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I'm hoping someone would be able to clear up a little question i had about the turbo and how it plays in to fuel economy.

Boost comes on at 2500 RPM...

Once boost comes on, does is automatically start sucking more fuel at low throttle?

Reason i ask is, if i cruise at 100km/hour, i am at about 2700 RPM or so and thus under boost and using more fuel. If i slow to 90, i am probably below the boost RPM and according to my elementary knowledge of turbo vehicles, my mileage should go up since its running ~ the same as the stock 2.3?

I may be way out to lunch here, please set me straight?
 

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I cruise at 80 MPH which is about 129 km/h. I show about 11 in. Hg vacuum - no boost at that cruise speed.

Unless you are driving up a very steep hill or towing, you should not be cruising under boost.

At cruise, the turbo has very little to do with fuel consumption.
 

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I'm hoping someone would be able to clear up a little question i had about the turbo and how it plays in to fuel economy.

Boost comes on at 2500 RPM...

Once boost comes on, does is automatically start sucking more fuel at low throttle?

Reason i ask is, if i cruise at 100km/hour, i am at about 2700 RPM or so and thus under boost and using more fuel. If i slow to 90, i am probably below the boost RPM and according to my elementary knowledge of turbo vehicles, my mileage should go up since its running ~ the same as the stock 2.3?

I may be way out to lunch here, please set me straight?
[/b]
u r correct, except about the stock 2.3 because that is an FWD and has a lot less weight. cruising at 55mph vs. 65mph can save as much as 20% in highway driving. and anytime u stay out of boost, u save gas. the more air u put in, the more gas it needs.
 

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cruising at 55mph vs. 65mph can save as much as 20% in highway driving. [/b]
Is this from direct observation on the MS6? It seems like an unlikely car to benefit from lower speed cruising since it's aerodynamic, low and geared to cruise at low (relatively) RPM. I'm sure an Explorer could easily see 20%, but an MS6? skeptical.
 

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I cruise at 80 MPH which is about 129 km/h. I show about 11 in. Hg vacuum - no boost at that cruise speed.

Unless you are driving up a very steep hill or towing, you should not be cruising under boost.

At cruise, the turbo has very little to do with fuel consumption.
[/b]
hmm... in theory, if the tb is open when youre at 80mph, ~3200 rpm im guessing, then the turbo should be compressing air, unless your wastegate is open, which it shouldnt be. i dont have a boost gauge installed, but my bov blows off if i let off the gas anywhere 2500rpm and above.
 

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I'm hoping someone would be able to clear up a little question i had about the turbo and how it plays in to fuel economy.

Boost comes on at 2500 RPM...

Once boost comes on, does is automatically start sucking more fuel at low throttle?

Reason i ask is, if i cruise at 100km/hour, i am at about 2700 RPM or so and thus under boost and using more fuel. If i slow to 90, i am probably below the boost RPM and according to my elementary knowledge of turbo vehicles, my mileage should go up since its running ~ the same as the stock 2.3?

I may be way out to lunch here, please set me straight?
[/b]
Hmmm, that's a good one. In theory, under light load part throttle cruise conditions the turbo motor should get slightly better mileage than a normally aspirated version with identical specs since the turbo serves to increase the dynamic compression ratio (cylinder pressures) and we all know that more compression equals better fuel economy. However, once you stick your foot in it and ask it to pump out some real power the fuel consumption goes to the moon. The key issue here is light load, steady state cruise. It should be more efficient at higher boost pressures relative to load. That's the payoff to a little bitty turbo. With a big turbo you need LOT'S of load to generate enough pressure in the exhaust to get that big wheel spinning and by then you're really sucking up the fuel. At light loads the thing is basicly parasitic drag. Ours is making a positive difference in the normal rpm and load range where it lives most of the time.
 

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Is this from direct observation on the MS6? It seems like an unlikely car to benefit from lower speed cruising since it's aerodynamic, low and geared to cruise at low (relatively) RPM. I'm sure an Explorer could easily see 20%, but an MS6? skeptical.
[/b]
no, its not an observation, just a general tidbit for most cars. i have no idea what you can save because i never fill up my tank. gas prices are so damn high here in cali.

and i think the ms6 is geared for passing w/o downshifting at highway speeds. originally, the ms6 had a smaller ratio for the 6th gear, but decided to make it larger in the end (for the US spec that is). had the gear been smaller, cruising at 65mph would have been <2500rpm instead of >2500rpm, and would have kept the car out of boost. essentially, you would save gas.
 

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Boost delivery depends on exhaust energy, not just engine rpm. If you are not burning enough fuel to produce enough exhaust gas to spin the turbine fast enough to spin the compressor to make boost, you wont get any boost.

It just doesn't take much fuel to cruise at 80 mph, so there isn't enough exhaust gas to spool the turbo enough to make boost.

Now, if you step on it a bit and try to accellerate, boost is insta there.

With the cruise control set at 80, I see the boost gauge hit 1-2 PSI going up hills then going back to vacuum as you get over the top.
 

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Hmmm, that's a good one. In theory, under light load part throttle cruise conditions the turbo motor should get slightly better mileage than a normally aspirated version with identical specs since the turbo serves to increase the dynamic compression ratio (cylinder pressures) and we all know that more compression equals better fuel economy. However, once you stick your foot in it and ask it to pump out some real power the fuel consumption goes to the moon. The key issue here is light load, steady state cruise. It should be more efficient at higher boost pressures relative to load. That's the payoff to a little bitty turbo. With a big turbo you need LOT'S of load to generate enough pressure in the exhaust to get that big wheel spinning and by then you're really sucking up the fuel. At light loads the thing is basicly parasitic drag. Ours is making a positive difference in the normal rpm and load range where it lives most of the time.
[/b]
has anyone read the road and track article about mpg btw?

http://www.roadandtrack.com/article.asp?se...article_id=3424

i posted this before in another thread, but i figure some people havent read it. theres a section about driving under WOT to get the best mileage in the city.

Boost delivery depends on exhaust energy, not just engine rpm. If you are not burning enough fuel to produce enough exhaust gas to spin the turbine fast enough to spin the compressor to make boost, you wont get any boost.

It just doesn't take much fuel to cruise at 80 mph, so there isn't enough exhaust gas to spool the turbo enough to make boost.

Now, if you step on it a bit and try to accellerate, boost is insta there.

With the cruise control set at 80, I see the boost gauge hit 1-2 PSI going up hills then going back to vacuum as you get over the top.
[/b]
ok, that makes sense then.
 

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Having the boost gauge does make the understanding of how the turbo works much clearer. Before I had the boost gauge I always thought whenever I stomped on the gas pedal so slightly above certain RPM then the turbo will automaticly spool on. And obviously it isn't the case. If I take off from the light with medium gas pedal engagement as oppose to WOT, the boost barely spool on and it actually just hit 0 on the gauge if lucky. So this is telling me the turbo hardly spin up. But if you want to have fun and WOT, then you will see the needle on the boost gauge goes crazy hit its peak then settle down to 1 BAR/ 15PSI or there about until the car reach its cruising speed and back to under 0 on the vaccuum side. But all this happens so quickly on the boost gauge that you barely have time to register the needle's movement.

I have replayed the boost gauge in its memory and in the 1-2-3-4 (every beep on this display is near the point that I change up a gear) gear change in the near WOT the boost went so quickly that you can hardly see it :

http://members.optusnet.com.au/m6666/boostlevel.mpg

Having said that. I am just puzzle why if I drive it sedately without the turbo spool on so often, yet the car's fuel consumption is still way more than the 6i that I used to have?
 

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Boost delivery depends on exhaust energy, not just engine rpm. If you are not burning enough fuel to produce enough exhaust gas to spin the turbine fast enough to spin the compressor to make boost, you wont get any boost.

It just doesn't take much fuel to cruise at 80 mph, so there isn't enough exhaust gas to spool the turbo enough to make boost.

Now, if you step on it a bit and try to accellerate, boost is insta there.

With the cruise control set at 80, I see the boost gauge hit 1-2 PSI going up hills then going back to vacuum as you get over the top.
[/b]
agreed....u can drive in a gear all the way to redline without ever spooling up
 

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agreed....u can drive in a gear all the way to redline without ever spooling up
[/b]
How is that possible, unless you were creeping to the red line, very, very, very slowly, barely pressing on the gas. By the time you hit the redline the car will be consuming lots of fuel because the engine is revving so high

Bottom line?

At what highway speed is the Mazdaspeed6 most fuel efficient? I assume this is in 6th gear?
 

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Its very possible, but as you said light throttle to get there...

I get very good gas mileage as 55 MPH in 6th with CC on...
 

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Do you think the MS6/MPS would employ a lean cruise routine?
[/b]
FYI, on Subaru's Legacy 2.5 turbo engine, those with the Cobb (access port, or some name like that, I don't recall exactly) have the option to load in a "fuel saving" fuel map mode... it turns out it barely gives 5% fuel economy compared to stock, so not worth it.
 
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