Posted by Troy Rudolph on June 29, 2001 at 23:05:28:
I had a custom exhaust built when the 3.5L engine was installed. It consists of a 2-into-1 performance muffler under the car, and a glasspack where the stock muffler goes. It resonated more than I wanted, but it was OK, or so I thought.
There was a noise that I assumed was backfire after getting back on the throttle after coasting. Unfortunately, that sound turned into a nearly constant rattle when the engine idled.
Then, it turned into an intermittent rattle. Here's a big clue, the car has no power when the rattle goes away. The power comes back when the rattle comes back. I checked the heat shields just for grins and they're OK. So, it's obvious that a baffle or something has come lose inside the exhaust system and occasionally causes blockage when it gets jammed in the wrong place.
So, I've been evaluating exhaust choices again. And, this led to some questions. For example, while I'm not planning to buy one, I took a look at the MM 300 system to see what it would take to make one of my own.
Then I began to wonder why it's such a big deal that the system flows 300 rpm. After all, the 3.5L engine sucks in around 600 cfm at 5000 rpm. Since there's no loss of mass in the combustion process, and the exhaust gases are much hotter than the intake gases, it seems that you'd need more exhaust flow than intake flow.
Clearly I'm missing something. Exhaust valves are always smaller than intake valves and all those engineers can't be wrong. I surmise that a partial answer is that it is easier to push the gases out than it is to draw them. However that can be all of it. Can any of you shed some light?
My current thinking on the exhaust is to use the dual-in/dual-out catalytic converter that I have with some early 635CSi downpipes, an '81 635CSi front muffler and a '78 635CSi rear muffler.
The '81 635CSi front muffler was designed to be used with 3-muffler system (front-center-rear), but it's nice and short and will fit the behind the cat. with no trouble.
The '78 635CSi rear muffler has 2-inch inlets and outlets and was designed for use in a 2-muffer system (center resonator-rear muffler). The center resonator in that system is just too long to fit behind the cat.