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Decrease, not by much though. While it is helping push air into your engine, the supercharger is constantly trying to compress air which takes a little bit of power from the crank, they never completely freewheel. Turbo's are the same way, producing an exhaust restriction, but it's not as big of of mpg loss (unless you do a bad job of matching the turbo or get an older design). The only exception to that is the roots supercharger since it doesn't compress air internally like centri & twinscrew, most companies put a bypass valve on them to reduce the drag to a fraction of a hp at cruise.
It shouldn't be any big drop in fuel economy; a few mpg at most, the drop will depend on the supercharger size & trim. Most people think it's a huge drop because of how much more they're in the gas after they get one.
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If you have to ask what it takes to get to 500 HP, you don't have what it takes to get there.
'89 Mustang GT
'86 Mustang vert
'84 TT 351w notch
'89 Turbo Ranger
'96 Musta
Decrease. There are two ways to increase power in an engine... Burn more fuel or decrease parasitic loss. S/C's increase power by allowing you to burn more fuel. They also increase parasitic loss. Both things negatively impact fuel consumption.
The only way to increase power and fuel mileage together is to increase the engine's efficiency. There are two ways to do this... increase combustion efficiency (hotter spark, increased compression ratio, perfect fuel mixture) and decrease thermal losses (decreased friction). You can decrease metal on metal friction and aero- and fluiddynamic friction (P&P heads, P&P intake, larger exhaust, more efficient water and oil pumps (electric)).
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Aaron B.
'08 Mustang GT/CS
'87 Nissan Hardbody - The "Mud Mobile" (450,000 miles and still counting)
With any big power adder that shoves air into your engine, you need more fuel in there too. If you shift before you hit boost it won't make a difference. But it's simple, being really fast requires fuel!
The only way to increase power and fuel mileage together is to increase the engine's efficiency. There are two ways to do this... increase combustion efficiency (hotter spark, increased compression ratio, perfect fuel mixture) and decrease thermal losses (decreased friction). You can decrease metal on metal friction and aero- and fluiddynamic friction (P&P heads, P&P intake, larger exhaust, more efficient water and oil pumps (electric)).
Larger diameter parts won't make your fuel economy better. The smaller ports & exhaust are favorable on low end. Making those bigger are going to make where the engine is most efficient at move to a higher rpm while taking away from lower rpm. long & skinny is ideal for lower rpm to keep velocity up, the problem with stock parts is while they are like that, they are also poorly made & could use some cleaning up like the crinkle bent exhaust.
If you wanted to you could make a 5.0 get incredible fuel economy if you wanted to build the engine for max efficiency at a lower rpm but you lose hp doing so.
As far as the supercharger route, if you're wanting to try to keep fuel economy & don't want to modify much internally, I would figure what your hp goal is & buy the lowest trim s/c you could that would get you there without getting out of it's high efficiency islands. Then run a standalone or piggyback & lean your a/f ratio out at cruise load as far as possible. That alone will put a stock fox 5.0 in the low 30's (only been tried on one so don't take that as a 100%). You should pick up about 3-6mpg which would help to negate the loss or make it not so noticeable. Switching to a standalone or piggyback isn't a bad idea if you're supercharging anyway for a higher power gain & depending on the air metering route you go you could remove some inlet restrictions. I ditched the air meter on mine in favor of speed density, but I prefer it if I have a way of controlling the ve table.
Your other option is a roots s/c instead of a centrifugal.
__________________
If you have to ask what it takes to get to 500 HP, you don't have what it takes to get there.
'89 Mustang GT
'86 Mustang vert
'84 TT 351w notch
'89 Turbo Ranger
'96 Musta
Larger diameter parts won't make your fuel economy better. The smaller ports & exhaust are favorable on low end. Making those bigger are going to make where the engine is most efficient at move to a higher rpm while taking away from lower rpm. long & skinny is ideal for lower rpm to keep velocity up, the problem with stock parts is while they are like that, they are also poorly made & could use some cleaning up like the crinkle bent exhaust.
When I say P&P, I mean just cleaning up and reshaping to reduce friction. Not hogging out. More air requires more fuel or you go lean and break s**t. The goal is just to decrease frictional losses.
__________________
Aaron B.
'08 Mustang GT/CS
'87 Nissan Hardbody - The "Mud Mobile" (450,000 miles and still counting)
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Would a supercharger give you more or less MPG
