catalytic converter question

[SamDaYarisMan]

Hey guys, i just read this article in an import car magazine and they did a test with a dynomometer that showed that high flow catalytic converters have a big effect on exhaust pressure, even more so than a cat back exhaust in most cases. They did the test on a civic si and were able to get 6horsepower from a magnaflow free flow cat. My question is does anyone know what the stock exhaust diameter is? I think i might pick up a catalytic converter because i want to free up some back pressure with out making the exhaust loud (been there, done that....annoying) Thanks

[paultyler_82]

It depends on the car, some have extremly restrictive cats, some dont, also in many cases, you'll need to modify the rest of the exhaust and the intake to realize much of a benefit from a high-flow cat, with the prices a Decent aftermarket catalytic go for, the price for what you gain isn't very good unless you're trying to free up every spare horsepower and you have a liberal checkbook. I know on the I30 that I have, changing out the Y-Pipes with aftermarket frees up some power because of two "pre-cats" on the pipe that get eliminated, but changing out the main cat will hardly net you any gain, but I don't beleive the 1NZ-FE has a 3way catalytic system like the I30

[mikeukrainetz]

You wont gain a whole bunch with just a cat change. The pipes are like 1.25 or something, they look tiny. Some of the high flow cats Ive seen can be about 100 - 300 bucks, if your going to spend that kind of money your better off either replacing the whole system from stem to stern or buy a cold air intake (which you will probably want to do anyway with an opened up exhaust)

[paultyler_82]

Are the pipes really that small?! I was thinking maybe 2-2.25"

[boxerboy]

The pipes are very small. My dic, aah is, well I think a 2"-2 1/2" exhaust with a header would yeild some power. I would not want to give up any gas mileage.

[riceboy]

Mike,

really... 1.25"???

[mikeukrainetz]

I dont have a set of calipers but they are tiny, just have a quick look underneath, like its built from straws. Even if you spend a hundred bucks on a cheap muffler and re-run the pipe from cat back with larger dia pipe. The exhaust from the header to the cat would be fine until you buy aftermarket parts to replace these. Without any intake mods even a million dollar exhaust wouldnt make a major difference.

[boxerboy]

Exhaust and intake won't ever make a major difference. Major will require major money. I think a little power maybe 5 to 7 hp would be good. Thatis if the cost is not much. Major power will need more fuel that means there goes the gas mileage.

[Snake]

do a 2.25 header back
remove all resonators and cats
install 1 high flow cat
you will open up a lot of horsepower
if you want it quiet, get a very low sounding quiet muffler

[mikeukrainetz]

Major difference when your talking about a car with just over 100hp is ANY extra HP. Considering the cost of any power producing add-ons there really is no cost effective way of making decent numbers with such a small powerplant. If the TS makes it to production buy a 1.8 as a swap. Youll probably spend the same amount of money as you would buying turbos / superchargers, exhaust, intake, Engine managment.. etc etc.
I digress...

Total exhaust mods, short of a header will probably net you like boxerboy wants / suggests 5-7hp over stock.

[BulletProofAuto]

i believe the stock piping is ~48mm
there are not too many catback exhausts out at this time which is really not too big of a deal with exception of the convenience. piping diameter dictates the velocity of the exhaust gases, you want max velocity which is found by enlarging the piping up to the point of the velocity being lost. in terms of backpressure, stock catalytic converter and stock muffler will disrupt the flow of the exhaust and cause the engine to work harder to pump out the exhaust gases. high flow cats have actually been dynoed to make more power then test pipes also(not too sure how).

[JustAnotherAsian]

Originally Posted by Snake

do a 2.25 header back
remove all resonators and cats
install 1 high flow cat
you will open up a lot of horsepower
if you want it quiet, get a very low sounding quiet muffler

2.25" might be a little too much... if the stock piping is 1.25". if it's 48mm (2") like john suggested, then 2.25 would be a perfect increase/upgrade.

anywaiz, john also got the "velocity" issue correct. the 4 stroke internal combustion engine sends gases through it's exhaust pipes in "pulses", and with every "pulse", there is a slight vacuum that follows it (we're talking about long pipes here from headers to puny exhaust tip, not open headers). the vacuum's intensity increases with velocity. each exhaust pulse therefore pulls the pulse behind it, like a trailer, or "drafting" (as i've seen in other sources). this vacuum effect or "pulling" is called scavenging (look it up). you can get the maximum efficiency with maximum scavenging at low rpms, but that means a small diameter pipe so the engine doesn't have to work as hard to "push" the gases out of the pipe. in high rpms, the pulses are practically non-existent and it's more of a constant flow, therefore, the engine now works to push gases out of that small pipe- this is why bigger pipes are better for high-rpm. so in other words, one would have to find the perfect balance between the smaller pipe for maximum scavenging, and larger diameter piping for minimal back-pressure so the engine can work on moving the car instead of pushing gases out of a pipe.

the exception is indeed forced-induction- but mainly just for turbo cars, since the gases's pulses going through the turbo are "equalized" and the scavenging effect is taken away after the turbo. bigger pipes are good in all turbocharged cases.

on supercharged cars, there is just more gases in each "pulse", therefore, the scavenging effect is still in place, but to a lesser extent because the "equalization" or (the point at which the "pulses" are practically phased to a constant flow) comes at an earlier rpm than of it's n/a counterpart. therefore, bigger pipes are also good for this application, but to an extent.

eh. sorry for the long post, but i thought it might prevent someone from slapping on that 3" full exhaust system on an n/a car.

[paultyler_82]

I hope the exsisting setup is two inch... I would like to fit in a Flowmaster 60 and the smallest they make is a 2" inlet, 2" outlet... Eventually, most of the pipes will be replaced anyway, but I don't want to deal with that just yet.

[Scotty]

Stock catalytic converters these days are so efficient these days that you wouldn't really notice a difference on the street, that and its completly illegal to replace it when its functioning properly. And please, don't remove it completely. Its not cool to have a slow and stinky car. I don't know about anybody else, but I always wonder why people complain about issues such as smog then they are driving around their stock street Hondas (just for example) with all emission components removed. Lame. I think your best bet would be to go with a good catback and maybe a header (of course, with CARB EO# ). Don't want to sound prude, but I guess Im just a hippie.

Scotty

[josh_trdsparks]

I can't see how taking one working cat off and putting another one is going to help the car in any way, shape, form. or idea. To pe perfectly honest there really isn't any such thing as a "High Flow Cat" anymore. The new cats are (I believe) a 3 screen design and replaced the old "bean" stlye cats from way backin the day. They flow better becuase they are bigger hence the name High Flow Cat. They are more efficient and better for emissions as well. Since all new cars come with these style cats there isn't really a point in changing the one you have.

Yes there are a few cars that may benefit for these aftermarket cats like an Si civic or even the Celica GTS. But these engines don't work off of back pressure they like or need to be as open as possible because of the V-tec and VVTI-L. These cars also make most of thier power at high RPM's. The yaris doesn't work in the same way but we all know that. If you open up the exhaust too much you will actually lose HP. I wouldn't out anything over 2.25 in on it personally and that's pushing it to me.