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Old 12-25-2004, 04:42 PM   #1 (permalink)
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For you supercharger guys- Does a supercharger help or hinder gas mileage?

I'm looking for a little more hp out of my 5.4 f150 motor and was having a debate if gas mileage would improve with more hp because the engine wouldn't have to work as hard for the same ouput. I understand the more fuel and more air equals hp so....
I'm confused.

thanx,

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Old 12-25-2004, 05:19 PM   #2 (permalink)
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You "shouldn't" get better gas mileage for two reasons: First, you will stomp it more that you used to because it feels sooo good; and it takes some power to drive the blower even when not in boost.

The best you could hope for is easing it along and seeing exactly the same MPG. I have built 3 blown engines, Chevy/B&M Rootes 144, 318 Dodge/Vortech centrifugel, and 4.3 Chevy/Camden 112 Rootes blower.

That said, maybe(in theory)if you keep the engine in boost a lot and short shift at a very low RPM and under load, your effective dynamic compression will encrease above the static 9.5 or so and give you slightly more thermal efficiency. But you have to weigh in the need to buy premium fuel for the blown motor which will offset any potential cost savings that might result from better MPG.

In reality, going down the highway on long trips and keeping it out of boost the mileage will be about the same. Around town it will be worse IMHO.

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Old 12-25-2004, 05:35 PM   #3 (permalink)
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I've never driven a blown car. But my Harley has a turbo on an essentially stock motor. Fuel mileage went from 35 to 45 on the highway. I think that's because it needs a lot less throttle opening to do the same job. Is there a turbo kit available for your truck?
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Old 12-25-2004, 05:48 PM   #4 (permalink)
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Turbos and a different animal. They don't use any engine power to compress the incoming air. It still boils down to this:

Its takes the same horsepower to move the vehicle with/without a blower and that's an inescapable fact. One way to encrease MPG is raise the effective compression ratio with a forced induction system. Turbos are the best and most efficient(read most expensive)method. If your engine is making boost it's also using more fuel than it was naturally aspirated.

Engine builders use forced induction on very small displecement engines to increase their thermo dynamic efficiency through high boost/high dynamic effective compression. IMHO a blower is not an avenue to better gas mileage on a large displacment lightly loaded engine but it CAN be a lot of fun!

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Old 12-25-2004, 06:54 PM   #5 (permalink)
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Chris,
Blowers get better mileage on a car by allowing the manufacturer to get away with a smaller engine for many applications but still allow the engine to put out a higher power level when needed. Understand, this post is VERY generic. There might be a situation where an engine might be so inefficient before the blower that it might actually get better mileage but it's not likely.
Just adding a blower to an existing engine can't reduce the car's need for a certain power level.
The real truth here is in Greg's first paragraph, you're going to enjoy the newfound power so much, you'll be stomping on it all the time!

d
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Old 12-25-2004, 09:07 PM   #6 (permalink)
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Turning a supercharger takes HP so for a given speed your car will need the hp it required before and the hp required to spin the supercharger. I would expect more fun and less gas milage. Scott
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Old 12-25-2004, 09:09 PM   #7 (permalink)
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If your pulling or more low end is what your looking for, then a screw type is more what your looking for. Check Kenne Bell web site.
I get full boost around 1800 RPM. Great for a truck.
No foot in it and the supercharger isn't even working.
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Old 12-26-2004, 12:38 AM   #8 (permalink)
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Thanx guys for the input. That is one sweet boat engine!
I have a '98 f150 4x4 and was toying with the idea of putting in a lightning motor, ya know- no cobra to tinker with anymore, and then started thinking about how anal I've been with the maintenance and a new lower end makes no sense. I tow a boat to the fla keys a few times a year and do some hiway mileage around the country. A new diesel f250 isn't in the scope of things and would really like to keep my truck, I've done some intake and soon exhaust which I think will help. Not easy finding a thru flow exhaust that isn't much louder than stock.
I appreciate any opinions on engine efficiency. Should I not waste my money? I was thinking of a powerdyne who makes a great bolt on with aftercooler setup for my truck.

thanx,
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Old 12-26-2004, 12:49 AM   #9 (permalink)
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lightning motor!!!!!!!!!!! you will love it.
I have a 2003 lighting daily driver (25k miles the first year)
Its faster than my cobra!
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Old 12-26-2004, 01:33 AM   #10 (permalink)
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From what I notice with my Powerdyne supercharger - the gas gauge moves toward E as I press the throttle toward the floor! [img]smile.gif[/img]

If I stay off it the milage is very good, but as soon as I push it more - like on I40 through NM where there is just no one around.... well the speed-o and the gas gauge have a direct connection... one going very quickly one way and the other just as quick the other!
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Old 12-26-2004, 01:42 AM   #11 (permalink)
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Thunderball,

Thanks for comment on the boat. It's a 196 Wellcraft Eclipse SS. Before the blower, 21pitch prop, 0 to 52 mph in 20 seconds. After the blower 24pitch prop, 0 to 60 in 12 seconds and no noticable decrease in cruising mpg because the larger prop dropped the cruise rpm by about 400.

For your truck, you could buy a better flowing exhaust system, and a chip designed for MPG/or reprogram the ECU but nothing will make very much of a difference. I have a 95 Dodge Ram (with a Vortech blower added) and I did not see any better gas mileage. I get about 15 on the highway at 70. There just isn't much you can do to a truck to improve mileage do its weight and wind resistance. A diesel is the only way to go for real mileage. Remember what I said in my first post about effective compression ratio. A diesel is about 25 to 1 static and with the turbo in boost ALL the time, the effective compression is somewhere around maybe 50 to 1. That translates into a very efficient thermodymic engine. A high compression engine will extract more HP then low compression engine from the same amount of fuel burned. A forced induction system not only increases effective compression it also increases effective engine displacement.

But the lightning would be a lot more fun!
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Old 12-26-2004, 02:43 AM   #12 (permalink)
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Chris,
I went from 23 MPG to 18 with the S.C.. That was hiway miles per gallon. I didn't even bother to check around town but it was significantly reduced!
Hope you and Richard had a Merry Christmas!
Thanks,
Ron
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Old 12-26-2004, 01:56 PM   #13 (permalink)
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A supercharger raises your volumetric eff.; therefore it should translate and overcome the input loss and generate a more fuel eff engine. The a/f ratio is a big part also. MOst people make the engine very rich 12.5:1 sometimes 11.5:1 to avoid any detonation, which is the most brake torque with a supercharger but much less efficent than a engine that is running stochimetric (14.5:1).
So, bolting on a supercharger would probably reduce your gas mileage. The computer would have to be designed specifically for a blown engine and be programmed to run stochimetric all the time.
Deisels run at about 14:1 compression ratio with 20-25 psi of turbo boost, which translates to a very volumetric efficent engine. Deisel is the way to go if you want good mileage. I have had three 2500 cummins rams so far and they have been great. Stock you will get 17 mpg, towing 4000+ lbs 15 mpg, and if you modify the engine and exhaust (free flow and more boost - higher Vf) you can double the torque and get 22 mpg.
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Old 12-26-2004, 04:10 PM   #14 (permalink)
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Increased gas mileage with a supercharger in an FFR ? ROFLMAO.

If you are driving your FFR for max fuel economy you must be the only one. You WILL use the extra available power and it takes fuel to make power. The more power the more fuel. At WOT a supercharger will increase volumetric efficiency but I thought we were talking about fuel economy? At part throttle you should see no, or minimal boost so basically the blower is a parasitic loss, kind of like driving with the air conditioning on.

If you can keep your foot out of it you should see minimal degradation in gas mileage. However, who can resist using the extra HP? Not me. I have got better than 20mpg on a highway run but the day I went for a dyno tune I got less than 8mpg. That includes the drive (highway) to and from the shop and 14 pulls.

The most efficient form of forced induction is turbocharging. However even turbocharged HP is not free. A turbocharger causes increases in back pressure in the exhaust that adds pumping losses to the motor. At light throttle these losses are minimal and far less than a typical supercharger. With a correctly designed turbo system even the losses at WOT throttle can be considerably less than a belt driven blower.
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Old 12-26-2004, 05:10 PM   #15 (permalink)
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I've never checked the mileage on my FFR so I'm not real sure what the blower has done for gas mileage. I can tell you that when I changed my daily driver from a XJ8 to an XJR which is essentially the same vehhicle plus 13# of boost from an eaton blower my average fuel consumption went from 19mpg to 13mpg.
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Old 12-26-2004, 07:44 PM   #16 (permalink)
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After re-reading the formulas in my engines book, I believe the only way to get a more thermally efficient engine is to increase the engine compression ratio - the difference in the volumes. A more thermally eff engine = better gas mileage. The volumeteric eff. has nothing to do with the engine fuel eff. Volumetric eff. just equates to more hp, since it is multiplied times the torque and RPM = Power. Boost pressure changes the density of the air and is not related to the compression ratio. A high compression NA engine will always be more eff. than a equivalent hp turbo or supercharged motor. Engine Thermal eff depends on the compression, and what cycle it is has been designed too (otto, deisel, brayton, etc).
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Old 12-26-2004, 10:53 PM   #17 (permalink)
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Thermal efficiency is another reason why turbos are typically more efficient as they extract a certain amount of otherwise wasted heat energy from the exhaust gas and turn it into energy that is used to compress the intake charge.

Heat reflective (low emmisivity) coatings on piston crowns, cylinder head combustion chambers and exhaust headers etc will also increase efficiency by retaining as much heat energy in the combustion process as possible.
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Old 12-27-2004, 10:34 PM   #18 (permalink)
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Now that's what I love about this place, thanx ya'll. Merry christmas Ron.

I guess it boils down to driving a shoebox, my stroked 351 in my cobra was around 20 mpg being naturally aspirated and yes, building my 351 with thoughts of mpg was considered and solved.
I guess I'll live with my shoebox of a truck.

thanx again for all the info,

chris.
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Old 12-28-2004, 01:19 PM   #19 (permalink)
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If your truck is a '98 you could also look at getting a set of heads and intake from a '99 and up f150, in '99 ford redesigned the intakes and heads on all the modular engine and came out with th P.I. which I believe stands for performance improved, it might be worth looking into.

HTH

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Old 12-28-2004, 07:35 PM   #20 (permalink)
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IMHO if you use your truck to tow I would stay away from the cent. type of blowers. Look for a roots or a twin screw, lots of torque in the lower rpm range. MagnaCharger make a nice supercharger that is intercooled. If your going to get a supercharger don't do the head swap with the PI heads. It will raise your compression ratio if you don't change your pistons.
There is also a single turbo setup for the 5.4 truck that just came out. Look over at www.f150online.com and check out there superchargers and turbochargers forum. Lots of info there.
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Old 12-28-2004, 08:47 PM   #21 (permalink)
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What kind of MPG are 347 stroker guys getting? How about 331 strokers?

My stock 5.0 (still in donor ('90 GT 5 speed w/ 3.08 gears)), will get 27 mpg on a long road trip if I stay below 80.

The reason I ask is because when I do the kit (hopefully ordering next fall), I was thinking of stroking it.

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Old 12-29-2004, 06:02 PM   #22 (permalink)
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ttt...?
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Old 12-29-2004, 10:18 PM   #23 (permalink)
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Kieran- you may want to start a subject asking that question. I got really good mpg with my stroked(383) efi 351w and I think a 331 would be decent as well if it was efi. With my limited carb experience it seemed that efi was better than carb in this regard.

chris.
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Old 12-29-2004, 10:31 PM   #24 (permalink)
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Hey Chris,
I've got a 98 f-150 with the 5.4 like you and I get 15mpg at best with a Vortech S trim. That's pretty close to what it was stock. It's a 4x4 so I'm probably loosing 1mpg or so due to that.

I agree with what some others posted about using a positive displacement type blower for heavy cars or trucks, especially if used for towing. You need some revs before the centrifugals start to come on strong.
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Old 12-30-2004, 12:23 AM   #25 (permalink)
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I put a Whipple SC kit on my 2000 Z71 and the mileage went from the 15-17 range (city/hwy) to the 12-15 range. BUT, when towing my Cobra in my 20' enclosed trailer the mileage is exactly the same - 8 mpg. But I don't have to keep kicking it down out of OD on the interstate. It cruises much more easily at 75 on the interstate than before.

My mileage around town is probably worse than most. I just love feeling that torque.
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