Removing cat convertor - SkyMan
Hi there,

And before you guys tell me I can't remove the cat because of MOT failure, let me explain...

I'm in New Zealand and they don't currently test emissions (although they will do in a few years time). Therefore its legal to remove the cat.

Anyway, I've got a 1995 Nissan Skyline 2.5GTS, and have got the car booked in next week to remove the cat convertor and replace it with a straight pipe. The Cat temperature sensor will be grounded to earth so the warning light won't come on.

All well and good, but then someone told me that it might make the exhaust note sound tinny, and I need a "resonator" fitting instead of a straight pipe.

The car has the original exhaust system fitted, which gives quite a muted sound.

I don't want to gut the cat (sounds painful) as I might want to re-fit it later when emissions are tested.

So, any thoughts on the exhaust note - will it change and sound like a boy-racers souped up Mazda pocket rocket? Do I need a resonator? - This sounds expensive, currently the job will cost me NZ$70.

Removing cat convertor - Aprilia
Removing the cat will change the note a bit because it will slightly reduce the amount of damping and the back pressure. Why not try it without first?

NZ$70 is only about £25, so its not expensive anyway (well, not by UK standards, but I guess NZ wages are about half UK rate?).
Removing cat convertor - SkyMan
You're right - $70 can be expensive - it depends on how far away payday is. Currently earning $50,000 which is a middle of the range wage here. Some things are more expensive (mortgages, food, etc.) but petrol is cheap: 95c a litre for 91RON ( the standard unleaded) and $1.01 to $1.03 for 96RON. Cos my Skyline was designed for Japanese 100RON I buy 98RON from BP for a $1.13 a litre.

Oh, and just to make you Brits sick, insurance costs me $320 a year fully comp (with 9 y NCD). I've got my house, contents, my car and my wife's car insured together for $85 a month :)

Anyway, back to the point - will the reduction of back pressure have any negative effects on the performance? I'm hoping it won't...

I also fitted a K&N filter a little while ago - that made a noticeable difference, so given that I'm running on lower grade petrol than its designed for, hopefully I'm close to the 190BHP quoted for the car when it was released in Japan.
Removing cat convertor - Aprilia
Actually NZ$50k sounds very good - I thought the average was something like $25k? Not a lot, if the house prices I saw in the better parts of Auckland are anything to go by (especially with a 7%+ mortgage rate)!

Anyway - your car's engine will have been designed to work with a specific back pressure. Removing the cat will have some effect - but I can't say what. It will certainly have an impact on the amount of EGR and change the gas flow in the head. I think you will just have to try it and see.....

K&N's (the 'oily' type) are bad news because oil droplets get onto the air-flow meter and eventually ruin it (don't think it throws a fault code on the Nissan - performance just drops off). A JDM AFM is probably about NZ$800+ new, so you don't want that to happen.....
Removing cat convertor - SkyMan
I've cleaned the AFM wire with contact cleaner, didn't think about the K&N oil gunging it up tho. I'll post a question on the Skyline forum about this possible problem and see if it does cause issues.

Does anybody have any experiences with K&N filters on cars with a hot wire AFM? Good thing I kept the old filter...

BTW - no-one can afford a house in Auckland anymore - thats why they're moving down to my part (Tauranga in the Bay Of Plenty). So house prices here are going thru the roof now!
Removing cat convertor - Aprilia

Take my word for it, the K&N oil WILL cause problems with the AFM - I have replaced a couple of AFM's on cars with K&N's. The oil droplets settle on the hot sensing element and carbonise, this alters the AFM calibration and eventually leads to sluggish response. IMHO K&N are just a pain, I don't believe they add any performance at all.
Removing cat convertor - v8man
4 stroke engines don't need back pressure - quite the opposite in fact. The free flowing exhaust gasses will need to be matched with a free flowing air filter. I use a Pipercross on my Rover Vitesse turbo. It is a dry foam type. The exhaust note will hardly change with the removal of the cat ( My car is de-catted until MOT time!) as the turbos mute the exhaust note anyway. What is a resonator? I have been tuning cars for years and have never heard of one.
Removing cat convertor - Peter D
The cat effectively replaces the expansion box of old which was to allow the free flow of gasses from the engine effectively smoothing out the individual exhaust strokes. This box also helps to stop the babbling noise on the overrun Removing this can cause a noisy exhaust note at the lower (less than 2/2.5 K rpm but is probably un-noticeable at 3k plus. Although you cay get some popping on the overrun. Go to
Exhaust section and look at what stand alone expansion boxes may be available from standard large car exhause systems. Good Luck. Regards Peter
Removing cat convertor - Aprilia
No, they don't 'need' back pressure as such, but the engine management system will have been mapped with the standard exhaust system in mind. Reducing the back pressure will have some impact, in particular if the engine uses EGR (which I'm sure this engine does) then reduced back pressure will impact on the amount of EGR (which could be a good thing, of course). Ideally, to make full use of these changes the system should be re-mapped.

As to the 'resonator' - well, all exhaust boxes are resonant at some frequency and are usually filled with a damping material to reduce the 'Q' (quality factor) of the resonance. Quite why it should be necessary to install a resonator box in this case I don't know.

Value my car