I'm having a set of 1 3/4" shorty headers custom built for my car. I have some questions regarding the secondary piping and the header collector. David Vizard has a formula that says the collector size should be 1.75 times the primary header pipe size. That works out to 3 inches for the header collector. Car has to be California smog legal. I can sneak by with the shorty headers. Cats have to remain.

Here is my current set up. I'm looking to make around 450+ horsepower at the flywheel with my next round of mods on my 355 CI motor. The car is a 3rd gen with dual 2 1/2" cats. The car currently has 1 5/8" headers with 2 1/2" pipes to the dual cats and then a Mufflex 3 1/2 inch exhaust system with a Magnaflow 3 1/2 inch muffler. Most cars I have seen have the 2 1/2" pipes or less from the headers to the cats.

I believe there is horsepower to be had with a properly designed secondary pipe system from the header collector to the cats and then on to the Y pipe. Here are the questions.

1. Would it be best to run 3" pipes from the headers to the cats and change the cats to 3"?
2. Maybe run 3" from the header collector for a certain distance and then next down to 2 1/2"? Sort of a "tuned" collector.
3. Maybe stick with the original 2 1/2" design with the pipe expanded to 3" at the collector?
4. Maybe a compromise system with a 2 3/4" pipe from the collectors to the cats.

Lots of knowledge on the board here and hopefully someone has already been down this road and can input their knowledge. Also does anyone have knowledge on a good 4 into 1 merge collector for the headers. Looking for something with low turbulance. Thanks in advance.

 

Just my 2 cents, I think your cats are going to make things interesting. There are a lot of variables in a non smog exhaust, like you said primary tube size, collector size, pipe diameter, pipe length to mufflers, muffler selection and pipe after the muffler as well as even the angle the exhaust leaves the mufflers.

I think your cats are going to add a huge unknown for a lot of people to work with. It might be easier to flange your exhaust before and after the cat, and have a setup for passing smog and a setup for making horsepower. That is just my opinion though.

 

Get some high flow cats and run 3" "Y" to 3-1/2"or 4' out the back With a 17" long shootout muffler.

 

From the research I have been doing it looks like a 3" system all the way to the Y-Pipe from the hheaders might be the best way to go at the 450HP level. The "Y" would then go into my 3 1/2" Mufflex system. Now just have to see if the 3" pipe will fit.