Car engines - some education needed.

[JPSmit]

So fellow forum members, I have puzzled over this question for some time but, would love to hear your thoughts. As long as I have been interested in cars, I have heard about some engines being better than others - or heard that various engines had worse gas mileage than others.

Setting aside fuel injection and computer management systems for a second - since that is its own type of voodoo.

Some parts I understand.

I understand that the thrust bearing are an issue with 1500 Midget/Triumph engines so they aren't as robust.

I understand that some cars have fewer crankshaft bearing making them less robust.

I understand that some have poorer castings and I even get that hemispheric heads burn more efficiently than non.

However, for instance, 80's vintage Jags have a reputation for poor gas mileage as do 70's AMC's and many Jeep Cherokees.

Some engines (Like the SBC) have a great reputation but others not so much.

Slant 6 is defined as Bulletproof.

so, what makes one engine better than another and especially why do one get terrible mileage while another not so much?

 

[DavidApp]

I am not an automotive engineer but here are a few thoughts.
Knowing how much time and effort people put into the intake manifold and head I imagine that the smooth flow in this area has a large effect on gas mileage. if this area is poorly designed could reduce the mileage. Also the exhaust system can be designed to get more power so I suspect a poor design would have the opposite affect.

Some of the gas mileage issues come from the size and shape hole the car makes in the air. So the same engine in different car models may get different gas mileage.

I have heard that the straight 6 is the ideal configuration, can't remember why though.

David

 

[SD Bugeye]

Seems that longer stroke engines produce better mileage
why?
guessing a longer time to more efficiently burn the charge?
6 cylinders come to mind.
they make great low end torque
the old stationary engines come to mind with massive flywheels lot of rotating mas and they run forever on small amounts of fuel torque is the real deal

 

[Bayless]

As I remember, the straight six is the best balanced configuration. And longer stroke usually produces more torque. For example, my Prefect engine is a flathead I4 with bore/stroke 2.5/3.64 for 1172cc. That small bore makes it 10hp under the then British tax rated system. It only makes 36 bhp at 4500 rpm but torque is 52 at 2500

 

[Deleted member 8987]

Federally mandated BS. Starting about 1974 and getting worse as time progressed...all supposedly to keep emissions down...yet the worst offender is the State of Confusion...errr...California, who mandated initially the most severe emissions controls and specs...and ended up using 10% more fuel per capita in the process.
Carburettor leanness, camshaft profiles, combustion chamber shape, piston head shape, heat riser flaps on exhaust, catalytic convers, vacuum retard on distributors, TVS to control everything (which required specific cooling system temperature and until that was achieved, milage double sucked) (and then they failed frequently).
I remember specifically 1976, 77, 78 full sized Lincolns with 460 and 4bbl that literally would not get out of their own way.
We discovered they had changed the cam gear timing (keyway notch was off). A "police interceptor" set of chain and gears from the local NAPA store caused both rear tyres to light up. Seriously.
Apparently Feds didn't like that as suddenly the sets of PI gears became unobtainium.

Combustion chamber and shape of piston heads...when we used to install those cut-out-of-scrap-car engines from Japan, locally the EPA threatened us with fines as the A) combustion area, B) the piston heads, and C) the camshafts did not meet Federal specs.
Regardless that Japand has far, far stricter specs....somebody sued and won, ane we were back to business as usual.
As long as we swapped all manifolds, carbs, distributors, TVS, smog devices to the new engine (which was fun sometimes and there wre no ports in the head for the shXt to connect to)

Sixes are not bulletproof. In line six throws have harmonic vibrations that can really be an issue at speed.
Look up a straight eight...those were BAD. I don't know how many I've seen with the starter hanging by the cables under the car because the 7 or 8 throw exploded through the side of the block.
V-8 have shorter length, same as 4, far less reflected harmonics.

Smoother inlet pulses.