2005 nissan altima exhaust setup



I have replaced the exhaust with a magnaFlow stainless steel performance cat-black exhaust system. I wanted to increase the performance of the engine without loosing mileage. I would like to know the best way to go to complete the system. what would be the best way to go as far as the catalytic converter to the exhaust manifold? I am currently getting 22mpg highway @ approx. 72 mph. do I just replace the cat or should I replace the cat and exhaust manifold? my check engine light had coded a exhaust problem. I have also installed a typhoon k&n cold air intake filter assembly. the exhaust was original so I figured it was overdo for replacement,but am I on the right track as far what I am doing now? also do I just clean up the sensors on the cat or replace with new? sincerely,backyard mechanic" slim shady"


If you increase performance…you will lose some fuel economy. You can’t have your cake and eat it too.


No real benefit to replacing the cat, and very expensive. There is very little to be gained with bolt on mods. Only a turbo or supercharger would get significant hp, but cost you mpgs.


I’m a big fan of exhaust mods, the right ones, as a way to help performance and mileage. A freer flowing exhaust helps everything, mileage and fuel economy, if you can keep your foot out of it.

The next best thing you can do is a set of long-tube headers to replace the exhaust manifold. They should NOT be racing headers, only use parts certified for street use with emission equipment. Since your convertor (at least one of them), I think, is part of the manifold, your choices may be a little limited.

You should also have the engine control computer re-flashed with a new “tune” to help optimize the changes you’ve made. Anything more is going to require serious parts (read $$$) that will consume much more dollars and fuel.


Your mileage seems a bit low to me,I’ve tried that cold air and free flowing exhaust nonesense-unless the old parts are defective and the manus did a horrible job of putting everything together,theres little to be gained,However I did notice Nissan did change the exhaust configuration on the Altimas(all the newer ones have duals for some reason) Ligenfelter had a thing or two to say about exhaust systems and I defer to Him(theres no replacement for displacement IMO) I expect you are trying to make a hi performance car out of a family car and thats one reason your mileage suffers,let off that throttle and your mileage will increase-Kevin


I think you might well get a little acceleration performance increase from all this. And you might get a little mpg increase. But not at the same time. You’ll have to drive conservatively to get the slight mpg increase that the exhaust mod may afford. And it won’t be much, and almost certainly won’t be cost effective from an mpg angle. Why? B/c if Nissan could have done the same thing, cost effectively, they would have as part of the car design, as cars with higher mpg ratings sell better.

Suggest you re-use the existing exhaust manifold and cat, unless you know of a reason not to.
The experts here say O2 sensors shouldn’t be cleaned. If something is wrong w/them, replace with an OEM part. You say you have a check engine code for the exhaust system. If you post the code, folks here may be able to offer some advice on how to address it. When did this check engine code appear? Immediately after replacing the cold air intake? Nissan designers use wind tunnel and computer modeling technology to design the air intake plenum to work with the air flow sensor so the ECM is able to determine an accurate air flow into the engine, then calculate the injector pulse time. Modifying the cold air intake could possibly confuse the car’s MAF reading, which might show up as a rich or lean condition in the exhaust stream.


It’s also possible that the K&N CAI system contributed to your cat converter failure. The oiled filter can allow oil vapors to be ingested into the engine and burned, sending carbon to coat the cat converter honeycomb’s precious metal coating and make its efficiency drop. You would not be the first to cause premature cat converter failure with an oiled filter.

If you keep the stock airbox and MAF setup, you can still create your own CAI using shop vac hose (or similar) and having it take its air from the outside rather than the fender cavity. I did this to my car, at the same time increasing the intake hose capacity by 87% and incorporating a ram air intake where a fake duct was. I also removed the carbon filter from the induction system. You should have one too, but you won’t find it on the parts list. The EPA requires it (via the DOT) to prevent gas fumes from wafting out the intake when you shut the engine down. There will always be one cylinder in the intake stroke no matter where the engine stops, which means there’ll be an intake valve open and an injector that was squirting. The improvement in passing power on the highway from these changes was surprising. Of course, if I drive aggressively I lose mpg. On a given engine, you gotta use more gas to get more power. Only major modifications, like lightweight pistons, rods, stripping the interior, dumping the dead body in the trunk, and other such costly mods, can alter that. And nobody does these things to get better mileage.


Let my throw something out there thats really strange,my brother was on a mileage quest on his late model Taurus and He changed the stock panel filter with a low restriction K&N or something-an exhaust leak showed up immediatley.Could the increased airflow have had any thing to do with that? A bit puzzling or was it coincidence?-Kevin


It was coincidence. There is absolutely no possible way that increased airflow can cause or even expose and exhaust leak.


Had to be,but weird non the less-Kevin (Actually I think the manifold was actually cracked)