After a little googleing I found this article explaining backpressure and it's effects. http://www.superchevy.com/technical/...exh/index.html I have always thought that you need a little back pressure to increase the scavenging effect in the cylinders... well, I guess I was kinda right but not completely... at least according to this dude.

 

Just about every Cherry Bomb Muffler will flow better than stock (and a lot better than a Flowmaster muffler) so that means one of two things. Either A) You are REALLY getting on it more than before because it sounds so good or B) The place you had the work done has no clue how to properlay do a cat-back exhaust and has created a restriction somewhere along the way.
My guess is it's a little bit of "A" and a good bit of "B".

Contrary to the way I've seen many exhaust shops do work, you can't just take a muffler and bend pipe arbitrarily to get it from point A to point B. There is a bit of a science and a lot of common sense that needs to be applied here...

 

One thing that should help if you are not using a tuner is to reset the pcm after any mod you do even after general maintenance such as cleaning the throttle body changing plugs etc. You could let the truck go along and relearn after a period of time but why wait its easy. On my 04 Hemi all you have to do is pull fuse #23 get in the truck turn key to start and wait for a chime turn back off put fuse back in and your down. Im not sure what fuse it is for others but it should say in the manual or under the cover.

 

From what I have gathered from years of reading on forums and other tech articles, a bigger engine benefits from a chambered muffler, while smaller mills need a straight-thru (IE: cherry bomb). I had straight pipes on my 2007 4.7, and I lost about 1 mpg. Now I have true dual Thrush 2 chambers with an X pipe and 2.25" pipe with tips out the back. I regained my lost MPG, plus a bit more with that setup. It seemed to have more low end grunt too, and it no longer sounds like a pulp truck.

 

Also remember it's winter and gas is blended differently, and that generally reduces fuel economy. Your exhaust setup is probably causing you to loose a bit of backpressure, and therefore a little extra gas, but combine winter gas + exhaust + you getting on it for the noise and you'll notice a drop in MPG.

 

how much did that set up run you? i had a flowmaster on my old truck that ran me just under 400. flowmaster is only for sound, i know they are horrible performance wise. so just wondering what a custom set like that runs?

 

Im having the same "problem". I'm contributing it to what the other member said, my foot getting put into it way more. When I first got the truck, I was babying it like crazy, trying to see how much mpg I could squeeze out of it (15.5 on average). While I was working out of town, the ol' lady snuck it down to the exhaust shop and got a 40 series thrown on it, just direct replacement. Wow. I am very impressed, so much so, I half-wot from every damn stop I come to, which is leaving me at 11.5 on avg. But, especially since prices are coming on down, its well well well worth it

Edit: $400?!?! Im assuming this is for a full dual setup, correct? My deal only cost 125, which isnt too bad imo. They did the D for 185, and thats re-routing out the side, purchase and install of the dual chromed tip, and everything welded vs clamped.

 

When you increase the pipe size you loss velocity. A high velocity will draw the next fuel charge into the cylinder during the cam overlap. If you wrap the exhaust to keep in the heat, you will increase the velocity and probably gain back some MPGs .....& ponies.

Mike

 

yea, i think it was like 385 or something for the full dual "flowmaster" branded kit off summit.

 

dang i wish i was getting 15mpgs. im totally stock with a light foot and getting 13.4