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Say you have two identical turbos, but one's turbine housing is 0.45 A/R and the other is a 0.90 A/R. Which would (theoretically) produce better gas mileage on the highway?

If the smaller A/R is used, then the turbo would possibly produce some boost, decreasing pumping losses, reducing the amount of work the engine has to do, improving gas mileage? But the larger A/R might reduce exhaust back pressure, also possibly improving gas mileage? Which would be the better choice?
 

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I've been wondering the same thing lately so I'm interested in hearing what others have to say.
 

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Brokedick Millionaire
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Say you have two identical turbos, but one's turbine housing is 0.45 A/R and the other is a 0.90 A/R. Which would (theoretically) produce better gas mileage on the highway?

If the smaller A/R is used, then the turbo would possibly produce some boost, decreasing pumping losses, reducing the amount of work the engine has to do, improving gas mileage? But the larger A/R might reduce exhaust back pressure, also possibly improving gas mileage? Which would be the better choice?
Ask VW/Audi and their tiny turbos......

Audi TT would get best mpg just on the edge of boost, but you had to be careful to not go into boost.
 

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95 CX Hatch
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^^ that. You should be in vacuum while cruising. Never had a tiny turbo so maybe they have the urge to try and spool a few psi?
 

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tiny turbo spooling low boost levels equals low air flow = little fuel

my turbo easily hits 1-2psi in 5th while cruising at 3500+rpm, you can tune for it with the right program. in 5th gear i'm at 13.8afr at 2psi, in 4th its at 13.2 and 2nd and 3rd 12.9 and first a very rich 11.2.... 1st gear gets run richer to get the turbo spooling.....

the things a proper tuner and program can do are amazing.
 

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95 CX Hatch
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I think most tuners fail to take the time and get part throttle and cruising dialed in. I think I made more Datalogs at light to moderate than wot. 300+whp and 32mpg (combined city/highway) makes me happy.
 

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Brokedick Millionaire
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a Audi TT (tiny turbo) does great for hills and mpg at "ground zero" as Mike called it, no boost but no vacuum in the manifold either, like those damn Krauts knew WTF they were doing!

KO3 or KO4 on a Honda SOHC? the tuning will be the hardest part but once done it will rock.

The issue will be the tuning though. Most Honda MPG tunes (NA) deal with the 10 in/mg range not the slight boost area. Slight change of thought process will be needed.
 
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