darkducati

I am looking for help or suggestions on exhaust design for my RX-8. Normally I would just buy one, but I believe that the aftermarket companies may be decreasing performance by using a dual setup like the factory did. I do not care much about how this looks, so a single will be fine. The other question is which piping diameter to go with. I am pretty new to rotaries, but have heard that they like the lowest level of restriction possible. Is this true? I don't care much about the bottom end, so whatever helps power from 5K up is what I am interested in. With these things in mind, I am thinking that a 3 inch system with a good straight through muffler will be the way to go. This will be a street driven car, so I will probably use a muffler that will allow the exhaust to make two passes. Of course this will do nothing if the manifold and midpipe are not addressed. So, I am thinking a midpipe and cat of 3 inches in diameter and a header that will have larger head pipes with a 3 inch collector. There is a wealth of knowledge on this board that I think will help guide me in the right direction. Thanks.

BTW Sorry for the length. I was trying to be as thorough as possible.

rotarygod

If you think you're long winded you've never read any of my posts!!! Here's a long one now:

This same argument always surfaces on the RX-7 forum about the 2nd gen RX-7. They also had a single exhaust split out to 2. The difference was that from the factory they had 2 mufflers. There are people on one side of the argument that say one muffler is lighter and therefore the car will be faster. They also say that added turbulence from a Y pipe also decreases performance. Here are some facts:

A good Y pipe is very smooth and not a source of turbulence. An improperly built one is. If you want to fab up your own system using a good Y-pipe Flowmaster has one for $35 from Summit Racing.

A Y-pipe that splits from 1-3" pipe to 2-2.25" pipes is showing an increase in area after the split. There is less pressure through the pipes after the Y than there was before it. It is actually beneficial to have a small area increase as you go back anyways.

2 mufflers are quiter than 1 when we are comparing mufflers of the same style. A smaller pipe through the muffler is quieter than a larger one. You propose using a single 3" through muffler. Unless you are using a chanbered muffler realize that all the muffling is done on the gasses that touch the outsides of the internal tubes since this is where the packing and damping material is located. Remember that 2-2.25" pipes has more flow than a single 3". Now also realize that there is more edge contact for better muffling capability in the smaller piped mufflers. 2 mufflers are quieter and better flowing. It's a win win scenario.

In order to get a single muffler to quiet effectively it has to be one of two things or both. It has to either be heavily packed and chamber effectively or it has to be big. A heavily chambered muffler can flow good but it would be best to use a larger diameter core design than the intake pipe you are using. This way there will be a smaller restriction. Remember that the amount of air passing through a pipe is directly related to gas speed. Slower air is less air and a larger pipe is slower air, therefore less of a restriction.

I think what you are going to quickly discover is that you have fabbed up a system that is much louder than any other system out there yet doesn't really have a power advantage over any of the others. None of the aftermarket systems are impressively better than stock and most of them have similar gains. Mazda just did a good job. If you want to do the single exhaust do it for looks since that is all that you will have that is different.

You also need to realize that most of the tuning of an exhaust is based on the primary pipe diameter. If you have a 3" after the cat, it doesn't really matter how large you expand it to 4 feet downstream if at all. Remember that after the tips it is in a space of infinite size. Removing the muffler completely probably won't change your powerband as long as there is a midpipe still in place. Since all of your restriction is at the cat, optimizing flow after it doesn't hold alot of merit. You'll see a few more horsepower but nothing spectacular. Also realize that the farther a muffler is from an engine, the less of a restriction it is. This is why that big keg under the back bumper really isn't that bad. The gasses leaving the engine and hitting the cat are of much more intensity. They are also much hotter. People think of exhaust gas energy as pure flow but the reality is that heat is also energy and the exhaust loses alot on the way to the back of the car. As the gasses cool down they also slow down since cooler air is denser air. Denser air takes up less space and hence anything in line farther downstream is less of a restriction than it was closer to the engine.

You're trying to functionally improve the shape of the wheel in a sense and you're not going to do it. What you will do is to create a visual alternative to the mass market.

darkducati

Thanks for the input. I guess I will try your method and see where it gets me. I already have two 2.25" inch stainless mufflers that should work well for this project. Thanks again.