ddd4114

Because they use commonly accepted terms and still understand the underlying concept?

Saying that backpressure is "non-quantifiable" and "unitless" in my opinion makes you look like an ignorant fool, even if it's not the 100% correct term to use.

limperatlinks

The idea that going to a larger piping will loose power in the low RPM's and gain power in the high RPM's, is false. If you look at any 3" vs 2.5" test, power is made EVERYWHERE in the powerband-THIS IS WRONG
You also quoted Fluid dynamics
The need for a 3inch exhaust has to be there, or else you will lose power! You wont see any power gain with a 3inch exhaust on a SOHC civic or go-kart, regardless of tuning!
Exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system! Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe.

limperatlinks

You certainly can go too big on both exhaust pipes and headers depending on what RPM range you want your torque to peak. However it is not restriction (back pressure) that changes the torque curve rather it is the exhaust velocity. With smaller pipes the exhaust is forced to increase in velocity so when the piston reaches near TDC there will be a scavenging effect from the higher velocity exhaust creating a slight vacuum effect at the exhaust port. If the pipes are too big then the exhaust will be slower so when the piston reaches TDC there will be little or no scavenging effect at lower RPM. The smaller pipes produce more torque at a lower RPM and depending on other performance perimeters this may be quite desirable but it is a trade-off in higher RPM power. This trade-off occurs because as RPM, and thus exhaust gas volume, increases the low RPM scavenging effect will be overcome and offset by the added restriction of the smaller pipes. Trading the smaller pipes to bigger diameter less restrictive pipes can result in power increases in upper RPM ranges due to less exhaust restriction but these too have to be small enough to create enough exhaust gas velocity to create the necessary scavenging effect.

Bigger is not always better and that is why sometimes it can be counter productive to go overboard on valve sizes, intake runner size and exhaust pipe size. By the same token smaller is not necessarily better either so the trick is to have a balance of parts and don't go "hog-wild" with the biggest of everything. When someone says that an engine needs a bit of back pressure they are confusing back pressure or exhaust restriction with scavenging, back pressure will rob an engine of performance but at lower RPM the scavenging effect created by smaller pipes will make more torque until the exhaust volume reaches the point that enough back pressure is created to cancel the scavenging effect. Back pressure is a power killer and in itself is unwanted so the trick is to balance the pipe size to put the torque where you need it, a smaller pipe will generally give high velocity with good scavenging at lower RPM but at the cost of power robbing back pressure at higher RPM, a bigger pipe will generally give less torque at lower RPM due to lack of scavenging effect but will make upper RPM power as exhaust velocity increases and scavenging is created but without the added restriction of the smaller pipes to create as much back pressure.

The bottom line is that back pressure itself is unwanted and the noticeable increase in bottom-end power that often occurs with a change to smaller pipes/header tubes is due to better lower RPM exhaust port scavenging and not because of added exhaust back pressure these pipes will cause at higher RPM.

ddd4114

You're correct in the context of primary sizing, but I've never seen a large exhaust after the cat/collector lose more than a negligible amount of power with back-to-back dyno testing (when tuning is included). If you have proof otherwise, please post it.

HdeucedeuceA

"Opinion is the median, between knowledge an ignorance." - Plato

I'm a realist, show me proof. Until then, all you say is just meaningless words.

ddd4114

Proof of what?

That it's a commonly accepted term? That should be fairly obvious or we wouldn't be having this discussion.

That ignorant, foolish engineers understand the underlying concept of backpressure, even if it's technically described with different terminology? Like I said, backpressure is just the gauge pressure at the beginning of the flow network. It describes the pressure differential created by energy loss in the system. This is fluid dynamics 101.

That it's neither a "non-quantifiable" nor a "unitless" term? If it describes a pressure, how could it possibly not have units of pressure? Plus, you can even buy backpressure gauges, and a gauge will inherently quantify a condition like a pressure differential: http://www.amazon.com/OTC-7215-Exhaust-Pressure-Gauge/dp/B0002SQWVQ

I don't understand how this is so difficult to grasp.

Swicked86

I figured this out a long time ago, people have used undersized exhausts to create reversion. If you need to go smaller on your exhaust to produce more torque you need to change your cam timing. This is probably most true for engines that were "over cammed" to produce a lopey idle and not a good enough bottom end or valve train to go with it.