Increasing displacement = Increasing MPG? (Theory)

[ksa8907]

Originally Posted by serialk11r

Don't worry, my highest gear is 2500rpm & 45mph, 1ZZ-FED. I don't know if you have a 1ZZ or the earlier 4E? (is that correct) but my 1ZZ bogs pretty badly below 1500rpm, so that downshift might actually be a good thing.

I missed the @ 80 part. My 3.2 does 3k at 90 mph, 1500@45mph. Im not sure you would get better mileage, but modifying the larger displacement to get the same should be attainable.

[ps2fixer]

Another random story of mine...

My dad built a 4x4 truck out of a OLD car front end (early 70s I think), v8 350 engine (from an olds cutlass, early 70s again), box custom made and very heavy ~1 ton alone, with large tires, duals in the back, 2nd 3x sized battery for backup/jump starts and he managed close to 20mpg with that thing. This was back when I was like 8, so don't remember a lot about it, but I do remember him saying it "idled" down the road, something like 1100rpm @ 55mph. I'm thinking at that rpm, the engine was loaded close to 80% to be efficient? I know for a fact the areo was not good besides the fact the cab was a car. If I remember correctly, the truck weighed in at around 6800lbs empty at the scrap yard.

Kind of an interesting read, explains a lot of the "storys" about the early 70s olds 350 engines in my family (used as truck engines a lot, good tourqe and last well 300k miles on some). Scroll down 1/2 way in the 350 section.

Oldsmobile V8 engine - Wikipedia, the free encyclopedia

[3-Wheeler]

Originally Posted by nbleak21

.....In theory I'm looking at a 7% increase in displacement resulting in a 12% (relative) decrease in TPS.....

1) A bigger engine will have more surface friction due to piston area, longer skirt length, and so on.

2) It will also have higher heat losses to the outside world because of the larger surface area.

3) A bigger engine will require more fuel because there is also more volume to fill, due to the larger displacement.

4) A larger engine usually weighs more, so there will be more losses when accelerating due to higher mass.

5) An engine with a lower TPS value will have more pumping losses because of the higher vacuum level in the intake tract. This is an inherent advantage of diesel engines; i.e. those with no throttle plate.

All things being equal, the larger engine should consume more fuel than a smaller one.

Jim.

[Daox]

All else being equal, bigger engines increase pumping losses at everything but WOT and therefore you have an efficiency penalty.

[nbleak21]

Thanks for all of your replies. Yes, as stated, My Transmission is already geared pretty low... I just haven't started driving more conservative MPH yet (because I am currently fine-tuning my VFAC for fuel economy before hitting the hypermiling techniques hard)

Originally Posted by 3-Wheeler

1) A bigger engine will have more surface friction due to piston area, longer skirt length, and so on.

Yes, I suppose there will be more surface friction due to the increased diameter (approx 1 percent [273mm dia vs 270mm] per cyl) though the piston skirts designed for the k24 engine are also smaller, and are coated to reduce friction so I feel those losses will be mitigated.

2) It will also have higher heat losses to the outside world because of the larger surface area.

I'm not quite tracking what you mean by this, can you explain further?

3) A bigger engine will require more fuel because there is also more volume to fill, due to the larger displacement.

I am running a VFAC, so I have the availability to lean the engine out at lighter loads.

4) A larger engine usually weighs more, so there will be more losses when accelerating due to higher mass.

The engine blocks are virtually the same in regards to weight as they are principally the same design, and the reciprocating parts (factory rods, and K24 pistons) are actually lighter then the factory ones in my F22

5) An engine with a lower TPS value will have more pumping losses because of the higher vacuum level in the intake tract. This is an inherent advantage of diesel engines; i.e. those with no throttle plate.

Under general principle, yes... but the design of the intake tract can greatly effect where the pumping losses have their greatest effect. In fact, the observation that sparked this thread in the first place was that I seem to have less pumping losses at 3100rpm then 2800 with my current engine, as the TPS requires less opening to maintain a higher speed then it does lower.

This leads me to believe that the sweet spot may of the IM may be at approx 120cfm, so the 150cc increase in displacement would drop that RPM for the same volume from 3100 to 2900rpms

[rmay635703]

Originally Posted by nbleak21

Under general principle, yes... but the design of the intake tract can greatly effect where the pumping losses have their greatest effect. In fact, the observation that sparked this thread in the first place was that I seem to have less pumping losses at 3100rpm then 2800 with my current engine, as the TPS requires less opening to maintain a higher speed then it does lower.

This leads me to believe that the sweet spot may of the IM may be at approx 120cfm, so the 150cc increase in displacement would drop that RPM for the same volume from 3100 to 2900rpms

The only thing that could explain that would be harmonics, not pumping losses. Other could be transmission oddities.