Exhaust Theory

So you think you know a little bit about exhaust systems and what performs the best?

I am using this discussion in order to show people that the old days of thinking back pressure are needed and that a lack of back pressure will show a power loss with too large of an exhaust.

take this comparison between the skunk2 exhaust and a 3" exhaust

This comparison was done with an h22 motor that is otherwise stock other than minor bolt ons and tuning. This chart shows the difference between the exhausts, however, the skunk2 exhaust was tuned and the 3" exhaust was NOT tuned. This tells me that through tuning, a little more could be brought out.

In past times, it was believed that every motor would do well with a 2.5" exhaust. Seemed to be the consensus, but no one really knew. You find people talking about it who really dont have a clue and have just read what others have said and followed the bandwagon. So because a turbo car likes a 3" exhaust, it is automatically assumed that 3" is too big for naturally aspirated.

This may be true for smaller 4 cylinder engines like the 1.5L D series and the 1.6L vtec motor. These engines likely would not respond to a 3" exhaust like a 2.2L H series motor would. Big engines need big exhaust in order to allow proper flow. You cannot generalize exhaust size based on it being 4cyl, 6cyl, 8cyl....etc. All engines are different and are going to require different exhaust flow systems.

I was reading over and thread on honda-tech where a member said he had read an SAE article about exhaust scavenging. This article stated that everything beyond the last merge in the exhaust was a restriction. Does this not make sense to anyone? It makes perfect sense! The last merge is typically at the end of the header before going through the cat-back so my interpretation of this statement is that the bigger you go, the more power you will make. But of course there are restrictions to this. As an exhaust gets bigger, you have less room regarding ground clearance. So obviously you cant put a 5" exhaust on your lowered civic and expect to get over speed bumps. A properly built exhaust for racing will not include a cat back. The header will be composed of 4 runners of equal length which will dump 4 tubes into one (4-1 merge collector), or the 4 runner will dump into 2 merge collectors which will result in dumping into one last merge. The purpose of merge collectors in conjunction with equal length runners is to use the scavenging effect. This effect is the process of each exhaust pulse that enters through the merge collector will end up creating X amount of vacuum behind it, which in effect pulls the next incoming exhaust pulse through the collector. This creates an effect which is continually making the exhaust velocity increase and engine RPM increases. After the last collector, there is nothing left to pull the exhaust. All that is left is restrictions (catalytic converter, exhaust bends, resonators, mufflers...etc). ONE BIG RESTRICTION. This is the main reason why you do not see purpose built drag cars that run a full exhaust. It simply doesn't make the power of an open exhaust.

So why are exhausts used if they are just a restriction in power? Well for one, exhausts are used for emissions control through the use of a catalytic converter which catches all the bad exhaust fumes and converts the harmful fumes into carbon dioxide and among other jobs. Another reason for using an exhaust is to keep noise levels to a minimum for street driving. The use of resonators and a chambered exhaust greatly reduces noise levels compared to running open header.

thats it for now, Blake

Additional Information:

The last graph is a comparison after the 3" exhaust was tuned. As you can see, there is a significant difference between the two. Especially after the 3" exhaust was tuned. And this is on a motor which is internally stock. Imagine the gains a built motor would see....hmmm.... knowledge is power.