390 build

[390owner]

I am thinking about having my 390 rebuilt by a machine shop that built it over 20 years ago. My question is about the gas mileage. When it was first done I could get around 12 miles per gallon. I am wanting more horse power so he is going to port and polish the heads, balance the lower end and put some kind of cam in it. Is this going to kill the gas mileage. He said it would make around 350 horse. I know is will depend on what cam but just dont want it to get 4 miles per gallon

[FElony]

I think you'll get more response if you include exactly what new parts are going into this build.

[FrozenMerc]

With that little bit of information, you might as well be asking Stevie Wonder what color that flower is.

Tell us the engine build details, what vehicle it is going into (the same 390 in a '76 F350 is going to get much different mileage then in a '34 Ford), drivetrain details, tire size, etc.

[390owner]

That is the problem I dont know what he is going to do. I just told him I wanted all the torque I could get for towing my camper. He said with some head work and cam and balancing it would be around 350 horse That is all I know. It is in my 79 bronco with a 4 speed and 350 gears and 32 inch tires. It used to get 10-12 and 7-8 towing. I may just call him and ask what he thinks. He also suggested aluminum heads but I dont think my headman headers will work with them

[cjshaker]

At that level of a build, in that vehicle, I don't think your mileage should suffer at all. I get 12 and 6-8, towing a 9500# trailer with a '70 F350 dumptruck with 4:11 gears and 28" tires. That's with an all iron 390 in a much heavier truck. I used to get 14-16 in my '68 F250 Highboy. Both were/are non-strokers with simple and mild, but solid builds. From what it sounds like on your build, your driving style will be a big factor in what your final mileage will be, but with modern cams and a good tune, it's common to pick up a big chunk of power and still retain the same mileage over stock, if not even improve on it.

[TJ]

At that level of a build, in that vehicle, I don't think your mileage should suffer at all. I get 12 and 6-8, towing a 9500# trailer with a '70 F350 dumptruck with 4:11 gears and 28" tires. That's with an all iron 390 in a much heavier truck. I used to get 14-16 in my '68 F250 Highboy. Both were/are non-strokers with simple and mild, but solid builds. From what it sounds like on your build, your driving style will be a big factor in what your final mileage will be, but with modern cams and a good tune, it's common to pick up a big chunk of power and still retain the same mileage over stock, if not even improve on it.

Agreed.  To 390owner, I expect about the same or better fuel economy and a bump up in power.  The thing to remember is the fuel economy will drop if you can't keep your foot out of the carb.  So if you spend much time enjoying the power you will see mpg suffer.

Don't forget to optimize your timing curve...that will help power and mpg.

[plovett]

I agree that with a mild performance build you might not lose any gas mileage.  A fresh engine will make up for any slight efficiency loss due to a slightly bigger cam.

So.....ask your builder what exact cam he is going to put in there, and let us know.  The exact compression ratio would be good to know, too.  Higher compression will help fuel economy as long as there is no detonation.

And.......add headers if you aren't already.  Headers help even the mildest builds get both more power and more economy.

JMO,

paulie

edit:  I see you have headers already.  Keep them.

[Falcon67]

That is the problem I dont know what he is going to do. I just told him I wanted all the torque I could get for towing my camper. He said with some head work and cam and balancing it would be around 350 horse That is all I know. It is in my 79 bronco with a 4 speed and 350 gears and 32 inch tires. It used to get 10-12 and 7-8 towing. I may just call him and ask what he thinks. He also suggested aluminum heads but I dont think my headman headers will work with them

That sounds about right for the combo.  Stock 460 in a F350 - 10 MPG unloaded, 6.75 towing.  It is what it is, I don't worry about it.  The modern 5.4L F150 gets 11 with 5500 lbs on the hitch.  I would not have him do anything to the head ports unless they need cleanup.  Torque production will be in the displacement and cam selection.  If you want more torque, add more CID.  That's what I'd do, and will do the our F350 when the time comes. 

[olman]

My 86 f-150 has a '69 429 converted to a 460 and C6 out of a wagon. DOVE heads edelbrock intake 750 performer, Doug Thorley tri ys and a good torquer cam Motorsport roller rockers with 3.50 gears and 16" wheels. Power galore but 10 mph up hill down hill fast or slow. Like it says "it'll pass anything on the road but a gas station"!
 
Then I had a 75 f-100 with a 428 out of a '66 Merc Merauder built with all the bells and whistles and it and got me an honest 15mpg on the hiway. It's only gas what the hey?