Bore size versus fuel mileage. 273 versus 5.2 magnum

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HankRearden

HankRearden

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So the price of gasoline has me rethinking the plan of a mild 5.9 8
In my arrow pickup.
Now I'm thinking since its the lightest vehicle I own I should make it my daily driver and build for a little fuel economy.
I have a magnum 5.2 that needs rebuilt but that got me wondering about finding an old 273 also.
I need it to be torquey because daily driving for me is in the hills of western pa.

But which do you think would be better for mileage and why?
 
Trailbeast gets some impressive mileage out of his 5.9 magnum. How much extra are you trying to get with the 273?
 
Slowswinger said:
Trailbeast gets some impressive mileage out of his 5.9 magnum. How much extra are you trying to get with the 273?
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I dont know. I've driver mopar stuff my whole life and fuel mileage was never a positive attribute. So I'm curious what I can put together.
 
Watching. Just my 0.02... a lot of fuel mileage boils down to efficiency and in some cases means more than just displacement. What gear do you plan on running? What trans? Do you plan on efi or carb? I would say that apples to apples, in a hilly area a stock 5.2 magnum will get over an incline under less stress than a stock 273, less stress equals less fuel consumption. I also think that for maximum fuel economy you will probably want efi to cover all types of altitude changes and other factors. Generally speaking, a 5.2 can most likely handle a taller rear gear as well to keep rpm down for the flats while not being bogged down during an incline. As Jay Leno loves to say when people ask how much power some of his cars have "Just enough power to get over the hill and save a little gas"
 
Smaller bore, like smaller chamber, promotes a more complete burn of the fuel. More compression & timing helps power & mileage, but you have the quality/octane of the gas to worry about.
 
My 273 gets almost 20 with a 833 and a 3.23 rear. It has plenty of torque. Mopar engines by design have torque probably because of the rod to stroke ratio. The Chevy guys always brag about building a 6" rod motor. Ya, my 273 has that and more. (Their jaw drops)
 
LA 318 with with Headers and a holley sniper EFI . use the 999 lock up trans on a switch, Or a 998. with around a 290 to 330 gear ratio depending on tire Dia.

The best of the best would be a cummins 4BT out of a mail truck. you can run Home heating oil, Diesel, or Kero.
 
The 273 with the smallest bore will have an efficiency advantage by way of reduced surface area of the piston and chamber. That's less area for combustion energy to soak into the block and head. But it's fairly minor in the greater scheme of things. The low hanging fruit is going to be cylinder pressure. If you want anything resembling fuel mileage, pick a cam that closes the intake valve before 60 degrees ABDC. That's going to mean a mild or "RV" type cam. No rumpy-rump idle.

If it were a choice between a relatively antique 273 or a readily available 5.2 magnum, the 5.2 would be my choice every time. I've never seen a 273 with multiport sequential EFI, like every 5.2 has.
 
Fuel usage is basically = Cubic Inches (smaller the better), RPM (lower the better), and volumetric efficiency (higher is better) at the RPM the engine spends most of its time.
 
To me building for economy is a waste of money beyond a basic tune. You would need to put a lot of miles on a car to break even nevermind gain. Not saying that shouldn't build with economy in mind when planning mods like cam gears etc.. Generally we all want a little performance even if it's like a 15-16 second car building different Cid to a similar performance level is gone require similar amount of fuel no matter if it's 440 vs 273.
 
The 318 magnum is superior in many areas. Better heads, more efficient chambers, smaller lighter valves, and a roller cam...all those things will help mileage. I have a stone stock junkyard 5.2 in a dart, with a dual plane intake, thermoquad, and a set of Doug's headers, 904 and a 3:23 rear. It gets 21 mpg on the highway easy peasy. No tricks
 
replicaracer43 said:
The 318 magnum is superior in many areas. Better heads, more efficient chambers, smaller lighter valves, and a roller cam...all those things will help mileage. I have a stone stock junkyard 5.2 in a dart, with a dual plane intake, thermoquad, and a set of Doug's headers, 904 and a 3:23 rear. It gets 21 mpg on the highway easy peasy. No tricks
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That's what I wanted to hear. A perfect combination. My 3.9 V6 Magnum in my 96 Dakota get's only 22 hwy. with the 500 o/d automatic. Why not have some v8 power!
 
toolmanmike said:
That's what I wanted to hear. A perfect combination. My 3.9 V6 Magnum in my 96 Dakota get's only 22 hwy. with the 500 o/d automatic. Why not have some v8 power!
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I recalled the dakotas I had experience with back in the day, the 3.9s got about the same milage as the 5.2s in similar equipped trucks. The 3.9 had to work harder
 
replicaracer43 said:
I recalled the dakotas I had experience with back in the day, the 3.9s got about the same milage as the 5.2s in similar equipped trucks. The 3.9 had to work harder
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Both engines are doing similar work and are similarly efficient so should burn similar amount of fuel.
 
Now if you lived in a place with really long flat open roads, like Wyoming for example, the 3.9 could potentially be better than the 5.2
 
The 5.2 magnum, Re drill the heads for LA Pattern Run a Sp2p intake with a 500Cfm Eddy get a MSD box. Get a RV Camshaft regrind 800-5,000 Rpm Range advance it 4 degrees. Recurve your distributor and run a Vacuum Can that pulls 22 Degrees at cruise. I have a a 88 318 Roller camshaft block with magnum heads and a regrind Cam Rpm Intake, i get decent mileage and power.
 
Hank, if your keeping the Arrow rear end, what ratio is in it? As i recall they had a pretty steep gear. If so, that will be detrimental to fuel mileage, especially with any V8 in that light vehicle.
 
273 with mag heads at .060 quench. Magnum EFI.
 
Many factors are going to affect engine efficiency apart from bore/stroke/cu in/rod length etc.
A convergence of efficiency factors [ eg, small ports, carb that atomises well ] will make a difference & improve economy.
 
OldmanRick said:
Hank, if your keeping the Arrow rear end, what ratio is in it? As i recall they had a pretty steep gear. If so, that will be detrimental to fuel mileage, especially with any V8 in that light vehicle.
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I haven't checked yet but I read they were all like 3.73 or something. I plan on swapping it out for something in the 2's as I cant fit much of a tire on it.


Making the factory efi work is out of my realm of expertise and patience. I'm thinking either a small carb , the old holley economaster would be great for this, or the fitech off of my CJ7.
I like the idea of the 273 with the magnum heads and an aftermarket roller cam . But the difference in cost between that and just using a magnum 5.2 is quite high and I kind of doubt there would be enough mileage improvement over the 5.2.
 
toolmanmike said:
That's what I wanted to hear. A perfect combination. My 3.9 V6 Magnum in my 96 Dakota get's only 22 hwy. with the 500 o/d automatic. Why not have some v8 power!
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I never got over 17 with my 3.9 in the 99 I had.
 
Since you already have the 5.2, That's the way I'd go. For the cost of 1- Finding a usable 273 (that ran on a carb its whole life, so most probably will need boring) and the parts to "273 with the magnum heads and an aftermarket roller cam", you will be WAY ahead of whatever mileage you MIGHT possibly maybe get by it being 45 cubes smaller, especially if, it being a pickup you put stuff in the bed or pull a trailer up & down those hills.
For the money you would spend on the 273, you might consider a bolt-on EFI, though the tank, plumbing, pump additions must be considered. I wouldn't expect huge gains over a well tuned (fix it anywhere) carb, but depending on what kind of elevation changes you expect, it might give you a little more drivability, quicker starts, etc.
 
How much fuel a year you'll use ?
How much gain in mileage do expect ?
How much will it cost ?
 
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