Question regarding exhaust pipe size?
 

I'm going to make my own mid pipe and was wondering what the advantage of using 3" piping if the outlet on the header is only 2.5"?

I'm running the Rev8 exhaust which has a 2.5" coupler to mate to the stock mid pipe but after that it is 3". I could replace that coupler since I'm making my own mid pipe, but that still leaves me with the problem of the header only being 2.5"

Is the extra trouble worth it or should I just keep it all 2.5"?

Thanks

unless you're going with a turbo

not rly.......

Don't hold me to this, but I believe it's because the header is built to handle much higher pressure than the rest of the exhaust, so you could get to the point where the same pressure is fine inside the header but not in the exhaust.

i'm confused

you aren't the only one :Eyecrazy:

The thing about larger diameter exhaust pipe is you will loose velocity of the exhaust. How fast the exhaust travels down the pipe.
This also plays a part in scavenging of the exhaust. There is a point where the exhaust velocity creates a vacuum effect and pulls more exhaust out. Instead of the engine having to push the exhaust out. Resulting in more power. Scavenging effectively removes more exhaust gases from the engine, basically pulling them out at higher rpms.

Or you can think or it like this,,,

ce = V + [A(p1 - p2) g / w]
where V is the velocity of the exhaust gases; A is the nozzle exit area; p1 is static pressure at the nozzle exit; p2 is ambient pressure; g is the acceleration of gravity; and w is the weight flow rate of exhaust gases.

When contemplating a modified exhaust system there are those who want the biggest diameter pipe that can be had. Their idea must be that fatter pipes are more effective at venting than narrower pipes. This sounds reasonable but it is not quite correct. Sure wider pipes have greater volume and higher flow capacity, but that is just half of the story. Capacity is one consideration but gas velocity is the other factor.

An experienced exhaust designer knows that the best exhaust is one that balances flow capacity with velocity. A given volume/time of gasses will travel faster through a 2" pipe than the same volume of gas passing through a 3" pipe. So when taken to its extremes we can see that a too narrow pipe will create backpressure (restrictions to positive flow) problems and a too wide pipe will cause a very slow flow with no backpressure.

The optimum is where the fastest velocity is achieved with the least constriction possible.



This situation will arise when the pipe is wide enough so that there is the least level of positive backpressure possible whilst achieving the highest exhaust gas velocity.

The faster the exhaust gas pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The scavenge effect can be visualised by imagining the high-pressure pulse with a trailing low-pressure area behind. The faster the high-pressure pulse moves the stronger the draw on the low-pressure gasses and the gasses behind that. The scavenge action is like (but not exactly) suction on the gasses behind.



The greater the clearance burned fuel from the combustion chamber the less diluted the incoming air/fuel mix is. Scavenging can also aid intake on overlapping valves (where the exhaust and inlet valves are open at the same time) by drawing in the intake. These are good things to happen.



So instead of going for the widest pipe possible we should be looking for the combination of the narrowest pipe that produces the least backpressure possible. In this scenario we achieve the least restriction on positive flow and the highest gas travel speed.



Exhaust pipe diameters are best suited to a particular RPM range. If we used a constant RPM engine this would be easy to specify. But a variable RPM engine will mean that not one size suits all. It is possible to vary the size of exhaust volumes according to rpm but it is very expensive (Ferrari has done it). The optimum gas flows (volume and speed) are required at the RPM range that you want your power band to be located. For a given engine configuration a small pipe diameter will produce higher exhaust velocities at a low RPM (good) but create unacceptably high amounts (bad) of backpressure at high rpm. If you had a car with a low RPM power band (2,000-3,000 RPM) you would want a narrower pipe than if your power band is located at 5,000-7,000 RPM.

EDIT: Damn it Easy, always gotta beat me to teh punch^^^^, then you kick me whiile i'm down by presenting the idea SOOO much more nicely and even show some of the science behind it.... :(

yeah, confusing....


RR afaik you are NA....

increasing exhaust diameter can get more exhaust gas out quicker, but you cant just make a 6" exhaust and get Fast and Furious results. for any given amount of gas flow, there is a "sweet" spot. any smaller diameter pipes and you are restricting the flow, but any larger and you've got such a large exhaust path that the gas slows and cools down.

basicaly in any application, you want the exhaust to get to the tips of your pipes as fast as possible. afaik the gains on a NA Renesis for 3" piping are nominal at best

best results seem to be with systems using 3" pipe

Ta Daaaaaaa !

See - didn't need all the explanation

Ok now I'm confused! :lol:

Basically what I mean to say (but eventually ADD'ed into a second thought) is that a larger exhaust will relieve the pressure off the manifold more quickly. More volume = less pressure = lower exhaust temperature. The manifold may be able to handle that pressure/temp, but the exhaust should try to dissipate that as quickly as possible, so bigger is better :)

Edit: I reserve the right to be completely wrong. Basic physics in regards to volume, pressure and temperature, but there may be other and/or better reasons for a larger exhaust.

I beg to differ Swoope.
There is scavenging in the Renesis as in all combustion type engines. The rotary needs it more than a piston engine with valves. The ports are open and there for will carry back internally any exhaust not scavenged by velocity. The Renesis will actually create vacuum in the exhaust port on deceleration. Just like the intake does does during acceleration.

Your missing the scavenging effect just discussed. Scavenging will remove exhaust gases faster and more efficiently. Thus reducing heat and improving combustion.

EDIT::: ^^^God DAMNIT EASY!!! ROFL :)



brettus might be right about 3" being better even for NA.... but thaty doesnt mean 4" would be better... at some point you peak out and start to loose velocity which is not good.

as far as scavenging, there isnt any scavenging in the traditional sense due to no port overlap like in previous motors..... i think what was meant is, if gas velocity is kept high enough - just before the port closes the gas travel will create a low pressure zone in/at the port that can actually pull a smal amount of vaccum in the chamber. this vaccum then allows more fresh F/A mixture to be introduced as the intake port opens.... these was debated/discussed/argued before somewhere, i remember it

BINGO!!!

There is a fine line between high rpm flow and low rpm flow. The larger the pipe the higher flow at higher rpms. The smaller the pipe the higher the flow at lower rpms.

You decide.

No header but I have a 2007 exhaust manifold and 2005 gasket on my car. After that is an SR Hiflow cat. It has the stock o-ring type gasket.
Like this,,

http://i59.photobucket.com/albums/g3...4212008009.jpg

Anybody seen Rodger? I think we confused him too much after post 4.

You guys are too funny. Thanks for all the input.

Tonight I spent some time looking over my options. I'm trying to make this sound as quite as possible. Below you will find a picture of my parts. My stock cat is still on the car.

http://i170.photobucket.com/albums/u...p/DSCF0360.jpg

Option one:
Cut the stock cat out and replace it with the extra stock resonator.

Option two:
Remove the connecting pipe from my Rev8 and create one long 3" mid pipe with out the center flange. This would give me plenty of room to use the two stock resonators and even add a 3rd. (the one from the test pipe)

Problems:
The stock resonator appears to be 2.75 inches ID. The stock mid pipe actually goes from 2.75 to 2.5 after the resonator.

comments?

check pm

beers :beer:

Sorry to bring this back from the dead but I am modifying my already modified stock exhaust and I am looking to replace the stock muffler with one single after market or two. Question is can I find one single muffler to fit the bill. Anyway is teh stock pipe 2.5" or 2.75" Most of the mufflers have either 2.5" inlets and outlets or 3". Not sure if that .5" will make a diff.

It's 2.37" (60mm)

2.5" is fine

Yup, which is what me and rotarygod were debating a while back, and I put into practise with my long-centre-branch manifold and 2.5" system, doesn't seem to have done my engine any harm, see sig!

Yeah, whatever .....

Thank you sir.

We've got a 380bhp engine on a single 3" exhaust too...

check with Eric Meyer on exhaust testing--he has done it all and normal theories do not seem to apply?
olddragger