westpak

iTrader: (19)

I agree, even if it is just 2-3 HP but you get the sound then I would spend 150 bucks over the 900-2000 for just 6-8HP, because at this point I don't think I will spend the money for the exhaust, not enough HP there and I don't think there will be much more, if they surprise me and they come up with 12-15 then I will rethink it.

If you do it, would you mind doing dyno after, I will chip in just to find out, although I wuould prefer to go the bolt on route.

NittoZ

I replaced it with a straight pipe and it sounds like A$$! It is very raspy and definitely louder. May just be me, but it was faster with the pre silencer on.

NittoZ

Apexi350z

iTrader: (3)

Yep, I went to a shop yesterday, cut off the pre-silencer, and replaced it with a straight pipe, it sounds more higher pitch, and it's also has more raspy sound around 3500rpm. I don't like it, going to weld the pre-silencer backon. I think I will wait for an aftermarket exhaust system.

It costed me $45.. Oh well, good experiment..

Them Bones

Dude, are you on crack? Cats do not store polution. What do you think is in there, a big sponge that absorbs smog? Cats attempt to fully burn the unburnt fuel that is exiting your engine. The byproducts of completely burned gasoline are carbon dioxide and water, both relatively harmless.

AFAIK, mercury is not involved at all in the process of burning gasoline. Lead was a major concern before the switch to unleaded gas.

Moral of the story - Leave the cats alone. They are not robbing you of anything, except pollution.

-TB

Boomer

I agree TB, I'm not too fond of having posters on this site describe their illegal activities as if it should be every day business. I sure don't support it! I think an addition to the rules is in order.

Boomer

^Tyr

um in indiana we have no smog / emissions tests period.. so removing the cats is not illegal in anyway as long as your exhuast doesnt go above X dB.

what i'd try instead is removing the cats and replacing w/ straightpipes and leave the silencer in ... the silencer is how they "tune" yoru exhaust pitch and get rid of the annoying irregularities such as teh raspiness and high pitch sound you heard.. leave it in there... the cats however do impede the flow of gas slightly because they promote a temperature change which changes the speed at which gasses move causing turbulence..

also for those of you who welded the pipe in.. heres your problem.. now that you cut and welded, thers not going to be a smooth surface on the internals of the pipe anymore, those slight disturbances actually create a lot of turbulence and will also impeded exhaust flow... i know a mufflershop that'll mandrel bend and weld on flanges and such.. replacements for cats w/ flanges usually run about 45-60 bucks depending on type of flange etc. also pick up the correct gaskets :P

Them Bones

Just because you won't get caught doesn't mean it's not illegal. Everything I've read says that tampering with a vehicle's emission controls is illegal, not to mention there's nothing to be gained from it on a newer car.

Buy high-flow cats if you want, but spare the rest of us the smog by not removing them altogether.

-TB

^Tyr

well,, there are so many cars on the road around here that just leave clouds let it be oil burning whatever,, i really doubt the already fairly clean emmissions from the z will hurt much,, but ok.. 5 hp doesnt meant hat much to me anyways.. i was just trying to state that in some states it is not illegal. or at least i guess from what you say there is not active monitoring or ticketing of people who do not follow the rules.

but in indiana there are no checkups of any kind and i've never heard of nor seen anyone ever pulled over or ticketed for bad emissions, granted im 1 person, but i have plenty of friends w/ cars that would be ticketed 5 miles into the border of the states you guys live in

plus since they would be bolted up w/ flanges and all, anyone w/ a lift could change them out in 15 min tops,, why wouldnt that be a good idea ofr track days and such?

roark

iTrader: (2)

Did any of you guys dyno your cars before/after you pulled the pre-silencer? I'm not only interested in peak HP gains, but the pre-silencer should also work as a resonator. If that is the case, you would see a HP dip in the 900-1400 RPM range.

kurt leonce

one thing you can do is take out the cat and bore it out and put just the shell of the cat back up, i did it you do feel the power

TRACK350z

I did that on my turbo Z (gutted cats). It did improve HP!!

But thats a turbo car!

I thought naturally aspirated cars NEED a certain ammount of back pressure, which the cats, pre silencer and muffler provide?

Any experts know if this is true?

Them Bones

I'm not an expert, but I know this to be true. The size of the stock exhaust is chosen very carefully. A correctly sized pipe will create a scavenging effect. As the exhaust pulse travels down the pipe away from the engine, it tends to create a vacuum when the next exhaust valve opens that helps clear the cylinder of the exhaust gasses. This HELPS power. If the pipe is too big, no vacuum effect. You don't get the same effect on a turbo car because the engine has to force the exhaust gasses through the turbine. You can generally go crazy with the pipe size on a turbo car. You have to maintain a careful balance on a NA car, though. After you've done enough mods on the intake side, you will have to increase exhaust capacity. Exhaust mods alone usually hurt power, unless the factory system is really crappy. Since we know Nissan's isn't, just leave it alone. Any additional flow you need as a result of non-FI intake mods can probably be had with a cat-back swap. Leave the friggin' cats alone. We'll all breathe easier.

dwnshift

iTrader: (1)

You guys have to look at the system as a whole. Throwing a $1500 exhaust system is going to just make your wallet lighter.
Step back and look at the whole picture. The presilencer is the least of this guys worries.
hint .................look at the T pipe................

^Tyr

um.. thembones,,, you do NOT want backpressure in an exhaust system.. what you are talking about is the scavanging effect the headers create from the exhaust pulses from different cylinders.. granted in the headers you will have slight backpressure.. but if you are talking about after headers,, you dont want backpressure,,

PistolPete

I might be on crack, but that has nothing to do with my argument.

Seriously, though, don't throw around accusations. It's not nice.

Do a bit of research. Howstuffworks.com has a simplified write up on cats and how they work. Catalytic converters do not burn fuel. They use a catalyst (hence the name) to chemically change some pollutants created by gasoline combustion. This catalyst (there are actually more than 1) is a chemical which is applied to screens inside the cat itself. The chemical reaction has byproducts which adhere to screens and the inside of the converter. When the converter is disposed, this contaminants can leach into the soil, and then groundwater.

Cats are a decent pollution control. My point was they are far from perfect, and needed less in computer controlled engines than they were in the past. They are definately restrictors to exhaust flow. Not every state currently requires them.

The best possible solution is for everybody to ride motorcycles. Smaller displacement means less pollution.

^Tyr

motocycles w/ fuel injection maybe.. but i can guaruntee that my cbr600f2 makes more pollution than a 350z does even missing a cat or 2... cbr runs fine (well its drained for winter) but maybe i'm wrong ... but i think more control over the combustion etc. is what really lowers emissions

Boomer

Yeah, or snowmobiles. The pollution level in our Nat'l Parks is in the toilet because of all the non-smog free snowmobiles. Rangers in the park are wearing respirators to be able to breathe. By all means, lets all drive as many vehicles as possible without smog controls so we can all wear respirators and further endanger the rare animals in our Nat'l Parks.

Yellowstone has worse pollution than L.A. in winter because of the snowmobiles. A senator, who is an avid snowmobiler, blocked legislation to require snowmobiles have smog controls like every other motorized vehicle and the legislation is dead for another year. The snowmobile manufacturerers screamed, they would lose money if they had to comply with smog regulations. I don't give a damn about them losing money, they are the worst polluters around our Nat'l Parks and they need to comply with smog regulations just like any other vehicle that transports human beings. And no, I do not approve of removing the cats on cars to gain 2-3 more HP. Wait until a good cat back system comes on the market, but please don't remove or gut the cats on your Z, you are polluting my air and everyone else's and we don't need any more **** in our air.

Boomer

N74DV

great..... yet another tree hugger

John

iTrader: (17)

Exactly... it's the restrictive collector are some of the most significant power drains on a car...