question...

[Dragon_Reborn]

Okay i thought the easier u let air come into your engine (intake, like the Injen CAI or AEM short ram.) and the easier u get the waste to exit (exhaust) the better it is for the engine. Now, what is all this talk about "back pressure," what is it, can somone explain it to me and why is it bad to not have any somone told me u need to have "back pressure," and that it bad to have a free flowing exhaust, could some one please clarfiy? :wtf:

 

[Sir Nuke]

its bad to have ABSOLUTELY ZERO back pressure......however even with a free flowing exhaust you are going to have SOME....

the back presure or flow of the exhaust gasses helps SCAVANGE the gasses from the other cylinders as they go through the combustion cycle...

for a better and detailed explanation....go to www.howthingswork.com and read up on the four cycle engine and how it works.

 

[1sty]

Your right on the intake you want the least restriction possible. SCC also did a long artical on this subject.
FOr exhuast on a naturaly aspired car you need back pressure, to be more exact a small, normaly not over 2.5", radius exhuast pipe that can flow alot of gas. However when someone goes to big the pressure starts to get too low and the velocity of the exhuats gas slows down. Exhuast gas actualy creates a pressure gradent and pulls more gas still in the chamber out. Basicly it creates a vacuum. When that vacuum has too large of an area to travel through the vacuum pressure drops and the gas actual flows less easily lowering horespower.

Anyone feel free to correct me here.

 

[njaremka]

Your right on the intake you want the least restriction possible. SCC also did a long artical on this subject.
FOr exhuast on a naturaly aspired car you need back pressure, to be more exact a small, normaly not over 2.5", radius exhuast pipe that can flow alot of gas. However when someone goes to big the pressure starts to get too low and the velocity of the exhuats gas slows down. Exhuast gas actualy creates a pressure gradent and pulls more gas still in the chamber out. Basicly it creates a vacuum. When that vacuum has too large of an area to travel through the vacuum pressure drops and the gas actual flows less easily lowering horespower.

Anyone feel free to correct me here.

pretty close on both counts.

both your intake AND exhaust need some kind of restriction to create max power. you want to size both to give the highest velocity. what this in turn does is alows for max charging through the intake, and best scavenging from the exhaust. sound a little confusing, but it is more involved than you think. also for consideration should be the length of the intake and exhaust when considering the diameter. (a really skinny and short pipe will give the same resistance as a longer and fatter one, just like electrical wiring)

 

[Nomad]

It's all about the velocity of the gasses as explained above.
The smaller the tube the faster gasses can move through with no air flow disturbance (to a certain extent). The larger tube will allow the air to start to swirl and eddy around corners and bends. This creates low and high pressure zones inside the pipe. You want the best combination of volume and velocity.

Rule of thumb for performance =
2 1/4" diameter for N/A engines below 2 liters displacement
2 1/2" max dia. for N/A engines above 2 liters.
3" min for turbo apps. (High boost apps can even run 3 1/2" to 4")

You'll generally lose some of the low rpm's torque but gains come in mid to high rpm's due to faster flowing gasses allowed to flow faster.

 

[Dragon_Reborn]

Thanks guys for the info, i understand now

 

[KYREDP5]

And for best results you wanna have a 2in exaust going into a coffee can sized muffler....j/k

 

[1sty]

Speaking of, I had the unfortunite luck of having to do an install in a civic with a similair setup. 1.5" stock pipe going into a 3" straight muffler with a 6" tip.
(rice)