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Greetings list,

I was thinking of ways to possibly/slightly improve performance of my 33 16v boxer motor that I have recently rebuilt as the engine by its nature doesn't have a great deal of torque at low rpm. I thought about ways to improve this a little, and exhaust back pressure seems to be one way of acheiving slightly more torque sub- 3500rpm. The flipside is that it is too restrictive at higher rpm - so here's my idea:

I wish to either find or construct a heavy duty butterfly valve which is operated by a servo or stepper motor, much like a throttle body. The butterfly will of course not completely shut off the exhaust but let exhaust through holes in the buttlerfly plate. My plan is to utilise a small microcontroller to variably open/close the valve depending on RPM (as my background is computer science, this will be the fun part!). I know that Ferrari have done something similar on the 360 Modena, so I would be interested to know how their system works.

Another aspect of the project is to simply have the car "quiet" below 4000rpm, but make a bit more noise when I push it a bit harder - so I only annoy the neighbours when I want to :)

I don't really know where the valve should be located - in between the engine headers and the resonator? between the resonator and the muffler?

And before I start hassling someone to use their mill, does anyone know where I can find a suitable 2" or 2.5" butterfly valve?

Your comments + criticisms welcomed :)

~Benjamin
 

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check summit and then e-bay..theres a whole bunch of good butterfly exhaust valve designs and a few high end ones with electric steper motors..all youd have to do is build a controler to adjust via rpm.......another option is one of the ford units wich is vacume controled


that said...what i would do is get one and then buy or make spare butterflys and drill holes in them larger and larger till you get the desired flow at idle...so that the valve can completly close as designed to keep carbon from over building around the valve
 

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I thought about ways to improve this a little, and exhaust back pressure seems to be one way of acheiving slightly more torque sub- 3500rpm. The flipside is that it is too restrictive at higher rpm - so here's my idea:
The key to more low end torque is smaller diameter exhaust runners. Increased back pressure is just an unwanted byproduct of a smaller exhaust.

Along the lines of your idea - My Nissan Maxima's muffler is designed with a spring loaded baffle. It is very quiet and supposedly not very restrictive at high rpm.
 

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Mad marky at exhaust tech do something like this im pretty sure. Not sure if theirs is motorised but shouldn't be too difficult. Dunno if you'd gain much performance but would be good to shut the little bastard up once your sick of the head ache :)
 

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I would go with more smaller butterflies, but as proven time and time again, butterflies are not actually a linear increase in flow, but its the initial opening of the valve the provides most of the airflow, the change becomes closer to linear after that. Also but the assy. as close to the collectors as possible.
 
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