Vaccum in relation to MPG - Fuelly Forums

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Old 03-22-2008, 03:45 PM   #1
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Vaccum in relation to MPG

I recently added a boost gauge to my otherwise stock WRX. Seems to work quite fine. It boost at around 14 psi at WOT (is it a sin in these forums? )

Funny thing is it's mostly reading vacuum in my day to day commute to work

Ok so on the highway I've set my cruise at 110 kmh (the speed limit here), I could notice, on flat areas of the highway the vacuum was around 20inHG, in slopes it would go down to around 25 and on uphills it would go between 5 - 10.

So from that observation, should we conclude in any given gear, a lower vacuum means better MPG? (the slower I would got the lower the vacuum would be). I know alot of vacuum is resulting in pumping loss, but we're talking about a small 2.0.

comments?
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Old 03-22-2008, 03:48 PM   #2
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Yup, the less mixture you're using the lower the vacuum. Vacuum gauges were the original "scangauge" for the mpg conscious.
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Old 03-22-2008, 06:38 PM   #3
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vacuum/boost vs MPG relationship is very different for turbo cars. the more boost/less vacuum you're running, the less the engine has to work to pump air in BUT you're also GETTING more air in thus more fuel so you have to balance the two.... one is a rising curve, one is a descending.. peak economy is where they cross.

on a related note: a friend of mine gets like 28 mpg highway in a toyota cressida with an 18 year old turbo supra engine. big exhaust, big intake, low spooling turbo, constant throttle.
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Old 03-23-2008, 12:46 PM   #4
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Yea, a 'friend' I know is getting about 43mpg in a 18 year old sports car with a turbo 2 liter motor.
Oh, OK it's really me

Another thing to keep in mind when trying to keep vacuum low; it's easier to keep vacuum low when the rpm's are high but high rpms will suck down more gas than if you use the same power at low rpm. So overall, try to keep rpm down.
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Old 03-23-2008, 01:09 PM   #5
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bigger displacement, bigger turbo, bigger car lol.

lol he is a friend. my cressida has the stock 7M and if you look at the mpgs you'll see part of why it's been sitting (hearing it and seeing how much parts cost is most of it lol) the turbo engine has the same 24 mpg hwy rating as the non-turbo but he's modded the exhaust and turbo (and other stuff...). he sits around 0 psi at any steady speed over 75 and as long as he keeps the gas steady, gets above mentioned mpgs with it.

My cressidas getting a non-turbo 2JZGE with a few mods... stock it's just better everything but same mileage.
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Old 03-23-2008, 06:17 PM   #6
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On my 1997 240sx, It came with an N/A 2.4L truck motor, I swapped a 2.0L Turbo and actually got about 2 mpg better and it was a heck of a lot faster.

(this was a time in my life where I could care less about mpg too, but I liked to check just for my own info.)
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Old 03-24-2008, 04:18 PM   #7
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Actually a high vacuum (low pressure) means more efficiency / MPG.

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