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Old 10-02-2014, 12:52 PM
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Default Bypass exhaust system

Hey all,
This is kind of premature as far as my build goes, however I like to get things worked out early before I get to the point of installing them.

What Im working on is a bypass exhaust system for my 2.0 WRX turbo motor. What Im trying to figure out is how to actuate the bypass valve based upon throttle change %/sec. The idea is via a cable and possibly a resistance actuator (similar to that of a screen door actuator but much smaller) the cable operated bypass valve will open under sudden throttle change, but return to the closed position when the throttle is held in place when your cruising along.
The effect will give the car a definite Aston Martin style growl when the throttle is depressed, and then it will taper away back to the normal muffled sound.

I have an idea of how to do this, but I am interested in ideas on how to actuate it. A vacuum based system is also another option. When the intake pressure drops from a sudden change in the throttle position it could cause the actuation as well.

Thoughts?
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Old 10-02-2014, 01:50 PM
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Technically grounds only, don't do it, no back pressure = drop in power, raise in fuel etc for no advantage bar the sound and you would soon tire of it. but if you must, a vacumn system with a spring return damped would do the job without too much complication
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Old 10-02-2014, 01:53 PM
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Its a turbocharged engine with two OEM subaru cats on it. So there is no issue with back pressure. This bypass system would be mounted after the cats, and would just bypass the muffler. So momentarily it would go to straight pipes, or through a different muffler with less baffling. All of the needed back-pressure is left normal before the cats regardless of what happens to the rest of the system.
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Old 10-02-2014, 03:40 PM
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I guess the question is can you come up with something that would react fast enough? Maybe a simple electric solenoid to swing the gate when the throttle reaches a certain point, but there would be no way to taper it without some sort of resistance feedback. Any way to tie into the tps?
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Old 10-02-2014, 03:56 PM
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Yamaha had something like that on the FZR's of the 90's called an X-up valve. More throttle = more valve opening.
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Old 10-02-2014, 03:59 PM
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Its really just a pair of y pipes, one will have the valve in it. Once i get the actuation part firgured out it would be more, "where can If it in".

If I could use a vacuum based system Icould just set it up to operate when a certain change in vacuum is present. However that could make the system stay open while driving at freeway speeds. I need something to detect only a massive change in vacuum.

A mechanical system would work if I could source up some kind of actuator that like a screen door resist throttle change, and actuate the system that way. Then it would slowly release the tension on the cable and go back to its resting position. But even that could have complications in its design.

There are key fobb remote control based systems but those only allow you to operate the valve on or off. Not a dynamic change on the fly. And they operate slowly.

Hmm, always challenges....

Yeah Nic, I have been playing with motorcycle performance muffs as an option. For a turbo they are plenty big, and free flowing. I was thinking of using one in the bypass side of the system. It would take the edge off the exhaust note, while still hopefully giving me what Im looking for in sound output. Even some of the shorty exhausts might be free flowing enough for this engine. I would need to get my hands on one to see for myself.

Follow up: the Exup valve requires an ECU so it may not be an option. Ill look into the wiring and control of it to see if it can be hacked.

Last edited by ydeardorff; 10-03-2014 at 01:19 AM.
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Old 10-02-2014, 09:32 PM
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Quote:
Originally Posted by Nic View Post
Yamaha had something like that on the FZR's of the 90's called an X-up valve. More throttle = more valve opening.
This is what your talking about right?
07 08 09 10 11 12 Honda CBR600RR CBR 600RR Exhaust Servo Exup Valve | eBay
I like this.... Good find rick! Ill have to look into this further to see how this is actuated.

Im not sure on how this works as both cables look to connect to the same barrel.

Last edited by ydeardorff; 10-02-2014 at 09:37 PM.
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Old 10-03-2014, 03:08 AM
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Electronicly you are looking at a stepper motor hooked up to the throttle in the same way an electronic throttle is and there are a load of different systems available or vacuum operated like an EGR with the spring producing the required gradual opening against vacuum.
Just remember 10%+ of factory engine development goes into the exhaust design.
Back pressure is built in to stop incoming fuel/air mix from exiting the exhaust before the valve closes and consists of many factors not just the muffler. On a two stroke it is absolutely vital to get right but even on a four strike it is important to avoid fuel loss and weak mixture. There are also various designs of insert that can produce different sounds from bass to ripping Italian sports car and turbo whistles. Induction roar can also play a big part in the sound of an engine, Fezzers and Lambos, etc would sound very different if they had different intake systems and much of the yowl from a motorcycle is induction not exhaust as they have to comply to the 98Db rule in most countries but if you are going for the deep bass note then large bore with a reverb would do, it's all about interrelated frequencies and harmonics and a very complex subject.
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Last edited by Peter; 10-03-2014 at 03:21 AM.
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Old 10-03-2014, 09:27 AM
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This might give you some more ideas.

8]
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Old 10-03-2014, 10:47 AM
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I like that dump valve idea for the partial open exhaust, of course it cannot pass all the gas, being a 60mm valve in a 75mm system but rather reduces restriction to allow higher through put and I don't think it would produce a 'nice' note, just a noisy one like you have a hole in your muffler, really need to 'tune' the outlet for a good sound, I would have taken a pipe from the valve outlet to a pipe tuned to a length to get a rasp. Be interesting to see what it ends up as though.
If money is no object a Vortex muffler will sound awesome. This is where there is a straight though surrounded by a cone and vanes so there is two speeds of gases. it produces a nice sound.
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Last edited by Peter; 10-03-2014 at 10:57 AM.
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