I recently asked my ford shop about putting in a flowmaster 40 series and they told me it wouldn't have enough back pressure to keep the o2 sensors hot enough on the 07 f150s with a 4.6. They said with all the new emmisions stuff it would cause my mpg to drop big time.i was always under the thinking i made the performance go up and better mpg not lower it. will it really cause that many problems?thanks

 

All EFI motors since about 1990 use heated oxygen sensors, so it really doesn't matter what exhaust is behind it. A lot of people still have misconceptions about performance parts and how they affect a motor, and don't understand that an EFI motor.. unlike a carbed motor, has the ability to adjust to changes in it's environment. So there is no danger of engine damage from exhaust modifications as long as the O2 sensor is retained, and if these changes increase airflow through the motor you will see increases in power.

 

The reason to keep some amount of backpressure is for cylinder scavenging. By having some pressure at the exhaust valves when they open will promote flow through the cylinder to help remove all of the spent fuel/air mixture so that will allow more fresh mixture in for better combustion. That's why everybody doesn't run around with an 8" exhaust.

 

You got the right idea, but backpressure isn't what helps cylinder scavenging, this is the misunderstanding I'm talking about. If the exhaust system is restrictive it will not promote effective gas movement through the motor, the exhaust system must however be sized appropriately for the engine, too big and it will loose low rpm power, too small and it won't make any HP. It's well documented that longtube headers significantly improve cylinder scavening, and they certainly aren't as restrictive as manifolds, or in other words, don't produce as much backpressure.

 

Very true. You don't need as much backpressure, but some is desireable. I used to have a list that was generated showing optimum exhaust piping diameter for an engine but I seem to have misplaced it. I'm sure a reputable shop can give a recommendation.

Plus the long tubes will definitely help with low- and mid-range power too!

 

I remember seeing a graphic here somewhere.... something like this one.... http://www.angelfire.com/my/fan/Exhaust_Pipe_Size.htm with tubing sizes and what HP level they support. It basically shows that the stock exhaust on these trucks should be about optimal size for the power the engine makes. The problem is these stock systems have excessively quiet(restrictive) mufflers and crimp bends in the tubing so the total system flows significantly less that it could. A system made from mendrel bent tubing and high flow cats and mufflers will produce more power even if the tubing is the exact same diameter, and that's simply because it produces a little better cylinder scavenging over a broader range than the stock system. It's all too common for guys to go WAY too big with aftermarket exhaust systems, they get more noise but actually loose some TQ and milage.

 

The ideal backpressure is zero, or even negative if that could be achieved. The loss of backpressure is not what hurts low speed torque and driveablility. What hurts is a loss of exhaust gas velocity. If you empty a 50 cubic inch cylinder that is full of exhaust into a small pipe it must travel quickly to get out of the pipe. This generates a column of moving exhaust gas that has a lot of momentum. That momentum causes exhaust to continue exiting the cylinder even after the piston passes TDC. Also, when the valve closes, the momentum of the moving gases causes a low pressure wave to form behind the valve. This low pressure wave resonates through the exhaust system and will hopefully find its way to the place right behind the exhaust valve just as the valve opens so there will be a nice low pressure area for the exhaust to flow into. When the pipe is too large the exhaust moves more slowly and has less momentum. Less momentum means there will not be as much "suction" to pull exhaust out after TDC and the low pressure area behind the exhaust valve after it closes will be of lower magnitude.

Backpressure is a function of pipe size and scavenging is a function of pipe size. That doesn't mean that scavenging and backpressure are related. If backpressure was needed to make good torque we could all run a 6" pipe from each exhaust port with a 1/4" hole in the end to allow the exhaust to escape and it would work fine. I think we all know that won't make any low end or top end power.

 

I stand corrected. I learned something new today...

 

so i shouldn't lose any mpg?