1st off the 360 is probably not right getting that bad of miledge but I still think there is gains to be had by building a 273 to get better MPG, cost effective though I can't say for sure. I do know the 273 can make as much power as a stock 318, as a matter of fact the 273 4bbl made more power than 99% of 318s only the rare 4bbl 318 made more and it was not by much. The 273 2bbl made more TQ than a 225 2bbl or 1bbl.

2nd the 273 heads might not be as bad as you think, depending on what year they are... They all had closed chamber heads until 68 and those closed chamber heads flow more than the 302 head and have smaller combustion chambers.

He is putting it in a truck so high compression is not really a good thing, all he needs to do is zero deck the stock pistons in the block if the bores are still good and use the felpro .039 head gaskets to get some quench, all stuff he would still need to do to a 318 or 360 so it is still the same dollars.

I think the real deal maker for a small motor in a big vehicle is cam selection. He will need a wide lobe seperation and even retard the cam a couple degrees so there is very little overlap and so every last drop of air and fuel has time to burn in the chamber instead of the exhaust. I would order a custom cam with about 180 degrees duration @ .050 and get the fastest lifting lobe you can get at that duration. Add 1.6 rockers to get as much lift as possible, you don't want the piston to work hard to suck the air and fuel into the chamber, this will reduce pumping losses and help make it up to the 230 hp that the 318 made and then some (I think 250 is not out of the realm of possibility).

Also since the 273 is going to make less power at the same throttle angle you will have to push the throttle harder to run the same speed, this means even less pumping losses because manifold vaccume will be lower.

For a carb you should run an 800 thermoquad on a performer intake if it is 67 heads (920 castings) or 302 heads you have, if your heads are 66 or older they have a slightly different intake bolt angle so get a factory iron 4bbl intake with an adapter, you should have plenty room under the hood.

You should be running headers for it to make the motor do less work pushing the gasses out, also if you run some very long small tube headers you can boost low RPM tq with reasonance tuneing.

I think you will get a few more MPG than you are now and it should easily make as much or even more power than a stock 318. It is not going to be a great performance truck but it will be fun doing it.

I kinda wish I had the money to put a scienced out 273 in my car to see how much more (or less)mpg it would make than the old 318 did. Or even do another 318 but with a custom cam and EFI but I really think the 273 could do better than most of the skeptics on here think.

Don't know that much about 273s (or 360s, for that matter), but I'll reiterate that technology is very good way to improve MPG. My '01 Durango R/T (5.9 - 360, basically) gets 19 MPG on the highway at 60 MPH. Granted, it has a K&N FIPK kit, Fastman TB, headers, and 3" exhaust all the way back (2.5" into a Y-pipe), but it's doable. The rest is factory through the 46RE and 3.92 gears. Full tank of gas and driver comes in at 5240 pounds.

Modern Vehicles get better fuel-economy in large measure due to computerized electronic fuel-injection.

Better fuel economy and drive-ability. Perhaps you could find used parts from a junkyard donor engine or an aftermarket EFI kit?

I have been thinking about a cam from Hughes Engines #Heh-1019AL rated for Rv, towing and 4X4's. It has 210/219 @.050 with.461/.489 lift on 112LSA. Dave is the first one to think this might work. I am not expecting 20 MPG or a powerful truck. I have bigblocks for that. I once wrote to a Mopar magazine and proposed this idea using a 273 block and a 360 crank ground to fit the mains. This would produce a 300 cubic inch motor with a long stroke for torque. The editor said it was a stupid idea and to stick with the 360. The Ford Crown Victoria police cars make good power and mileage with a 4.6 liter V8. 273 is 4.5 liter. The Ford 4.6 has a bore of 3.55 X 3.55 stroke. Have a LITTLE FAITH IN THE LOWLY DISRESPECTED 273. I think with a few modern parts it might meet my goals. I intend to do this and will report back.

i think i remember that ? in the mag. Go for it and please report back. good luck

Powerwagondude. You're getting some good replies here and I will totally agree with many of them.

Vacuum advance, overdrive and no 273. Mileage can be obtained with cubic inches. I just went from a 3.5L LHS to a 3.8L Park Ave. Same everything but better mileage.

My truck's original 318, 2 barrel could easily get 18-21 with 25mpg straight highway recorded but it was probably an 18 second truck that could barely tow anything.

It also depends on how much power you are willing to sacrifice.

Now I went with a bigger engine, mileage is down slightly 16-19 average with a peak of straight highway at 23mpg but it's much faster, 14.8 @ 91.5 and it has enough power to make me question my tie down straps.
It's a 5.9 Magnum, carbed, stock cam and a 5 speed manual.

On the higway in 5th gear.

Just enough power to still be a fun truck.

Not knocking you a bit.Give it a try and let us know what happens.
I will say 210* @ .050 is still pretty big for a 273.Just a thought.......

If you think the 4.6 wil get good mileage in a truck you will be sadly mistaken. We have several, about 10, F150's. Some with the 4.6, they get maybe 13 mpg and are fairly gutless. My work truck has the 5.4 and it get's 18 or so, plus or minus .5 mpg depending on if I have a tail or head wind.

Good luck on your project but I suspect you willbe disappointed in the results. Your truck has the aerodynamics of a brick wall and the 273 will have to work harder to overcome that. Simple fact there. It takes so much power to overcome it. The 360 will make that power at a lower rpm than the 273, take so much gas to make a level of HP that regardless you will still be burning that gas, or more, to push your truck around.

Don't listen to the naysayers, true it will not be a hot rod but you never said you wanted one. I will also tell you I hate fords but the 4.6 aint that bad, not great but not bad and you gotta consider he said he mabey averages 40mph so aero dynamics is not going to play a big part.

I think 20mpg is easily possible if you are not running pedal to the metal all the time.

2 benifits you will have over the bigger motor is pumping losses will be significantly lower and friction will be a lot lower.

As for the compression comment I should have presented it with more other input. Yes you want as much compression as it will handle without detonateing itself to death but in a heavy truck with super short duration cam and small cubes you will find that limit at a lower compression than a bigger motor with a bigger cam. I would shoot for something like 9.5 max and be prepared you might need premium fuel.

Go back to basics.

If 3 otherwise identical PowerWagons are driving on a highway at 40 mph,
all of the same weights,
tires and gearing,
but one has a 273,
one a 318
and one a 340 V8
why does the 273
get better MPG at this steady speed situation?

The answer (mostly) has to do with MAP
(manifold absolute pressure)

Is a picture worth a thousand words?

You should however prefaced that by stating for instance a "2.73(?) cu in" would likely get poorer mpg. There is a sweet spot and you picked 40 mph steady speed for your example, the OP uses is likely different and therefore has a different sweet spot.

The 675 and 308 are going to work against you for MPG and HP

You should either use the 302 heads or your original heads, I think they are a 315 casting looking at the pics.

can you offset grind the 273 crank to get the stroke you want? might beable to use a set of chevy rods to save $$

all 273's had forged cranks, i think all 360's had cast.

you cant go nuts with the heads because bore strouding in the bore will be an issue.

does anyone know how much over bore the 273 can take?

cost vs. benefit for building a mileage motor, I still think you're better off using a 318 with KB167's vs a 273....

I had a 1975 3/4 ton with a factory 225 slant six. It got 13-14mpg.

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I dont expect a truck to get the same mileage as a lightweight car. I realize a truck is heavier. However Mopar did build 225 cubic inch trucks. Those sixes did move the trucks and also got better mileage than the V8s. Yes they had less power. But my goal is to make as much power or possibly more than a 318 (which is available in trucks) and try to get better mileage than a 318. Perhaps this is not possible. Thats why I am asking if anyone has had experience with the little 273. Many of you say the 360 will get better mileage. I know for a fact that the 360 get really bad mileage. I have had dozens of them. I have had many 318s also which get better mileage than the 360s. The brown truck is a 1/2 ton two wheel drive. It weighs 3800 lbs. Thats not much more than your average B body. Thanks again for your help

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