actually back pressure is required to keep the required vacume pressure on ur intake manifold in order to get gas into ur car, as is the same for any naturally aspirated car. if u have a turbo or supercharger u can just dump the exhaust into a huge pipe, but honda's very much rely on the correct back pressure to operate efficiently.

 

What kind of car do you have. A two stroke. Intake and exhaust are controlled by two different valves.
Engine 101:
Cam opens intake valve, piston moved down sucking in air and fuel(most fuel injection is supplyed just on top of the valve), Cam closes valve, piston compresses and you get power at ignition causing the piston to be forced back down, cam opens exhaust valve , piston goes back up pushing out exhaust gas.
At what time does backpressure come into play. If you have to much pressure, the exhaust gas could not leave the cylinder and your performance would turn to shit. Maybe vacuum from the air leaving the exhaust.

 

You could probably give a class on ricer 101 too huh?

 

the idea that backpressure is necessary for performance/torque/whatever is a total myth. Maybe back when the dinosaurs roamed, you needed backpressure for some pushrod behemoth, but nowadays, the less backpressure the better. period. the whole point of a smooth mandrel bent straight through exhaust is to speed the exhaust gasses and evacuate them as fast as possible. Sound like backpressure to you? No. The reason you don't want a huge 3" exhaust on a N/a car is because its actually slower unless forced out by a trubine. But in either scenario, the whole point is to reduce backpressure and speed exhaust gas evacuation.

sorry:blah: h:

 

you APPEAR to contradict yourself at least once in this statement....(NTO beign a dick)

so here you are saying you DON'T want ANY back pressure at all in any case unless its an old domestic

here you are saying NO pressure is not good

here you are saying it is good again...

ill be real honest im confused on the topic... i KNOW ive driven certain cars with and without exhaust.. and in a LOT of cases the one that are faster without exhaust (open headers) are BIG domestics making like 350+ hp typicaly what ive seen is unless you are making the power open exhaust actualy seems to SLOW cars like ours down... im not sure what teh problem is or why it happens.. BUT thats what ive seen..

 

the reason a 3" isn't optimal for smaller/no-turbo engines is exhaust speed. The exhaust moves faster through a smaller chamber, which dosn't necessarily have anything to do with backpressure. I guess I had 2 separate points: backpressure = not necessary, exhaust speed = good. I just kinda lumped them together, sorry for the confusion. THe whole old car statement was just in reference to the folks i know at autocrosses and stuff who swear by their supertrapps.

 

they are all relative.... if you have a smaller vessel and are forcing MORE air through it it will create MORE velocity if you have a HUGE vessel (no exhaust) and more air it will create like normal velocity sinse you are not forcing it through a small channel.. blow through a straw then cut the bottom off a soday bottle and blow through that with EQUAL force... the air coming out of the straw will be traveling at equal or greater velocity..

 

i recently had an experiance that can prove how back pressure is necessary. At the beginning of my 3 hour drive Sunday, my exhaust pipe cracked and split apart due to a lack of flex.




this created an "open header" exhaust that many 400hp+ domestics use. I have a DOHC B16b motor with an exhaust manifold measuring 2 1/2in in diameter so I clearly make more power than a 1.5L dx and have plenty of "flow". I have had this setup now for 5 months. My point is - when my exhaust pipe cracked and left me with an open header (no backpressure) i noticed an extreme loss of torque and low rpm horsepower. My 3 passengers who have never heard the word "back pressure" also noticed a dramatic loss of power. Without the back pressure provided by my Greddy SP exhaust, i was unable to make torque and unable to out accelerate minivans. I know my experience involves much more power and much less backpressure, but the rule stands. Civic 101: You're Japanese economy car needs backpressure.

 

exactly oc.... sorry if I'm not really making any sense. The whole purpose of exhaust tuning is to find the right diameter pipe that will facilitate exhaust speed (as you just described) while adding as little backpressure as possible(like a perfectly straight straw). Inevitably, you need to have a little backpressure in order to get that speed, but the goal is balance it just right so that pressure is minimized and speed is increased.

 

when my muffler fell off, I removed the entire pipe. Didn't notice a bit of difference (aside from the noise h: ) But then again thats only a 90 hp car. But its dyno proven, cars with open headers and no exhaust make more power, although youre right, they can loose a little down low. But you guys are right, undoubtedly you've got to have a little bit of backpressure, but generally lower levels (like the 5-15 lbs you get from a good aftermarket exhaust) are better than higher levels (like 25-30 lb pressure of a baffled/louvered muffler or crunch bends).

I guess I should have said not much backpressure is necessary, not no backpressure. Really, its impossible to have no backpressure anyway.