Hollowed Cats

[cubanman2]

Will hollowing my cats reduce power? Will my car have enough back pressure? I have a 93 SC400.

 

[JBrady]

Do not just hollow out the cats. It will sound terrible and performance will not be helped. Low end power will be lost and you will be lucky to not loose top end power.

 

[cubanman2]

The reason i did it is because they were clogged and i did not have money to buy new ones.

 

[Celsiorous]

JBrady what size pipe would you replace the hollowed cats with? 2.25?

 

[JBrady]

2.25 is a good choice. You can actually have a shop fit a correct pipe inside the hollowed cats. That is probably the quickest and easiest fix. It will "look" like it has cats.

Of course the seemingly obvious choice would be used cats from a salvage yard but in the USA it is illegal for the yards to sell the cats. Seems stupid to me but that is the law.

 

[Celsiorous]

Thanks JBrady, the pipe inside the cat is a good idea, do you think that this will still be noisey?

 

[400ZED]

it will still be noisey but should take out a bit of the tinny ness produced by the thin wall cat. if they are strict over ther as they are here then i houlw fill the gap between the cat and the pipe with something so that when you tap them they don't "ting"

 

[Celsiorous]

Thanks 400, cats arnt checked in NZ so I dont need to worry about that part of it at the moment. I basically just want to get rid of the sound as cheaply as I can untill I get some headers.
The noise coming from the cat is driving me mad, if I was to replace the cats with some small resonators could I remove the sound all together?

 

[JBrady]

Thanks JBrady, the pipe inside the cat is a good idea, do you think that this will still be noisey?

If you still have the stock exhaust you should be fine. A great deal of the noise from hollowing out the cats is the walls of the cats resonating. The pipe inside should do the trick. Much cheaper than trying to fit a resonator in place of the cats.

 

[cubanman2]

JBrady, i found that the 2.50 or 2 1/2 pipe fits perfectly in the Cat. The 2.25 is to small. I know because i just did it today.

 

[JBrady]

JBrady, i found that the 2.50 or 2 1/2 pipe fits perfectly in the Cat. The 2.25 is to small. I know because i just did it today.

That is surprising because the inlet outlet size on my 1999 catalysts are 60mm which is 2.36". I do know that the stock 93 used 50mm (1.97") pipe after the catalysts so I would still keep the pipe inside the cats small. Actually a tappered pipe from the outlet ID size of the manifold to the ID size of the pipes would be ideal. Any time you increase and decrease the pipe sizing you loose velocity and risk reversion. While the 2.5" pipe you used is MUCH better than hollow cats it is still larger than what I would have used.

That said congratulations on doing this. Did you have hollow cats before? Please post you comments about sound level, quality and power.

 

[cubanman2]

Yes, i had my cats hollowed. The sound level seems about the same. Performance is the same.

 

[Celsiorous]

I have had my hollowed cats removed and replaced with 2.25 pipe, the exhaust guy didnt want to spend the extra time fitting the pipe inside the cat casing. He had to flare the ends of the pipes a bit but it the fit was fine.
It is definately louder than stock but far quieter and nicer sounding than the hollowed cats. It has a slight V8 rumble now and when you put the foot down it sounds much more like an 8 than before, there is still a bit of a resonation sound.
Power wise I have noticed that there is more low rev torque, the only way I really know this is because there is an intersection that I often floor it out of and with the hollowed cats I just took off, now it lights up and gets a bit loose

Thanks again for your advice JBrady