Back pressure is a negative, period, some can't be avoided but ideally a system would have zero back pressure and properly sized and shaped pipes to keep maximum velocity. Exhaust gas velocity is the primary factor.

 

I thought some back pressure is needed or you could actually harm your engine?

 

Dragsters run open headers... Backpressure indicates a restriction to flow. Simply having pipes, you are going to have some...... its just unavoidable. Sizing your exhaust to your application, and using good flowing pipes is the hot ticket.

 

no way.. backpressure is bad..

a lot of folks confuse the term with scavenging- which is good..

the overall premise is to have pipes large enough with soft enough bends not to create obstacles where the pulses bunch up.. but small enough to get (and stay) hot- which keeps the flow of air moving..

each pulse will have a leading edge of warmed air, that encounters the trailing edge of the previous pulse which has cooled off- so it has to push the cooler air in front of it out of the way.. If the trailing edge of the pulse can stay hot, it is less dense and can be pushed fairly easily.

having as small a diameter pipe as possible while still allowing the engine to breathe unimpeded is the ticket because it gets and stays hotter- hotter creates areas of pressure lower than ambient atmospheric, which creates a vacuum- the effect is called scavenging, and scavenging keeps the pulses moving by tugging them when the energy of the cylinders 'push' has expired..

an engine with backpressure isn't allowing the engine to expel all it's spent volume, which means it gets reused.. less o2, less power extracted on the power stroke.. but it revs faster.. revving faster can be good, but it is wasting energy w/o drawing the proper amount of o2 with each intake cycle..

I didn't aim to get this involved, but: as the cylinder expels spent volume, and meets resistance it creates a pill of compressed air that stays in the cylinder.. compressed air rebounds quicker, and assists the piston on it's intake stroke- but there is only so much volume allowed into the cylinder, so an equiv amount of fresh air/fuel doesn't get to enter the cylinder.. it revs faster because it is pushing off that pill of compressed air, even if at the expense of another cylinder on an opposite cycle..

so even though the engine is revving faster, the power it could be producing is limited because there isn't as much fresh a/f to blow up.. so you lose power.. all on the account of backpressure..

I'll be the first to admit this isn't exactly accurate, there is a lot more to it, but it is the general idea as I understand it.

too open an exhaust doesn't get warm enough to create scavenging- and the pulses will get back up.. and create a tinge of backpressure.. to tight is too restrictive and will create it by not moving them out of the previous pulses out of the way fast enough for the following.. it'll back up..

air in/air out.. if you increase one and not the other, you'll have issues somewhere.. it's a balance..

/end rant