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no muffler= horrible gas mileage?

no muffler= horrible gas mileage?

Postby intimidatorapg [OP] » May 30 2012, 5:25am

i put some shirty headers on my 06 Z71 and cut the muffler off. It sounds amazing. my only problem is....im pretty sure my fuel mileage dropped a few MPG's. does anybody think this is due to lack of the muffler and backpressure? I love how loud "The Hussy" is, but i like my money too. does anybody know how i can keep my truck this loud if not make it louder with dual straights and an "X" pipe, but get my fuel mileage back? maybe install some single chamber flowmaster 10's? thanks in aadvance for the help.
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Re: no muffler= horrible gas mileage?

Postby apache30 » May 30 2012, 3:59pm

Maybe you just like the sound too much and can't keep your foot out of it? :lol: The lack of back pressure could have something to do with it. Maybe get a tune and see if that helps. Most people on here use the DiabloSport Intune.
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Re: no muffler= horrible gas mileage?

Postby GMCFanatic2010 » May 30 2012, 4:23pm

Not a mechanic but I know your engine does need a certain amt of backpressure to function at optimum performance. I would say it definitely has at least something to do with the lack of the backpressure .... not real sure how to fix that without putting pipes back on but either way best of luck and if you get a chance perhaps post a vid or something for us curious type people :) Id love to hear how it sounds
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Re: no muffler= horrible gas mileage?

Postby tcarr25 » May 30 2012, 4:25pm

If you were to put 10 series it would still be loud. I had true dual 3 inch exhaust on my old 6.0 with 10 series and that thing roared.
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Re: no muffler= horrible gas mileage?

Postby Voodoo » May 30 2012, 4:31pm

If you look at a lot of high performance cars, such as auto cross, they all have complete exhaust systems. Engines tuners and engineers will tell you that to get it all working in harmony the system must be in tact. There may be better components to the system than what the factory uses that will help performance, but my belief is when you change the system by removing components, performance is adversely effected. I would contact whomever you got the headers from and ask them what the bestparts would be for what you're trying to accomplish and go from there. They probably have ton of dyno sheets.
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Re: no muffler= horrible gas mileage?

Postby lilmagoo1 » May 30 2012, 5:01pm

how loud would my truck get or how different would it sound if i cut my tips off?
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Re: no muffler= horrible gas mileage?

Postby dirtydeeds » May 31 2012, 8:23pm

There is a lot of mis-concepton floating around on this subject. Hopefully I can help.

The size of the tubing has to meet the intended exhaust flow. I know you've heard that smaller tubing is good for low end, low rpm's and big tubing is good for high end, high rpm. Having a restrictive muffler causes backpressure which makes the system act like a smaller system which promotes low end scavenging and low end torque, and better fuel economy because you don't have to step on the throttle as much. Then at higher rpm the engine powers through the muffler and still makes power, but not near as much as it would if you were to uncork the restrictive muffler.

By un-corking without doing anything else to promote low end scavenging you create huge pumping losses. There is no velocity which means there is no low pressure 'wave' behind the high pressure pulse. Simply put no scavenging.

Change the scenario now starting from the beginning: Stick in a big cam with intake that can flow with it, your cylinders are much more full and have a much bigger explosion after ignition. That creates much more pressure that exits the exhaust valve at a much higher pressure. If the header length and diameter are properly sized that pressure will turn into velocity. With that velocity comes a low pressure 'wave' behind it. If the length of the header primary is correct that low pressure wave will pull out the next high pressure wave. Then the chain reaction begins. If you can optimize all of this to happen as effeciently as possible and make it coincide with when the intake and cam is doing exactly the same thing only with the intake then the low pressure wave will actually not only pull out the next high pressure pulse, it will pull in the next intake charge. That's why a good cam will have some overlap. Both intake and exhaust valves are open at the same time for a split second allowing the scavenging of the exhaust system to pull in the next charge.

Even without a big cam this still happens. Your intake was designed by Chevy to 'flow' just like a good exhaust system does. The length of the intake runners and the plenum size all account for when the peak power level occurs. Without going into peak volumetric effeciency theory etc. I can tell you that even your stock cam will benefit by installing long tube headers. A big cam would only exaggerate this.

In most oem's they've done a great job tuning the intake, sizing the cam etc. The exhaust is the only place that they lack. It's not by choice though, their hands are tied. A big cam with lots of overlap may make power, but some of the fuel/air charge is expelled through the exhaust system and exit as hydrocarbons making less economy and more emissions. Long tube headers will also scavenge some of the air/fuel charge with the same above problems. Running true duals doubles the cost of the system and when you're making millions of them that equals a lot of money. Also, they have a federal drive by noise standard that they have to meet. That's why there is a huge trash can aka muffler in the stock system. Notice how I started in the front and went down the line in all of my above correspondence?

Now you're asking, where does this leave me? Well you have to ask yourself, what is the intended use of the vehicle? Based on this answer, what is a realistic plan? Does your budget support such a plan? I know of a lot of guys with some disposable income who will tend to go with shorties only because they can have em now and they don't have to wait until they save enough for long tubes. I know this personally as I was once that guy.

After installing shorties to fix an exhaust leak on my first V8 I was pretty disappointed when the car actually felt slower. All of that work. Circumstance happened and a set of Long tubes were thrown my way a year or so later. After installing them I was amazed at the extra hp/tq. With my last truck, I went straight for long tubes, I actually dyno'd before and after to realize a gain of 32hp over stock!

You can generally categorize this as there are 3 types of truck owner:

1. 85% of owners keep their truck stock

2. 13% want a little more power, or a lot more power, but either way their budget only allows for or they are only willing to spend enough to buy a decent compromise, not a full kill exhaust system. These guys are the biggest market for the exhaust system companies. These guys are also most likely to make a mistake. They usually think bigger is better and buy a system with tubing that is too big, or in other ways mismatched to their vehicle.

3. The last 2%, these guys either have a ton of money and just do what their speed shop says, or they've figured it out through experience. Or they know to save their money until they have enough to accomplish their goals. These guys know that simple easy stuff won't give them the power that they're looking for. They're willing to spend the money to do it right but also don't want to waste any either.

I'll refer to type 2 and 3 guys. Type 1 guys don't care about this.

Type 2 guys don't have a couple grand to do a full kill system so it's really important that they do it right the first time. Buying that bolt on system gets them those pretty pipes that are bigger and so it should make way more power, not. Also, the shorties being easy to install are appealing too. Problem is, big pipes slow velocities and all the stuff we talked about above is out the window. They tend to over think something that is really simple. The most restrictive part of a stock exhaust is the muffler. Second to the muffler is the manifolds. The cats and tubing are the least restrictive. Moral of the story, if you don't change out the manifolds, that leaves only the muffler. Now it's important to do this right and I'll get into this later.

Type 3 guys already have a plan. They're going to replace the stock manifolds for Long Tube headers basing the size of the primaries on what the manufacturer says it should be based on the other parameters of the engine. The rest of the system is based on whether they need cats or not. It's hard to go wrong after the long tubes unless you just go way overboard either way too big or too small.

Back to the type 2 guys:
There are a few different ways to quiet down an exhaust system. You can accomplish it by 1.restriction, 2. reflection, 3. absorption. Guess which makes most power? Absorption, guess which is cheapest? Restriction. Most OEM's use Restricton. Most high end cars like lambo, porsche, ferrari, where money is no object, they use absorption. But guess what, absorption mufflers also act like a restriction muffler, the difference is that restriction goes away at higher rpm, right where you want it to, here's why:

At low rpm that high pressure exhaust pulse travels down the system until it reaches the absorption style muffler (aka 'flow thru' muffler). It enters the muffler and immediately expands through the perforated tubing and into the case of the muffler. When it expands it slows down. By this time the next high pressure pulse has hit that one since it slowed. Then the following pulse hits the last, and the chain reaction begins, this is back pressure. Everybody thinks that Magnaflow, Borla, etc. have no back pressure, well this isn't true as I explained just now. But here's where you get your high end performance, as the rpm's increase those high pressure pulses turn into more of a high pressure steady flow. This flow maintains pressure in the muffler case. If you size the case properly the pressure in the case is equal to the pressure in the exhaust tubing just as the engine enters peak volumetric effeciency, right when it needs the most scavenging, and least back pressure. Now the exhaust flows straight through the muffler, it doesn't expand and slow. In short, it's a street muffler at low rpm, and a race muffler at high rpm. That's the best of both worlds right there.

I just wrote this on the fly to try and help out. Hopefully this all made sense. I actually make mufflers and bolt on exhaust systems. This week has been slow so I'm browsing forums seeing where I can get into trouble.

Next week I'm building a couple of exhaust systems for my buddy's motorcycle salvage business. There's a 2011 6.0 and a 2012 5.3, he wants true duals on an otherwise stock truck and I'm trying to talk him into running a single exhaust (even though that means less money for me) but the 2012 he wants to go full kill. I'll start a build thread and post a link here. In the mean time here is another thread that should help you decide whether to put a muffler back into your system:

http://www.tundrasolutions.com/forums/performance-exhaust-systems/187499-how-properly-built-exhaust-system-works/
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Re: no muffler= horrible gas mileage?

Postby camfire87 » Jul 30 2012, 8:52am

Wow, let's give this man a big high five for that article. Excellent writeup! Beware of writing things this well though, because if you do there is a good chance that guys like me will be hounding you for info and advice...
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no muffler= horrible gas mileage?

Postby MossOutdoors » Jul 30 2012, 2:36pm

I had 3'' true dual cherry bomb extreme mufflers and that set up was the loudest in Katy, Tx! She gave me head aches for the first week!
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Re: no muffler= horrible gas mileage?

Postby dirtydeeds » Jul 30 2012, 4:45pm

camfire87 wrote:Wow, let's give this man a big high five for that article. Excellent writeup! Beware of writing things this well though, because if you do there is a good chance that guys like me will be hounding you for info and advice...


I'm happy to help. :mrgreen:
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