I have had a look now on a few car forums and found that if you keep the cats on you keep a good amount of torque

 

but if you remove them you lose torque and gain HP

 

Everyone knows that the more torque the quicker your car will accelerate. Hp is top end

 

So why do we fit decats or have i totally got this wrong. I am tempted in putting mine back on if this is the case and hopfully tone the noise down a little.

 

Please disscuss :lol::lol:

I can think of no reason why keeping the cats would improve torque, it doesn't make sense. It's like saying, "stuff something up your exhaust and get more torque".

I can think of no reason why keeping the cats would improve torque, it doesn't make sense. It's like saying, "stuff something up your exhaust and get more torque".

 

me neither.. I would stick with the decats.

Are they really that much of a hindering? I remember reading someplace that Nissan's cats actually flow pretty good, and that removing them really didn't do all that much...

 

I'm just asking because on the Primera I replaced the cat and didn't notice any difference performance wise. Just a lot louder exhaust, wich became annoying after about 1 hour so I stuck on the cat again...

 

I'm not saying the two cars are directly comparable in that way though. Seeing as one is turbo and one is N/A

As far as I understand, back pressure is only required for two stroke engines to prevent the unburnt fuel/air mixture from passing straight through the combustion chamber into the exhaust. As all engines we'd be concerned with are four stroke, by the time anything is in the exhaust it's considred waste and should be expelled with as little resistance as possible.

 

Besides! we have practically THE most restrictive component possible in our exhaust system....a turbo.

Are they really that much of a hindering? I remember reading someplace that Nissan's cats actually flow pretty good, and that removing them really didn't do all that much...

 

I'm just asking because on the Primera I replaced the cat and didn't notice any difference performance wise. Just a lot louder exhaust, wich became annoying after about 1 hour so I stuck on the cat again...

 

I'm not saying the two cars are directly comparable in that way though. Seeing as one is turbo and one is N/A

 

was the prim sr20? cuz i did the same on my mera gti and got sod all difference, due to the huge precat assy on the downpipes. when i replaced the header the difference was great! :-)

 

in my experience doing away with cats has allways been a good thing :-)

 

Besides! we have practically THE most restrictive component possible in our exhaust system....a turbo.

 

ever seen a corsa with a spud up its pipe???

When I put a straight-through system on my sierra the difference was immense. A lot better mid range and top end too. I've never heard of cats boosting torque and I've been a car nut since I could read :shifty:

 

All I know is, if I take the cats off my Zed its likely I'll get clouds of smoke - but I'm pretty sure that doesn't boost torques or horses either :lol:

From what i know, when removing a Cat from a low CC engine it removes the backpressure as murmur said, but your BHP figure should still increase as your torque curve peaks earlier in the rev range thats what is effected, once the engine has gained the revs and the backpressure isnt required the gains are shown which is why the BHP is increase as the bhp curve is a lot later in the rev range than the torque curve. Well thats what i thought anyway :D im still going to decat mine!

 

Hollings.

was the prim sr20? cuz i did the same on my mera gti and got sod all difference, due to the huge precat assy on the downpipes. when i replaced the header the difference was great! :-)

 

in my experience doing away with cats has allways been a good thing :-)

 

Yes, when I replaced the stock manifold(with precats) I noticed a big difference, I also upped the ignition to about 20degrees and put on a CAI.

But upon removal of the main cat, nothing... just more noise :P

 

 

I was just on conceptZperformance site and replacing your cats with test pipes gives you a 10-14hp, but I would like a dynosheet thanks :smartass:

Ive no cats just hks back boxes and pulls like a train,cars dont need cats anyway just a money spinner for government,has anyone in club had dyno test with and without?

i read it on the evo and scooby forum.

 

Every car i have ever decatted i have lost bottom end grunt and gained it up top a little hmmmm dont know what to do?

i read it on the evo and scooby forum.

 

Every car i have ever decatted i have lost bottom end grunt and gained it up top a little hmmmm dont know what to do?

 

yeah but scooby's and evo's have piddle little 2.0l engines with huge turbo's.

yeah just wondering how it would affect us, when i did my decatt i cannot remember if it made any differance? just more noise iirc

 

But i did make 330hp on the dyno with stock boost ?

yeah just wondering how it would affect us, when i did my decatt i cannot remember if it made any differance? just more noise iirc

 

But i did make 330hp on the dyno with stock boost ?

 

I think the only way to tell for sure would be to dyno it before and after tbh. But I would be surprised if you lost torque.

Well id assume it would allow our little turbo's to spool up a lot faster because of the greater pressure difference on either side of the turbo. But that would be the same for the evo's and scoobys. Every evo owner ive met runs a decat as far as im aware anyway :mellow:

Never mind the power, it's all about the sound....!!

Found mine to be more responsive when de catted !

exhausts and pressures can alter a car dramatically. Its all about playing about with it and testing. like one exhaust can br made for low down power others top etc. Have you not seen wangan highway lol

I can think of no reason why keeping the cats would improve torque, it doesn't make sense. It's like saying, "stuff something up your exhaust and get more torque".

 

LOL! Fit 2 or 3 cats to really increase your torque!! :lol:

 

besides bhp = torque X rpm/5252 so the 2 are linked anyway!

Dont confuse Turbocharged and NA exhaust flow requirements, very different.

 

In each case backpressure is an undesirable side affect of decreasing the exhaust flow velocity, its never wanted.

 

With a TT - after the turbocharger you want the gases gone as quickly as possible, minimizing the pressure downstream of the compressor, Bigger the better, although after a certain point going bigger wont have any benefits.

 

While decatting a TT does offer some power increases its more about the response and spoolup improvements, and its noticable.

 

With an NA its all about the flow velocity - too large and you slow the velocity which reduces the scavenging affect of exhaust gas pulses at low revs. Which can result in a small torque loss at lower rpm. Scavenging is far less beneficial at higher rpm though.

Edited February 2, 201114 yr by Yowser

Dont confuse Turbocharged and NA exhaust flow requirements, very different.

 

In each case backpressure is an undesirable side affect of decreasing the exhaust flow velocity, its never wanted.

 

With a TT - after the turbocharger you want the gases gone as quickly as possible, minimizing the pressure downstream of the compressor, Bigger the better, although after a certain point going bigger wont have any benefits.

 

While decatting a TT does offer some power increases its more about the response and spoolup improvements, and its noticable.

 

With an NA its all about the flow velocity - too large and you slow the velocity which reduces the scavenging affect of exhaust gas pulses at low revs. Which can result in a small torque loss at lower rpm. Scavenging is far less beneficial at higher rpm though.

 

Absolutely Spot on Dave.

This post shold be made a Sticky. It sums it all up very nicely without going too deep in to the subject.

LOL! Fit 2 or 3 cats to really increase your torque!! :lol:

 

besides bhp = torque X rpm/5252 so the 2 are linked anyway!

 

Another blindingly simple fact that seems to escape many people/discussions.

Dont confuse Turbocharged and NA exhaust flow requirements, very different.

 

In each case backpressure is an undesirable side affect of decreasing the exhaust flow velocity, its never wanted.

 

With a TT - after the turbocharger you want the gases gone as quickly as possible, minimizing the pressure downstream of the compressor, Bigger the better, although after a certain point going bigger wont have any benefits.

 

While decatting a TT does offer some power increases its more about the response and spoolup improvements, and its noticable.

 

With an NA its all about the flow velocity - too large and you slow the velocity which reduces the scavenging affect of exhaust gas pulses at low revs. Which can result in a small torque loss at lower rpm. Scavenging is far less beneficial at higher rpm though.

 

 

Ah righty ho, ta very much. dont know why the other site states it, must be because it full of cavs:blink: