I just put in a K&N Gen II intake system and a Corsa Dual Rear exhaust on my 04 silverado. I was expecting to gain some gas mileage but it turns out i've lost about 40 miles to a tank. Someone please tell me what is going on? The dude at the local tire pros says its prolly because i went from a single to a dual exhaust. is this right? oh yeah, it isn't because my foot has gotten heavier, i am driving at the same speeds as before.  

Yep, He's telling you the truth. I'm not too sure why it works this way but I believe it is because you have less back pressure.

Most of the time you will also lose low end power too.

I remember when I first replaced my restictive factory exhaust system from a 2.00" to a 2.25" size pipe with a high flow muffler. I ran it for about 2 months with no tailpipe and I lost low end power. As soon as I had a muffler shop bend a new tailpipe up for it, it ran great.

The so called dual exhaust systems that they have now aren't true dual exhaust anymore. All of the cat. backs just use a dual outlet muffler witch to me are worthless. The only thing that's good about them is the nice sound that they produce, but they are not worth the performance loss or the lower miles per gallon loss.

If you have a V-8 engine your better off with a 2.25" to 2.50" max. single exhaust system with a quality high flow muffler.

...okay, yeah that makes sense, the tips are 4" but they are pretty shallow tips and the tail pipes to them are a lot skinnier....so i should look for a single exit performance exhaust with smaller tips...anyone got reccomendations?  

The exhaust tips wouldn't have any effect on power loss or gas milage as most are short in lenth. How about using a muffler like the Flowmaster 40 series or a muffler that I have used on my last two trucks, its called the Flowtech Terminator and is made by Holley. It is much cheaper than the Flowmaster and produces a nice throaty sound and gives good performance gains.

The only place that I know of that sells this muffler is Pep Boys, but other places might sell it too. I bought one last year for my truck and it went from $50 down to $34. It's a well made muffler that is made from heavy aluminized steel and will most likely outlast the rest of the exhaust system.

Flowmaster 40s really are a quality product, I love them. As fot the mileage, the loss in backpressure is the problem. And a "more restrictive" muffler can help. (not that flowmaster makes a restrictive product) You can also replace youe exaust system, or portions of it, with smaller diamiter pipe. But this would mean paying for the same thing twice and I HATE that.   

THINK about it since you got more air coming and going the computer is telling it to richen up to keep the right fuel ratio. it don't really matter want the ads say they usually say better gas milage and performance its hard too have both  

True titan up. Either your engine will run too lean and get hotter, or it'll put more fuel in. With more fuel and air you should be making more power, generally speaking.

However I remember reading about camshaft selections in magazines like CAR CRAFT and heard stuff like lopey cams will bump up the RPM's where your HP and torque peaks are, but by just changing the cam alone for more mid-range and top-end power usually makes the low-end torque and torque at idle, as well as low-end RPM mileage to drop.

I think the stock exhaust is around 2.5" diameter. How about a 3" single exhaust?

Yes Sir i would agree 2.75" or 3"..  as far as cams is concerned its to do with intake , gear ratio, compression, and alot more and what RPM you want the power to come on at like the reeds in a 2 stroke engine. i look at it like this if you want gas milage you need to build alot of TQ. down low and thats where most every day 4x4's run anyway TQ gets the body moving alot easyer an faster less strain like they say HP is how fast you hit a wall TQ is how hard you hit the wall .. but anyway i think thats why a desiel gets much better milage would you agree ? but to each his are her own. but i do know you can't pump all that air in and out on a computer controlled truck or car and get more power and gas mileage its one are the other..thats just  my opinion..  

Computer controlled cars are trying to do the thinking for you. Trying to force you car run to factory specs unless you install a mod chip. Hell, my my '02 S-10 won't even start when the hose from the Cold Air Intake is loose!

I got twin Flowmaster Super 40's on my '77 K5. True Duals no cats, and 2.25 pipes from the Flowtech headers back. I got more low-end torque after I installed the H-pipe in front of the S 40's. I got so much power from that mod alone it was un-real!  ...and the exhaust growl is like no other!  Just say no to electronics!!

I run a torq tube on my camaro in front s. 40's it makes a bigg differance in tq. now im having a problem of  blowing out the casings on the flowmasters. do you have the flowtech afterburners.

a computor controlled car is ment to be more efficient and to tell the mechanic whats wrong with it. its more efficient on keeping the gas at the correct mixture. were as you can tune a carb one minute and the weather changes it an't got to be much change and its out of tune. while you been gone there has been questions on how to get better gas milage and more horse power to me its one or the other it an't just putting on a catback or a high flow air box thats hp not gas miles. if people would just think about what has got to happen it an't brain surgery. the more air in & the more air out the more the computor has to richen up. they say more milage that is almost impossable unless you ease off  from a stop and let the extra tq. work. but explain this my 69 nova has a 355 with a 512 lift @ 50 duration cam a stealth intake with a demon carb 205int. 1.6 exe. 200cc runners 300lb springs the orignal powerglide with 342's in the rear it gets alot better gas milage than my titan 4x4 or anythang else i have except my honda 4 stroke on my carolina skiff.

I've heard that before about the flowmasters at high hp. What size pipes are you running ? also Mandrel bent or pipe bent ? 

I do have the flowtech afterburner headers on it. - again due my finances. I wish I could have gone better but the money wasn't there. The quality on these holley headers is crap. I had to grind down and finish some of the welds on it before I installed them. Tijuana made parts - no kidding!

That's what I mean about computer controlled cars & trucks. They are not designed for high hp mods. That little electronic 'brain' will fight you all the way back to it's original specs and fuel economy (efficiency)  - unless you install a custom burned chip for high performance. The last thing I'm looking for in my beast is fuel economy or efficiency! I'd have bought a yugo or a neon ( made from recycled milk jugs) if I wanted that.  My beast is not my DD truck. ...and I don't try to make my DD truck high performace. - Like you said, you cant have it both ways.

Carbureted cars do not perform well in high or low temperatures, that's just one of the laws of nature, but the trade off is not worth it in my opinion. I'm old school - obviously. Carbs are also cheaper to fix and maintain ( if you do it yourself ) than electronics and fuel injection.

I've heard that too about older performance cars and trucks having better fuel economy than a new factory vehicle of equal power.

Did you put in and X-pipe or H-pipe?  

I'm running 2 3/4 out of the header's to the flowmaster's and 3" from the flowmaster's out the rear.

As far as the flowtech headers i wanted to try something diffent and i put them and the flowtech afterburner's on my nova and as far as the finish on the hedders as soon as i fired it up the finish started burning off and had to modify the driver side to go around the shift linkage but have been impressed with the performance they are like step tube header and give performance in the low, mid, and high rpm range. they need to make them in a heavier guage steel and a better finish.

Ya, I had a similar problem with the finish burning off on the first trip when they got hot. I didn't need to make any mods on mine ('77 K5 - 350ci ) - I also don't have the problem of working around the A/C compressor. It didn't work when I bought the truck so I ripped it out, and a lot of the other dead weight in there that went to it. I must have got rid of more than 100 lbs of dead weight in the engine compartment.

I sprayed my headers flack black - 1500 header degree paint. I wasn't sure if the paint on top of the coating f*cked it up ininially, but then I heard the same symptom form others who did,'t paint the same type of header. Crappy Holley quality again. They were available in a more expensive coating, but I didn't opt for that - lot more expensive. I wouldn't be surprised if it flaked off just the same though. I do agree about the performance and thicker gauge steel though. It would also be nice if they were not rusting in the box when I received them!

I felt improvement along all ranges low med & high. The H-pipe is what really made the low-end torque more noticable, especially due to the 750 cfm double-pumper holley that's known for it's low-end weakness unless you have a high horsepower engine to bolt in onto. Everyone says "it's overkill for this application They're made for 1/4 mile cars" - I can't help it, I love over-kill !!

What's the horse-power on your Nova ?

H-pipe's make more low end torque and a kick ass sound? You sold me. I want an H-pipe too! *grin*