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Old 02-07-2007, 11:11 AM   #1
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Question 13% mileage increase by relieving exhaust back-pressure?

I was looking up my car on www.cartalk.com and found this to be interesting:

Quote:
February 2005

Dear Tom and Ray:

I have a '95 Geo Metro with a 1.0-liter engine and a five-speed transmission. I recently took my car four-wheeling and broke a few things. OK, OK, I broke a lot of things -- the catalytic converter being one of them. I have no idea where it is now; it's just gone. Anyhow, when the converter was there, I got 52 mpg, but now I'm getting just over 60 mpg! I want to know why I'm getting better mileage. And would I be evil if I just never replaced the converter? (I have a straight pipe in there now, all the way to the back.) Thanks. -- Steve

RAY: The reason you get better mileage is because you've eliminated most of the "back pressure" from your exhaust system. In addition to protecting us from air pollution (converter) and noise pollution (muffler), those two devices also restrict your exhaust to a certain degree. And that decreases your mileage. It's the price we pay for what we call civilization, Steve. You should consider joining it sometime.

TOM: But the back pressure also protects your valves. Without sufficient back pressure, the hot exhaust will blow past your engine's valves so quickly that it'll burn them up, and eventually you'll need to replace them. Let's do the math.

RAY: You're getting an extra 8 miles per gallon. Which, over the course of 12,000 miles, saves you about 30 gallons of gas. So, without the converter and muffler, you save $60 a year.

TOM: A valve job on this car would probably run you about $600.
I took off my Fiero's catalytic converter way back in 2003 and found a noticeable increase in acceleration (with no noticeable noise increase), but I never would've thought that it could increase FE. However, if it would make my engine wear out faster maybe it's not such a good idea.

Still, it's something of interest for those of us who never have to pass an emissions test.
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Old 02-07-2007, 11:25 AM   #2
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Whether or not we have to pass an emissions test, I think we should all carefully consider our roles as citizens of a world, with a fragile environment in need of our protection.

But yakno, whatever. Anyway, his results are most likely BS, I added a second cat to my car and saw no difference, actually, there was an improvement, but I realized it had nothing to do with the cat, or at least, nothing quantifiable.
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Old 02-07-2007, 11:30 AM   #3
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Originally Posted by SVOboy View Post
Anyway, his results are most likely BS, I added a second cat to my car and saw no difference, actually, there was an improvement, but I realized it had nothing to do with the cat, or at least, nothing quantifiable.
I was thinking of that too. I never had a real noticeable gain in FE in my Fiero (granted I wasn't as precise in measuring FE then). I looked at other threads on this site and it seems that elimination of back pressure can hurt FE.
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Old 02-07-2007, 11:32 AM   #4
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If the cat was clogged or burned out any vehicle would see a gain when the cat was removed.
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Old 02-07-2007, 12:08 PM   #5
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Erm, dunno about the valve thing. I don't think dirt-trackers have this kind of problem. The exhaust flow is moving plenty fast and plenty hot. Add one more reason to the "engines need back pressure to work right" myth...

Possible the cat was partially clogged, and I'm certain that the stock exhaust was at least a size or two less than the car would really like and with cruddy bends. A new exhaust with larger tubing from the engine back and a hi-flow cat would likely keep most of the mileage gain.
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Old 02-07-2007, 12:39 PM   #6
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I'm just confused about MPG and exaust restriction. I know that some racers remove the exaust system to improve power, but more power doesn't neccessarily mean more MPG. Or does it? I'd sure like to read more about it.
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Old 02-07-2007, 01:00 PM   #7
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Engine RPM range

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Originally Posted by Peakster View Post
I'm just confused about MPG and exaust restriction. I know that some racers remove the exaust system to improve power, but more power doesn't neccessarily mean more MPG. Or does it? I'd sure like to read more about it.
From the trends observed, it seems like engine RPM has a lot to do with backpressure and FE: with the racer -- during high RPM operation, less restriction is required for maximum power...but for FE, the best economy is attained at lower engine speeds -- which typically need a good dose of back-pressure to operate efficiently, and consequently a bit more restriction.

Each vehicle's exhaust requirements are so unique, that it would be hard to make a generalization. In this department (since I haven't found, or performed an experiment with this regard) I would be inclined to stick with the stock dimensions.

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Old 02-07-2007, 03:01 PM   #8
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rh77 -

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Originally Posted by rh77 View Post
From the trends observed, it seems like engine RPM has a lot to do with backpressure and FE: with the racer -- during high RPM operation, less restriction is required for maximum power...but for FE, the best economy is attained at lower engine speeds -- which typically need a good dose of back-pressure to operate efficiently, and consequently a bit more restriction.

Each vehicle's exhaust requirements are so unique, that it would be hard to make a generalization. In this department (since I haven't found, or performed an experiment with this regard) I would be inclined to stick with the stock dimensions.

RH77
Does a car want the same back pressure across all RPM/MPH combos for good MPG, or are there "different restrictions for different situations" for good MPG?

I was thinking that maybe one good experiment would be to progressively restrict the tailpipe outlet by attaching smaller and smaller conical hi-temp steel tubes onto the exhaust pipe and seeing what happens to MPG. Even better for a flexible experiment would be an "exhaust choke" dial on the dashboard that would control a hi-temp butterfly valve in the exhaust pipe.

I am only talking about tailpipe exhaust narrowing because that is the easiest mod I can think of.

Here are some gizmos that claim to do something in the exhaust area, but I think they have been shot down :

The Infamous Turbolator
http://www.jcwhitney.com/autoparts/P...001736/c-10101
http://www.performancepeddler.com/se...tor&x=0&y=0#a3
... and it's bastard son, the MINI EXHAUSTONATOR
http://www.jcwhitney.com/autoparts/P...haust+tip+MOpG

... hmmmm, maybe a powered Turbo-Mini-Exhaustinator?

YL(aughter)MV

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Old 02-07-2007, 03:47 PM   #9
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What theclencher said. There are situations, such as high output modded engines that can benefit from open exhausts, but even then, high flow cats and mufflers are made, and are usually suitable. In the past a manufacturer would tune an exhaust such that it would resonate at some rpm, generally at around peak power, because it would help volumetric efficiency and boost peak power output a wee bit. What happens is that at certain engine speeds, depending on the design of the exhaust, the pulse from a previous exhaust valve opening will bounce back from the muffler and reinforce the pulse from the next valve opening, which would provide better scavenging (pulling the exhaust gas out of the cylinder), and the equivalent of a couple psi of boost compared to the usual scavenging. If we ever hear muscle car enthusiasts talking about matching the intake and exhaust, this is what they mean. An exhaust can have this scavenging effect at some rpm depending on how it's designed, and longer runners on the intake help out with low end power, while shorter help with high end power. If we have a long runner intake with an exhaust that'll scavenge best at a high rpm, the engine will be kinda flat. There are also butterflies on the exhaust, and intake runners with different lengths that open up depending on rpm. Newer cars have variable valve timing, and can fiddle with the timing so that scavenging occurs at a much wider range of engine speeds. This, combined with drive by wire, aka lots of throttle tip in, helps out low rpm efficiency w/o any change needed on the driver's end.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 02-07-2007, 04:00 PM   #10
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Hello -

Based on theclencher's post, I was googling "car throttled exhaust system" and found this :

TIGERS: Exhaust Gas to Electricity for Reductions in Fuel Consumption
http://www.greencarcongress.com/2005..._exhaust_.html

I am sure we have talked about this before, but it seems germane to the thread in a tangential sort of way.

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