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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Agreed, the lower the back pressure in the muffler the better. I stand by my note on the factory muffler and heat exchanger. It is a very good system w/o the combo cat/muffler. The remainng muffler is not a big bend. It is a very open chamber on a tail pipe hooked to it.

There are no dought better muffler systems. My fav is to dump into a flow through muffler and exit the pasanger side with very little bend.

Of cource the best turbo exaust is very short (apx 10"), straight, and with the proper flair to it to keep the gasses from slowing down when they cool but not so much they delaminate against the walls of the tubing.

The 76/77 turbo had a vac sensing WUR. It would reduce control pressure when manifold vacuum went away. The WUR's after that take something like .5 bar of boost before the control pressure is lowed. After the exhaust past the turbo is opened up the next bigest thing that is contributing to turbo lag is the restrictive metering plate. Reducing CP earlier reduces this lag in a noticable way.

I used to reduce my CP to almost zero electronically with a frequency valve and controler to get the metering plate moving. Then I would bring back some to the CP to keep my fuel ratio where I needed it.

As to the CBV, on a stock C2T the compressor bypass valve works only under significant vacuum between shifts. If you do not hear it hiss between shifts or if your boost drops a bunch between shifts, it is bad. The do seem to fail quite often.

CIS trubos are unique as they can not use a Blow Off Valve that vents to atmosphere. The air must be circulated around the compressor wheel in a way to keep the air flow through the metering plate flowing at the correct rate. A larger Compressor Bypass Valve allows more air to pass between shifts and keeps the compressor spinning at a faster rate so boost comes up faster.

I put an after market adjustable CBV on my C2T that had a higher flow rate for less stall between shifts. Many people do this.

However, I figured out that if one could keep it open the compressor wheel would have to spin faster as air is alowed to escape around the compressor in a loop. Thus, if the compressor wheel was spinning faster most the time. This gave me ready reserve and instant boost when I needed it instead of waiting for the turbo to have to spin up.

This was noticeable on the freeway when I needed to pass and off the line when I cracked the throtle.
Old 12-04-2008, 02:14 PM
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