Eliminate Secondary Cats!!!!

[986Jim]

Originally Posted by Brucelee

Why do you believe that eliminating the second cat will release quite a bit of HP?

Any restriction in the exhaust system of a NA car robbs HP. You may loose on the bottom end, but overall in the peak where HP matters to go fast, the best exhaust system is a totally open one.

This has been proven on the dyno over and over. Many all motor cars that race in that class benefit from funny looking headers that dump right behind the motor with no cat or exhaust system. They produce 15 more peak hp than the best exhaust system can. Mid range may be affected, but for racing it all about peak HP.

In thoery eliminating the secondary cats will eliminate restriction in the exhaust and give more hp around the HP curves peak (where ever that is on a boxster). Best woudl be to eliminate both, and have a straight through muffler for the most peak HP possible. That however would be really loud and probablly negatively effect mid range which is nice for the street, but would be better for top end power.


This is a popular race header for the B-Series honda motor. many have gone 300whp all motor with a header like this. They could never do it with a cat and exhaust system. They suffer from no mid range, but make crazy top end power.

Here is our All motor drag car,

It made 278whp and 199ft/lbs of torque at 9900rpm lol. Yeah great for racing but no good for daily driving obviously. See the header? The old school thinking was we needed back pressure, but this is simply not true ever no matter how you slice it. Back pressure is ALWASY bad under every circumstance on any motor ever built. This used to be the train of thought until header started to be tested on cars at the dyno and showed that the best NA exhaust system is just like a turbo car, which is no exhaust system, just a straight pipe with a nicely designed merge collector and nothin else.

All motor power is one of my specialties because coazing 278whp from 2.2L is not easy as you can imagine but 10,000rpm redline sure helps.

[edit] BTW, the car runs 10.74 @ 123mph incase your wondering

[eslai]

Originally Posted by 986Jim

Any restriction in the exhaust system of a NA car robbs HP....

I highly doubt Bruce was looking for Backpressure 101, I think he was just questioning why anyone would think that removing the secondary cats would make a significant amount of power.

[986Jim]

Originally Posted by eslai

I highly doubt Bruce was looking for Backpressure 101, I think he was just questioning why anyone would think that removing the secondary cats would make a significant amount of power.

8hp is a significant amount of power. Because you get 8hp from headers and 8hp from exhaust and 8hp from intake etc.. So all the sudden you made say 20hp from the extras.

The thing is, you can feel that HP increase even when it's small, you will also have increased throttle response because the engine is now more efficient pumping air in and out with less restriction.

It's not Backpressure 101, however people seem to lose sight that any gain is a gain. When building a race car weight is a huge problem. We do everything to take weight out, and more than you think. Eventually we get into ounces of weight saved. We cut metal from the trunk and doors just to save 10 ounces of weight. Anything and everything you can do. Eventually all those ounces you pull add up to 10 lbs of extra weight you pulled, makeing the car better at it's job. So each HP even if it's only 1 is a increase, you just add it to the other small increase you work to get and they all add up.

Race a car for real with a track only car and you will understand what I'm talking about. Guys use different weight oil for different temperatures because a 0-20 will give 6 extra hp over a 10-30 if it's not very hot out.

[eslai]

I have raced many cars on real tracks and yes, what you say is true--for a race car.

On the street, simply removing the secondary cats is not going to make a car significantly faster. In my experience any increase under ten horsepower in a car that weighs a ton-and-a-half is not significant even to the butt dyno.

Now, if you replace the entire exhaust system and tune the ECU, sure you can get some noticeable gains, but that's not what we're talking about here and even so, I can't believe you're doing Mod Math.

Anyone chasing single-digit HP increases on a street car should really just learn how to drive better. You can't even accurately measure such small gains on a dyno--it's all within statistical error.

[Brucelee]

"Anyone chasing single-digit HP increases on a street car should really just learn how to drive better. You can't even accurately measure such small gains on a dyno--it's all within statistical error."

Good point. Reminds me of the guys who buy the "superchargers" off of EBAY!

[986Jim]

Originally Posted by eslai

I have raced many cars on real tracks and yes, what you say is true--for a race car.

On the street, simply removing the secondary cats is not going to make a car significantly faster. In my experience any increase under ten horsepower in a car that weighs a ton-and-a-half is not significant even to the butt dyno.

Now, if you replace the entire exhaust system and tune the ECU, sure you can get some noticeable gains, but that's not what we're talking about here and even so, I can't believe you're doing Mod Math.

Anyone chasing single-digit HP increases on a street car should really just learn how to drive better. You can't even accurately measure such small gains on a dyno--it's all within statistical error.

See this is where you almost had it. You were sooooo close....

You said, if you tune the ecu and do the entire exhaust system. Well this is all part of that entire exhaust system. Generally most people don't do all the mods at once, but as they can afford it.

Yes he may not feel the cats on the street (however for 8hp I think you would) but with the headers and cat bypass, and muffler and intake and chip after all these are done you would feel a different for sure.

It would be much more noticable if you did it all at once as you pointed out, for sure I agree with you there. Slap on 20hp worth of mods all at once you will know, but then you don't really know what made the best difference and dont get to feel each mod as you do it either.

In this case, better driving wont help you as were talking about straight line acceleration on the street, so basically while your already moving and standing on the gas. That takes not driving skill, however with a few extra mods you will notice that acceleration gain.

At the track you can physically measure lower time around a lap, but many guys don't make it to the track and just want more wooosh feeling during hard acceleration in a straight line. (Wrong car really buy a Z06 for that) but we all want what we want. For 986silverbullet that is what he's looking for, and he will get it with those mods if he completes them all.

Honestly your posts telling him not to mod the car in a thread asking about doing a particular mod really are not helping.. If you have some positive information to add here please by all means. Simply saying "you wont feel 8hp in the butt dyno" is not helping him.

Regards,

[eslai]

No, i think I got it right. He won't feel the power gains (if there are any) if he does things incrementally, so what's the point?

Think about how much it costs to do it all piecemeal too. It's much cheaper to drop the car off and say, "hey, yank the exhaust and put on this new one" than it is to go back and have them put the car on a lift a bunch of times once you realize you need a new muffler, new main cat, new headers, etc. Also, what if someone convinced him to retune his ECU each time to take advantage of the new mods? The bang-for-the-buck is just not there, especially at Porsche prices!

I'm saying that he'd need to do the whole kit and kaboodle to feel anything, for his car to make him go, "Wow! That's so much better!" If he's not going to do that, there's no point. Go for the gold, or go home.

[Brucelee]

It is great to see that no one has strong opinions about this subject!