Moparts
MPG 318 vs 360
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=297On 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.
Posted By: Anonymous
Re: MPG 318 vs 360 - 08/17/10 11:33 PM
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.
Posted By: None2Slow
Re: MPG 318 vs 360 - 08/18/10 01:37 AM
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.
Posted By: Supercuda
Re: MPG 318 vs 360 - 08/18/10 03:32 AM
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.
Posted By: HotRodDave
Re: MPG 318 vs 360 - 08/18/10 07:07 AM
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.
Posted By: 360view
Re: MPG 318 vs 360 - 08/18/10 11:47 AM
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.
Posted By: patrick
Re: MPG 318 vs 360 - 08/18/10 11:59 AM
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.
Posted By: zrxkawboy
Re: MPG 318 vs 360 - 08/18/10 12:51 PM
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.
Posted By: Magnum
Re: MPG 318 vs 360 - 08/18/10 03:28 PM
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.
Posted By: 5thAve
Re: MPG 318 vs 360 - 08/19/10 06:18 AM
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.
Posted By: mark7171
Re: MPG 318 vs 360 - 08/19/10 06:59 AM
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.
Quote:
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.
You know I thought about that too but, then the 318 is #s matching to my car.
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!
Posted By: a12rag
Re: MPG 318 vs 360 - 08/19/10 11:43 PM
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
Posted By: Supercuda
Re: MPG 318 vs 360 - 08/20/10 01:50 AM
Quote:
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
Those US or Imperial gallons, cause if they are Imperial gallons that mileage is horrible. For those that don't know, Imperial gallons are 5 qts, US are 4.
Posted By: volaredon
Re: MPG 318 vs 360 - 08/22/10 02:57 PM
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.
Posted By: SattyNoCar
Re: MPG 318 vs 360 - 08/22/10 05:05 PM
Quote:
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.
100%
I had a '71 Chrysler, and a '76 PU with factory 360's in them. The truck was the worst of the two, NEVER got above 12, and usually averaged in the single digits around town. Compression checks, timing checks, different carbs, nothing increased the mileage of that 360.
Contrary to popular belief, with the truck, I swapped the truck's 360 for a (stock) 440 and did see a mileage improvement.
But, the OP's question was 318 vs 360, so, I won't get into it. 
Modded, the 360 may do better, but, stock, I'd go with a 318.
Posted By: gdonovan
Re: MPG 318 vs 360 - 08/22/10 05:50 PM
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..
Posted By: HYPER8oSoNic
Re: MPG 318 vs 360 - 08/23/10 01:27 AM
Quote:
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..
I think you just hit on something here. The 5.2
version of the Magnum motor may be the ticket
to higher mpg. It's already blessed with a low-friction valvetrain and very efficient heads with
decent sized valves. All it needs is a good "torque type" cam to get the job done.
Posted By: gdonovan
Re: MPG 318 vs 360 - 08/23/10 02:04 AM
Quote:
I think you just hit on something here. The 5.2
version of the Magnum motor may be the ticket
to higher mpg. It's already blessed with a low-friction valvetrain and very efficient heads with
decent sized valves.
It may have less friction out of the box compared to LA shaft rockers but the roller rockers on the magnum are an upgrade that is typically good for 10-15 hp on the chassis dyno due to less friction.
Posted By: HYPER8oSoNic
Re: MPG 318 vs 360 - 08/23/10 02:38 AM
Quote:
Quote:
I think you just hit on something here. The 5.2
version of the Magnum motor may be the ticket
to higher mpg. It's already blessed with a low-friction valvetrain and very efficient heads with
decent sized valves.
It may have less friction out of the box compared to LA shaft rockers but the roller rockers on the magnum are an upgrade that is typically good for 10-15 hp on the chassis dyno due to less friction.
My thoughts exactly!! 
Posted By: Magnum
Re: MPG 318 vs 360 - 08/23/10 02:01 PM
Quote:
I think you just hit on something here. The 5.2
version of the Magnum motor may be the ticket
to higher mpg. It's already blessed with a low-friction valvetrain and very efficient heads with
decent sized valves. All it needs is a good "torque type" cam to get the job done.
The cam you are looking for is already in there. They are ridiculously small, I think even smaller than a LA318.
Posted By: patrick
Re: MPG 318 vs 360 - 08/23/10 04:44 PM
Quote:
Quote:
I think you just hit on something here. The 5.2
version of the Magnum motor may be the ticket
to higher mpg. It's already blessed with a low-friction valvetrain and very efficient heads with
decent sized valves. All it needs is a good "torque type" cam to get the job done.
The cam you are looking for is already in there. They are ridiculously small, I think even smaller than a LA318.
similar duration, more lift.
if you want a good cam for a roller 318 that is a good balance of power and economy, get the stock cam reground by bullet cams, and use their HR259/316 lobe for both the intake and exhaust. pulls very hard in my magnum headed 318 roller motor.
Posted By: wedgeheaded
Re: MPG 318 vs 360 - 08/23/10 08:56 PM
I missed seeing you at the NATS, but I guess you saw me leaving. My 75 Doba has a 360 and swap meet intake/4bbl, (there when I bought it) and was getting me around 13mpg. I put on dual exhaust and got just over 10 driving to the NATS. I got to looking and found that the swap meet intake is single plane, I never noticed it before. I'm on the look for a dual plane intake now. I'd be happy to be getting mid teens but I think there's more there. It has just over 70K miles and has lots of oil leaks and needs valve seals.(winter project) I'm intrested in following this thread to see what I can glean from it. Best to ya, Tony.
Posted By: feets
Re: MPG 318 vs 360 - 08/23/10 10:29 PM
I think the 360 ports are just too large for cruise economy.
The smaller 318 heads will velocity up at the low rpm. That's going to give you much better cylinder fill at low throttle opening.
The Magnum heads are simply a better design but may be a tad large for maximum economy.
Posted By: gdonovan
Re: MPG 318 vs 360 - 08/23/10 11:31 PM
Quote:
The Magnum heads are simply a better design but may be a tad large for maximum economy.
My 5.2 Durango 2WD would pull down a confirmed 20.5 mpg (calculated from fillups) while cruising through Ohio and Kentucky at 70 mph.
Not bad for a heavy brick with no mods other than a Hooker max-flow muffler.
Posted By: HYPER8oSoNic
Re: MPG 318 vs 360 - 08/26/10 03:04 AM
Quote:
I think the 360 ports are just too large for cruise economy.
The smaller 318 heads will velocity up at the low rpm. That's going to give you much better cylinder fill at low throttle opening.
The Magnum heads are simply a better design but may be a tad large for maximum economy.
The Magnums are just fine for the 5.2/5.9 motors.
The key is to KEEP the velocity up, and a good
design intake and cam WILL do the job. Careful
selection of duration/overlap on the cam and a
dual plane or torque-type intake is neccessary.
Posted By: HYPER8oSoNic
Re: MPG 318 vs 360 - 08/28/10 09:08 PM
Quote:
Quote:
I think you just hit on something here. The 5.2
version of the Magnum motor may be the ticket
to higher mpg. It's already blessed with a low-friction valvetrain and very efficient heads with
decent sized valves. All it needs is a good "torque type" cam to get the job done.
The cam you are looking for is already in there. They are ridiculously small, I think even smaller than a LA318.
I beg to differ since there are MANY of these "torque-type, efficiency" cams on the market. Half ARE an improvement over the stock (factory) design.
Posted By: HYPER8oSoNic
Re: MPG 318 vs 360 - 08/28/10 09:10 PM
Quote:
I think the 360 ports are just too large for cruise economy.
The smaller 318 heads will velocity up at the low rpm. That's going to give you much better cylinder fill at low throttle opening.
The Magnum heads are simply a better design but may be a tad large for maximum economy.
Well said!!
Posted By: Mad Max
Re: MPG 318 vs 360 - 08/31/10 07:28 PM
I'm going through a similar quandry with my '93 Ramcharger 4x4. Just got it locally, really nice condition, about 150,000 on the clock, has the 3.55 gears and 30" tires.
Discovered the overdrive isn't engaging, and my highway rpm has been a nice 2400-2600 rpm, and I've been getting ~ 15 mpg - not too bad really. But that's with the tranny in 3rd, not overdrive. When I get the od functional my 65 mph cruise rpm will be 1650-1850 rpm, and I can't help but think those rpms will pull the 318 out of it's sweet spot, and likely my mileage will go down.
I've always felt the small blocks 'enjoy' a bit more rpm, and I'm thinking seriously about replacing the 3.55s with 4.56's.
With 4.56s and a 31" tire, 65 = 2250 and 75 = 2600, and that sounds pretty good to me.
I can't help but think the 4.56's would be a better gear ratio, especially considering I do a lot of around town driving and the gears should help get the truck around lots easier.
What rpm range do y'all think a Magnum 318's 'sweet spot' is?
Posted By: BossRide
Re: MPG 318 vs 360 - 08/31/10 07:39 PM
My '73 has a 904 with a 318 torque converter, 2.73 gears, 275/65/15 tires, a rebuilt '79 360 from a Volare wagon, a 650 Holley on an Offy intake, a semi-lumpy cam, a decent valve job, TTI headers with dual exhaust and an H-pipe, all the A/C stuff and power steering, and I drive like a lead-footed monkey - regularly cruise to Birmingham and back at 80-85mph... I get between 15 and 19 mpg whenever I check it.
Posted By: HotRodDave
Re: MPG 318 vs 360 - 08/31/10 09:13 PM
Your truck will get better MPG with the 3.55 than the sub 4 something gears you want. Nothing at all wrong with running a 318 magnum 1800 RPM. You will have less vacume in the intake for the pistons to pull down against. It will also make less friction inside the motor. You will be takeing a step backwards. If the truck would get better MPG with the lower gears the factory would have put them in it.
2600 RPM is way beyond the "sweet spot" for MPG(I am guessing you mean tourque peak, but even that don't make sense because you don't cruise at WOT so that tourque peak means nothing at cruise).
Posted By: Mad Max
Re: MPG 318 vs 360 - 08/31/10 09:38 PM
ya my 'sweet spot' query was more along the lines of 'where is the optimum rpm range for a 318 for maintaining speed', not for max power. More like, where in the rpm band is 'good/enough' torque generated.
I really wish there were dyno results generated where the engine isn't at full throttle, IOW a half-throttle dyno run showing how much power is generated in a 'normal' driving condition verse WOT. WOT isn't what I'm concerned with - I want to see the power levels for an engine in the rpm range where it spends all its life.
Thanks for the reply. If 1800 rpm is an efficient/maintain speed rpm for a mag 318 then I'll be happy to leave the gears alone. Conveniently, 1800 rpm is also the sweet spot for a 4bt Cummins....
Posted By: gdonovan
Re: MPG 318 vs 360 - 08/31/10 10:04 PM
Quote:
ya my 'sweet spot' query was more along the lines of 'where is the optimum rpm range for a 318 for maintaining speed', not for max power. More like, where in the rpm band is 'good/enough' torque generated.
My 5.2 magnum 1500 Ram tools down the highway at 2000 rpm.