Into the weeds engine design for fuel efficiency discussion.

mgoblue9798 said
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.

A lot to consider with that. First is what are you cconsidering towing and how often. Back in the '60's people that towed the larger Airstream holiday trailers all drove large cars with big block engines, preferrably in the 450 CID range. Trucks at the time were plain work vehicles with few options, not like the gussied up dancehall girls they are today. If you intend a lot of towing, a big block is the way to go. A small block can be stroked and built to produce the torque required for towing, but there just is not the beef needed for longevity. There is a reason big industrial engine blocks are built from cast iron 1" to 1 1/2" thick. They have a high duty cycle, meaning they run at max or close to max design power for weeks. Our automobiles operate at light duty cycles, small throttle openings for cruise most of the time with short bursts of max power to pass or merge.
If your intended use is fairly "pedestrian" most of the time and tow occasionally and not real heavy, a stroked small block may be the best choice. A 4" stroke 5.9 Magnum may be a good choice with a fairly short duration, high lift cam to build torque.
Flat top pistons with 2 valve reliefs tend to produce better burn capability. The piston crown, depending on your head gasket, should protrude out of the block about 0.005". Chevy Gen 3 are supposed to be 0.008" above the deck surface. The Chevy rings are about 1.5mm wide or 0.060". That is 1/16". 1mm or 1.2mm rings generate less friction and you want them up fairly high.
Back to the original question; open chamber heads are prone to detonation, slow burn rates and poor emissions. In early emissions engines NOx was being looked at and increasing combustion chamber volume was a quick and inexpensive way to drop the compression ratio. High compression tends to generate the higher combustion temperatures that create the NOx. EGR was also used to reduce combustion temperatures.
Since moving on to sophisticated fuel and ignition control systems this can be mitigated. Now port and combustion chamber design that promotes swirl is encouraged. Whether the chamber is heart shaped or D shape, a good quench is desired. Porting of the intake should open up the cylinder wall side of the port. Any work on the cylinder center side of the port should be minimal, mostly cleanup of ridges from casting. The port bowl should have a bias or angle compared to the valve of about 10° pointing toward the cylinder center. This promotes swirl. There is a do not touch area of unshrouding between the chamber wall and intake valve from about the center of the valve head up to the quench pad. Any work there will tend to cancel swirl and work against you.
For the exhaust, angle the port bowl the opposite direction to the intake, as that is where the flow wants to generally go. If there is a bit of a quench pad on the sparkplug side, round that edge of to let the exhaust follow around and into the port. On the main quench pad, lay back the edge to direct exhaust into the port.
For a street oriented engine with fairly restrictive exhaust ports, unshrouding the valve and cutting the valve and seat for a 40° seat can aid low lift flow. The intakes will benefit with a 25° or 30° back cut.
Not sure about incorporating the Somender Singh grooves. If you do decide to try them, I think 1 to 3 angled to enhance the intake induced swirl as the piston is aproaching TDC and the plug is sparking may be beneficial.
If it will fit under the hood, a dual plane airgap manifold works for a broad torque curve. If you go with an airgap, use a carb with annular discharge boosters like the AVS2. The Rochester Quadrajet triple booster functions well also.
For exhaust, a tri Y header improves low and mid torque.
I would recommend watching David Vizard's Powertec 10 Youtube videos on porting. Yes most is Chev and Ford, but the principles apply and you adapt differences. He gets fairly detailed on the Ford E7 head and 289 for valve unshrouding and where to and not to.
Two Powertec 10 videos to watch are Ep80 and Ep86. These pertain to the UTG initiated Mission Impossible project on a 318. Ep80 discusses some of what I related about open chamber heads. DV is investigating milling the head 0.100" to get some quench.