How-To - Tuning Your Exhaust
Interesting article I read from Domestictunerz.com... just tought I might share with you all...
Do you REALLY know how important your exhaust system is? Tuning your exhaust isn't about SOUNDING cool. It's about going fast.
I've learned my fair share of tips and tricks over the years, from many people who've been in the racing business for decades. So now it's time to talk exhaust.
Lets start from the engine and flow back (pun intended).
Cylender head- to port or not to port?
The first place your exhaust sees when it's leaving the engine is the cylender head. Your car can only suck in as much air as it can pump out, remember. Although not necessary for engines that are not severely modified, a good port and polish will give you a little HP. A port and polish (AKA P&P) opens up the exhaust ports to a larger diameter (port) and smooths them out (polish) for best flow. Most people take their heads in to have them P&P'd, but if you're willing to take the risk you can do it yourself.
Cam Timing- where being retarded can be better
In order for your exhaust to leave the engine, the cam has to open up the valves so the piston can push it out. Most cars are timed a little bit advanced, which allows for better fuel economy, better low-end torque and cleaner emissions. But (unless you're FastFireTwoTwo) you want to go for top-end power. An adjustable cam gear should do the trick for most cars. Play around with the timing a bit and see where your car likes it best. When you're at the track, make several runs at different degrees of timing and see what gets you the best ET. Also note that when adjusting timing you will want to change your shift RPM slightly (a little higher when you retard it, a little lower when you advance).
Stock manifold vs. Street header vs. Race header
Your stock manifold has one thing in mind- low end torque. While it makes the car more drivable, it also robs you of high-end power. One thing you can do is get your stock manifold ported and polished. This will give you a little more high-end power while not sacrificing much low-end torque. A street header is the best option for a dual purpose (street and race) vehicle. It offers good high-end power and decent low-end torque, keeping the car drivable on the street, yet making it more powerful when racing. A race header has one thing in mind- top-end power. It sacrifices a lot of bottom-end torque while giving you massive top-end power. However it's not very good for a dual-purpose vehicle, because you have to rev higher to start moving.
Your catalytic converter
This one here is actually a no-brainer. Cats rob power. If you can get away with it, get rid of your cat. However if you get caught it's a $1500 fine from the EPA (I'm not responsible for your actions!). If you need a cat, however, go with a nice high-flow cat. In your Summit Racing catalog, look for a good high-flow cat. I know they are available for under $75, and will make a drastic improvement over your stock cat. I know someone who had a stock cat on their car and ran a 17.1, then a month later ran a low 16 with a high-flow cat. So yes, the cat makes a huge difference!
There are LOTS and LOTS of mufflers out there. Don't be fooled. Replacing your stock muffler(s) with a big chrome one doesn't do anything but make noise. IT WILL NOT INCREASE YOUR HP, even if it claims to. Your stock exhaust system was designed to make low-end torque, not high-end power. A high flowing muffler without a high-flowing system is like a band without a conductor... it might make some noise, but it won't do anything productive.
When looking for a muffler to go for power, look for a muffler that you can see straight through, and the muffler body should not be much larger than the inlet and outlet. Also look through the muffler. Does it look like a screen in there or is it louvered? A "screen" muffler will flow a lot better than a louvered one. Another good option (if you have the money) is a Supertrapp muffler. These mufflers sit on the end of your exhaust system (like a tip) but are tunable. You can add discs to them to make them more free-flowing (top-end power) or remove them to make more backpressure (low-end torque).
When making a FULL exhaust system, make sure all your components line up. Most street cars don't need any more than a 2 1/2" exhaust system. A good rule of thumb is to make your new system 1/4-1/2" larger than your stock system. So if your stock ststem was 2", get your (header/manifold) to be 2 1/4"- 2 1/2"... and make everything else the same size (or slightly larger as you get farther back).
When making/buying a Cat-Back system, make sure everything after the cat (or including the cat) is the same size (or getting slightly larger farther back). A Cat-Back system is nice but often very expensive. Check with your local muffler/exhaust shop before ordering anything. They might be able to make you a system for cheaper. Sure, it won't be stainless steel... but if you're REALLY going for power, that doesn't matter.
A little tip I learned from a Pro-Vee racer
If you REALLY REALLY want power I have a tip. Note this may be illegal. Take a can of cheap spray paint and paint your exhaust system white from the cat back. Drive around for a day or two then look at the system. Most of the system will be black (the paint burned off). Where the paint stops turning black is where the exhaust is mostly cooled and is just "along for the ride". Now get your hacksaw and cut your system off just after that point. Bolt on a high-flow muffler (if you removed all your mufflers). Then target that exhaust right out the side using a 90 or a 45 bend and some straight pipe. This is what I did. It cost me all of $40, made my car sound nice, and made my exhaust flow like the Mississippi.