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Old 04-26-2011, 11:26 AM   #1 (permalink)
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high flow exhaust = 3-5% mpg gain?

What no exhaust modifications. Switching to a high flow exhaust muffler or system(ie magnaflow , borla) IS a magic bullet 3-5% increases or more are often seen and pay back can be less that a year of driving.

Also Plus 0 tire increases (ie 225 75 15 tire for 1 205 70 15) will increase mileage because the larger tire rotates slower thus the engine rotates slower. keep in mind that on FWD cars you are limited to how much taller a tire you can install. you must also recalibrate your odometer as it will be turning slower than actual.

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Old 04-26-2011, 11:32 AM   #2 (permalink)
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Hi incautious - split your post out into a new thread...

I'd be interested to see some quality evidence (not anecdotal) of a fuel economy benefit from exhaust mods.
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Old 04-26-2011, 11:39 AM   #3 (permalink)
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I've always been under the impression that high-flow exhaust could only add benefit (mpg gain) over a certain RPM/speed. When I sourced out the exhaust for my car, the guy doing it put all of my info into a calculator to decide what I needed (stuff like header diameter, # of cylinders, rpm threshold, HP, etc).
I could very easily be wrong, but I've always thought that reducing "back pressure" would hurt on the bottom end more then it would help on the top end (for economy anyway).
Would love to see some actuall verifiable results either way though.
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Old 04-26-2011, 12:18 PM   #4 (permalink)
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One of my cars had last silencer such that pipe entered and exited from same end, replacing that with freeflow type did improve throttle response and also 0.5l/100km decrease in fuel consumption, which I could see happening right after exhaust replacement and it was permanent drop.

However I did not do any other testing than just noting consumption of each tank, would be interesting to see some proper testing with different kind of vehicles and different type of driving.

One thing that might easily affect to results here is sound, driver can sub-constantly use less throttle when car is louder, which can be also affecting freeflow air filter testing. Less fuel saving mind might actually push pedal deeper because of same reason, so testing should be done with cruise control, I think it is more crucial with these mods than with others.

NA motor exhaust pulses are important when trying to make most power at least, those are used to flush out all exhaust gas in performance motors, for that reason measurements are rather important.

It should be possible to calculate exhaust to have best gas removal for certain rpm, as it is possible to calculate intake runners so that at certain rpm there is kind of 'boost' from recoil pulses which fills cylinders much better, exhaust must then match to intake and both need to match to cam etc. That way it is possible to get quite few % more power from engine, perhaps also better economy, but changing only one of the components is not giving much improvement if there was nothing wrong with tune up in first place.

All those + modifying cylinder head should give gains to at least older cars, where swirl is not best possible and intake manifold runners are not so good. Cylinder head modifications especially are beneficial at part throttle situations, but to get best it have to be looked as whole package.

That is at least what tiny understanding I have been given from subject.
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Old 04-26-2011, 01:01 PM   #5 (permalink)
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Quote:
Originally Posted by jtbo View Post
NA motor exhaust pulses are important when trying to make most power at least, those are used to flush out all exhaust gas in performance motors, for that reason measurements are rather important.

It should be possible to calculate exhaust to have best gas removal for certain rpm, as it is possible to calculate intake runners so that at certain rpm there is kind of 'boost' from recoil pulses which fills cylinders much better, exhaust must then match to intake and both need to match to cam etc. That way it is possible to get quite few % more power from engine, perhaps also better economy, but changing only one of the components is not giving much improvement if there was nothing wrong with tune up in first place.

All those + modifying cylinder head should give gains to at least older cars, where swirl is not best possible and intake manifold runners are not so good. Cylinder head modifications especially are beneficial at part throttle situations, but to get best it have to be looked as whole package.

That is at least what tiny understanding I have been given from subject.
Well, you stated that about 1 million times more elegantly then I did. I totally agree, its the whole system that modifies performance towards a certain goal. Not just slapping on a new muffler.
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Old 04-26-2011, 01:23 PM   #6 (permalink)
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Pardon my ignorance, but how could decreasing exhaust restriction negatively affect fuel economy?

I just got a car with a performance exhaust and a silencer and I was planning on taking the silencer off, but if it hurts at low rpm I won't, since all my driving will be at low rpm.
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Old 04-26-2011, 01:52 PM   #7 (permalink)
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A re-done exhaust properly tuned to optimize flow and power at lower RPMs can help mpg. Just any higher flowing system, however, may or may not.

For example, appropriately sized long tube headers of the correct length, followed by a high flow cat and decent flowing muffler, with appropriately sized (read: not as big as you can fit) piping can definitely help. The gains may not be significant though.

John - if the exhaust system is made too large, it's poorly tuned for low rpm, and will have turbulence and poor flow that doesn't clean up until much higher rpms.
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Old 04-26-2011, 02:34 PM   #8 (permalink)
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Interesting. I don't suppose there's any easy way to test without special equipment?

The only thing I really need to know is if it's better to have the silencer on or off though, so just doing a quick ABAB once I get a MPG gauge should do it I guess.
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Old 04-26-2011, 02:39 PM   #9 (permalink)
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I'd be surprised if the silencer makes any noticeable difference on vs off. Backpressure isn't the key (unless it's excessive), fast, linear flow is what matters.

There are calculations that can be done to find the optimal header size and length, optimal pipe size, etc. Autolounge.net | Tech | Exhaust
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Old 04-26-2011, 02:42 PM   #10 (permalink)
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Sounds like it's time for A-B-A on a dyno!

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