[quote=“andrew”]
What do we need clean air for?
:rolleyes:
old cats fall apart like that and partially melt …so replace it or remove it …an old one is shit , do something about it either way , thanks all I have to say about it
I remember having the hugest problem on my friends Cefiro when his 3-1/4 racing cat fell apart and created a total blockage in his exhaust pipe and we had to find a way to remove it all and now things are better than before without it ,
What are you talking about “all the backpressure you NEED”!? This whole discussion is about exhaust flow rate and not backpressure…read the article I posted. I am definitely not an expert but I trust Mike Kojima’s opinion.[/quote]
If your running a test pipe that is the same diameter as the rest of your exhaust there will be no drop in exhaust flow rates. Your would be hard pressed to find any race car that can get away with running a straight pipe deciding to run a cat just to achieve some kind of gain in this area. Pull your shirt up to your mouth and take a deep breath. pull your shirt down and do the same. Which is easier. Maybe I will add this to my list of things to test on the dyno this summer. I am a big fan of actually testing things, mostly because I don’t believe stuff that can’t be backed up with math. I would also like to point out that a lot of people end up running a exhaust system that is bigger then they should be. Which is where a lot of the problems come from. Blaming it the cat removal seems to be a way to make them feel better about the money they spent on their overly expensive incorrectly matched catback system. No, I don’t write for a magazine, and I know that because you read that in a published document it gets more credibility then me posting on the Internet. But realistically pretty much all those guys writing for magazines are just tuners like us, with cooler jobs. I have found errors in a couple magazines where the author made a assumption that was wrong based on what they were seeing on a dyno.
Exhaust choice just like pretty much anything else you buy for your car should be based on the amount of air your engine is capable of pumping and where you want to make the most power.
I need to get a larger diameter exhaust on my silvia … I need 90mm as my exhaust system is chocking right now with 80mm with the turbo setup I have on my Silvia , I see people running 80mm on their NA 240sx and that is overkill but sometimes there is huge gains when you open up the ehaust and sometimes you loose alot of low end torque , someone correct me if I am wrong
You are absolutely correct, but don’t confuse backpressure and flow rate.
I don’t believe everything I read but Kojima does have some damn good credentials. I agree with you 100% that testing is the way to go and that if the math doesn’t lie then one knows the change is a good one.
I will readily admit that you likely know far more about this than I do. All I was trying to say to the guy that asked for advice initially was not to simply gut his cat and expect more hp and I referenced the article as a source to learn more about it.
Well where do you want to make your power. What your saying is accurate, depending on what your going to call a huge gain. Most street cars don’t see really high RPM operation very often so you would likley be better off with a smaller exahust system because the car would opperate better and me more enjoyable to drive with the extra midrange torque. If your planning to roadrace or drift you car which can mean a lot more hi rpm you would likley benift from having a more open system on the car. Also like I said before, when your car is turbocharged the turbo itself provides back pressure so going to a 3 inch exhaust is often a good idea because you will see a large drop in EGT.
so to clarify, there is no O2 sensor positioned at the cat, therefore nothing to give me any problems with mixed signals to the ecu? I’ve made up my mind on how i’m going to get rid of the cat converter, i just want to know if i’m going to need to deal with any sensors.
It shouldn’t change anything if your car is OBD1, realistically the cars that usually throw codes as a result of cat removal are all OBDII or newer. Your car will become considerably louder as a result of cat removal also. I have found that pretty much every Honda that is running no cat will sound raspy on the top end… I don’t know if this is true for Nissan’s as well but I would guess its likely.
MyGTR,
If your looking to improve flow rate and have a aftermarket exhaust system its always a good idea to take your header/turbo manifold & Downpipe and get a pencil grinder or cone stone and remove any excess weld or casting marks from the inside. You likely know this all ready but many people don’t even think about it. A lot of the times cheaper aftermarket products are actually based (measurement wise) on a better brands design. The only difference comes from ether higher quality materials or in most cases just someone doing clean up after the part is produced. Spending a half hour could net you up to 5hp… its not much but that’s the kind of thing that allows certain people to out compete everyone else. :thumb
uhhhhhh Thanks but I wasn’t asking for any advice
My understanding of the mechansim for low end torque loss with big pipes is that lower exhaust gas velocities decrease scavenging efficiency. More burned gasses are left in the cylinder and there’s less room for fresh air and fuel.
Because most exhaust systems are of a fixed flow resistance, back pressure typically goes up with increase gas velocity so it looks like back pressure is helping you.
I suspect this is only true at low revs. When things really get screaming you’ve got all the exhaust gas velocity you need and less backpressure means more power.
I want a variable diameter exhaust manifold!
The R34 GTR had a silencer that closed a port at lower revs and opened up as exhaust flow increased. I can’t see this increasing exhaust gas velocity so my understanding may be flawed. :?
I’d love to see dyno curves for cat vs no cat. (I hate cats, but I like clean air.)
Dan