First you say they were 1/4 mile and then you say they weren't, make up your mind.

 

For just a street car, I'd keep the cats in. Maybe replace them. If you take them off and all you want to do is drive around a street car, it's going to cause more headaches than it's worth. You'll get a few HP and some great sound, but the cops will give you HELL!!!

 

I removed the pre-cats when the headers went in. Later removed the main cat guts and ran it for a few years. Definitely made it considerably noisier. Plus you do get that sulfur smell as the others have stated.

When I had the exhaust changed out to true dual, they added a pair of hi-performance cats and the noise went down and sulfur smell went away. No performance change.

 

Don't have $300 for a magnaflow cat....cheaper to buy a $20 piece of pipe and do some fabrication

 

Allow me to explain. The 1/4 mile time is derived from my 2nd gear pull based on a model validated with 5 different cars ranging from my '72 Firebird with a 455 (brute low end torque) to a '92 Audi S4 running 24 lbs of boost - lots of HP, not tons of torque. I will e-mail you the model if you like. It takes into account gearing, drivetrain loss, wind resistance, shift times, obviously the torque curve of the engine - which can be derived by doing the second gear pulls that I have done on numerous cars.

I am simply trying to help dispatch myths and rumors with hard numbers. I was not saying that replacing with a $300 catalytic on a stock engine would not be a wise choice that may not limit your HP at all. The OP's question was should I remove all catalytics? I was simply saying what it does to performance, I was not even saying he should or shouldn't, just giving him some facts rather than opinions. I am simply saying that removing the 1991 catalytic converter with 100k miles on it, even one in very good shape, will pick up time. Here are the raw numbers for comparison between the two second gear pulls - draw your own conclusions if you like. The RPM numbers are calibrated via datamaster compared to my tach, so that is why they look funky. BTW, these numbers are at approx. 5500 ft elevation. As one would suspect the gains get larger at the upper end of the rev range.

Stock Run
RPM Seconds
1627 0.00
2033 0.63
2440 1.30
2847 1.97
3253 2.57
3660 3.23
4067 3.90
4473 4.70
4880 5.67

Removed Catalytic Run
RPM Seconds
1626.7 0.00
2033.3 0.63
2440.0 1.27
2846.7 1.90
3253.3 2.47
3660.0 3.10
4066.7 3.77
4473.3 4.50
4880.0 5.40

I also have runs for the Magnaflow Catback exhuast that eliminated another 0.2 seconds over the same run, getting it down to 5.2. I specifically did this in stages so I could measure each stage, as it obviously would have been easier to do the whole exhaust at once, but I really wanted to be able to provide data to folks on the forum instead of speculation, because when I was researching it all I could find was based on "feeling" and speculation. No hard times or data.

Anybody got hard numbers saying anything different, i.e. that they saw no gain when they removed their stock catalytic? How about a comparison between the random tech cats and straight pipes on a stock engine? HP numbers, times of any sort? I am all ears and eyes and would love to see the data. I am just trying to share data and I feel like I am getting a little bit attacked because the data does not match folks pre-conceived notion of what should happen. It's just data, it has no dog in the fight, but one should not push it aside just because it contradicts their opinions. If I did that in my profession half the hardware that I have designed that is sitting on and orbiting other planets would not be there - because there have been many times when my speculation did not match my test data - which meant my speculation was wrong and now I needed to understand why, not just say the test must be bogus.

*steps off soapbox*

BTW, I did not notice any sulfur smell and this is still with my precats on.

 

You can find them for less. Not an excuse. It's justifying what you want to do IMO.