 600HP, its being pushed and its fighting back some. under those conditions the drive pressures are getting very high and the turbo speeds are very high aswell - i would be suprised if it doesnt vibrate some. its a volvo and the charge air systems suck.