[LettuceB]

I've been considering a downpipe and I have 2 general conclusions from my research...

1. Catless downpipes are going to smell. Given the fact my car already smells a little funky if standing near the exhaust I'd hate to make it worse.
2. Catted downpipes are junk until you pay good $$$ for one. To do it right there has to be adequate quantity of a high quality catalyst... that isn't cheap.

There seems to be a good solution to this issue, however... either add an external or internal cat bypass which opens under increased pressure. This way the exhaust should retain the factory low-odor behavior at idle and still provide better flow for hard acceleration / high-boost scenarios. I would love an internal bypass pipe which could be dropped in a hole drilled through the cat.

So, has anyone attempted this? I'm not a material scientist / mechanical engineer... what sort of pipe material could handle cat temperatures and thermal cycles? Any thoughts on a simple flap / valve inside the pipe that would open under boost? I'm thinking 100% mechanical... it could have loose tolerances and leak at idle so long as the majority of exhaust goes through the cat.

Cheers!
LettuceB

[Phyrexia]

The complexity and cost make it more effective to simply do the labor to swap the DP for a CC when you need to test.

[RSicks]

Nothing like making your O2 sensors freak the f-ck out. You'll be throwing CEL's every time you got on her.

Don't try and reinvent the wheel. Either go catless or go catted.

[LettuceB]

Most new inventions aren't from original thought, they are from reinvention of that proverbial wheel. Whether reinvention is worthwhile for this is hard to say, but blindly saying it isn't without asking the question is not the mentality of an engineer.

I'm not sure the complexity of what I propose is ridiculous... In fact I know it's not. A high flow cat is certainly a simpler option and for a low-cost solution (relatively speaking) it can't be beat today. A catless downpipe cannot be beat performance-wise and is cheaper yet.

For the individual who would like the best of all worlds, though (oem or better cat performance under normal driving conditions, no / lowest smell, high-flow or better performance under acceleration) it would be interesting to explore. As for the O2 sensor, it's just signal gain and interpretation. Pipe design to control flow near the sensor could also assist here.

This is intended to be a theoretical discussion, don't be afraid to open your minds. Because I own a n55 it seems like a good place to start the discussion. Who cares about cost, how easy it is, etc. If it makes sense, offers improvements over the current techniques and doesn't get blocked by those in power, cost and manufacturing issues will work themselves out.

[FastF30]

https://www.ecstuning.com/b-ecs-part...pdk=AAAABgQEAg

[mihail.b]

Originally Posted by LettuceB

Most new inventions aren't from original thought, they are from reinvention of that proverbial wheel. Whether reinvention is worthwhile for this is hard to say, but blindly saying it isn't without asking the question is not the mentality of an engineer.

I'm not sure the complexity of what I propose is ridiculous... In fact I know it's not. A high flow cat is certainly a simpler option and for a low-cost solution (relatively speaking) it can't be beat today. A catless downpipe cannot be beat performance-wise and is cheaper yet.

For the individual who would like the best of all worlds, though (oem or better cat performance under normal driving conditions, no / lowest smell, high-flow or better performance under acceleration) it would be interesting to explore. As for the O2 sensor, it's just signal gain and interpretation. Pipe design to control flow near the sensor could also assist here.

This is intended to be a theoretical discussion, don't be afraid to open your minds. Because I own a n55 it seems like a good place to start the discussion. Who cares about cost, how easy it is, etc. If it makes sense, offers improvements over the current techniques and doesn't get blocked by those in power, cost and manufacturing issues will work themselves out.

I think the initial question you should be asking is whether this idea is something that people will want/need.

No point spending time researching a solution if only a handful of people may want it.

Personally, I think it will be a lot of effort, for little to no gain. I think your o2 sensor will be sending mixed signals to the dme, which sure, you could compensate, although you'd need some sort of logic to tell the computer that system is in cat byoass mode etc

Add to the above the extremely limited space you have to work with in that area. Trying to fit in another 3-4" catless pipe + a mechanical butterfly is gonna be a pain in the ass.

[thejeremyman9]

I have thought about this setup before, mostly in the context of my 3000gt.

In my mind the easiest place to achieve this would be near the back of the car where the exhaust splits, but i have doubt that the cat would ever get hot enough to actually function efficiently. That's why they are as close to the engine as possible. I always envisioned a setup where you would have a cat on the passenger side of the exhaust after the split, and a straight pipe on the other side with the stock valve in place. Catted and quieter in comfort, catless and louder in sport. Easy to achieve but questionably functional.

Another alternative is just go catless downpipe, then install a exhaust cutout under the car, then install a cat right after that in the straight run of exhaust (more like how 90s cars do it). You would then just manually open the cutout when you wanted to be catless or somehow tie it into the sport selection. This would of course bypass all mufflers as well and may be excessively loud. Also probably not worth the expense and effort.

In both of these cases you would get the CEL for no cat.

[RSicks]

Originally Posted by LettuceB

Most new inventions aren't from original thought, they are from reinvention of that proverbial wheel. Whether reinvention is worthwhile for this is hard to say, but blindly saying it isn't without asking the question is not the mentality of an engineer.

Let me help you. There are 2 O2 sensors and upstream and downstream. They work together to make sure that both the engine and catalytic converter (if there) are working correctly. If you were to bypass one or both of these sensors, your car would run like utter shit, throw CEL, and go limp mode.

So I'm not blindly making up things, I am simply saying it will not work.

Engineering: Without a designated piggyback ecu to adjust a total of 4 -O2 sensors - shutting off 2 sensors and immediately turning on the other 2 each time you decide to gun it. Your ecu will see that downtime in between the switch and throw a CEL as it would see this as a major emissions problem. May not be on the first pass, but it will as the emissions systems are very precise.

Mechanically: There is no room near the downpipe to add 2 more O2 sensors let alone a second downpipe. Moving the cat farther back would diminish the output signal to the downstream O2 sensor which would also throw a CEL saying the cat is not functioning properly.

This is the best engineer/mechanical answer I can give you without a full breakdown of what each O2 sensor reads and does and why you do need both even with a catless downpipe.

I hope this helps you understand what I am getting at. I am sorry if you felt I came off short, but a lot of times in forums... people just randomly say ideas without even doing research. I've just came to the point of saying its not going to work.