Theory: Increased Engine load leads to better fuel economy
Theory 1: Less load better fuel economy
General consensus says the more load the less fuel economy, the less load the better fuel economy is. This is how old school MPG gauges (vacuum gauges) used to work, the more you open your throttle, less vacuum, worse fuel economy. It makes sense on why, the more load (air), the more fuel is needed to keep the mixture from leaning out.
This is why increasing ignition timing can increase gas mileage too; by increasing timing you get more torque (until you reach the knock threshold), by increasing torque car has more power thus you use less throttle to keep the car going at the set speed.
Theory 2: More load better fuel economy
Metrompg.com did a warm air intake (WAI) test, one of the theories behind WAI is quoted below. Gasoline Engines are most efficient at wide open throttle and least efficient at part throttle.
Originally Posted by metrompg
Reducing the density of the intake air by heating it effectively reduces the power of the engine at a given throttle opening (relative to the same engine with a CAI). To do the same work as the CAI-equipped engine, the throttle of the WAI engine must be opened wider at a given RPM. This increases engine efficiency by reducing throttling or pumping losses:
"The air is less dense, so you get less horsepower at the same throttle opening, thus, you have to open the throttle wider to let in more air and get the horsepower that you need. That increases the efficiency because one of the primary causes of the well-known part load inefficiency of gasoline engines is the throttle loss." - (source)
Theory 3: Ignition timing
On my 9.5:1 Roller Rocker DE+T (with low-port intake manifold), I've noticed a rather weird trend too. The less timing I ran the better my MPG got. I started out at with 38-40* timing for highway speed cells. I got down to 30* before I ran out of road to test on (12 hour trip), I do plan on going lower then 30* on my next big trip. This is completely backwards of my 8.5:1 SR20DET (with Ported intake manifold), I kept on getting better MPG the higher I went, leaving it 40-42.
While both motors are SR20's, they are completely different animals internally, Roller Rocker is much more efficient internally then a DET. The more efficient the engine the less timing is required to get the same amount of torque out of each cycle. Having too high of timing would mean the mixture would finish burning while the piston was still pushing up, at WOT this would cause detonation, at partial throttle it will increase resistance, the same way it becomes harder to pump tires up the more pressure they have. What's odd is, on the dyno Roller Rocker's best torque was at 36-37* for that rpm range.
Hill climbing vs MPG
I occasionally get out of my small town hidden in a valley, ~1,400 ft above sea level, and drive to Fairfax area, ~300ft. If I fill up after a descending drive only, I will get around 30mpg. If I were to just turn around and drive back, ascending to 1,400ft, my MPG would be much higher, closer to 34mpg. I noticed the same trend when I went to Richmond (practically sea level). I always found it interesting that going 64 west over West VA mountains kept my gas mileage pretty high too, one would expect all of the hill climbing would hurt the gas mileage badly.
Now there are two possible explanations to this, either it's theory #2, or it's the fact that higher load columns will have less timing then lower load columns, thus decreasing the timing advance in the lower load would bring the MPG (less resistance from over advancing, ie pumping tires up example).
General consensus says the more load the less fuel economy, the less load the better fuel economy is. This is how old school MPG gauges (vacuum gauges) used to work, the more you open your throttle, less vacuum, worse fuel economy. It makes sense on why, the more load (air), the more fuel is needed to keep the mixture from leaning out.
This is why increasing ignition timing can increase gas mileage too; by increasing timing you get more torque (until you reach the knock threshold), by increasing torque car has more power thus you use less throttle to keep the car going at the set speed.
Theory 2: More load better fuel economy
Metrompg.com did a warm air intake (WAI) test, one of the theories behind WAI is quoted below. Gasoline Engines are most efficient at wide open throttle and least efficient at part throttle.
Originally Posted by metrompgReducing the density of the intake air by heating it effectively reduces the power of the engine at a given throttle opening (relative to the same engine with a CAI). To do the same work as the CAI-equipped engine, the throttle of the WAI engine must be opened wider at a given RPM. This increases engine efficiency by reducing throttling or pumping losses:
"The air is less dense, so you get less horsepower at the same throttle opening, thus, you have to open the throttle wider to let in more air and get the horsepower that you need. That increases the efficiency because one of the primary causes of the well-known part load inefficiency of gasoline engines is the throttle loss." - (source)
Theory 3: Ignition timing
On my 9.5:1 Roller Rocker DE+T (with low-port intake manifold), I've noticed a rather weird trend too. The less timing I ran the better my MPG got. I started out at with 38-40* timing for highway speed cells. I got down to 30* before I ran out of road to test on (12 hour trip), I do plan on going lower then 30* on my next big trip. This is completely backwards of my 8.5:1 SR20DET (with Ported intake manifold), I kept on getting better MPG the higher I went, leaving it 40-42.
While both motors are SR20's, they are completely different animals internally, Roller Rocker is much more efficient internally then a DET. The more efficient the engine the less timing is required to get the same amount of torque out of each cycle. Having too high of timing would mean the mixture would finish burning while the piston was still pushing up, at WOT this would cause detonation, at partial throttle it will increase resistance, the same way it becomes harder to pump tires up the more pressure they have. What's odd is, on the dyno Roller Rocker's best torque was at 36-37* for that rpm range.
Hill climbing vs MPG
I occasionally get out of my small town hidden in a valley, ~1,400 ft above sea level, and drive to Fairfax area, ~300ft. If I fill up after a descending drive only, I will get around 30mpg. If I were to just turn around and drive back, ascending to 1,400ft, my MPG would be much higher, closer to 34mpg. I noticed the same trend when I went to Richmond (practically sea level). I always found it interesting that going 64 west over West VA mountains kept my gas mileage pretty high too, one would expect all of the hill climbing would hurt the gas mileage badly.
Now there are two possible explanations to this, either it's theory #2, or it's the fact that higher load columns will have less timing then lower load columns, thus decreasing the timing advance in the lower load would bring the MPG (less resistance from over advancing, ie pumping tires up example).
Last edited by Vadim
on 2013-09-30
at 20-52-01.
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