Okay I have a 60mm piping apexi ws2 catback on my eg. I was wanting a 2.5" catback, but am i missing something here because i did the math and it came out to be 2.36 inch.

60mm=2.36 inch

63.5mm=2.5 inch

Also when talking about 2.5 inch catback, I would measure the ID (inner diameter) of the exhaust right and not the OD (outter diameter). Or does the metal/piping it self, that is between the ID and OD plays a part in the measuring to?

Thanks fellaz...

 

Exhaust systems are made from tubing, measured by O.D.

 

2.36 inches will support 400hp there is no need for 2.5 or larger piping unless you are making more power than that.

 

How do you know?

 

2.36 should be plenty for anything under 400 hp and mabe even more than 400 hp you want some back pressure in every exhaust you will actually lose hp if you are running too large of an exhaust

 

No you don't.

 

Ok, your right

 

Thats just funny.

 

In japan they use metric sized tubing. 60mm OD is just one of their standard sizes. You're not missing anything as long as you see that 2.5" is just a little bigger than 60mm. Were you expecting it to be bigger or something?

 

It's funny 'cause it's true.

 

you don't want back pressure . if you don't know why a stock d series loses low end when some ******* puts a 3" exhaust on it don't post an answer in this thread or any other in regards to air in a tube and flow. it all has to deal with http://en.wikipedia.org/wiki/Bernoulli's_principle

 

To some extent, yes. However, the assumptions made by Bernouilli's Principle don't really apply to an exhaust system. Compressible, viscous, unsteady flow can't really be evaluated with one or two equations. Acoustics further complicate things.

Though I haven't tried this myself, I've talked to others that installed large exhaust systems and regained lost torque after a tuning session. In fact, they gained more torque everywhere.

Whatever the case, the whole "you need backpressure to make torque" is an old myth (or at best, a half-truth).

 

piping needs to be sized to fit the output of exhaust gasses ... adding "backpressure" has never helped get air and fuel into a motor . using a smaller pipe in a section to speed up flow may have.

 

This doesn't speed up flow. It only creates a bottleneck that decreases power.

 

Kind of a nit-picky correction, but the speed will increase (in most circumstances), but the flow rate will probably decrease from the energy loss.

 

it increases air speed. faster moving air exerts less pressure on a surface than slow moving air. this is a good thing to a point . same is true for making a pipe bigger to allow more volume of air to pass.. you need to find the balance between the two. the correct sized pipe will keep the speed up FPS and flow CFM all within the desired limits. what size that is,IDK but im sure someone will chime in. more factors than I am going to point out.

the whole back pressure thing is false . you want to get as much exhaust out of the motor as possible and as fast as you can. a pipe too big will slow the air down to a stall and that creates a high pressure zone in an exhaust = not wanted.

I saw a post related to this where someone said anything after the collector should be as large as you can get due to the scavenging being done all ready in the header. this makes more sense if you look at it as the header being in open air.

 

The majority of your argument only applies to header design.

I suppose a smaller tube will increase velocity, but its going to kill the volume. I have never seen an engine loose power from using a larger exhaust, when the engine was tuned after the exhaust installation. Even people that run open header make more power than with an exhaust system. The only time I could ever see exhaust velocity, post header, posing a problem is under very light acceleration. Who cares about that anyway.

Something else to think about is that many Honda four cylinder engines respond very well to a 3" tuned length intake. You are going to have more gasses coming out of the engine than are going in. How could something smaller than a 3" exhaust be sufficient.

Btw, I run a 3" exhaust on a 1.8L and have no issues, even with a less than ideal header.

 

why does everyone reccomend a 3' diameter piping for k20a swaps

i got 220whp on an eg w/k20 swap with just a 3"catback and boltons