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Old 05-19-2006, 09:20 AM
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MPG with free flow exhaust?

I am wondering what kind of fuel mileage improvement you really get with headers and free flowing mufflers? The small power increase is always nice but I am not going to dump $300 on my daily driver for 10 horses or whatever. However these days if it improved the fuel mileage enough it might be worth it. Does anybody have any real numbers or experiences with there car?

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Old 05-19-2006, 09:46 AM
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It depends on your engine set up. A stock engine will not respond to headers and much as an engine with extra compression and camshaft will.

However, even a stock engine will pick up at least 20 hp and 30 ft lb with a set of headers. And deliver about 5 to 10% better mpg. So if you are getting 12 mpg now then expect 12.6 to 13.2 mpg.

You can also pick up 20 hp on a stock engine with a simple bowl smoothing job on a set of stock cylinder heads. But the heads would need to come off and the valves removed to do the job.

Also, headers are noisy. The engine noise in the car will double!
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Old 05-19-2006, 10:56 AM
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5 to 10% increase in fuel economy for any type of engine? I am wondering about smaller engine late model fuel injected vehicles in particular. I have never run a nice exhaust set up on say a v6 or 4 banger for that matter. I Have always used headers and high flow mufflers on my v8s and I know its worth the money for the bigger engines but what about the smaller ones?
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Old 05-19-2006, 11:03 AM
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pick the wrong header.... it can cost you torque and mpg normal driving...more pedal from a stop sign or up a hill

that's why most new cars have just "shorty's"..... you need back pressure to about 4,000 rpm....it's torque that accelerates the car
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Old 05-19-2006, 11:04 AM
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You just really have to make sure that you match up the numbers right. Getting the right size headers and exhaust is the key. If you get to large diameter, you're obviously gonna lose low-end power where mpg is the key. But getting headers that are relatively equal length vs. stock manifolds will help each cylinder breathe more efficiently and evenly.
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Old 05-19-2006, 11:05 AM
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Shorty headers are a joke, I can't believe the price some people are paying for that 10hp "peak gain".
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Old 05-19-2006, 11:07 AM
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The amount of MPG increase you'll get obviously depends on your right foot, too. I know that when I put a cold air intake on my Impala SS I lost mileage because I loved the sound of wide open throttle too much.

Assuming you drive it exactly the same, the amount you'll gain is based on how much pumping loss you gain from the free-flowing exhaust, how much the mufflers themselves were the restriction, and (assuming your car makes more power after) if you now drive with less right foot to drive in the same fashion.

In general, simply replacing mufflers and headers won't do much. You also have to take a look at how you drive the car. The engine won't return peak MPG until its at full operating temperature and headers often make it take longer to reach those temps. If you're just around town driving, you might spend a proportionally larger time below operating temp which may cost you fuel. If you're on the highway cruising, that extra cool time might be offset by some fuel savings.

But as far as exhaust giving you good returns on MPG for your dollar, its a crap shoot. Some might, some might not.
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Old 05-19-2006, 11:39 AM
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(The amount of MPG increase you'll get obviously depends on your right foot, too. I know that when I put a cold air intake on my Impala SS I lost mileage because I loved the sound of wide open throttle too much.)

Thats a really good point I did not even think about. It sure is hard to drive like an old lady even without the throaty pipes and intake. My daily driver is the last thing I need to be hot roding!
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Old 05-19-2006, 12:09 PM
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The increase in mileage and/or performance on an engine will also depend on the type of engine and the quality of the stock manifolds. On an old Chevy, the stock manifolds aren't very good and headers will make big gains.

On imports it will be model specific. For example, old Z cars have pretty good stock manifolds and installing a header on a stock engines doesn't do much more than make more noise. However, add a cam and raise compression and the header will help.

Go to ChevyHiperformance.com and look at their old tech articles. There is a article called "What are Headers Worth". They test 7 or 8 different headers on a SBC with a 292H comp cam and full length headers were worth 50hp on this engine compared to the stock manifolds. I haven't read the article in a few years so I may not be 100 percent correct on my numbers.

That is right, you need a full length, small tube header for low speed torque and mpg. And of course, if you drive like a grandma, your mileage will be better. And you will be healthier if you don't smoke, drink, and chase women, just in case you didn't know.


here is the website article that I wrote about above:

http://chevyhiperformance.com/techarticles/49178/

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Old 05-19-2006, 12:26 PM
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Sorry that wasn't the article I was talking about but it still was pretty good.

Maybe that header comparison was in a Super Chevy magazine? It has been 4 or 5 years.
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Old 05-19-2006, 12:40 PM
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Yeah that is a good article. Still they are testing v8s. Will my 3.5 horse lawn mower benefit from a long tube header? LOL I have always wanted to do that..
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Old 05-19-2006, 01:43 PM
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weirdbeard,
note on 454's article....dyno run starts at 2600rpm=5HP/11ftlbs gain at 70 mph typical.....
5HP/3400lb car=.00147% power/weight gain, 11ftlbs/3400lb car=.00323% torque/weight gain max

gain will be nothing below 2600 for intown driving

for "shorty's", if it is a 350, find some used Vette shorty pipes,
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Old 05-19-2006, 03:58 PM
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I would do some model specific searching before you decide. I know the late model Dodge 4.7 in the Dakota's and Rams only gain approx. 7 hp with headers, not worth it for 5 to 6 hundred (or more) dollars. My brother has an Acura and he found some independent research on Honda's that on a stock engine headers were worth minimal gains, similar to the Dodge 4.7.
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Old 05-19-2006, 06:33 PM
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Yeah I dont think its going to be worth it unless I could score a deal off ebay or something. Even then if it hurt the power an MPG (which is not uncommon on a sensor laden vehicle) It would be disaster. Now back to the header pipe on my lawn mower... Just kidding
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Old 05-20-2006, 04:41 AM
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header pipe for the mower= "jr dragster" pipe by Hooker, see attached.....your neighbors will just love it...LOL...mount a chrome glasspack on the end of it!

actually you can improve the torque on most "elcheapo" mowers by extending the exhaust like the motor picture on the right and using a much bigger muffler....muffler is designed for "quiet" not pulse flow
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