Into the weeds engine design for fuel efficiency discussion.

[mgoblue9798]

Point of this thread is discussing engine building strategies for both fuel efficiency and running on cheap gas.


@mpgmike I am hoping you might be willing to share some of the latest info on the topic.


Part 1: Combustion chamber design.

Assume a couple givens:

Good quench is a given in this hypothetical, no matter the type head chamber involved.

Also the theoretical engine will be a street engine. Jack of all trades 1500 to 5000-5500 rpm. Some would consider it a truck motor based on the rpm, or daily driver, call it what you will.

1 A: Cylinder head chamber design.

What (if any) cylinder head chamber type offers the most in terms of efficient burn? All things like compression ratio and quench being equal, is there an advantage to a heart shaped closed chamber (IE magnum small block head),a bathtub closed chamber (think 60s big block 516 head), and a regular open chamber like a 906 or J head. Assume each has appropriate piston to achieve quench.

Which chamber shape could run on the lowest octane fuel all else being equal? Or asked another way, which
could run the highest compression ratio on 87 octane?

1 B: Piston top design. Is the flat top closed chamber combo the best for efficiency and low octane tolerance, or would a step head piston set up for quench in a closed chamber be? Not talking about the 80's giant pop up dome pistons here, think KB pistons with the step designed for quench.

1 C: Mods to be considered to 1A and 1B: Given is to relieve all sharp edges. What about Signh grooves? Are they beneficial for either fuel mileage or octane tolerance? How many grooves and what part of the chamber should the grooves be pointed towards?

How about dimples in head chamber or on piston top? What if any benefit would chamber relief cuts around the valves or a 15 degree top cut in valve job have in efficiency terms? Does either effect swirl in the chamber and if so in what way?

Related article: A New Diesel Piston That Increases Power And Reduces Emissions


If there is any interest in the topic, part II will be about the rotating assembly- ie bore v/s stroke, rod ratio, pistons and rings, etc.

 

[CFD244]

Sounds interesting.........

 

[junkyardhero]

318 MAX fuel economy builds?

So I'm looking into buying a long-distance capable daily driver. I have my eyes on a Dodge Diplomat and Chrysler 5th Avenue. Both have 318s and I thought it may be of my own interest to buy one as a daily as they are things I can work on instead of things I would need to take to a shop. In the...

www.forabodiesonly.com

 

[toolmanmike]

Point of this thread is discussing engine building strategies for both fuel efficiency and running on cheap gas.


@mpgmike I am hoping you might be willing to share some of the latest info on the topic.


Part 1: Combustion chamber design.

Assume a couple givens:

Good quench is a given in this hypothetical, no matter the type head chamber involved.

Also the theoretical engine will be a street engine. Jack of all trades 1500 to 5000-5500 rpm. Some would consider it a truck motor based on the rpm, or daily driver, call it what you will.

1 A: Cylinder head chamber design.

What (if any) cylinder head chamber type offers the most in terms of efficient burn? All things like compression ratio and quench being equal, is there an advantage to a heart shaped closed chamber (IE magnum small block head),a bathtub closed chamber (think 60s big block 516 head), and a regular open chamber like a 906 or J head. Assume each has appropriate piston to achieve quench.

Which chamber shape could run on the lowest octane fuel all else being equal? Or asked another way, which
could run the highest compression ratio on 87 octane?

1 B: Piston top design. Is the flat top closed chamber combo the best for efficiency and low octane tolerance, or would a step head piston set up for quench in a closed chamber be? Not talking about the 80's giant pop up dome pistons here, think KB pistons with the step designed for quench.

1 C: Mods to be considered to 1A and 1B: Given is to relieve all sharp edges. What about Signh grooves? Are they beneficial for either fuel mileage or octane tolerance? How many grooves and what part of the chamber should the grooves be pointed towards?

How about dimples in head chamber or on piston top? What if any benefit would chamber relief cuts around the valves or a 15 degree top cut in valve job have in efficiency terms? Does either effect swirl in the chamber and if so in what way?

Related article: A New Diesel Piston That Increases Power And Reduces Emissions


If there is any interest in the topic, part II will be about the rotating assembly- ie bore v/s stroke, rod ratio, pistons and rings, etc.

What kind of mileage are we striving for? What year and model of car and what engine should we concentrate on?

 

[mgoblue9798]

What kind of mileage are we striving for? What year and model of car and what engine should we concentrate on?

Chrysler big or small block. . Has to be capable of towing when needed. Not interested in hemis, more about lower rpm efficiency. This is specifically about the engine ,but if forced to pick a vehicle lets say a 91 club cab d150. There is no mileage goal per se, just looking for latest tech regarding engine design in terms of efficiency and high compression on crap gas.

For example part 2 I was going to ask what engine would be more efficient in terms of fuel mileage- a small block stroker 4.040 bore 4.0 stroke 6.123 rod and modern heart shape chamber , or a big block 4.18 bore with 3.75 stroke 6.76 rod with bathtub chamber heads? Again same compression, similar cam specs, intake type, etc. How do rod ratio and bore size play into efficiency and octane requirements? Bores and strokes mentioned are just a reference to keep engine cubic inches similar for comparison sake.

 

[mgoblue9798]

318 MAX fuel economy builds?

So I'm looking into buying a long-distance capable daily driver. I have my eyes on a Dodge Diplomat and Chrysler 5th Avenue. Both have 318s and I thought it may be of my own interest to buy one as a daily as they are things I can work on instead of things I would need to take to a shop. In the...

www.forabodiesonly.com

Lots of good ideas. Some specific to the 318, and some general. I am looking at more of a comparison between a small block stroker and a smallish big block of about the same cubic inches. Which architecture is better for fuel mileage and knock resistance and why?

 

[ACME SS]

I'm kinda confused since every car manufacturer in the past 20...oh maybe 35 years has been forced by government to raise fuel efficiency and in doing so they have come up with all kinds of technology such as knock sensors, variable valve timing, so on and so forth. I looked up the Diplomat and they were built between 76-89. Maybe I'm missing something but perhaps purchase a newer car?

 

[mgoblue9798]

I'm kinda confused since every car manufacturer in the past 20...oh maybe 35 years has been forced by government to raise fuel efficiency and in doing so they have come up with all kinds of technology such as knock sensors, variable valve timing, so on and so forth. I looked up the Diplomat and they were built between 76-89. Maybe I'm missing something but perhaps purchase a newer car?

If only I could have thought of that...

 

[toolmanmike]

Power and torque for truck use is easier to find. When I saw efficiency I was thinking more mileage. You can't have it both without technology.

 

[Phreakish]

There's lots one can do to old engines to get them closer to modern consumption.

Looks at late model engines and the trends are clear:

Smaller ci will always improve mpg. Smaller bores burn more completely. Shorter strokes lose less to piston friction. There's a reason for so few big-bore engines outside specialty vehicles.

Thinner rings and lower ring tension, which requires more precise bores, BTW. Better chambers, but also lower crevice volumes - so you want head Gaskets with bores that nearly match the block, high upper rings, etc. The valve job will probably have more effect than the exact shape of the chamber. Squish, quench, whatever you call it does matter but with a reasonable compression ratio (9.5-11, imo) it'll happen anyway so it's not worth sweating over.

Then there's the lubrication side.
Thinner oil is easier to pump, better lubrication and solid films on piston skirts help reduce friction too.

High static compression improves efficiency and requires precise timing control and octane requirements, which is where knock sensors come in. High compression can increase pumping losses though, and so the exact right ratio will depend on the cam to a great extent.

You can reduce pumping losses by closing the intake late but obviously this can also kill torque down low, which is where variable valve timing comes in. Without it, you get to pick either a high torque under 5k, or a screamer that has to cruise at 3500. Similar to the lower ci deal, lower revs will always use less fuel so choose your operating rpm range wisely. A low peak power rpm will come with a low HP number too. Built right, you'll be able to sustain highway speeds uphill no problem, but your 0-60 is probably going to be double digits even with no load or trailer.

Even the cooling system is a huge deal. I remember an ASE article circa 2005 talking about molded plastic coolant diverters that Audi was putting into their open deck cooling jackets to even out the temperature of the cylinder wall. They realized something like a 4% reduction in consumption across the power band due to better ring seal. Also, hotter temperatures tend to operate more efficiently but are also less tolerant of mistakes. 205-215deg thermostats are not uncommon in modern engines. Also, dual acting thermostats, which my or my not be adaptable using an external housing on a Mopar v8.

Fuel delivery is a whole book unto itself. Port injection is a great compromise because it's still far enough from the valve to cool to intake charge and atomize the fuel, but not so far as to cause a lag in mixture correction when conditions change (passing, or sudden additional commanded power) and not so close as to wash the cylinders down or cause valve deposits. Having injectors in the ports also makes it possible to trim each cylinder individually.

We can also posit that additional mpg and efficiency could be found in reducing reciprocating mass like the rods and pistons. The OEMs don't make them thinner and thinner each year just to save on metal costs alone..

Reducing rotating mass can help, but less so than reciprocating masses. But every single time a car accelerates it has to spend energy getting the crank and flywheel and trans up to speed again.. So they do still matter.

Speaking of transmissions. There can be a lot of sophistication built into them beyond just a lockup converter and extra gears. Newer transmissions do the job far better than any of our old hot rod parts.

Its not an easy task to accomplish a lot of this. It takes either tons of experience or learning a ton of the math that's already been developed. You'd have to pick the brain of an automotive Engineer to even start finding resources on that. Eng-tips.com often has plenty of gray beards willing to tell you all about many a subject, so that may be a good place to search out info.

 

[Bewy]

Nope, smaller cu in doesn't necessarily mean better economy. Every engine has a 'sweet spot',
where all it's parts work together to give best mileage or best power, & rarely both....
A small engine has to lug. A big one doesn't.

It is very difficult to give an accurate answer to the original question because of so many variables, other than use a carb with smaller pri bores [ eg, a TQ ] & a high velocity dual plane intake. Good ign system like HEI that will fire a 060 plug gap.

I see a lot about piston crevice volume [ CV ]. Some people even use it in their CR calculations! Need to think why there is a CV: the top of the piston runs hotter where the rings are because it is closer to the combustion heat. If the piston diam was not reduced in this area [ CV ], it would seize as it heats up. The CV is all but gone at operating temp.

 

[Phreakish]

Nope, smaller cu in doesn't necessarily mean better economy. Every engine has a 'sweet spot',
where all it's parts work together to give best mileage or best power, & rarely both....
A small engine has to lug. A big one doesn't.

It is very difficult to give an accurate answer to the original question because of so many variables, other than use a carb with smaller pri bores [ eg, a TQ ] & a high velocity dual plane intake. Good ign system like HEI that will fire a 060 plug gap.

I see a lot about piston crevice volume [ CV ]. Some people even use it in their CR calculations! Need to think why there is a CV: the top of the piston runs hotter where the rings are because it is closer to the combustion heat. If the piston diam was not reduced in this area [ CV ], it would seize as it heats up. The CV is all but gone at operating temp.

Show me an instance where a larger engine gets better mpg for a road going vehicle.

Larger engines create excess power which is a waste in every regime except acceleration and ascent. Longer strokes, bigger bores, and large cam profiles all waste energy in order to gain power outside of the cruise regime, but average mpg is dominated by cruise.

When cruise mpg is the goal, minimizing ci is the lowest hanging fruit.

OP says he wants to tow - but does tow mpg matter as much as unladen? Without those details it's impossible to figure out which end of the displacement spectrum to be on. Knowing a max grade, weight, and operating elevation would make figuring out the minimum requirements a lot easier.

Also, typically a smaller engine with a turbo will out power and out mpg the larger na engine in almost every case. It can maximize torque when needed and use fuel sparingly when it's not. It's no secret why oems have had a love affair with boost in ecino cars for some decades.

 

[mgoblue9798]

Guys I appreciate the input but I am really only looking for answers to the specific questions I mentioned in my posts above.

All the general stuff mention I have a good handle on already. I didn't mean for this thread to be a catch all for fuel mileage ideas.

I am specifically looking for input on the types of parts I outlined in terms of fuel efficiency and octane tolerance.

 

[jos51700]

The issue with grooves and such is that they collect and build carbon, and then you have pre-ignition issues with such.

So far as fuel economy in pistons and heads, just open up any late model motor and copy that. The OEM's will hire you and pay you handsomely if you're capable of gaining them 0.1mpg.

 

[mgoblue9798]

The issue with grooves and such is that they collect and build carbon, and then you have pre-ignition issues with such.

So far as fuel economy in pistons and heads, just open up any late model motor and copy that. The OEM's will hire you and pay you handsomely if you're capable of gaining them 0.1mpg.

Good point on the carbon buildup. Considering a water/methanol injection set up that would take care of that problem.

 

[ACME SS]

So back in 79-81 I drove a Datsun 1200 sport coupe. I LOVED the car and wish I had never sold it. Anyways, I used to carry a timing light and tools with me and would advance the timing on foggy or humid days and I got phenomenal mileage. I would then have to retard the timing on dryer days as I heard the knock or pinging. Interesting how variable timing does just that these days.

 

[AJ/FormS]

As for the engine:
Cylinder pressure is your friend. Pressure makes heat. Heat makes power. The more efficient your engine is, the less fuel it will take to make a given output AND fuel-economy is a good measure of it.
> Power allows less rpm. Less rpm makes fuel-economy.
>but; Very low rpm runs into difficulties with;
1) a late closing intake valve pressurizing the intake, followed by
2) a lack of adequate fuel-atomization with a big carb
3) short power-stroke duration allows energy to be wasted by sending hot hi-energy still expanding combustion gasses straight out the tailpipes
4) cams with long overlap periods, allow headers to pull raw A/F mixture into the exhaust system
5) a lack of Cruise-timing, with factory type timing systems

The problem with running a very high cylinder pressure, on cheap gas, is the inescapable onset of detonation....... which is aggravated by low-rpm.
My experience in this area, is that at 65=1600 rpm (my car), the power-timing wants to be 18/20 degrees at WOT, but 50/56 degrees at cruising. That means I need a vacuum advance can of say 53 less 18= 35 degrees. AFAIK, one does not exist. The best I could get with the modified stocker was 20 degrees; so no matter how you slice it, my engine will be running about 15 degrees retarded at 1600 rpm and cruising. I solved that problem with a dash-mounted dial-back timing module.
The thing about that is, I gotta remember to dial it down when coming off the hiway.
In the end, I found so little fuel-efficiency difference between 1600 and 2200, that I settled on a gearing that gets me 65=2240; for which the cruise timing wants to be 60 degrees, but the factory parts can only be modded to about 40 degrees, add the 15 in the dial back=55* of cruise-timing, and close-enough.
My solution to the detonation problem was running alloy heads, and an AirGap intake with fresh cold-air..
This 360 combo has run cylinder pressure as high as 195psi on 87E10, at a power-timing of 34*, all in by ~3400.

I gotta tell you tho;
The slower you run the rpm at cruise rpm, the more closed the throttle is likely to be. Depending on the cam, there comes a point where reducing the cruise-rpm any more, will start to require more power, and therefore INCREASED throttle opening.
No matter, the point is this; Even tho your engine may boast 11.3Scr (mine), and may crank 195psi at 350rpm with the throttle at WOT;
those numbers, by themselves, are almost meaningless when cruising with the throttle barely open. Because,
if your body/chassis requires say 40 hp to go 65 mph, you will adjust the throttle until 40 hp is being produced. The trick to getting it right, is to select a cruise-rpm that meets the power-requirement with the smallest amount of throttle opening and with the right amount of cruise-timing that the engine actually asks for. After that you just lean out the carb until she loses speed and or it starts requiring more throttle opening again, to get the required fuel to maintain your chosen speed.
IMO any SBM can make enough power at 1600rpm to cruise on, but the bigger it is, the less throttle-opening it will take. And also IMO, the longer the stroke is, the more energy can be extracted out of the fuel, before the exhaust opens.
But I also found out that 65@1600rpm was not at all better than 65@1800, which was only a lil better than 65@2240.... lol.
That 367 combo easily out-performed any 318 ever produced in terms of hiway fuel economy, yet that combo went 12.9@106 in the quarter with; 3.55s, 2.45 street tires, and @3650 with me in it, spinning for a long,long way down the track.
IMO, that combo was strikingly efficient.

So let me rephrase this.
Cylinder pressure is your friend. Pressure makes heat. Heat makes power. Power allows less rpm. Less rpm makes fuel-economy.

It's not about the max pressure by itself.
It's about the whole combo.
Which is "Effective-Pressure", which is what your engine can make at various throttle-positions; and NOT at WOT.
The more pressure that you start with the easier it is to tickle the maximum number, until you hit the detonation wall.

OOPs I guess I shouldda read ALL the comments ..........

 

[Bewy]

^^^^^He gets it.

 

[rumblefish360]

MPG Mikes YouTube channel should help some.

https://m.youtube.com/@ecoceptor3894

 

[jos51700]

Just build it for max torque at a reasonable RPM and gear it to stay at that RPM. Peak torque is peak efficiency. Rhoads lifters, tuned length intake, high compression, polish the chambers on a set of Magnum heads, roller cam with WIDE lobes, etc.

 

[rumblefish360]

The Rhodes lifters, I have used them before, they are a great band aid to the wrong cam but I’m not entirely sure I’d build an engine with those as part of the plan. Perhaps if I had to meet a vacuum rule.

 

[mgoblue9798]

As for the engine:
Cylinder pressure is your friend. Pressure makes heat. Heat makes power. The more efficient your engine is, the less fuel it will take to make a given output AND fuel-economy is a good measure of it.
> Power allows less rpm. Less rpm makes fuel-economy.
>but; Very low rpm runs into difficulties with;
1) a late closing intake valve pressurizing the intake, followed by
2) a lack of adequate fuel-atomization with a big carb
3) short power-stroke duration allows energy to be wasted by sending hot hi-energy still expanding combustion gasses straight out the tailpipes
4) cams with long overlap periods, allow headers to pull raw A/F mixture into the exhaust system
5) a lack of Cruise-timing, with factory type timing systems

The problem with running a very high cylinder pressure, on cheap gas, is the inescapable onset of detonation....... which is aggravated by low-rpm.
My experience in this area, is that at 65=1600 rpm (my car), the power-timing wants to be 18/20 degrees at WOT, but 50/56 degrees at cruising. That means I need a vacuum advance can of say 53 less 18= 35 degrees. AFAIK, one does not exist. The best I could get with the modified stocker was 20 degrees; so no matter how you slice it, my engine will be running about 15 degrees retarded at 1600 rpm and cruising. I solved that problem with a dash-mounted dial-back timing module.
The thing about that is, I gotta remember to dial it down when coming off the hiway.
In the end, I found so little fuel-efficiency difference between 1600 and 2200, that I settled on a gearing that gets me 65=2240; for which the cruise timing wants to be 60 degrees, but the factory parts can only be modded to about 40 degrees, add the 15 in the dial back=55* of cruise-timing, and close-enough.
My solution to the detonation problem was running alloy heads, and an AirGap intake with fresh cold-air..
This 360 combo has run cylinder pressure as high as 195psi on 87E10, at a power-timing of 34*, all in by ~3400.

I gotta tell you tho;
The slower you run the rpm at cruise rpm, the more closed the throttle is likely to be. Depending on the cam, there comes a point where reducing the cruise-rpm any more, will start to require more power, and therefore INCREASED throttle opening.
No matter, the point is this; Even tho your engine may boast 11.3Scr (mine), and may crank 195psi at 350rpm with the throttle at WOT;
those numbers, by themselves, are almost meaningless when cruising with the throttle barely open. Because,
if your body/chassis requires say 40 hp to go 65 mph, you will adjust the throttle until 40 hp is being produced. The trick to getting it right, is to select a cruise-rpm that meets the power-requirement with the smallest amount of throttle opening and with the right amount of cruise-timing that the engine actually asks for. After that you just lean out the carb until she loses speed and or it starts requiring more throttle opening again, to get the required fuel to maintain your chosen speed.
IMO any SBM can make enough power at 1600rpm to cruise on, but the bigger it is, the less throttle-opening it will take. And also IMO, the longer the stroke is, the more energy can be extracted out of the fuel, before the exhaust opens.
But I also found out that 65@1600rpm was not at all better than 65@1800, which was only a lil better than 65@2240.... lol.
That 367 combo easily out-performed any 318 ever produced in terms of hiway fuel economy, yet that combo went 12.9@106 in the quarter with; 3.55s, 2.45 street tires, and @3650 with me in it, spinning for a long,long way down the track.
IMO, that combo was strikingly efficient.

So let me rephrase this.
Cylinder pressure is your friend. Pressure makes heat. Heat makes power. Power allows less rpm. Less rpm makes fuel-economy.

It's not about the max pressure by itself.
It's about the whole combo.
Which is "Effective-Pressure", which is what your engine can make at various throttle-positions; and NOT at WOT.
The more pressure that you start with the easier it is to tickle the maximum number, until you hit the detonation wall.

OOPs I guess I shouldda read ALL the comments ..........

Thanks AJ, lots of good info.

Much more helpful than being told to buy a small car which is what my post about specifics was in response to.

I am curious as to thoughts on bore v/s stroke. Bore sizes for newer v8 LS and Hemi are all close to 4 inches. Is this due primarily to emissions, fuel economy, or do both go hand in hand because of a more complete burn?

 

[AJ/FormS]

Is this due primarily to emissions, fuel economy, or do both go hand in hand because of a more complete burn?

I do not know.
But I'm always willing to guess;
A four inch bore, give or take a lol, makes enough room for typical valve sizes, and for them not to be shrouded. Any smaller and you soon have to give up either valve diameter, or inter-valve support, or they get to be shrouded.
Long strokes allow more power-stroke time, at cruise rpm, to extract energy.
An old 318 had a cam of 240/248/112. In at 2* advanced this was 130 degrees for compression and 122* for power.
Now, 122* for power extraction is a stinking long time in terms of crank rotation. By the the time the exhaust valve opens, and with an SCR of sub 8/1 , the pressure of those hot exhaust gasses are almost back to atmospheric pressure, having release all they had to give. This is great for fuel economy. Even for making torque.
But those same 122* of power-extraction, if you want to make power, steal a lotta time from the exhaust cycle, and so those are usually the first to get sacrificed./ A typical power stroke for making street-power will be close to or less than ~105*..
If you think about this;
When the piston gets to 122* ATDC, it is slowing down and is just 58* from the Bottom of it's travel. My guess is that in a 3.315 stroke engine, this is less that 1" from the bottom.
But say you had a 3.58 stroke engine, this same 122 degrees would put the piston further from the bottom..... Which means, you can run more exhaust duration, and still have a similar amount of power extraction clock-time. And if you think about that long enough;
you might get to thinking about comparing the fuel economy potential of two same engines except one with a 3.315 stroke and the other at 3.58.
But if you did think about that, rightaway you might think what if I put a one-size bigger cam into the 3.58 engine. With say 7* less power-stroke and those 7* given to the exhaust stroke.
Rightaway the 3.58 stroke would make more power and torque at low-rpm, by virtue of the greater cubic inches. But with the bigger cam, it will also make more midrange and hi-rpm power.
but with a similar amount of clock-time spent in the power-stroke while cruising, should make similar fuel-economy.
And if you think about this long enough, there's a good chance you will conclude to never again build a 3.315 stroke SBM, but if you did; it sure as chit would not be at 8.0 Scr, lol, Nor with a pizzazz 240/248/112 hydraulic cam.
From the decks down, there is nothing wrong with a 318LA. I mean spec-wise it is almost the same as a 340, save the overbore. And it is almost the same as a 360, save for a bit of of bore and stroke.
What the 318 has lacked since 1972 has always been compression.
People say it doesn't matter that much, and under racing conditions they are sorta right. Once the rpm is up, cylinder pressure, or lack of it, becomes minor compared to the operating rpm.
and, while cruising, it is again almost a non issue, at typically posted speed limits.
Where/when cylinder pressure matters is exactly where a streeter operates. Namely; on/off the throttle, banging back and forth between stall and shift-rpm in First gear, and especially when getting into second.
And it's not just the pressure I'm talking about. It's about the activity that happens in the intake manifold.
The higher your cylinder pressure is, by deduction, the smaller the total chamber volume has to be. And the smaller it is, the quicker it will respond to the falling piston on the intake stroke.... because..... a big chamber acts like an Air-spring; you can stretch it a long way before the plenum even knows it's there because it is busy listening to 3 other air-springs on every revolution. It be like, the plenum is saying" yeah yeah, I'll get there in a sec." Whereas with a small chamber, the falling piston puts a good pull on the plenum, demanding attention like, right NOW dammit!
This translates to an engine that is always ready to rock, just stab the pedal and hang on! As you can see, this has ZERO to do with the Compression stroke! Yet the only way to get this kind of throttle-response, Normally-Aspirated, is with a hi-compression ratio.

IDK anything about emissions, and pretty much don't care,
cuz IMO, it ain't cars that are a problem.
IDK anything about what factories think.
or IMO, even if they think at all. lol.

Oh almost forgot; B/S ratios;
273 is 3.63/3.315= 1.095, baseline
360 is 4.00/3.58= 1.117, this 1.02% bigger than 273
318 is 3.91/3.315= 1.179, this is 1.056 bigger than the 360
340 is 4.04/3.315= 1.219, this is 1.034 bigger than the 318

If a bore to stroke ratios have anything to do with fuel mileage,
a 360 should, when similarly equipped........ make BETTER fuel economy than any 318 ever built, and even rival a 273, the difference going to internal friction, which we can do something about. And you may know this; my 367 was once combo'd to exceed any factory 273 fuel economy spec ever published, for a car at 3650 pounds (me in it); I mean smashed it.
So there is that, .......... but it wasn't similarly equipped, lol.

 

[rumblefish360]

And done with a gear vendors overdrive correct?

 

[Kern Dog]

Show me an instance where a larger engine gets better mpg for a road going vehicle.
Larger engines create excess power.

A large engine operated at 3000 rpms with a slight throttle opening will get better mileage than a small engine at the same speed at WOT.
Put a slant six in a one ton truck and try to haul a load up a grade while getting more than 8 mpg.
A 440 can idle up the grade and beat the mileage of the slanty.