New York mechanic
Hi, I'm Andy and I've been fixing and modifying cars for a long time. I've also been working on improving fuel economy on some of the cars I've owned.
About 1988 or so I had a 1972 Ford Mustang with a 351 Cleveland engine. I paid $300 for it. I took apart the engine and found a worn camshaft and some bad valves. I got a similar pair of heads from a 400 engine and did some porting work with a grinding stone to improve airflow. Then I had them milled 0.060 inch to increase compression to 10:1 or so. I bought a higher performance camshaft. I put everything together, retarded the ignition timing enough to stop knocking, filled up the gas tank and drove it over 100 miles from somebody's yard to where I was living. It was very powerful but when I filled the gas tank I calculated that I got only 9 MPG. I was driving about 55 MPH most of the time. I decided I had to do something to avoid wasting my money.
I noticed that the camshaft seemed to be advanced about 6 crankshaft degrees and decided that I would like to see what happens if I retard it. I bought a special crank gear with multiple keyways to allow many different cam timing settings. I retarded the camshaft by 12 degrees compared with how it was with the old gear. I readjusted the ignition timing and later installed a Rochester Quadrajet carburetor. When I took a long trip I measured 18 MPG. However, it didn't feel powerful anymore. After using it for a few months one of my valve lifters started rattling so I had to take the engine apart again. Instead of just replacing the bad lifter, I decided to do an experiment. I took out the pushrods from every other cylinder in the firing order, including the one with the bad lifter. Then I clamped all the unused valve lifers with hose clamps so they would not touch the camshaft. I didn't remove them because I didn't want to cause an internal leak that would reduce the oil pressure. I also decided to advance the camshaft by 4 degrees to improve low end torque. After tuning the ignition timing and taking another long trip, I measured 24 MPG. So I learned about three things that improve efficiency: high compression, retarded valve timing, and cylinder deactivation.
Many years later I got a Honda Civic CX as payment for some engine repairs. The fuel economy was about 35 MPG on the highway. Eventually I realized that it was running rich because of the smell of the exhaust gas. I think the problem was that the old timing belt was retarding the camshaft which lowered the airflow and confused the computer which was measuring absolute pressure in the intake manifold instead of airflow. I tightened up the timing belt tensioner and connected a resistor from the output of the MAP sensor to ground in order to lower the voltage sent to the computer a little bit. When I took a a trip, I was able to go 500 miles on 10 gallons, so it got 50 MPG. I still have this car in storage.
I think there must be lots of other cars on the road with this problem. Their owners probably take them to the shop to get a new oxygen sensor. After a few weeks the oxygen sensor probably fails from too much carbon buildup. It makes a lot of sense to use the MAP sensor trick until the there is an opportunity to replace the timing belt.
Now I'm driving a 1988 Honda Accord coupe. To improve the fuel economy I've done these things:
tighten the timing belt tensioner
align front and rear wheels (by myself - I don't pay for car repairs)
unstick the distributor advance weights with some WD-40
advance the ignition timing at idle
get the engine to breath warm air near the engine instead of outside air
fix the vacuum advance on the distributor
I also tried running the engine on only two cylinders for a few hundred miles but I decided it wasn't worth it because local fuel economy didn't seem any better and the shaking was destroying exhaust pipe parts. The power was still adequate to go 60 MPH in high gear and it could hold the speed on most hills by downshifting to third gear. On steep hills I could downshift to second gear and go 50 MPH. I think that it was probably running too rich at low RPM so maybe that's why the fuel economy wasn't great.
I measured about 35 MPG on the highway running on four cyliinders but that was before I fixed the vacuum advance so I'm hoping for 40 MPG on my next highway trip. The car has plenty of power and great handling on curvy roads.
I also like to figure out the mathematics of fuel economy. I think this is my most important formula:
MPG=D*V/(BSFC*HP)
D: density of fuel, about 6.25 lb/gal for gasoline
V: velocity in MPH
BSFC: brake specific fuel consumption of the engine in lb/hp-hr
HP: engine horsepower needed to maintain speed
As an example lets assume we can go 25 MPH on 3 HP with an engine that is burning gasoline at 0.5 lb/hp-hr. These numbers seem reasonable in a small car with a very small engine, like about 200 cc or 12 cubic inches.
MPG=6.25*25/(0.5*3)
MPG=156.25/1.5
MPG=104.17
It looks good to me. Maybe we could add lawnmower engines to our cars for low speed driving. A normal car engine would not have this good efficiency at only 3 HP. Supposedly it is best to run the engine at about 2500 RPM and 2/3 load to get the highest efficiency. A 2 liter engine would be running at a very low load.
A variation of the formula lets you calculate MPG when you know BSFC and the force acting against the motion of a vehicle.
MPG=375*D/(BSFC*F)
This makes it clear that there are two things you need to do to maximize fuel economy: reduce the force resisting motion, and make the engine and drivetrain as efficient as possible (reduce the BSFC). You can also try to buy gasoline when its density is high by waiting for a cold day.
It is also useful to know how to calculate power when you know force and velocity.
P=F*V/375
This can also be rearranged a couple of ways.
F=375*P/V
V=375*P/F
The F is force in pounds, V is velocity in MPH and P is horsepower.
So how fast can you climb a vertical cliff when you have a 3000 lb car that has 150 HP at the wheels?
V=375*P/F
V=375*150/3000
V=18.75 MPH if the car has traction (maybe a high wind is helping)
Hopefully I'll be helpful in contributing some fuel economy information.
Well that's enough for now. I'm getting hungry.
Andy
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