power gains with cat bypass pipes

[The Radium King]

in a tuned exhaust you use unequal length pipes and a specific back pressure to facilitate exhaust gas scouring and increase torque at low rpm.

equal length pipes and reduced back pressure (high flow mufflers, cat delete) will improve large volume air flow and increase hp at high rpm.

so, you increase high rpm hp at the sacrifice of low rpm torque. i have yet to find a dyno of an aftermarket exhuast that didn't make less torque than stock in the 2500-ish rpm range.

porsche decided that most folks drive in the low to mid rpm range and designed the exhaust accordingly. ask yourself how often you really run your car over 6000 rpm vs how often you are tooling around at 2000 to 4000 rpm. that should dictate whether the cat delete will be of any benefit.

[Johnny Danger]

I didn't experience any loss of low end torque after I by-passed the secondary cats on my vehicle. In fact, dyno results later revealed a slight blip in hp when done conjunction with an aftermarket cat-back exhaust . Although some degree of back pressure is essential, keep in mind, at 400 plus cells, the secondary cats are extremely restrictive.

JD
p.s. The weight reduction is a plus also.

[jmatta]

I have Fabspeed secondary cat bypass pipes and a "Pedro Style" sport muffler (also running the Evo Hi-Flow intake). Though I can't vouch for any increase or decrease in low/mid torque, the engine revs much much freely, without the cats. Before installing, the engine (e-gas) would always hesitate on downshift throttle blips; once the bypass pipes were removed, throttle blips are near instantaneous, which would likely be due to loss of restriction.

Oh, they sound great, as well!

[The Radium King]

i will agree that the throttle response on my '00 S feels 'disconnected'; i've always attributed that to the e-gas. further, any improvement in throttle response does not necessarily mean that you are making more hp/torque. your car sounds like it is a lot for fun to drive, however.

to jd i would ask (a) the hp increase was where in the rpm range, and (b) any hp effect that a cat bypass pipe is +/- say 5 hp on a 200 hp vehicle; can any of us say that we can 'feel' a 2.5% change? there is a larger variation between two stock engines, or two engines at two different altitudes, or a hot vs cold engine, or a driver with a fat girlfriend vs a driver with a skinny girlfriend. and what effect does the audio component have on how you feel the power response (ie, it 'sounds' more powerful; the 'sound' makes you want to rev higher)?

all i know is what i've seen, and all dynos of aftermarket exhaust show a minor torque reduction at lower rpm (if they show lower rpm at all; often the dynos start at 3000 rpm).

not being argumentative; just being conversational and trying to distill the facts given the sometimes difficult nature of electronic conversation.

trk.

[spongebob]

I'm not worried about loosing any torque as the engine is equipped with only two cats in many countries including Germany. Probably the powergain wont be noticable either. The main goal is to shave off some weight from the rear engine mounts which hopefully will reduce the annoying vibrations at cruising speed.( I know that the mounting height of the mufflers alters the characteristics of the vibrations)
If sound and throttle response is improved its a great plus.

[BYprodriver]

Originally Posted by spongebob

I'm not worried about loosing any torque as the engine is equipped with only two cats in many countries including Germany. Probably the powergain wont be noticable either. The main goal is to shave off some weight from the rear engine mounts which hopefully will reduce the annoying vibrations at cruising speed.( I know that the mounting height of the mufflers alters the characteristics of the vibrations)
If sound and throttle response is improved its a great plus.

Usually the cheapest way to dampen vibration is to ADD weight!

[Johnny Danger]

The secondary are completely superfluous . Its merely a case of Porsche having a "belt and suspenders" approach to emissions/OBDII compliance. They're like having an extra set of thumbs. Get rid of them !

[spongebob]

Originally Posted by BYprodriver

Usually the cheapest way to dampen vibration is to ADD weight!

I agree but, changing the weight in either direction will change the freqency and amplitude of the vibrations hopefully out of the " critical rpm" that this engine seems to have. Also putting less load on the transmission mounts will give them more room to dampen the vibrations.
This is just my theories but the will be tested later when the white stuff on the ground is gone.

[Johnny Danger]

Originally Posted by The Radium King

i will agree that the throttle response on my '00 S feels 'disconnected'; i've always attributed that to the e-gas. further, any improvement in throttle response does not necessarily mean that you are making more hp/torque. your car sounds like it is a lot for fun to drive, however.

to jd i would ask (a) the hp increase was where in the rpm range, and (b) any hp effect that a cat bypass pipe is +/- say 5 hp on a 200 hp vehicle; can any of us say that we can 'feel' a 2.5% change? there is a larger variation between two stock engines, or two engines at two different altitudes, or a hot vs cold engine, or a driver with a fat girlfriend vs a driver with a skinny girlfriend. and what effect does the audio component have on how you feel the power response (ie, it 'sounds' more powerful; the 'sound' makes you want to rev higher)?

all i know is what i've seen, and all dynos of aftermarket exhaust show a minor torque reduction at lower rpm (if they show lower rpm at all; often the dynos start at 3000 rpm).

not being argumentative; just being conversational and trying to distill the facts given the sometimes difficult nature of electronic conversation.

trk.

My "journey" toward creating a better exhaust system for my vehicle began with the installation of an aftermarket cat back exhaust. My first impression was, that this upgrade produced a much more uniform engine response, and of course better sound. Subsequently, I deleted and by-passed the secondary cats. Again, the vehicle just felt "better", with no sense of loss with regard to low or mid range torque. It wasn't until I began experimenting with the header/pre-cat setup that I experienced varying results. To make a long story short, the design that ultimately lead to the best results for me was a set of custom fabricated headers that incorporated longer, equal length primaries that flowed into a well devised merger collector - followed by a set of 200 cell tri-metal HJS cats. Dyno results later showed consistent gains in hp through out the entire power curve.

[The Radium King]

ok. most dynos i've found are for (a) headers, and (b) cat back (ie, both mid pipes and muffler). again, very few perform on paper.

if we look at the variation in cats between usa and row cats, we could assume that the secondary cats on usa cars are an afterthought (required for emissions) and were not considered in the initial tuning of the exhaust. so, we can return to the intended tune of the car by going to catless mid pipes (as well as reduce some lbs and improve sound - cool).

if that assumption is true, then the meaningful tuning comes from the headers and the relevant back pressure comes from the exhaust. i have a set of ebay 'knock-off' headers and would note that, while they are presumeably a copy of fabspeed or somesuch, they are very rough at the manifold flange and could really benefit from an old-school polish. as you state, varying results. a longer header will reduce the negative effect of equal length primaries at low rpm, but also reduce the benefits of tuning for low rpm. it gets down to fuzzy lines on inaccurate dynos. seat of the pants is your best measuring stick.

[Johnny Danger]

Originally Posted by The Radium King

ok. most dynos i've found are for (a) headers, and (b) cat back (ie, both mid pipes and muffler). again, very few perform on paper.

if we look at the variation in cats between usa and row cats, we could assume that the secondary cats on usa cars are an afterthought (required for emissions) and were not considered in the initial tuning of the exhaust. so, we can return to the intended tune of the car by going to catless mid pipes (as well as reduce some lbs and improve sound - cool).

if that assumption is true, then the meaningful tuning comes from the headers and the relevant back pressure comes from the exhaust. i have a set of ebay 'knock-off' headers and would note that, while they are presumeably a copy of fabspeed or somesuch, they are very rough at the manifold flange and could really benefit from an old-school polish. as you state, varying results. a longer header will reduce the negative effect of equal length primaries at low rpm, but also reduce the benefits of tuning for low rpm. it gets down to fuzzy lines on inaccurate dynos. seat of the pants is your best measuring stick.

For the most part I agree with your assessment. Upgrading the oem system involves a delicate balance of reducing restricted air flow, and at the same time maintaining a necessary amount of back pressure . I found this to be especially true throughout my experimentation with headers/pre-cats. Ultimately, the most successful application for me involved following the basic design principles of the oem lay-out, however, improving upon them in the ways previously mentioned. The quality of materials, smoothness of welds, precise mandrel bending and the particular elements used in the cats were all extremely crucial to the overall success of my exhaust project . Once this was accomplished, the end result yielded an impressive increase in performance .