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Old 07-18-2013, 06:23 PM   #1
tbox56
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Question Exhaust for Porsche 914 swap

Several years ago I swapped an EJ25 into my Porsche 914, and have recently had time to pick the project up again. Right now I am working on designing an exhaust system, and trying to do my research to get it done right the first time, rather than having to rebuild it in another year.

The current plan is to do a dual exhaust all the way back (only a few feet, it's a mid engine car) using a pair of sweet thunder mufflers. Passing an emissions test is not an issue where I live, so I am not putting any catalytic converters in. There are a couple of things that I need to figure out at this point. 1: How should I handle the O2 sensors? There was originally one in front of the first cat and one between the first and second. With no cats and two pipes how should I handle this? I know that the sensors produce a varying resistance as the exhaust gas composition changes, so I cannot wire two sensors (one for each side) in series or parallel, because that will screw with the readings, either doubling the resistance, or halving it respectively. Has anyone tackled this problem before?

The other sensor issue is how to eliminate the second sensor, originally between the cats. I have read that it is used to determine if the cat is at the correct temperature, and will actually force the engine to run slightly richer if it is not to heat it up. At this point, I am planning on determining the ideal resistance that does not affect how the engine runs, and 'fake' the computer so it always sees that resistance. The other issue is that of the heating circuit for that sensor. Presumably that circuit will change resistance as it approaches operating temperature. Does anyone have any experience with this?

Finally, I need to decide if I am going to install a crossover in the exhaust system. My research has told me that the largest gain to be found by using a crossover is in sound quality, and that much of the engine drone can be eliminated with the use of a crossover. Theoretically there can be some power gain as well, but I think with how short my exhaust system will be any gains will be negligible. Does anyone have any thoughts on this topic?

Thanks for you time!
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Old 07-20-2013, 02:41 AM   #2
Teryn It Up
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If you want to run 02 sensors for separate banks you will need to use an ecu that is compatible with a dual bank ignition system. In most 4cyl subarus there is only one bank for fuel trim control. The 02 sensor ahead of the cat will read the oxygen content of the exhaust stream and the ecu will either enrich or lean out the fuel trim. Some vehicles, typically ones with more than 4 cylinders, have separate fuel trim control for both banks separately. The 02 sensor post cat is purely for monitoring the cat. Thank you C.A.R.B.

So if you want to run true dual exhaust you will need a new ecu.

My suggestion would be to use a stock or borla or custom header and then split the exhaust into dual exhaust afterwards.

It may be possible to just monitor one bank of the motor. But I would think the ecu would recognize the reduced number of cylinders in terms of reduced exhaust gas. Then think it has a lean condition and either enrich the fuel mixture or throw a dtc for a lean condition.

One more idea, and you may have to talk to someone who actually tunes cars to verify, is that you may be able to tune your way around the problem, with a wide band controller or something of the sorts.
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Old 07-21-2013, 12:50 AM   #3
tbox56
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Thanks for taking the time to respond.

As you probably assumed, I have the stock ECU, and like you said, a single O2 sensor and a single point of control. Unfortunately, last I checked, the 04 2.5x forester ECU was not tuneable, so that takes 'tuning it out' off the table. I looked at aftermarket computers, but for now would really like to stick with the stock computer, more for cost and simplicity sake than anything.

The problem with a Y pipe and then splitting them is space. I only have about 3 feet from the exhaust port on the engine to the back of the car, with nowhere to cross or merge the pipes without adding a rats nest of pipe. My original thought was an X pipe and put the O2 sensor right in that, but space became a huge issue.

I think at this point my best option is to look at it from an electrical perspective, not a mechanical one, and determine how to reproduce the readings of one sensor with 2.. It may be a science project, but it should be fun
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Old 07-21-2013, 03:09 AM   #4
Teryn It Up
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Well then your best bet would be to splice two sensors together in parallel and see what happens. My guess would be the frequency would look slowed to the computer and it will think the sensor is not responding. Or the 5v reference draws too much amperage and frys the ecu. Or the heater circuit pops the fuse every time, unless you install a larger fuse and see how long the power wire lasts.

Again I would take the mechanical route. I would use a turbo exhaust manifold and build off from the up pipe flange. That would direct the exhaust from the left bank in front of the motor and connect to the right bank then toward the rear of the vehicle then run the exhaust to the rear along then trans.
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Old 07-22-2013, 04:25 AM   #5
panel
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tbox56....have a look here.........lotsa info

http://www.914world.com/bbs2/index.php?showtopic=110990
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Old 07-22-2013, 10:23 AM   #6
Jaxx
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Quote:
Originally Posted by Teryn It Up View Post
Well then your best bet would be to splice two sensors together in parallel and see what happens.
noo!!!!

the o2 sesnors are not the same
one is a wide band the other is a narrow band

i don't know that you are going to be able to make this work
an 04 forester ecu is going to be looking for things like the evap purge and other things i can't think of off the top of my head
the ecu will start but bein limp mode
it may be time to look at and standalone ecu
most of the non subaru bodies swaps use older pre odb-2 (1.8/2.2)ecus or ecus that can be flashed

look at
http://www.outfrontmotorsports.com/e...management.htm
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Old 07-22-2013, 01:22 PM   #7
Teryn It Up
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Quote:
Originally Posted by Jaxx View Post
noo!!!!

the o2 sesnors are not the same
one is a wide band the other is a narrow band
We were talking about making two bank 1 sensor 1's to work together. Not a bank 1 sensor 1 and a bank 1 sensor 2
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Old 07-24-2013, 12:09 PM   #8
Jaxx
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they are not bank sensors
they are primary 0-5v (long term fuel trim)
and secondary 0-1v (short term fuel trim)
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Old 07-24-2013, 12:19 PM   #9
tbox56
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Quote:
Originally Posted by panel View Post
tbox56....have a look here.........lotsa info

http://www.914world.com/bbs2/index.php?showtopic=110990
I forgot about that post! Thanks for reminding me!
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Old 07-24-2013, 12:22 PM   #10
tbox56
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As far as the splicing of sensors, I am not going to just cut the cables and hook them together, but I should be able to make a custom circuit board with some AND/OR logic that would emulate the signal that a single sensor would make based on the input of the two sensors. It shouldn't be too tough, since we are just playing with variable resistance. Powering the sensor heaters can be done with a transistor to ensure the current draw does not exceed what the computer is supplying (I will have to verify if this even goes through the computer to begin with.)
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Old 07-25-2013, 03:04 AM   #11
Teryn It Up
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Quote:
Originally Posted by Jaxx View Post
they are not bank sensors
they are primary 0-5v (long term fuel trim)
and secondary 0-1v (short term fuel trim)
That is the measurement of b1s1.

Quote:
Originally Posted by tbox56 View Post
As far as the splicing of sensors, I am not going to just cut the cables and hook them together, but I should be able to make a custom circuit board with some AND/OR logic that would emulate the signal that a single sensor would make based on the input of the two sensors. It shouldn't be too tough, since we are just playing with variable resistance. Powering the sensor heaters can be done with a transistor to ensure the current draw does not exceed what the computer is supplying (I will have to verify if this even goes through the computer to begin with.)
The signal is not a "variable resistance". The flow of the exhaust into the sensor element creates a chemical reaction that produces voltage. There is no resistance measured by the ecu.
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