I am looking to replace my exhaust on my 4.0L ranger. I would like to put something on that would improve or at least wouldn't hurt the gas mileage. What would running a straight pipe and a cat after the manifold pipes do? or would it be better to go with no cat and just a flowmaster muffler? I have no knowlege about exhaust whatso ever and am looking to learn about backpressure and it affects on the engine, fuel economy, etc,... thanks in advance..

 

Im not a fan of how smaller engines sound with no mufflers, so I cant help you there. IMO, 2.25"-2.5" single with a good muffler using mandrel bent pipe would be the best bet. For an easier solution, look at some cat-back systems for a bolt on and go setup.

The only thing to know about backpressure is that it isnt needed, or wanted. The engine will not blow up, burn valves, loose all of its torque or otherwise kill small children if you lose backpressure, like so many people will try to tell you. Even a straight pipe setup has 1-5psi of backpressure from bends, etc so you can never loose all of it. Exhaust velocity is what you want. Look up some internet sites and do some reading to learn about the 2. In fact, read several and make up your own mind about it.

In short- backpressure = restriction. Less restriction = higher efficiency from your engine = more power, better mileage. However, going too big is a waste. Bigger pipe is heavier and more expensive and you will never get good exhaust velocity. Velocity is caused by each exhaust pulse moving without restriction and causing a small vacuum behind it as it passes through the system and cools. The vacuum helps pull the next pulse out of the engine and make room for a cleaner air/fuel charge on the next stroke and reduces pumping losses. More air/fuel = more power. A stronger engine uses less fuel at a given speed because it doesnt have to work as hard to keep that speed.