Sleepless in Seattle

Im still learning my way around the car but i was doing alot of searching reading things here and there and Since im keeping my car NA, from what i understand if you use to big of an exhaust you lose backpressure which actually does not net you any extra power??

so. In searching for an exhaust system, what should i be looking for?

is 2.5 in piping ideal?


i dont know much bout what determines how loud an exhaust is besides size, but what is one that is quiet uder 3k rpm?

jjwalker

There is an exhaust narrative on mazdatrix that answers your question quite nicely.

http://www.mazdatrix.com/e1.htm

2 inches, as they say. Also, I'd like to point out, it isn't backpressure you are worried about. A bit of backpressure is great for reciprocating engines because it keeps the exhaust vavles from burning up. On the flip side, the rotary has none so that is moot. The key here is exhaust gas velocity. Too big a pipe, the exhaust pressure isn't great enough to move the exhaust out of the system fast enough, as it will start to cool and become harder to move before it exits the system. Obviously too small a pipe, pressure is too high and that becomes the restriction. It is all a nice balancing act between exhaust pressure and velocity. There are some good wikipedia articles that explain pressure and velocity that you should read up on. They don't speak of exhaust specifically, but the general premise will make sense.

SoloII///M

iTrader: (13)

My exhaust is all custom, and the guy who made the header has dyno results showing that his stepped header makes more power across the rev range than others. I haven't tested my car on a dyno yet as I'm still working on the tuning so I'll have to wait to prove that out.

Primaries are 1 7/8" stepped to 2", with a 2.5" collector into a 3" megaphone. Two 3" cats and 3" stainless mandrel bent piping to a resonator and large rear muffler. It's on the loud side but livable. I'm going to replace the straight through rear muffler with a "turbo style" muffler to cut sound a bit more.

Should have dyno charts in about a month.

jjcobm

iTrader: (18)

Whoever did your exhaust knows what he is doing. He optimized the exhaust to have maximum exhaust gas velocity (by stepping from small dia. to large dia). You should be really happy you have a guy that hooked you up like this.

If he went further, he could of possibly optimized the length of the primaries and entire exhaust to optimize it for scavenging effects.

SoloII///M

iTrader: (13)

He did really good work.

I'm happy to provide his contact info via PM to anyone interested, so long as anyone contacting him mentions that I recommended him. I don't get anything from him in return but just want to help him out.

Sleepless in Seattle

see. this is the knowledge i want to gain.

the things you dont learn in tech school.


I want to learn the whys and hows. not the "this is how you replace this"

I want to learn fluid dynamics. the specifics of the exhaust system as mentioned earlier. etc.
i want to understand all of it. haha.

i kno it may take me years but im still young and i think its a reasonable goal.


i like learning the things majority people dont know.





wow...my post was really off topic just now.

anyways thanks for the replies guys. so 2 inches shall be decent for my NA.

rotarygod

iTrader: (1)

Keep the primary runners at 2" or slightly under. Collect them to no larger than a 2-1/2". I know that 3" has already come up but there is FAR more to exhaust design than just size and a few steps. For a standard street car 2-1/2" is about the practical limit and all that is really necessary. There were people who swore by the expansion chamber type design from SDJ and I argued against it doing what most people thought it did. I was vindicated when a forum member pulled it off and ran a standard collector and didn't lose any power and in fact gained some midrange.

Don't overthink an exhaust system. It's far easier to mess them up than it is to get them right.

Sleepless in Seattle

ok so let me get this straight for the sake of learning..

Primary runners is the y pipe? or does that include the downpipe?

also side question....why is there 3 cats on the stock pipe? is that necessary? or can you fab one up and and straight pipe till the end and put a cat before the tip?

SoloII///M

iTrader: (13)

There are three cats because that's what was needed to meet the emissions standards of the time. The first cats light off closer to startup than the large main cat, reducing startup emissions.

Depending on your local regulations you can pass a sniffer test with one single main cat if the car is otherwise running correctly. I highly recommend not running without a cat as it makes the car extremely stinky, pollutes the air unnecessarily and doesn't provide any tangible performance benefit to an otherwise stock (or near stock) motor.

John

Sleepless in Seattle

ok i see.

yea figured that its pointless to go catless unless its a race car.

referencing to my previous post what is the terminology of the exhaust system that majority use on here?

and this is a diff topic but seeing as i have someone way more experienced than i, kinda want to pick your brain a bit. From what i understand, to power an NA You can upgrade exhaust, Air intake, Port the housings and lean the fuel a little. without the porting part, is it still reasonable to have a power goal of 180 at least? or is that chasing clouds? I know that my S4 in tip top shape puts out 145? 35hp gain seams do-able.

Evil Aviator

The primary tubes attach to the engine with a flange. Their length is generally considered to be measured from the engine flange to the point at which they collect or terminate. Optimum inside diameter is around 1 3/4".

Not in any particular order:

1. Short Primary Collected, usually 22-25" for performance
(The Racing Beat header shown below is actually a little too long)
http://www.mazdatrix.com/Pictures/ra...gleExhaust.jpg

2. Long Primary Collected, usually 120-125" for performance
(Racing Beat calls theirs "Street Port" to indicate that it works well with street ported engines).
http://www.mazdatrix.com/pictures/racing/16422.jpg

3. Uncollected (aka "True Dual" on some internet forums)
http://www.mazdatrix.com/pictures/racing/16-16128.JPG

Edit: Of course, after I wrote all that I found this link that explains it better, lol:
http://www.racingbeat.com/Tech/Tech.Exhaust.2.htm

Yes, a non-ported 6-port 13B can hit 180bhp with an aftermarket (EFI or carb) intake and exhaust system.

The engine in this article is street ported, but you can get the general idea of an aftermarket EFI or carb setup:
http://www.twminduction.com/ThrottleBody/carb_vs_fi.pdf