Toymods Car Club Inc: Tech & Conversions => Exhaust Myth... Fact or Fiction?

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oldcorollas
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Kita-Ku, Sapporo, Japan
Registered:
January 2003

Re: Exhaust Myth... Fact or Fiction?

Thu, 10 February 2005 05:22

OPPY wrote on Thu, 10 February 2005 13:16

I know this may sound silly, but back pressure IS required on most Normally aspirated cars.. not all.

please stop assisting the myth Wink

backpressure is NOT required IF the engine has been designed properly.

Quote:

The reason it is required is acually to assist in drawing more air from the next cylinder after it fires, if there is no back pressure, there is no vaccum behind each pulse of exhaust flow, therefore not sucking it through, slowing down the exhaust flow and decreasing the efficency of your valves. Why does it vary from pushrod to multivalve.. simple, more valves = more exit points for the gasses.

i don't follow you. as i tried to explain earlier.... when the exhaust valve opens, a POSITIVE pressure pulse travels down the primary at the speed of sound (higher in a hot gas), and when it reaches the primary/secondary junction, a negative pressure pulse is reflected. it is this reflected negative pressure pulse that assists in cylinder scavenging.

with back pressure, we are talking about a higher pressure in the REST of the exhaust, as a result of a restrictive system.

more valves = more exit points for the gas? how does this affect things when the ports merge in the head, and then you have a single exhaust primary per cylinder?

for anyone who seriously wants to know what is goin on, go and borrow the book "the scientific design of intake and exhaust systems" by Phillip Smith.. it is held in the UNSW library, and is also available elsewhere. that book explains the concepts and some mathematics behind the design of manifolds...

Quote:

What I have seen, is that Not all n/a cars require it. A good example is the majority of Honda VTEC engines, for whatever reason, they work much better without back pressure.
Another example of the oppisate is the genIII LS1 motor, the ideal exhaust setup was actually the one that nobody purchased.. everyone wanted the big bore twin 2 1/2 inch.. rather than the system that worked best.. twin 2...

Less back pressure is required on multi valve engines than push rod V8's

no shit....

the honda motor is designed properly with an efficient head, and SENSIBLE cam choice for the motors rpm capability.

with the V8... too big a cam for the rpm capability of the motor requires backpressure to keep the cylinder filled properly at lower (ie less than 6000) rpm.

i can't stress this highly enough. ANY TIME YOU INCREASE THE BACK PRESSURE WITHIN AN EXHAUST, YOU WILL DECREASE THE AMOUNT OF GAS ABLE TO FLOW = POWER.
YOU CANNOT MAKE MORE GAS FLOW THROUGH A MORE RESTRICTIVE SYSTEM BECAUSE YOU ARE DECREASING THE POTENTIAL DIFFERENCE BETWEEN THE MOTOR AND THE EXHAUST, RESULTING IN A LOWER DRIVING FORCE FOR THE EXPULSION OF EXHAUST GAS. THE ONLY REASON BACKPRESSURE WORKS IS BECAUSE THE CAMS ARE NOT SUITABLE.

the bike is a perfect example. cams are designed for high rpm. at low rpm, you restrict the exhaust to reduce the harmful effect of the large valve overlap at lower rpm. at higher rpm, when the overlap is working effectively, you remove the restriction.

read the book i have now mentioned twice, and tell me that is incorrect.

Cya, Stewart

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