We currently have a tired 184k mile 318 in a 68 B-Body with 2.76 gears and 275/60/15 tires out back. We drove 232 miles to the Nats and average 20 mpg driving. The 318 has a Weiand Action Plus/Summit Headers/Firecore50 wires/Mancini Racing Ignition setup/1406 Edelbrock/Summit Racing K6900 cam currently.
The 318 is getting slightly tired and leaks/burns oil.

We have a spare 79 360 minus heads/intake out in the garage that would need a rebuild but, I am thinking of keeping this on the budget side of things. That being said the 360 would have slightly off the line torque. My question being if I built the 360 I was thihking of using the above top end package off the 318 for the car.

A couple of changes I am thinking is we do have a TQ 800 cfm with an electric choke that has been rebuilt. Also, I am thinking of this cam for the 360 since we have a heavy car and 2.76 gears.
http://www.lunatipower.com/Product.aspx?id=2340&gid=297

On the 318 I could rebuild with higher cr pistons but, I am thinking the same cam for it or maybe a factory spec 340 cam.

My goal is mpg here along with some peppy power and run 14.9s with the car? Just wondering what the opinion is here. Also, we can get a set of factory LA heads cheaper than the Magnums so we would use the 1.88/1.62 heads. What do you guys think? TIA.

20mpg is pretty darned good!

The larger valves/ports of the 360 heads are definitely more efficient than the 318 heads.

I did a 318 with the 1.88 intake 360 heads and it easily broke into the 14 second range @ 95mph at the track in a street driven car. I got 17 mpg but had big single plane intake, headers, .480 Comp cam, 3.23 gear etc.

Years ago, maybe 15+, Car Craft took a 63-64 Nova and pulled the straight 6 and put in a very respectable 350. They dramatically increased their fuel mileage. They say it's because they needed less throttle input to move the car, which equates to less fuel. I believe it was a 194 and a powerglide that they replaced with a 350/350 combo. You gain more torqu going with more inches which helps get you off the line without having to step on the pedal so much.

I've run a 318 and 360 in my 87 Diplomat. @.94 GEars, A833 trans, P255/60R15 BFG's.

318 was mostly stock, with MP electronic ignition, added a 4bbl intake and 1406 carb to it. 25 mpg highway, routinely.

360 was a 79 engine, 340 cam, same intake, carb and ignition as the 318. low 20's highway.

Took the 360 heads off, put the 87 318 2bbl heads (302 castings) on the 360, rest the same as the above 360 combo. Substantially more low end torque, no change in high rpm limits or power. I would routinely peg the 125 mph speedo.

Swapped a Comp XE262 cam in and more power. No real change in mileage to speak of.

Best combo for my setup was the 360 short block, 318 heads, OEM iron 4bbl, XE262 cam, 1406 carb.

Plenty of power, low 20's for mileage, good top end. A real nice combo.

Don't buy into the big cubes = better miledge bologne. There is a whole lot more to it than all that. You want me to throw out some numbers, my 318 I had in my cuda got a best of 29 mpg the 360 in it now barely will get 20 mpg. If bigger motors got better MPG then a prius woul have a 512 hemi under the hood. If you take two similar engine lie say a 3.9 v6 magnum and a 5.2 magnum and put them in the same car the V6 will get better MPG UNLESS you force it to accelerate the vehicle as fast as the 5.2 did, for exaple running 75% of the engine power when you are trying to merge instead of 50% of the 5.2 power, if you push it to get the same performance it will use the same fuel, the 5.2 has the potential however to use 25% more fuel at WOT than the V6, people just don't push them that hard and so every one claims the V6 uses as much fuel as the V8 with is only true if you expect them to perform the same, drive the V6 like a V6 instead of trying to demand V8 performance than it will get better MPG. I have owned plenty of both vehicles and my V6 dakota gets better miledge than any of my V8s ever dreamed unless I am hard on it. If you force it to down shift more it revs more and you lose the MPG improvement.

Higher throttle angle means less vacume in the intake resisting down ward movement of the piston and costing you miledge. That is one reason why higher gears help MPG. Higher RPM also makes more firction.

It only takes mabey 20 hp to maintain reasonable highway speed in an average car, if your motor is capable of makeing substantially more power at your cruising RPM you are wastin energy. The reason car makers do not set cars up to run that low RPM is because then you have to down shift to go up a hill and people don't like there car to shift, that is why we have MDS and dual VVT (both technologys make the motor more efficent by reduceing intake vaccume at cruise) motors now combined with higher gears, the new motors make more low RPM tq so they don't have to shift as offten with the higher final drive ratios.

In my experimenting high compression and quench make better MPG along with less overlap and high flowing small ports, then gear the heck out of it and watch your cruise MPG go way up. If you get the compression too high retarding your cam can reduce intake vaccume, reduce cylinder preasure, give you combustion more time to press on the piston and you can get even more MPG, in it's exagerated form this is called Atkinson cycle, another technology used in modern cars for increased MPG.

If
"the goal here is mpg with peppy power"

then my opinion would be to
sell the parts you already have,
and buy a complete
1992-1995 5.2 Magnum V8 package
out of a van that had the 36RH transmission.

I know the tendency to hold on to older machinery,
I have that too,
but you would probably give up at least 4 MPG
at steady highway speeds.

my A/B comparison:

car is an '86 5th ave, with A500 OD tranny, 3.55 rear, 27" tires, ~4200 lbs with driver.

first engine: reringed 360, comp XE262, redone 360 heads, home ported by me, headers, eddie RPM air gap, eddie 1406 carb: 15mpg mixed, best of 18mpg all highway.

current engine: reringed '86 short block, redone magnum heads, home ported by me, reground roller cam (259/259 adv, 208/208@.050, .506/.506 lift), same intake, carb, & headers as the 360. nearly identical power via the butt dyno, but 17+ mpg mixed, haven't done a tank of all highway, wouldn't be suprised if it was over 20mpg.

if you want to do it on the cheap, do the 360 with 318 heads. if you want the most efficient motor, I'd do a 318 with KB167 pistons at 0 deck and magnum heads. brand new EQ magnum heads are $725/pr with LA intake pattern from hughes with their #1110 springs. once you redo a set of used heads and get the proper springs for that cam (comp 901's or the above hughes), you'll probably be into them for ~$300, probably $450 if you need to replace the valves.

magnumminded-
I like your plan. I would use the 360 for the build, along w/ the 340-spec cam you mentioned (or similar) and the TQ.

My numbers are pretty similar to Supercuda and Hotroddave. Mid 20's with ease but from a very well worn and weak stock 318 in my 87 Ram.
Swapped in a 360 Magnum, w bolt ons. 670 Avenger, M1 dual plane and small headers to 20mpg.

As for power, the first stage was just the engine swap. So my numbers was about 5mpg loss but it went from (guessing) a 18 or 19 sec et to a low 15.

Later swapped the rear gear and overdrive manual transmission. Gained 3 mph and now in the high 14's and straight highway is 23 mpg.

With no other changes, bigger engines use more fuel. I don't care about fuel used when accelerating. Most of your average mileage is from driving down the road. During that time smaller is better.

My 318 5th Avenue used to get about 27MPG on the highway and was OK in the city. 5th Avenue with a 360 got pretty good on the highway too but probably not as good but it was horrible in the city. If you're going after mileage Id stick with the 318, if you don't care so much and rather have the power go 360.

This is sad. Come on..

My 323" 340hp cam, 3.23 , 4 speed a body gets 14/16 mpg highway/city .

My 93 FI Magnum Ram Charger new got 12 , that was claimed as on the sticker .

You want performance and 35mpg mileage get a 2000-02 4 door Geo Prizm 5 speed a must. 40 honest MPG city. The engine is the NUMI joint GM-toyota venture 1.8L vvti rated at 125hp/125lbs , it obviously has more. expect 7 second 60mph sprints and 15.2 secong 1/4 mile. say what! 40 mpgs...Prizm is the King.

Most common 40 mpg city car is the Saturn 1.9L SL2 i4. the twin cam 1.9L boasts 127hp/125lbs, with 5 speed 7.5sec 0-60 and 15.90 1/4 miles are happenable. Any Sl2 5 speed , from any year.

The up till 2001 hondas' are nice but don't get 40 mpg.

what about putting a cummins turbo diesel in it? Change the turbo to twins, get a giant intercooler and a different cam, you can have massive power and great mileage!

My 74 Duster360, still has the original 360 in it, with one rebuild on it. All stock for 74, including the cast iron 4bbl intake and TQ. 727 with factory stall convertor (believe it is 2800rpm), and have changed out the 8.25 rear end with sure grip and 3.55:1 to 8.25 open with 2.45:1 ! - why you ask ???

In 1999 I drove to Carlisle and back (sheesh, like over 3,400 miles round trip ??) . . wanted some mpg and keep the RPM down while cruising . . . here are the REAL world numbers for my car - 70mph all day long, 2600rpm and 22MPG !!! Not to mention LOADS of mid-top range power - gotta love passing in this car !!! . . .

I think if you did a rebuild of the 360, with the 340 cam, used the TQ, had a bit of stall convertor and kept your gears you have now, you would be happy camper !

Just my

Mark

I have had 318s get me mid 20s on the highway (even my 87 B 250 van got 21 great for a brick)
but I have never ever had a 360 in anything, that got over about 13.

Sort of an extreme example but I'll throw it out there-

I went from a 119,000 1974 318 with a 4-bbl and duals to a '94 roller cammed 5.9 with H.S. roller rockers and the MPG got better.

Much less throttle is needed to get the car up and moving and the later model motor has much less friction with the roller cam and rockers plus lighter pistons and rods with a windage tray too.

Same carb, exhaust manifolds, distributor.

Of course at WOT, that is another story..

Been several weeks since I did the swap, very happy as I expected to get punished bad by the MPG gods but it didn't happen..