Need a tutorial on boost and its effect on mpg

[hpwjr]

First, how and why does being in boost affect mpg, and then, if boost is a detriment, what strategies should I use to maximize mpg by minimizing boost? I consider myself a reasonably conservative driver, one of those who generally pays attention to maximizing mpg, if for no other reason than it’s fun. I really watch acceleration. But towing is difficult to get better mileage. Many have emphasized staying out of boost. I can monitor that with my OBDlink. I now typically lock out 9 and 10 when I tow. Is the rise in rpm and the consequent wear on the drivetrain offsetting than gain in mpg?

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[Gros Ventre]

First, how and why does being in boost affect mpg, and then, if boost is a detriment, what strategies should I use to maximize mpg by minimizing boost? I consider myself a reasonably conservative driver, one of those who generally pays attention to maximizing mpg, if for no other reason than it’s fun. I really watch acceleration. But towing is difficult to get better mileage. Many have emphasized staying out of boost. I can monitor that with my OBDlink. I now typically lock out 9 and 10 when I tow. Is the rise in rpm and the consequent wear on the drivetrain offsetting than gain in mpg?

The issue is preventing cylinder detonation. The method commonly used is to enrichen the mixture. This then puts a hit on mpg. The common way of staying off the turbos is to go to manual shifting and select the appropriate gear,

 

[hpwjr]

Do you shoot for zero boost or just try to minimize boost?

 

[Gros Ventre]

Do you shoot for zero boost or just try to minimize boost?

When I do that I try for zero boost unless it means too low a gear and really high RPM.

 

[v8440]

There is a tradeoff; staying in a higher gear means running at lower rpm, which reduces friction. It also sometimes means more boost, which uses more fuel at that same rpm than less or no boost at that rpm. So, you have two opposing things fighting each other. If you find that raising boost to keep rpm low increases fuel usage over selecting a lower gear and avoiding boost then what you're saying is under the circumstances you find yourself boost costs more fuel than higher rpm. That doesn't mean this is always the case, it may be a matter of degree to determine which one wins.

 

[Porpoise Hork]

Anytime you dip into the turbo the mpg takes a nosedive. More air means leaner air/fuel mix so more fuel is needed to maintain the ideal 14:1 mix. Gears 8-10 are all overdrive for the 10r80. Try locking out 8th as well when on surface streets or up to 55 mph. The use of lighter throttle allowing the electric motor to do the heavy lifting off the line instead of dipping into the turbo will help a lot. Doing this I regularly see 26-30mpg on surface streets. Then on the highway I won't unlock 9-10th until I'm doing at least 65-75 mph. Doing tall this prevents the engine from lugging.

 

[Lowvez]

Boost is just the turbo increasing the amount of air (oxygen) into the engine. With this increased density of oxygen, you need to add more fuel to keep the air fuel ratio in check. So essentially, you're just burning more fuel when you're in boost. The same way you would burn more fuel when you floor it in a vehicle that doesn't have a turbo. (Or two)

 

[Gros Ventre]

The turbo doesn't just "increase the amount of air" pushed into the cylinder, it also raises the peak cylinder pressure. The turbo boost means the cylinder pressure is the turbo output pressure. The engine shifts from drawing air into the cylinder by the downward motion of the piston, to the turbo pushing air into the cylinder. The cylinder pressure when the intake valve closes is then higher. The peak pressure as the piston approaches TDC is higher by the amount of boost. If the engine isn't set up properly (a hot summer day raises cylinder temperature and can be bad) detonation can occur as the piston approaches TDC. If it's bad enough the engine will destroy itself before you can lift your foot off the acclerator. The prevention for this is to enrichen the mixture beyond what a stoichiometric measure would call for. In turn this is why mpg drops seemingly more than might be expected. This is why Ford recommends 91 octane if you're towing or fully loaded, particularly on that hot summer day on a long hill climb... Enrichment plus 91 octane provides more margin from detonation as the engine ages.

 

[Buyer2021]

You can use the un-calibrated relative boost gauge like an old-school relative manifold vacuum gauge to maximize fuel economy. Less boost (higher vacuum) = greater relative fuel economy for any given instant situation. You'll trade-off power / rate of acceleration for economy in certain situations, which may or may not be acceptable for you.

Experiment with the gas pedal (gentle launches vs more aggressive launches) and manual gear selection (usually lower gear with somewhat higher RPM / less boost when on hills or level cruising into a strong headwind) to see the effects.

The gains are small increments at any instant in time, to gain a meaningful increase in fuel economy over the long-term you'll need to be diligent and consistent in using 'hypermiling' driving techniques.

One side-effect of driving for lower boost is generally lower relative engine temperatures. Personally, I don't manipulate things to minimize relative boost for fuel economy but do sometimes use the technique to minimize engine temp when towing.

YMMV

 

[bhamilton]

Boost go up...mileage go down

 

[dochawk]

Boost go up...mileage go down

true, but sure sounds like a late career boxer talking about tomorrow's opponent!

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