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Old 10-08-2012, 08:39 AM   #1
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Top Speed Bypass Pipes For 2.5L

Last week I picked up a set of the Top Speed bypass pipes for my 1997 2.5L Base model. Yesterday I took a look under the car to plan out the install and noticed something I hadn't before ordering them, both O2 sensors are mounted to the mid pipe/ cat pipe. The Top Speed bypass pipe only has one O2 sensor mounting point.

So my assumption is that they simply want you to leave the other O2 sensor off the car. This of course would produce a check engine light so I thought what if there was a second mounting point on the bypass? After some research is seems that on the early models that had a single cat, the system looks for a different reading between the O2 sensors since one is before the cat, and the second one is after, therefor creating a different reading from both. Being a straight pipe, this wouldn't work.

The other options seem to be to accept the check engine light and ignore it, but then what happens when you get a check engine light from something else, you won't know.

Someone also mentioned flashing the cars computer, but that's a bit too far for me.

So has anyone in the same situation come up with a different work around that leaves both the computer and check engine light in tact?
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Old 10-08-2012, 09:32 AM   #2
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Mike,

You can't delete the cats on a 2.5 without triggering a CEL by virtue of the fact there's only one set of cats for that model boxster. By eliminating them, the vehicle will be render catless, and therefore you'll have to re-map the ecu in order to get rid of the CEL. The potential loss in low to mid range torque is another discussion.
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Old 10-08-2012, 09:53 AM   #3
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You are correct about your secondary O2 sensor. If you install the TS bypass, your only options to avoid a CEL is to either have a new cat custom spliced into the system or install the later model headers with a cat incorporated into the header (not sure if this is direct on), or get a ROW flash which eliminates the need fo a second O2 sensor.

If you decided to drive it as is with a CEL, I'm not sure what that will do to your engine management system. It may go to factory set mixture settings utilizing throttle position and MAF to determine mixture.

Let us know which direction you go...
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Old 10-08-2012, 07:05 PM   #4
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Some people have had success simply mounting the rear o2 sensors out in the air and the fronts into the mounting holes. Didn't work in my case but worthy of a try.
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Old 10-08-2012, 07:27 PM   #5
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Will cutting into the pipe and using a spark plug arrestor for the downstream O2 sensor work? Did this successfully for several Subaru's, but since readings are taken from the connector with a Porsche, I presume this will be a problem. It's something I've been considering when I pick up some eBay headers and bypass pipes.
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Old 10-12-2012, 02:39 PM   #6
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Quote:
Originally Posted by Okanebm View Post
Will cutting into the pipe and using a spark plug arrestor for the downstream O2 sensor work? Did this successfully for several Subaru's, but since readings are taken from the connector with a Porsche, I presume this will be a problem. It's something I've been considering when I pick up some eBay headers and bypass pipes.
Let us know how that install goes. Top Speed says you can use an adapter that moves the O2 sensor ½" - 1" out of the normal bung hole….question is, will that throw a CEL? Someone who has been successful please feel free to chime in.

Waiting to get my headers put on BUT WANT TO KNOW WHAT I AM GETTING INTO. How hard is it to re-program with a flashy thing? Where?
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Old 10-12-2012, 03:14 PM   #7
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Quote:
Originally Posted by Homeboy981 View Post
Let us know how that install goes. Top Speed says you can use an adapter that moves the O2 sensor ½" - 1" out of the normal bung hole….question is, will that throw a CEL? Someone who has been successful please feel free to chime in.

Waiting to get my headers put on BUT WANT TO KNOW WHAT I AM GETTING INTO. How hard is it to re-program with a flashy thing? Where?
I don't think anyone does ecu mapping for 2.5's. Maybe Pedro ?
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Old 10-12-2012, 06:00 PM   #8
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I've had success in the past with the o2 sensor adapters on another car..I had built a turbo system years ago (not on a Porsche unfortunately) that had deleted the primary cat, leaving only a secondary..that little o2 adapter worked great, no check engine light. Might work for your application, worth a shot they are cheap. Most of the cost will be having a muffler shop weld a bung in the pipe for you.

This is a pic of what I'm talking about..not this actual one but good reference
http://www.bigdaddiesgarage.com/images/o2-straight-adapter-install.gif
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Old 10-12-2012, 07:37 PM   #9
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Since your bypassing it anyway, couldn't you just put a 1M ohm resistor across the O2 leads, this should eliminate the CEL..
http://zero-signal.org/o2/O2bypass.html.htm
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Old 10-13-2012, 04:54 AM   #10
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Originally Posted by tony_fury View Post
Since your bypassing it anyway, couldn't you just put a 1M ohm resistor across the O2 leads, this should eliminate the CEL..
O2 sensor bypass
I should caveat my post.
I found this circuit on the Internet and not certain if this circuit would work on your P-car. I take no responsibility for this.

I have no experience with this type of mod on the O2 system of a car. I have experience bypassing O2 sensors on motorcycles, but not using the circuit that I posted the link to. I used strictly a resistor and for the life of me, I cannot figure out why they would use a capacitor in parallel with the resistor.
This combination of resistance and capacitance in parallel is known as a RC tank circuit. The capacitor will act as a charging tank, while the resistor will drain the charge.
It will provide as a great voltage filter in this scenario.

The oxygen sensor is positioned in the exhaust pipe and can detect rich and lean mixtures. The mechanism in most sensors involves a chemical reaction that generates a voltage drop. (Resistance variable)
The engine's computer looks at the voltage level to determine if the mixture is rich or lean, and adjusts the amount of fuel entering the engine accordingly.The reason why the engine needs the oxygen sensor is because the amount of oxygen that the engine can pull in depends on all sorts of things, such as the altitude, the temperature of the air, the temperature of the engine, the barometric pressure, the load on the engine, etc.


When the oxygen sensor fails, the computer can no longer sense the air/fuel ratio, so it ends up guessing. Your car performs poorly and uses more fuel than it needs to.

By bypassing the O2 sensor and providing a set resistance, you are essentially fooling the computer that the AFR is at this level always. It is only providing a fixed voltage drop.

Here is a good read on what the O2 sensor does.
how an oxygen sensor works

T
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Last edited by tony_fury; 10-13-2012 at 05:50 AM.
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Old 10-13-2012, 09:00 AM   #11
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I've pretty much decided to sell the pipes instead of having to deal with check engine lights, resistors, bungs, etc.

I'll probably post them in the for sale section for about $125
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