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the truth about backpressure

rileyrs

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i just posted this in my exhaust thread, about finishing my system, and was explaining my backpressure info, then i did a search, and realized that tons of people are actually confused with this topic, so i though id post this to try to clear up some stuff about backpressure.
my exhaust thread:
http://www.explorerforum.com/forums/showthread.php?t=256037

first some basic exhaust theory:
http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm
http://www.thumpertalk.com/forum/showthread.php?t=659727
also if any of you read petersens 4wheel and off road magazine, in the october 2005 issue there was some great info about intake and exhaust, and it explains backpressure clearly.

the thumpertalk one, focuses on backpressure only, but explains it well.

Here Goes.
no car ever needs backpressure, not even the sohc, despite what people say.
backpressure is always bad.
what is good, is a condition that creates backpressure which is the higher pressure within the exhaust, than the atmospheric pressure. also know as the exhaust pulses, which pull the gas out, but this simultaniously creates back pressure, so many people confuse these.
also gas velocity is needed, and smaller pipes yield higher velocity, often people use too large of diameter pipes, and their gas velocity is now so slow, that is also kills the exhaust pulses, and they loose power, and it leads them to falsely believe that they need backpressure.
but if your system is done well these pulses will work together to pull themselves out, so the engine doesnt have to, which keeps gases flowing quickly. headers will help do this, since they work with the exhaust pulses from each cylinder, rather than muffle them, and kill their velocity like stock manifolds.

i know this is a commonly confused issue, but its just not true that a free flowing system is bad.
many people just think the sohc needs backpressure because they put a flowmaster, and 3 inch piping, in their stock system and it loses power, but thats simply because they didnt do the rest of the exhaust, or enough reading, and the cats are destroying that exhaust pulse, and making the engine work to expell the gases from the combustion chamber.
but if the whole system is done right, then there is no back pressure, but good solid pulses... and in turn More Power!!!

anyway, hopefully that explains backpressure a bit, its kinda hard to explain, but ive read tons of stuff on exhaust theory, and this is the best way i can explain it, since its kinda hard to put in writing.

if anyone has anything to add that i missed, please do so, im just starting this thread to try to help people with their backpressure confusion, and in general to build their exhaust systems, correctly.
 






rileyrs

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i just added this to my exhaust thread, but i thought it would be useful here too. this doesnt really apply to the v6's since they dont have this issue with two cylinders firing in one bank and creating uneven flow, so an x or h pipe really isnt needed in a v6, but surely couldnt hurt, just to make sure everything is equalixed, but this mainly is for the v8 guys out there.

here is info about the V8 and why people believe they need backpressure.
a segment form a post on mustangforums, by Professor Wizard about H pipes:
http://mustangforums.com/forum/2005-...explained.html

Start Quote:
The firing order of all production V8s, regardless of make, has one cylinder in each bank that will fire within 90 degrees of crankshaft rotation of another cylinder in the same bank. This occurs twice during completion of the entire firing order. These two cylinders will be exhausting almost simultaneously into the same exhaust manifold system.

Full-length four-tube headers help separate these pulses until the collector is reached. If this is a full race car running "open exhaust", you will notice the collector dumps into a short open pipe at least 2.5 times the size of the header pipes, or the header pipes dump direct without a collector. This is done to avoid the conflict of pressure caused by the timing of the 2 counter firing cylinders, which will create back pressure and degrade torque, horsepower and general performance, especially at higher RPM.

On a full exhaust system, after the header tubes dump into the collectors, the two close firing cylinders are fighting each other for space in the collector and exhaust pipe. The result is reflected pressure waves traveling back up the exhaust system, backpressure, lost power and poor economy.

At the same time two cylinders exhaust in one bank, there is no activity in the opposite bank. The traditional H-pipe equalizer allows some of the excess pressure to bleed over to the 'quiet side' of the exhaust system, resulting in some low and mid-range torque improvements. At high RPMs, however, in traditional exhaust systems, the gases cannot bleed across the H-pipe fast enough to help power significantly. Performance systems with the H pipe design, attempt to over come this by using a shorter cross over pipe which is also slightly larger in diameter as the main exhaust, then would be used in a standard exhaust.

To overcome the power loss of "over loading" the H pipe design, Exhaust manufacturers came up with the X pipe design, which features a tangentially Siamese crossover junction to synchronize exhaust pulses. The X-pipe concept is to split the flow in the crossover junction, so the pressures on both banks will be equal and pulse-free after the crossover, regardless of the rpm. Volumetric efficiency and power are therefore improved at all engine speeds. The negative aspect to the X pipe design is, because of the crisscrossing of the flow stream, harmonic pulsations will develop on some systems at certain RPMs, which will be perceived as a buzzing or humming sound.
End Quote.

so the reson people lose power in their v8's is because they have a cat back exhaust, and there is now an uneven flow of gas due to the firing order having higher pressure in one bank than the other, and these gases are forced to merge into a Y-pipe and they slow down while conflicting, and then can escape the exhaust system fast enough, and the engine has to work to expell them, and there is a loss in power.
so once again, if a FULL exhaus system is done correctly, for the motor that you are workin with, then always the best option is one with little or no backpressure.
but if you only do one part of the system as a free slowing setup, and leave the rest, then you can indeed loose power.
 






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