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Unread 02-22-2007, 05:02 PM   #1
jeepunlimited
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Exhaust leak causes bad gas mileage ?

hey guys i read in another post on jeep forum that exhuast leaks could cause bad gas mileage with a 4.0L ? is this true, i know i have a cracked exhuast manifold...but i dont have the time nor the $$ to fix it right now ? soo if anyone can, can somebody explain how a exhuast leak would cuase bad gas mileage ??

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Unread 02-22-2007, 05:10 PM   #2
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With your vehicle being OBDI (Same basic motor as my YJ), you only have one O2 sensor, which is located before the catalytic converter.

Unfortunately for us with the 4.0, the manifold is most likely to crack where the collector meets, which means the exhaust gases can escape the system before passing by the O2 sensor.

Typically what will happen is the O2 sensor will get a false lean reading and tell the ECU to add more fuel to combustion chamber. This will result in worse gas mileage, and if bad enough can take out the O2 sensor and potentially clog up your catalytic convertor. Of course how long this takes depends on how bad your ECU is compensating for the lost exhaust gases and how long it has been that way.

Most people will recommend that you just use a stock manifold replacement, such as the one found on the upper left corner of this website. I recently changed mine out and went with an aftermarket design manufactured by PaceSetter. I was very happy with the fit and quality of the product I received, plus the price was right as well (compared to other aftermarket manifolds).

Good luck!
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Unread 02-22-2007, 05:11 PM   #3
ras2001wj
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I would say exact opposite, exhaust leak relieves back pressure allowing for better flow and thus saving a few mpg's, i mean by all means correct me if i'm wrong...., but i would be careful with exhaust gasses leaking, dont poision yourself

Edit- ths post was being typed before the above was entered, and i didnt think of it that way.
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Unread 02-22-2007, 05:19 PM   #4
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Quote:
Originally Posted by ras2001wj
I would say exact opposite, exhaust leak relieves back pressure allowing for better flow and thus saving a few mpg's, i mean by all means correct me if i'm wrong...., but i would be careful with exhaust gasses leaking, dont poision yourself

Edit- ths post was being typed before the above was entered, and i didnt think of it that way.
Yeah, another thing to consider is that your exhaust system requires a certain amount of backpressure to function correctly. Given, too much backpressure is a bad thing and will rob you of power but so will too little.
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Unread 02-22-2007, 05:37 PM   #5
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I've run a vehicle with open headers, it ran like a bat and was loud as all get out. I then had to add mufflers (legal reasons) and for fun I timed my 0-60 and quarter times both with pipes and without. With pipes I was faster. Open headers will acutally decrease your torque as your valves require some backpressure to help them seat correctly. Without it, your engine may not run efficiently.

Racing engines are designed to be run with open exhausts so they put stronger valve springs in, plus they aren't meant to last 250K.

Removing your exhaust may sound cool, but check your times or dyno your rig and you will find that you actually loose.
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Unread 02-22-2007, 05:50 PM   #6
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Quote:
Originally Posted by Woody
I've run a vehicle with open headers, it ran like a bat and was loud as all get out. I then had to add mufflers (legal reasons) and for fun I timed my 0-60 and quarter times both with pipes and without. With pipes I was faster. Open headers will acutally decrease your torque as your valves require some backpressure to help them seat correctly. Without it, your engine may not run efficiently.

Racing engines are designed to be run with open exhausts so they put stronger valve springs in, plus they aren't meant to last 250K.

Removing your exhaust may sound cool, but check your times or dyno your rig and you will find that you actually loose.

Yeah......that's what I meant
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Unread 02-22-2007, 07:06 PM   #7
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you would have to change your valve clearances for it to run right and who the heck would want to deal with that
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Unread 02-22-2007, 07:50 PM   #8
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The way I have been taught is: engines need no back pressure per say. There is no back pressure on many engines (race vehicles, lawn mowers, etc)

But because of noise, emissions, and intoxication a long pipe is needed for the Cat, muffler, and to release the gases away from under the vehicle. This presents a dynamic flow issue. The engine emits exhaust flow in pulses (due to the nature of a very inefficient combustion engine). It is these pulses that create an issue as you want a smooth fast flow rate. If the lag time between each wave front of the exhaust is increased then the wave fronts catch up and collide; and then the flow rate is slowed down. Therefore it is critical for an exact pipe diameter to be selected. If a 4 inch exhaust pipe is use on any regular motor vehicle engine, then the flow rate will be decreased as the wave fronts will always be crashing into one another, with each exhaust stroke.

If you remove everything from the throttle plate forward (no intake plastic) and the entire exhaust system, then I believe one would have the maximum capacity of the engine.
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Unread 02-22-2007, 07:55 PM   #9
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Quote:
Originally Posted by greasefingers
The way I have been taught is: engines need no back pressure per say. There is no back pressure on many engines (race vehicles, lawn mowers, etc)

But because of noise, emissions, and intoxication a long pipe is needed for the Cat, muffler, and to release the gases away from under the vehicle. This presents a dynamic flow issue. The engine emits exhaust flow in pulses (due to the nature of a very inefficient combustion engine). It is these pulses that create an issue as you want a smooth fast flow rate. If the lag time between each wave front of the exhaust is increased then the wave fronts catch up and collide; and then the flow rate is slowed down. Therefore it is critical for an exact pipe diameter to be selected. If a 4 inch exhaust pipe is use on any regular motor vehicle engine, then the flow rate will be decreased as the wave fronts will always be crashing into one another, with each exhaust stroke.

If you remove everything from the throttle plate forward (no intake plastic) and the entire exhaust system, then I believe one would have the maximum capacity of the engine.
That's theoretically true, but that isn't how vehicles are designed. Like Woody said, some backpressure is necessary to make the valves seal correctly.
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Unread 02-22-2007, 08:33 PM   #10
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I am not sure that I believe that. If I was the engineer, then spring pressure should be correct for that.
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Unread 02-22-2007, 08:41 PM   #11
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Quote:
Originally Posted by greasefingers
I am not sure that I believe that. If I was the engineer, then spring pressure should be correct for that.
No offense, and I don't want to get in a pissing match over it, but are you an engineer and did you design it? I'm not saying that I designed it but being a Mech. Eng. I have pretty decent idea of how things are designed and function.
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Unread 02-22-2007, 08:43 PM   #12
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Quote:
Originally Posted by greasefingers
I am not sure that I believe that. If I was the engineer, then spring pressure should be correct for that.

As true as that may be, you also need and exhaust system to aide in scavaging of the combustion chamber (this is another affect from the engine pulses as well) and the extraction of heat.

You would need a lot of air flow around the motor itself to keep from burning up exhaust valves from excess heat in the engine bay or on the dyno stand.
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Unread 02-22-2007, 08:45 PM   #13
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Quote:
Originally Posted by ajmorell
No offense, and I don't want to get in a pissing match over it, but are you an engineer and did you design it? I'm not saying that I designed it but being a Mech. Eng. I have pretty decent idea of how things are designed and function.
No offense taken. I have a BSME but I am not an automotive engineer. I work with chemists.
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Unread 02-22-2007, 08:53 PM   #14
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Quote:
Originally Posted by greasefingers
No offense taken. I have a BSME but I am not an automotive engineer. I work with chemists.

Fair enough. Like I said not trying to get in a pissing match and I respect your opinion and thoughts, but I just think that they heads and valves were engineering to flow a certain way and that the rest of the system was taken into account.
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Unread 02-22-2007, 09:30 PM   #15
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Let me see if I have this. Valve spring causes the valve to close. Back pressure tries to keep the valve open. Some how back pressure is needed to reduce the spring force so that the valve closes properly. Can someone explain how that works? Especially since back pressure can only be a few psi.

BTW, you don't have to post a copy your diploma before you answer.
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