My mpg went down after I removed catalytic convert

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Hey guys I had a custom exhaust done on my car and the cat was removed in place of straight pipe and I noticed my mpg has gone down why is this? 88 nissan 200sx 3.0.
 
Your car probably needs back pressure to run like it was designed. What were you trying to accomplish by removing it?
 
Originally Posted By: boostedtsiawd
Hey guys I had a custom exhaust done on my car and the cat was removed in place of straight pipe and I noticed my mpg has gone down why is this? 88 nissan 200sx 3.0.


Does that car have a carb?
 
Its like how a vehicle consumes more fuel when there is a hole in the muffler,you have created a "void" in the system,possibly even hampering low end torque,the throttle is making up for it.That and the higher flow rate is causing new fuel to fill the cylinder completely as the exhaust is all "gone",little residual burn left behind.
 
Originally Posted By: NHGUY
Its like how a vehicle consumes more fuel when there is a hole in the muffler,you have created a "void" in the system,possibly even hampering low end torque,the throttle is making up for it.That and the higher flow rate is causing new fuel to fill the cylinder completely as the exhaust is all "gone",little residual burn left behind.

Would that be an issue with a newer OBDII vehicle or would the computer compensate based on O2 sensor readings?
 
ODB II systems have O2 sensors after the cats in addition to the O2 sensors before the cats to ensure that the cats are functioning and cleaning up the exhaust. if you eliminate the cats on an OBD II system you have to get creative to keep the MIL off. additionally, you would not pass emission inspection around here.

and while all ODB systems (ok, FI systems) can compensate for changes in backpressure to keep the fuel to air ratio correct, they cannot modify the torque curve unless they also have control over valve timing and intake runners.
 
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Without the right amount of backpressure your EGR system doesn't work right and without the right amount of EGR you knock, pull timing, and lose thermal efficiency.

You can also get "scavenging" with a properly tuned exhaust system, though that's usually more at the headers, but anything's possible to screw up.
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i noticed long time ago that if you have exhaust leak, the engine power noticeably goes down. it least in carburated cars. i haven't had exhaust leak in more recent cars, though.
 
Are you sure it's not your ears deceiving you? If the engine is making more noise but producing the same amount of power, you'll get the impression that engine power is down when in actuality it's the same just producing more noise.
 
Originally Posted By: tomcat27
ODB II systems have O2 sensors after the cats in addition to the O2 sensors before the cats to ensure that the cats are functioning and cleaning up the exhaust. if you eliminate the cats on an OBD II system you have to get creative to keep the MIL off. additionally, you would not pass emission inspection around here.

and while all ODB systems (ok, FI systems) can compensate for changes in backpressure to keep the fuel to air ratio correct, they cannot modify the torque curve unless they also have control over valve timing and intake runners.



The post cat O2 sensors are only to monitor cat is functioning and doesn't affect fueling whatsoever. A tuner will turn them off eliminating the cel.
 
Originally Posted By: Clevy
Originally Posted By: tomcat27
ODB II systems have O2 sensors after the cats in addition to the O2 sensors before the cats to ensure that the cats are functioning and cleaning up the exhaust. if you eliminate the cats on an OBD II system you have to get creative to keep the MIL off. additionally, you would not pass emission inspection around here.

and while all ODB systems (ok, FI systems) can compensate for changes in backpressure to keep the fuel to air ratio correct, they cannot modify the torque curve unless they also have control over valve timing and intake runners.



The post cat O2 sensors are only to monitor cat is functioning and doesn't affect fueling whatsoever. A tuner will turn them off eliminating the cel.


Yup, Ford even calls them "cat monitors" on the parts end of things. You can also stack 2 spark plugs non foulers to fool the sensors.
 
You should have put in a Race Cat from Magnaflow. If you pulled the cat, the system senses increased flow thus increased fuel. Always modify after the cat unless race only is your only worry.
 
Originally Posted By: Clevy
Originally Posted By: tomcat27
ODB II systems have O2 sensors after the cats in addition to the O2 sensors before the cats to ensure that the cats are functioning and cleaning up the exhaust. if you eliminate the cats on an OBD II system you have to get creative to keep the MIL off. additionally, you would not pass emission inspection around here.

and while all ODB systems (ok, FI systems) can compensate for changes in backpressure to keep the fuel to air ratio correct, they cannot modify the torque curve unless they also have control over valve timing and intake runners.



The post cat O2 sensors are only to monitor cat is functioning and doesn't affect fueling whatsoever. A tuner will turn them off eliminating the cel.


On the OP's car, likely, but not every car. On my previous STI, the fueling tables were still affected by the post-cat 02 sensor, IIRC. I was actually pretty surprised to see this.
 
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