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Old 04-07-2010, 09:22 PM   #1 (permalink)
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Will an Exhaust mod help fuel economy?

Some say modding the intake ups fuel use because the increase in air causes more fuel to be injected, in a FI car for instance. I would have thought that for a given throttle opening, more air gets in with less pumping losses when you mod your intake. I figured you'd get more power per unit of fuel burned (due to lower pumping losses), and so as long as you drove properly and conservatively, you would increase fuel economy....but, at least a couple of people have noticed that modded intakes always tend to decrease fuel-economy. Ok, for the purpose of arguement, if we assume modding the intake for higher-flow (as in the typical 'better performance' mods) always increases fuel consumption, then what about exhaust mods?

What about modding the exhaust? I know I could get a header and a slighty larger diameter exhaust, complete with mandrel bent pipes and straight-through muffler etc. All else being essentially equal, will modding the exhaust alone produce an increase in fuel-economy? Or is there another approach to exhaust mods that would increase fuel-economy?


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Old 04-07-2010, 09:33 PM   #2 (permalink)
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Well, it kind of depends what KIND OF CAR you have, haha.

Your theory is right on, reducing pumping losses should always lead to better FE. The problem is that if you start fooling the vehicle airflow or O2 sensors, you may end up over fueling for no reason (computer is fooled). Also, slapping a big bore intake and exhaust system could actually reduce volumetric efficiency and thus raise pumping losses at certain / lower rpms. Because engine friction rises with rpm, and fuel conversion efficiency is better at lower rpms, this can mean a loss of mpg in everyday driving. So if you have a N/A car I would avoid a completely free-flowing exhaust. If it didn't hurt your FE, it still sure as heck wouldn't pay for itself in fuel savings.

Now, if your car is turbo / super charged, I would say go for it. They nearly always respond well to bigger pipes.
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Old 04-07-2010, 10:13 PM   #3 (permalink)
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Naturally Aspirated 2001 Corolla CE. 1.8L VVTi and no mods other than a drop-in K & N air-filter, semi-synthetic manual tranny fluid and synthetic engine oil. If I modded the exhaust, it would be a mild mod for smoother flow, not a huge increase in diameter etc. You're right about the increase in size possibly messing with flow and volumetric efficiency at lower RPM's. I'll do some research and plan a mild mod with possibly slightly increased diameter mandrel bent pipes as the main feature for improved flow. I actually wish I could make my car have better MPG on average, but also be able to go fast occasionally, but if that can't be done, I'll lean towards better fuel economy. For a grocery getter, the little Corolla goes fast enough when necessary, which, in reality, is almost never, especially when driving for fuel-economy. I'm glad I found this forum. I feel less crazy now when I see how many KM's I can coast with the engine off....lol...

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Old 04-07-2010, 11:35 PM   #4 (permalink)
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Don't bother with larger diameter pipes. You'll see a drop in low-end power from the reduced flow velocity. You're better off going with a higher-flowing muffler and catylitic converter and leaving the stock pipes in place. That's the setup I have.

As it's been stated, reducing pumping losses helps both power and economy.
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Old 04-07-2010, 11:41 PM   #5 (permalink)
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It's all about math, lucky for you someone figured out the formula for the correct size for you already!
Autolounge.net | Calculators | Exhaust Pipe Sizing
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Old 04-07-2010, 11:55 PM   #6 (permalink)
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Dumb auto engineers must not have access to that formula since they persist in putting too small pipes on everything
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Old 04-07-2010, 11:59 PM   #7 (permalink)
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Quote:
Originally Posted by Ryland View Post
It's all about math, lucky for you someone figured out the formula for the correct size for you already!
Autolounge.net | Calculators | Exhaust Pipe Sizing
That's not a bad site. The figure that I got is almost the same as what is installed in mine.
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Old 05-16-2010, 09:11 PM   #8 (permalink)
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Intake Restriction and Economy

I know exhaust was the main question of the post, but I see this a lot about the intake, and it's rarely really addressed, so I'm moving from lurking to posting around here...

The effects of intake modifications on pumping loss is very commonly misunderstood. The common assumption is that reductions in intake restriction will lead to reduced pumping losses. This is entirely FALSE at all engine loads other than WOT. On the typical NA engine, a certain amount of intake restriction is CRITICAL to running the engine, otherwise you'd be running at full throttle, all the time. In order to decrease power output from WOT at a given RPM, you need to decrease the pressure outside the intake valve, thereby allowing less air and fuel to enter the combustion chamber, which will then produce less power than would be done at WOT. The only remaining way to alter power output without changing pressure at the intake valve is by altering the fuel/air ratio. And since the cylinder/piston and intake valve essentially create one big vacuum pump, the way to reduce the pressure outside the intake valve is to create restriction in the intake path. That is THE function of the wonderful device we call a throttle: creating restriction.
So, say your car runs at 1800rpm and 10% throttle at 55mph. If you then go and modify the intake to be less restrictive, you may now need only 8% throttle to maintain this same condition. And the condition that you are maintaining is a certain pressure outside the intake valve. Therefore, in order to maintain the same output you have not changed pumping losses at all, you've simply altered how much restriction is being created by each individual part of the intake. So you are correct in thinking that decreasing intake restrictions would reduce pumping loss at a given throttle position, but what you need to consider is that a reduction in pumping loss on the intake side means more air is now entering the cylinder, and you're producing more power, using more fuel. So to maintain the same RPM/speed/load etc. as before your intake mods, you need to reduce the throttle opening to get back to the same total restriction you had before the mods. If you apply these principles to the performance end of things, the reason intake mods can improve top-end performance becomes clear. If you can now move the same amount of air at 95% throttle that you used to move at 100%, you now have headroom to increase power. (note: intake modification can improve performance below WOT, however any improvement in performance below the throttle position that moves the same amount of air as the pre-modded WOT will be due to harmonics, not reduced flow-restriction)

I would question the assumption that reduced intake restriction hurts fuel economy. Any reduction in economy with such mods is not directly related to the reduced restriction, since as we see above the TOTAL intake restriction must remain the same at a given load. Other factors come into play though; A) intake mods will often be CAI-related, which may reduce economy. B) Throttle position at load is affected, more importantly, it's reduced. Therefore your accelerator is more sensitive, and you may be giving more effective throttle than you used to be without even realizing it.


As mentioned, pipe size is critical in determining MPG loss/gain with exhaust modification. Proper scavenging lowers the pressure beyond the exhaust valve and can increase efficiency. Improper sizing can have the opposite effect, but it is all dependent on the desired efficiency range in terms of RPM's.
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Old 05-16-2010, 10:43 PM   #9 (permalink)
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^^^ I agree, that is very well said.
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Old 05-16-2010, 10:55 PM   #10 (permalink)
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What you need to consider is how the exhaust pipes are bent. If they are press-bent, then the pipe diameter is reduced (sometimes dramatically). mandrel bent tubing is the best way to go. It keeps pipe diameter constant, which keeps exhaust velocity constant.

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