Im new member and am building an 82 f150 as a dd and could use some advice. i am going with the 300,np435, and 2.75 9" rear end 4x2. my original plan was small cam p&p the head 4bbl intake and headers. 470 holly and a D.U.I distributor. maybe some jet work on the carb. the goal is to get mid 20s mpg and bump up the power just a bit but just enough the keep up on the highway any advice on this would be appreciated thanks

 

Thats a hefty goal. I would recommend focusing on reducing weight, pumping up tires, skinny tires, ensure wheels turn freely, gearing for low rpm, coasting in neutral. You think a higher CFM carb is gonna give you more MPGs?

 

One of the problems with the six cylinder configuration is fuel distribution. Engines will not burn raw liquid fuel, it has to be atomized sort of like a "fog". That's one of the main drawbacks to carbs and throttle body fuel injection, the wet intake manifold and keeping the fuel in a fog like state. The six especially has a problem in this area, since the intake is so long. It's very hard to give the end most cylinders the correct balance of air and fuel.

Keeping the volume of the intake very small keeps the velocity up, which helps keep the fuel atomized. The original intake sits right against the exhaust manifold. This heats the intake, heat also helps keep the fuel atomized.

One way around all this would be to fuel inject the engine.

Keep the carb and the intake very small for good fuel mileage. You want the engine to be turning very low rpms.

 

Why the NP435? I haven't driven a six with the NP an 2.75's but I wouldn't think that would be a fun combo since you really only have three gears in normal use. I would consider the M5OD which would give five gears to use daily, although you would probably want to change the final drive ratio. Ford sold a "high MPG" combo back then and, IIRC, it used the M5OD. And it had a small chin spoiler.

 

In 83 Ford promoted the F150 six as a high MPG model....package included the 4OD tranny, and Overdrive name plate on the tailgate, and 41 psi tires.

 

thanks for all the replys the thought on the higher cfm carb was that the secondarys would stay shut till i really got on it but i have been thinking about the 390. i spend most of my time doing about 65. the efi was something that i thought about but thats a bit complicated for me. as for the trans i did not want to give up the the granny gear and that low revers and the cost of a 5sp is a bit out of budget at the time at some point i thought about the zf 6sp when i drove the truck home i also fond that i would not pull an over drive it originally had the 3+od trans and i put it in od doing about 70 and it bogged out.
my last truck was a 300.np435,4x4 with 3.08s and it did pretty good till about 65 then it would start drinking gas.
also i have read that back in the 70s ford advertised 300 making 170 hp and getting up to 30mpg dont know how true it is or if they just had better gas back then.

 

No way one of these bricks going down the road is going to get 30mpg. A truck's aerodynamics are terrible, and they are heavy. A ranger pickup would be a better start to try to achieve 30mpg. You might get there with a 4 cyl.

 

Despite the advertising hype about these trucks being aerodynamic, they are about as far from it as is possible. Just look at the changes Ford made in the 87 and later models that surely must have been for aero as they killed the looks. They moved the headlights flush with the front edge and rounded the corners - among other atrocities. (Sorry, Dave.) so that's why you are seeing the MPG go south above 65 - the wind drag has taken over as the significant energy consumer and its value goes up with the square of the speed.

 

the trans r/a you are wanting to try. It (NP435) may be a 3 spd but it at least does have an ultra low both forward and reverse that could be useful in many situations.

Keep rpm's low, and speed under 60...

Heat your intake of choice..(I like the Clifford cuz' the water heated portion seems to be long underneath the floor)

I would use a smaller Venturi 2bll carb, or for the uniquesnnes of it maybe even a BG or Eddy Stromberg copy. Small ventruri, high velocity..(welcome flames here) or, of course a 1bll of your choice.

Use skinny tires inflated well...tape a raw egg to the skinny pedal and drive very light footed..

Personal testimony of mine..non-ethanol 93 works best for me.

 

A 4 bbl with vacuum secondaries can be a decent choice for mileage. It has smaller primaries than a 2 bbl, so as long as you don't open the secondaries it keeps the intake velocity high, which is the goal. But you don't need to go as big as a 470 cfm. That would feed a 300 at 5400 rpm (assuming perfect scavenging). a 390 cfm carb will still get you up to 4500 rpm, and if you're going for mileage in a 300 you probably aren't going to spin it any faster than that.

 

I have always wanted to mess with and learn about weber carbs. They are supposed to be VERY adjustable and adaptable. But they have always been out of my price range.

 

well like i said i do not know how true that 30 mpg thing was as i have a 2.3 ranger i drive now and it struggles to get 28 and thats when i dont shift at 4 grand.
as far as the carb goes i spend most of my 100 mile daily drive doing 65/70 do yall think the secondarys would stay shut at that with the 390? from what i have calculated i sould be running between 1800 and 2300 rpm at that speed.
also do i have anything to lose doing p&p on the head? thanks for all yalls input so far it is looking like there are not a lot of people doing what i am trying or if there are they'er not putting it on the net so all help is appreciated.

 

1800 to 2300 is a huge range. The six wants to run at the very bottom of that range, so you need to gear to put it there.

The secondaries are adjustable so you can set it up such that they won't open at any speed. But the factory setting might have them coming in around 70 under certain conditions, like the 30 MPH headwind I drove into yesterday. It felt and sounded like we were going 100, but when we turned around and drove with the wind it was so quiet it was spooky.

It is impossible to answer the question about porting and polishing because I don't know what you are going to do. It is quite possible to hurt the low RPM performance with porting, like making the intake runners big or mis-matching the head and intake ports. But within reason you probably won't hurt anything - nor will you gain anything on MPG. I say that because I went to great lengths porting the heads on a 351W, and doing things that wouldn't hurt low-end performance, only to find absolutely no increase in MPG. None. Nada. And that was very carefully measured over many tanks and exactly the same course and conditions. However, it made a big difference in the top-end performance. The engine wound like it had a 4bbl on it in spite of have a little 2bbl. Apparently the big help was on the exhaust side, and I had gone from a single system to a true dual system at the same time as the head work.

 

stringbeanf200,

I am a little late in the conversation, but welcome to the forum! You have been given advice from some very knowledgeable people! Pay attention to what they say.

I will try to recap what you have asked and said and go from there. Your original post said your goal is to build the 300 for mpg and then you added that you also wish to bump up power to be able to keep up on the highway.

Later, you mentioned that in the 70s, the 300 engine had a horsepower rating of 170. That figure was the gross horsepower. Automotive manufacturers switched to net ratings and the published figures were immediately lower.

You then mentioned having a 4X4 with 3.08 gears and that after 65 mph, it began drinking gas. As has been mentioned, these trucks are like a brick going through the air and the faster one drives, the more power [fuel] is required. In reality, 55-60 mph will return the best mileage with a moderate compromise in speed. Gary Lewis mentioned the drag and coefficients doubling with the speed. Look at it this way, @ 50 mph, 50 X 50 = 2500; @ 60 mph, 60 X 60 = 3600; @ 70 mph, 70 X 70 = 4900. At 70 mph, it requires approximately 96% more horsepower [think gas mileage going downhill!!] than while driving 50 mph. That is a substantial difference.

You mentioned that your commute is 100 miles @ 65/70 mph and that your engine speed is 1800-2300 RPMs. Gary pointed out that is on the high end for the 300 engine. For gas mileage, it is good to be right at the maximum torque value of the engine. That would be approximately 1600 RPMs for most stock carbureted 300s.

The 300/4.9L engine thrives on pulling at low engine speed. I previously owned a 1983 F150 with 4.9L, 4 speed manual O.D. and 2.47 rear gears. It would return 26+ mpg @ 60 mph. At 60 mph, the engine was turning 1375 RPMs. It ran just fine on relatively flat ground, but quickly required a downshift to climb a hill or pass.

My current 1986 F150 with 4.9L, 4 speed manual O.D. and 3.08 gears returns ~22 mpg @ 60 mph. At 60 mph, the engine is spinning @ ~1700 RPMs. It runs well on the highway and rarely requires a downshift.

Keep in mind, the previous two examples have O.D. [yours does not] and the O.D. ratio is 0.71:1. That makes the 2.74 gearing like having 1.95 gears without overdrive. My 3.08 with O.D. is like 2.19 without O.D. My point is that when Ford Engineers planned for mileage, they came up with the big six cylinder engine turning slowly and breathing through a one barrel carburetor. The one barrel maintained the desired high velocity of the incoming air/fuel charge and by lowering the RPMs, the throttle plate will be opened more, thereby reducing pumping losses.

We all encourage each other to do what makes you happiest since, after all, it is your truck. What you might consider doing is to calculate what it costs to make your drive with your truck now. Then, guestimate what mileage increases you might realistically obtain with any suggested changes and then calculate how far [how long?] you will have to drive to recoup your investment?

For little expense and quite possibly some surprising improvements, try some or all of the following: remove all unnecessary weight, remove anything increasing wind drag [i.e. driving lights, tool boxes, unusually mounted spare tires, oversized mirrors, etc.], use stock sized tires [highway tread, not aggressive] with proper inflation, tune up engine with new air filter, use of correct temperature thermostat, proper alignment, make sure the choke and preheating of manifold as well as the preheated air to the air cleaner snorkle are working, if your 1982 had a cold air intake from the radiator support, it should be hooked up, use of synthetic oil and grease helps a little. I am sure I have missed something, but others have and will suggest more.

The biggest improvement will come from reducing your speed [55-60?] and driving like an old man [someone suggested placing an egg between your foot and the gas pedal.

I know this is a l-o-n-g post, but I sense that you really do want to get better mileage and I share the same desire for my truck.

Hope I did not put everyone to sleep. ZZZZZZZ

 

David - I would rep you for that were I not in jail. Well done!

Stringbean - There's a guy with hands-on knowledge of the 300 in a couple of different configs and what mileage they delivered. Take heed as he gave you two benchmarks of RPM and MPG that point out both how low the R's & MPH need to go to get good MPG as well as how fun they are to drive in that guise.

So, if you want to stay with your NP and 2.75 gears plus drive 65-70 then you won't come anywhere near what David/1986F150six achieved. I'm not saying that is a problem, I'm just setting your expectations. And, as David implied, for the price of changing everything to get that high MPG you can pay for a lot of gasoline.