guku0701

i have a 92 sc400.the previous owner has hollowed out the cats and almost straight piped it.only the 2 silencers remain.does this process affect the power of the car and my gas mileage?
any help will be appreciated

DJ XO

iTrader: (1)

Stock to stock, it won't be a huge difference, BUT you stand to lose some back pressure, which if you're really in tuned with your car will make it feel slightly sluggish from a dead stop. On the plus side, you MAY experience a tiny bit of top end improvement. Doubt it though. Lol.

megamax

iTrader: (9)

N/a applications stand to gain less than turbo applications from no cats. The piping bends and diameters are the key to n/a power gains.

Subjug4tor

iTrader: (1)

Back pressure is a myth when it comes to modern cars... What matters is pressure vs. flow of the exhaust stream. You want a system that gets the gas out of the system as soon as possible, without causing high pressure. For example, a 1" pipe will have a very high flow rate, but also a very high pressure, while a 4" pipe on a stock motor is too large, allowing the gas to cool and contract in the system which is no good for flow. You want to find the ideal balance of the two, which for a stock 1uz motor is dual 2.25-2.5" pipe from the headers, through an X all the way to the rear. Smaller and you will lose top end power due to pressure, larger and you will lose bottom end due to low flow in the system, making the engine work harder to push all the exhaust gasses out.

A hollowed out cat can reduce power, because the sudden increase in pipe diameter and then the sudden restriction back to the tubing causes turbulence. Instead, have a straight pipe welded in place of the cat. Alternatively, if you want the exhaust to pass a visual check for cats, remove the cat and weld in a pipe inside it, then put it back on the car. Looks stock, but you gain a *tiny* bit of power removing the cats.

durtysc300

Don't forget that it is federal law to keep emissions control equipment on the car. I believe it is a 10k fine

MattStarr

Some cities don't require emissions and I highly doubt a federal officer is gonna find out. Just sayin....

durtysc300

Oh I know but its just a heads up

millertime

I dont car about the whole backpressure vs not debate,.

heres what i know, i have a 1995 sc400.

I removed the resonators and put in straight pipe = no power gain just louder

i removed the 2nd cats = still no real power gain but louder

installed IMF headers, 2 1/2" pipe with x and magnaflow mufflers, = SLOW CAR

sent the computer to jet chips, = car idles lower, car has no torque but all kinds of top end/ vs my buddys stock sc400 i gut him on the highway but not in the 1/8 mile he takes it

Took walker 2 1/2" x 11" cats and put them in after the headers. Its a car again! sooo moral of the story is. that particular engine/ecu, requires a certain amount of backpressure to operate with acceptable torque for what i feel is needed for everyday driving,

guku0701

do you know if the setup i have affects the gas mileage?i almost get around 18mpg highway and thats not normal.

Subjug4tor

iTrader: (1)

It shouldn't. I get about 18mpg combined highway/city, but about 23-24 strictly highway. How old are you o2 sensors and ignition parts (plugs, wires, cap and rotors)?

guku0701

plugs rotors and caps have been changed.is there a way to check if the o2 sensors are bad?i think the previous owner has taken off the check engine light coz he took off the cats.im thinking this is the case.

MattStarr

Punched out cats won't reduce gas mileage. We did the same thing to my cousins SC300 and his gas mileage never changed. On the other hand, bad o2 sensors could. I've never seen this happen but considering that they take part in regulating the a/f mixture, its definitely a possibility. As for checking them, I would just go ahead and replace them anyways. They're bound to fail eventually anyways.

Subjug4tor

iTrader: (1)

x2 on replacing the sensors if you havent already, it cant hurt. to see if the check engine light works, just turn the key to 'run' but dont start the engine, all the dash lights should come on.

INTIMAZY

iTrader: (5)

Dynos or it didn't happen




I have yet to see dyno proof of "backpressure" ever increasing power at any point of any motor's powerband.

Subjug4tor NAILED it. Velocity is the key word and cats kill velocity regardless how modern or high flowing they are. On older cars from the dawn of emissions era, cats are an absolute cockblock for making power.

Smaller diameter piping can shift powerbands but just adding cats has never been a solution to suddenly making power.

guku0701

is there a common o2 sensor that usually goes bad?coz i might have to take it to the dealers to find out which one is bad