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Originally Posted by marin8703
the engine has nothing to do with how much its going to burn relatively. Its called the conservation of energy. If you have the same mass, and try to move it with a smaller engine, it will be at higher rpm, but use less fuel per revolution because of the smaller size. A larger engine would do it at less rpm (thus seems easier) however because of its larger size uses more fuel per revolution. 3.5L to 6L almost double per revolution. Although its more complicated than that, i just wanted to make the point.
Tires however could be an issue.
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The size of the engine has everything to do with how much fuel it consumes, assuming everything else is equal. Power= PLAN where P=Pressure in the cylinders, i.e. air pressure past the throttle body, L=Length of stroke, A= Area of the piston head, and N= number of times. Since the air fuel mixture reamians constant reguardless of the throttle position or RPM, for every given volume of air going into the cylinder there is a proportional amount of fuel. Air pressure in the cylinder is regulated by the throttle plate. When it is closed, and the engine is at idle, very little air makes it past the plate and you have low pressure in the cylinders. This shows up as high vacuum on a vacuum gauge, or low pressure Manifold Absolute Pressure (MAP) gauge. When the throttle plate is fully open, then there is high pressure, almost full atmospheric pressure, in the cylinders. This shows up as very low vacuum, or high pressure on the MAP.
Now, to put this in prespective of the Hummers, the 3.5L is at a certain throttle position and a certain RPM producing a certain amount of power during cruise. Put the 6.0L at that same RPM and throttle setting and it will produce more power because of it larger size. Thus, the 6.0 can perform more work for the given RPM and manifold pressure (throttle position), but at a cost of more fuel. If you want it to consume the same fuel as the 3.5L at the same RPM, you must change the throttle setting to one with lower manifold pressure to match the power output of the 3.5L (That may be physically imposible depending on the RPM, target fuel consumption, and the brake fuel consumption of the 6.0L). If you increase the load on the 3.5L, it will require a higher manifold pressure (throttle open wider) and consume more fuel to maintain that same RPM. So, if the H2 cruises with a certain aerodynamic load, mass load, and rolling resistance and gets a certain fuel economy, it is entirely possible for the H3 to increase its aerodynamic load, mass load, and rolling resistance to the point that the engine is working hard enough to consume more fuel that the larger underworked vehicle.
As to gearing, assuming everything is eqaul, and the engines are producing an idential percentage of their total power, then yes, the smaller engine would have to have gearing to work in it's favor, and it would have a higher RPM. But, by changing the tire size without regearing the axles, you have changed the effective gearing to something unfavorable to the smaller engine, thus increasing it's load and throttle position just to maintain the same RPM.