Despre evacuari - Backpressure

[yulasinio]

Multi schimba evacuarile fara sa se ghideze dupa nimic, sau dupa principiul ca cat mai mare cu atat mai bine. Am gasit pe un site un ghid despre ce inseamna backpressure si daca avem sau nu nevoie de ea.

Backpressure: The myth and why it's wrong.

Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

Some basic exhaust theory.
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

Backpressure and velocity.
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

So how did this myth come to be?
I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to upgrade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

Conclusion.
So it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

[Achim]

foarte bun postul merci

[CristianD]

super fain articol
si logica ideea, pacat ca nu scrie si o dimensiune care sa fie ok in RPM mici si super buna pt RPM mari.

[Achim]

e logic ce zice el dar in realitate am obs ca nu se intampla asa
adica la mine atunci cand aveam serpi 4-1 -> decat -> tseava (nu prea groasa) -> finala cartush aveam senzatia ca pierd Nm si nu merge mai tare
si alta chestie ... Toda si Invidia am observat ca au tot felul de elemente care folosesc la backpressure (inclusiv finalele nu sunt cartush, exceptie face N1, dar care totusi are intermediara) )

[cioli17]

am experimentat pe pielea mea treaba cu backpressure si este cam FALS ce spun baietii.
un motor aspirat are nevoie de backpressure chiar daca merge in 8k rpm... se poate monta un manopetru pe galerie ca sa iti masori presiunea de gaze... dar in mare spus '' teava dreapta '' NU e ok pt. un motor aspirat.

[sKyNeT]

ns dar eu de cand am pus "teava dreapta" am impresia ca am castigat ceva in ture mari. ce-i drept nu am marit diametrul decat cu 9 mm

[yulasinio]

Nu vorbeste nimeni de teava dreapta. Este vorba de backpressure si daca este benefic pt un motor NA sau nu. As vrea si niste argumente la ce spuneti voi. Daca schimbi toba si suna mai tare nu inseamna ca si merge mai tare deci numai cu un dyno ai putea cate-de-cat compara evacuarile

[lover23]

ca sa inteleg si eu mai bine
daca ii schimbi teava de 55 cu una de 57 si ii anulezi intermediara adica ii pui teava in afara faptului ca o sa urle un pic mai tare o sa pierzi sau o sa castigi putere adik cuplu
nu sunt din acela care crede ca un filtru si o finala iti dau 15 cai dar totusi as vrea sa stiu care e rezultatul
e mai bine sa ii lasi diametrul ei sau se poate mari un pic ???
am inteles ca daca e prea mare atunci pierzi putere considerabil.

[cioli17]

Originally Posted by yulasinio

Nu vorbeste nimeni de teava dreapta. Este vorba de backpressure si daca este benefic pt un motor NA sau nu. As vrea si niste argumente la ce spuneti voi. Daca schimbi toba si suna mai tare nu inseamna ca si merge mai tare deci numai cu un dyno ai putea cate-de-cat compara evacuarile

Poi cum nu ? Daca pui 'teava dreapta' pierzi backpressure,logic.

Backpressure-ul se poate pierde ori din cauza traseului care poate fi de diametru prea mare ( dar la noi nu prea e cazul cred ca,ca nu are nimeni catback de 3 inch pe civic ma gandesc ) sau din cauza faptului ca nu este deloc restictiv ( nu are cat sau intermediara care sa ''mai opreasca din gaze'' )

Argumentul este ca un motor aspirat ca sa mearga cum trebuie are nevoie de contrapresiune pe sistemul de evacuare si asta mai exact poti vedea montand un manometru pe galerie de ex. iar presiunea unui motor n/a trebuie sa fie undeva pe la 0,1-2 bar.Sa nu uitam nici ca presiunea e invers proportionala cu volumul,nu ?

Eu am observat la mine scaderi de putere dupa ce am montat catback-ul custom care era format din galerie 4-1 si teava pana la finala ( 60mm ,. 2,25 inch ). Am simtit din start o pierdere iar cand am ajuns la timeslip mi-am confirmat ca NU era in regula.

Galerie 4-1 sau 4-2-1 la alegere,teava 2.25 inch,intermediara sport,si o finala sport.In opinia mea este cel mai bun combo pt evacuarile unor motoare aspirate cu puteri relativ apropiate de cele de fabrica.

[filip-s]

trebuie sa sustin ce a scris cioli mai sus, pe "pielea" mea am simtit-o

[yulasinio]

Eu nu sustin ideea ca o teava dreapta din galerie pana in finala e un lucru bun, mai ales daca e custom dintr-un tub lung Acum daca CAT-ul sau intermediara conteaza e cu totul o alta idee.
Am vazut foarte multe dyno-uri in care B-urile sau comportat foarte bine la de-cat. Eu urmez sa ii dau jos cat-ul dupa ITP si sa tai jos filtru de pe intermediara si sa folosesc finala mugen si o sa vedem atunci ce o sa iasa

[Achim]

pai...
eu am avut ca si Ciolac race headers 4-1 cu de-cat, apoi traseu marit cu 6 mm (dar pe aceeasi forma ca originalul, fara intermediara si cu backbox din carbon de tip cartush ...
masina suna excelent dar clar nu era niciun fel de imbunatatire a felului in care tragea, poate dimpotriva. oricum, dupa de-cat si alti sherpi ar fi trebuit sa fie ceva in plus... asa ca era de la backpressure ... zic eu. comparatia am facut-o vis-a-vis de alte masini cu care m-am dat in repetate randuri ... spre exemplu... cu config mai sus amintita, stateam egal egal cu BMW 330Ci ... ceea ce nu e taman ok pt un EP3 cu full exhaust si admisie BMC CDA ...

"eniuei" ... eu am senzatia ca masina mea merge mult mai ok acum cu Toda catback ...
si apropo de backpressure... eu cred ca de aia Mugen foloseste TwinLoop'ul, pentru a crea un asa efect... si tot de asta Toda, nici macar la catback'ul race pe care il am acum nu are finala cartush ...