I'm talking about a V8 engine that gets 20-25+ miles per gallon)
Well, many V8 engines get up to 25 mpg on the highway so you aren't stretching too far. My 88 Corvette with the L98 (350 cu in) V8 and 4+3 manual trans can get as much as 29 mpg on the highway. A lot of that depends on the profile of the vehicle the engine is put in, my Corvette sits low and is aerodynamic, put the same engine in a tall truck and the mileage won't be as good.
Fuel efficiency is dependent on many factors. Displacement, intake and exhaust design, combustion management,gearing,vehicle weight,aerodynamics (as mentioned above). The number of cylinders for an engine of the same displacement doesn't mean much except for what the engine was designed to do in one vehicle or another. Why have the complexity of a V8 if you can get the job done with a V6 with the same displacement? It depends on what you want it to do and how much money you have....
How realistic is it to want to get 25 mpg out of a 1987 mercury grand marquis that weights aprox. 2 tons? The engine in it currently has 164k on it, so i think i might be building a new engine soon. Previous owner didn't take that good care of it. As far as gear ratio, its a stock automatic. Would a manual be more fuel efficient?
you need to look up the axle code on the driver door and tell us what it says above the AX area. that tells us the rear gear is. you can get better mileage out of a few tune-up type things. some basic maintenance and the car should pick up on the mpg. Grand Marquis in 1987 can get 20+mpg also... that is what i drive most of the time... also have a 1986 also here that belongs to my girlfriend's son...
i do lotsa modding to the cars here and make them better... biggest killers are the extra drag from smog and a/c and if you have a vacuum leak.
Well, if it's a 351 with a carb you would be doing good to get 20MPG, if it was running right. If it's a 302 with FI, you may get a little better. I used to get 23 from an '87 Caprice with the 305/4bbl and 700R4 overdrive trans at moderate highway speeds.
Those are old pushrod engines with dated induction and engine management systems... I'd save the expense of rebuilding an engine that old and get a newer vehicle with a 4.6 unless you are totally in love with the '87. A car that size and that old just isn't going to get very good MPG, no matter what you do.
a 5.0L is cheaper to build up. i personally prefer the 5.0L and rather work on that. at least with a 1986-1991 you can get performance parts cheaper than a 4.6L and many people have take off stuff from mustangs that bolt on... examples include headers, h pipes, x pipes, intakes, cylinder heads, etc.
Oh, yeah much cheaper if you re-do the whole thing. Since he wants MPG more than performance (I guess), that's just an opinion on the newer vehicle. I know most of the old small blocks are great engines. I've had several Mopar 318's and 360's in work vans and most of my friends' work trucks were Fords that ran forever...especially those 300inch sixes.
I have a '91 B250 Dodge van that is completely rust-free and that I love. I will probably rebuild the 318 before I buy a newer van...but it shouldn't need it for a long time.
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