Trying for gas mileage but still want a V8

I want to build this 273 for gas mileage in a truck. I want more power than a 318 and better mileage. Here is my plan: fresh bottom end, Comp Cams #20-210-2...260 dur/.440 lift 110LSA, rated 1200-5200 RPM, dual exhaust maybe headers, 3.23 gears 31 inch tires,Either Edelbrock 500 four barrell or Holley two barrell#2300. Any input is appreciated

My 360 gets bad mileage

Oh I forgot. I know the 273 heads suck. I plan to use 318 small port heads for port velocity #675,#308, or #302

you might get better mileage out of a 360, way more TQ, so your foot doesn't have to be into the pedal as deep as with a 273

I have the 360. It gets 10mpg on a good day if I baby it. It is a 1975 with a Holley four barrel. Plenty of power but I want better mileage. I drove a 273 Barracuda to San Diego and got 25mpg. 360s get terrible mileage. Maybe the fuel injected 5.9 magnums are better but I am building a 70's truck. The 273s were rated at 180 hp. The 318s were rated 235 hp. I am only looking for 240-250 hp. Mileage is my goal. The 273 is 45 cubic inches smaller than the 318. That should help mileage. Anyone try this before??

Slap an overdrive in it and have the best of both worlds.
Seriously,by the time you buld a motor and swap it you have spent more than enough to install an overdrive tranny.Keep your torque and have your mileage too.

when will you get a return on your money buy removing the 360 and installing a rebuilt 273 to gain maybe 5 mpg? i would remove the 4bbl on the 360 and install a 2bbl set up first and see what happens you can probably find someone that will give it to you for free, next thing i would look at is your timing to make sure your running full vacuum advance in the area of 50 deg total at light cruse, make sure you have a clutch fan, my 4.7 03 dakota only gets about 19 mpg highway with the 5 speed auto turning 1,800 rpm at about 60mph. are you sure the barracuda got that milage was the gear stock? tires stock, speedo working right? Not tring to be negative, but i would try to maximize what i have before i undertake all the work and $$$$

The overdrive is a good idea. However it is only a gain at highway speed. My commute is all below 40 mph. I used to have a Dakota wit the V6. It got good mileage but was too small for me. I need the full size truck. I am not worried about the cost of upgrades or the work involved in swapping motors. I just wanted to experiment with Chryslers smallest LA V8. I have a motor lifter on the back of my truck and do this all the time. P.S. the tow truck also has a 360 and gets really bad mileage but LOTS of power. Thanks for the input guys

You can't expect a power wagon to get the same milage with the 273 as the barracuda. Your talking 1500lbs difference at least. Not to mentiuon the rear end ratio. The 360 will get better milage than the 273 will in that truck by a good bit. By the time you build the 273 to get anywhere near enough power to move that truck you've blown the gas milage all to crap.

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

When i had a 360 in my 74 D100 longbed got 16-18 mpg. You won't get that with the 273.The sixes also had a trans and rear end geared for milage and they didn't get all that much better mpg. Put a 2.76 or 2.91 under the truck and you will boost the milage.

[I know for a fact that the 360 get really bad mileage. I have had dozens of them.] well go ahead and build the 273 , I have a 360 about 425hp runs very low 12's, single plane 750 race holley, that gets about 12 mpg with a 3.55 gear in a 3450lbs car. something is not in good working order on your 360, have you tried to tune it up?

I was about to say with my 360 based 408 stroker ,od auto trans and 3.91 gear I get 14-15 on the highway and 12-13 in town. My truck weighs 5000lbs.

i bought an aspen from a guy in colorado who had build a late 70's crew cab. it had a 500 inch motor. the build was designed to make as much torque as possible. he was telling me that he would get around 18mpg on the hwy. torque is the key. your combo with the tall tire and tall gear will eat you up. the slant motor makes more torque than the 273 will.

Put a thermoquad on the 360 along with a wide band A/F meter ($200 for an innovate LC1 with digital gauge) and tune ignition and carb for mileage.

don't bother with a 273. pistons are too expensive to get high compression/0 deck. copy hotroddave's 318 build...

the key to mileage is high compression and little overlap. you'll get neither with a stock bottom end 273 and that cam.

or another option is his short block (KB167 pistons at 0 deck), magnum heads, lunati voodoo 60400. the magnum heads will make significantly more power than the 273 or '302 heads.

or use an '85 up 318 (roller cam motor, 302 heads or magnum heads) with the KB167's, and get the roller cam reground by bullet cams, probably with the HRL256A lobe on the intake and exhaust if using magnums, the HR259/316 on the exhaust if using the 302's...if you're maximizing MPG, use the HR247/306 lobe--similar adv. duration as a stock cam, but about 10 more .050 dur than a stock LA 318 cam, and a lot more lift (almost as much lobe lift as a stock cam's valve lift!) use that lobe for intake and exhaust and you'll have 23 degrees overlap on the stock 112 LSA

my combo is a stock '86 short block, pistons .055 in the hole, magnum heads home ported by me, magnum roller lifters, pushrods, and stock mag rockers, thin head gaskets (9:1 comp measured) RPM air gap, headers, eddie 600, and the roller reground by bullet with the HR259/316 lobe for both intake and exhaust.

on the butt dyno, engine has as much power as my old XE262 equipped 360, but I'm getting 3mpg better (18 vs. 15) in my 3900 lb 5th ave. the great thing with roller cams is the aggressive profiles...my cam has 10 degrees less .050 duration than the old XE262, but only 3 degrees less duration at .2", and more gross lobe lift. area under the lift curve is the same or better than the old cam, but with 13 degrees less overlap.

I predict worse mileage with the 273. A small motor trying to push a big lump equals poor mileage.

You'd be better off buying the LM1 someone mentioned and tuning your current 360. 360's can get good mileage, but not with an off the shelf carb, especially a Holley generic type carb.

I had a 273 in a 67 coronet. So more weight than the barracuda, but not as much as the truck.
got around 11 mpg around town. Just too big to move with that little motor.

these hyper-mileage 273 threads turn up every now and again, but there is never a follow up thread, because building a 273 for mpg does not make sense.

honestly, you drive a truck its only going to get so good with a vintage powertrain under the hood.

i would go along with the TQ swap mentioned above. smaller motor doesn't necessarily mean better mileage - my 78 D100 only got about 9 mpg with the 225 /6 1bbl and a 4 speed. the worn-out 440 in my car was getting 13 at that time. i always wanted to put a 360 or 318 in my truck to get better mileage, thinking the slant 6 made it too under-powered, thus working harder to get moving

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

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He is putting it in a truck so high compression is not really a good thing,

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A very thorough and valid response IMO, except for the above comment. When attempting max mpg, the highest compression possible for the combination is the goal. I agree on the headers and cams, plus with a mild hyd roller, piston coatings, windage tray, light oils, stock oil pump, dual 2.5" exhaust, multi spark ignition, 190F? thermostat, and mainly a very healthy vacum advance. The most significant mpg predictor is the drivers right foot, and the extra 45 cubes combined with bang for buck IMO might make a 318 a better solution. I have a mild street built 273 installed by previous owner in 62 b body, but my right foot negates any mpg +'s.