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Discussion Starter #1
Is it a fair statement to say that intake mods generally increase mileage and exhaust mods decrease mileage? If not, why?

Shawn
 

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Any mod that decreases "pumping losses" will increase fuel economy. There is a certain point where some exhaust backpressure is good for fuel economy, and so you can go too far. I am starting to think that my MBRP exhaust that deleted the 2nd cat did not provide the positive effect on my fuel economy that I was looking for. I don't know the exact reason why- and it could be due to a lack of sufficient backpressure to give the 2nd O2 sensor a correct reading. A low reading would cause the ECU to think that it needed to increase the fuel air ratio. You are probably experiencing the same issue, if you have an Xcal, you might be able to get a tune that uses only the upstream 02 sensor for ECU calculations and that might solve the issue.

Another reason exhaust mods would decrease fuel economy would be due to your foot getting heavier so you can make that cool sound. [race]
 

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whynotthinkwhynot said:
. I don't know the exact reason why- and it could be due to a lack of sufficient backpressure to give the 2nd O2 sensor a correct reading. A low reading would cause the ECU to think that it needed to increase the fuel air ratio.
The o2 sensors dont read back PSI they read AF before and after the CAT , you should have the rear o2`s turned off if you have a tune , the 2.5 and the 3 inch with mods both do great for power on the Dura engine low and high in the RPM

Tom
 

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I get 35mpg on the highway in my car.

Any mod that makes more torque at your cruising speed lets you use less throttle.
 

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1turbofocus said:
The o2 sensors dont read back PSI they read AF before and after the CAT , you should have the rear o2`s turned off if you have a tune , the 2.5 and the 3 inch with mods both do great for power on the Dura engine low and high in the RPM

Tom
I don't know what I was thinking, perhaps that opening the exhaust up and removing the 2nd cat was not giving enough time for a complete reaction to happen at the 2nd O2 sensor, and so that sensor was giving was producing less voltage than it should be for what was actually in the exhaust.

Lower temperature exhaust gasses would do that too, since I think the heaters on those sensors only work during start-up.

Anyway, I'll see about getting a tune to delete the 2nd O2 sensor from the loop.

Thanks
Alex
 

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Discussion Starter #6
1turbofocus said:
The o2 sensors dont read back PSI they read AF before and after the CAT , you should have the rear o2`s turned off if you have a tune , the 2.5 and the 3 inch with mods both do great for power on the Dura engine low and high in the RPM

Tom
Are both the O2 sensors before the flex pipe? I did not install my exhaust. I have really liked the increase in power that the 2.25 MBRP exhaust gave me and will will not be taking the car past exhaust, intake, TB, Xcal2 tune and stage 1 cams. Did I make the right choice with the MBRP and should I have the rear sensor turned off?
 

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Yes both the sensors are before the the flex pipe on a D20. One is just before the manifold cat where the 4 primaries come together, and the other is just after the manifold cat and just before the flex.

Just so you'd know, our exhaust does not have a seperate flex pipe like the early D23. There is no flange, and it does not bolt off. The PZEV motors, even the new ones, have a seperate flex pipe- and much larger manifold cat.

Look under your car to see how the exhaust was installed. It is possible that the person who installed the exhaust did not remove the 2nd cat due to regulations. I'd think they would've given you the resonator back- but maybe they didn't so you wouldn't ask "Why wasn't this installed?" The MBRP pipe should be nice and shiny. The resonator section should be visible in the center of the car if you can stick your head under there. I have a little head with a dinosaur brain so I can do it. You'll notice a flange in line with the front door hinges at the center of the floor, behind the flange is where the resonator should start. If they flipped the resonator to the back of that tube and installed the factory cat behind the flange- you should see two humps- one shiny, one not. The resonator is long, about 20 inches, and the cat is only 10 inches.

Do you have an SCT XCal2? I do, but no tune to over-ride the 2nd O2 sensor.
 

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Discussion Starter #8
whynotthinkwhynot said:
Yes both the sensors are before the the flex pipe on a D20. One is just before the manifold cat where the 4 primaries come together, and the other is just after the manifold cat and just before the flex.

Just so you'd know, our exhaust does not have a seperate flex pipe like the early D23. There is no flange, and it does not bolt off. The PZEV motors, even the new ones, have a seperate flex pipe- and much larger manifold cat.

Look under your car to see how the exhaust was installed. It is possible that the person who installed the exhaust did not remove the 2nd cat due to regulations. I'd think they would've given you the resonator back- but maybe they didn't so you wouldn't ask "Why wasn't this installed?" The MBRP pipe should be nice and shiny. The resonator section should be visible in the center of the car if you can stick your head under there. I have a little head with a dinosaur brain so I can do it. You'll notice a flange in line with the front door hinges at the center of the floor, behind the flange is where the resonator should start. If they flipped the resonator to the back of that tube and installed the factory cat behind the flange- you should see two humps- one shiny, one not. The resonator is long, about 20 inches, and the cat is only 10 inches.

Do you have an SCT XCal2? I do, but no tune to over-ride the 2nd O2 sensor.
I live in Toronto, Canada and had the MBRP put in at Canadian Tire. Before they took it off the lift they let me walk under the car and have a look. They did not alter the MBRP at all and it starts at the flex and goes striaght back. As well they gave me my old exhaust system back seeing as it is a brand new car and the stock exhaust is only 5 months old.

I do not have an Xcal 2 yet. How did you like it and did you notice a large difference in power?
 

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Yes I like the XC2, and it improved my fuel economy by 2-3 mpg, as well as made the car much easier to drive in top gear at lower speeds. There will be less of a need to shift into 4th to make quick highway passes. It also gives you the ability to take your car to a tuner with a dyno and figure out what the best map is for your set-up. It's really a must have if you're going to do any future modding in the way of engine internals like the cams you listed.

I would definately get one if you're planning on doing more mods in the future. You should also consider locating a local tuner who can tune your car on a dyno for maximum effect. Dyno time can be pricey, so wait until you have your cams if you intend to install cams. It's not something you would want to do twice. Sure, a generalized tune can be purchased for your motor with those cams, but that tune wouldn't be as agressive as a tune that was done with your car on the dyno.
 

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Discussion Starter #10
whynotthinkwhynot said:
Yes I like the XC2, and it improved my fuel economy by 2-3 mpg, as well as made the car much easier to drive in top gear at lower speeds. There will be less of a need to shift into 4th to make quick highway passes. It also gives you the ability to take your car to a tuner with a dyno and figure out what the best map is for your set-up. It's really a must have if you're going to do any future modding in the way of engine internals like the cams you listed.

I would definately get one if you're planning on doing more mods in the future. You should also consider locating a local tuner who can tune your car on a dyno for maximum effect. Dyno time can be pricey, so wait until you have your cams if you intend to install cams. It's not something you would want to do twice. Sure, a generalized tune can be purchased for your motor with those cams, but that tune wouldn't be as agressive as a tune that was done with your car on the dyno.
I see, so basically what you are saying is a tune will smooth things out and give me a bit more grunt down below around 2800-3500rpms? That sounds fantastic! I'll definatly pick up a Xcal ASAP! and then get the car professionally done on a dyno somethime very soon after I put the cams in. This sounds like a plan![thankyou]
 
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