Fuel Economy...

This is not an easy thing to answer.

First off, just changing to regulr gas will not improve fuel economy any farther than the tank of gas will cost less. If your fuel economy is poor, you are not going to see an improvement by switching fuel grades. It's obvious that you have other issues to fix before you will see mileage improve.

Secondly, where did you get this idea from?

"How "multiple bends + 1 hole" in the muffler/exhaust system can affect the Fuel Economy in a given car?"

A common myth is that a freer flowing exhaust will improve performance. That is just not true. Not all cars benefit from this. A Geo Metro is not one that would. The reason you see trucks and cars that do benefit is most likely because they have a large engine that is stifled by an exhaust system that is small...for a reason.

That reason is backpressure. Talk to anyone and they will tell you that backpressure is bad and it is, if you are looking to get as much power out of the engine as possible. However, without backpressure, the exhaust system does not work. If the exhaust system does not have enough backpressure, the engine will not be able to pressurize the exhaust system with exhaust gases. It can't pressurize the exhaust system becaus ethe engine does not produce enough volume of exhaust gases to fill the tube so that the exaust gas pressure wave seals the tube. It is essentially blowing out into the atmosphere at that point.

The problem with no backpressure and that problem behaving like the open atmosphere is that the exhaust gases lose velocity too quickly. If they lose velocity, they do not move out of the chamber fast enough to extract all of the spent gases out of teh combustion chamber. If it doesn't get all of the gases out, the engine has to pump them out. That takes power away from the engine that the engine uses to move the car. Less power to move the car means it don't go so fast. It also means that the engine has to work harder and longer to move the car so fuel mileage goes in the dumper too.

The principle I just explained about the exhaust gases creating a vacuum is just like a siphon. To demonstrate, take a hose, submerge the whole thing is a tub of water, cover one end, pull it out and uncover the end. The reason this happens is because the air in the atmosphere is pushing on the surface of the water in the tub. Because it is pushing on the surface, it is exerting pressure. The water can't move though because the tub is holding it in there. It has no place to go...that is, until you uncover the end of the hose that is outside the tub. At that point, water will come rushing out until the water level in the tub gets low enough to let air into the tube. Then the siphon breaks because the vacuum in the hose no longer exists. Why? Because the water's pressure wave was no longer large enough to seal off the inside of the tube. Exhaust gases are just like water. If you can get them to move out of the chamber quickly on thier own, they will create a vacumm inside the chamber. This is called scavenging. The benefit this provides is that when that intake valve opens up, the pressure variance inside the chamber will be great enough to actually start sucking air in before the physical movement of the piston can start creating a vacuum itself by moving down the cylinder and expanding the chamber before the air can fill it. That gets you more power because you are then filling the cylinder more than the mechanical work of the engine can do by itself. If you can exceed the cylinder filling point beyond what the machine is capable of doing itself, you just increased efficiency.

That is why less bends and larger/more holes will offer better mileage. However, your engine has to be operating at a level greater than what the exhaust system is handling. Most small cars are operating at that level but they are doing that for a reason. The reason is that backpressure helps build torque. Bigger engines like the V8 in a Mustang will benefit from less backpressure because they have such a high volume that a freer flowing exhaust will help them with the scavenging mentioned above.

The problem is though that small engines do not make a whole lot of torque down low in the RPM range. This hurts driveability. This is also the reason that those "hot rod" Civics need to rev the piss out of the engine to get it to go anywhere. They hurt thier low end performance because they put on a high-flow exhaust and dropped backpressure. Losing that backpressure killed thier torque rating.

Torque is a measure of how much work an engine can do over a given distance. Horsepower is a measure of that work over time. If your engine's torque output drops, it can do less work over a given distance. Since that number drops, it takes the engine more time to do the same amount of work it did before. This decreases horsepower too. Know what else it decreases? Efficiency. That correlates to suck **** gas mileage out of a weenie little 4 cylinder engine.

So why does backpressure help build torque? Simple answer, combustion chamber pressure. Backpressure helps an engine build cylinder pressure. The more you compress that air/fuel mixture, the stronger the explosion will be. That makes more torque. Torque is a good thing. If you relieve that backpressure, you are only going to hurt that ability. The bends, twists and muffler designs in a exhaust system are designed to help that as much as possible because it improves small engine efficiency.

Kind of confusing huh? Backpressure hurts the performance of larger engines but helps the performance of smaller engines.

Bottom line, if you plan on doing some serious engine work in the way of larger valves, more aggressive cam profiles and larger fuel/air delivery parts then a freer flowing exhaust is in order. On a stock Geo Metro engine, it won't help you, only hurt you.

This also explains why when you go to a race track and see the kids with the Honda Civic Si's that are mid 15 second cars stock, hop them up and actually run slower than stock. They will spend 5-6K to go slower!

BTW, the only time when no back pressure is beneficial is when you are running a turbocharger. That is why cars with high performance turbo systems will have large exhausts with multiple pipes or one large pipe that looks like a sewer pipe. Turbo'ed engines built power with boost. Those boost levels directly correlate to the efficiency of the turbo. If teh exhaust system has too much backpressure, it buils up pressure and waste gases in teh exhaust system. This causes the turbo impellars to cavitate and that drops efficiency.

Hope all that made sense. It sounds good in my head. I'll correct speeling mistakes later. Right now, I gotta go home.