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Old 22nd February 2008, 21:11   #31
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Does exhaust gas reversion raise cylinder temperatures? Or rather, reduce duration of cooling cycle?
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Old 23rd February 2008, 11:56   #32
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i dont know if the exhaust gas reversion could raise cylinder temperature.. can you elaborate a bit ?
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Old 23rd February 2008, 12:01   #33
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Quote:
Originally Posted by headers View Post
i dont know if the exhaust gas reversion could raise cylinder temperature.. can you elaborate a bit ?
OT - for a moment i thought this thread was about U !!
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Old 23rd February 2008, 13:03   #34
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Originally Posted by ram_hyundai View Post
oh ,is it the heat that was destroying the valves.isnt heat generated almost proportional to the rpm at which the engine is running,more heat-more fuel-more rpm.like more fuel is fed at lower gears when power is required and the engine rpm is also high.and more heat is also generated and used.so does it mean that a guy travelling at high rpms can damage his valves even without a F.F.E.
and does it mean that a guy with a F.F.E.driving at sedate rpms is safe and he wont have his valves blown.
ram

Constant high RPM usage can bring down the life of an engine

A badly designed header can produce high back pressure which robs the engine of power. Back pressure is not good as it makes expelling the gases harder. What is needed is correct gas flow velocity at the desired RPM range. A correctly designed pipe can actually make an engine run cooler.
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Old 23rd February 2008, 21:24   #35
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ok,where are the gurus to give us a clear picture ,or is it that the query made do not deserve a proper explanation.
ram
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Old 23rd February 2008, 23:24   #36
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Serioulsy RAM,Not a single guru has replied!!! And the replies are really scary!! I am using Headers on a daily drive car and i am still waiting a proper explaination
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Old 25th February 2008, 08:08   #37
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Hi guys, Sankar has already explained in simple words the use of a headers:

What a correct / well designed headers could do and what wrong ones do!

"A badly designed header can produce high back pressure which robs the engine of power. Back pressure is not good as it makes expelling the gases harder. What is needed is correct gas flow velocity at the desired RPM range. A correctly designed pipe can actually make an engine run cooler."

cheers
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Old 25th February 2008, 09:48   #38
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a perfect calibrated backpressure is always needed for proper functioning of the engine. backpressure causes scavenging of the exhaust gases, as well as it helps in low end torque. also the exhaust valves get the cushioning effect due to this backpressure. Extra backpressure will cause the intake charge to dilute and the power drop will be drastic. a small backpressure helps this intake charge for a better combustion as well as perfect valve timings.

a FFE header which is not designed to deliver this will always affect the engine life.

i have put a ffe on my Bike. i ended up screwing my exhaust valves completely within few months. the valves had got serious banging. also the valve seats had got damaged. now i am back on my OEM exhaust after changing the exhaust valves , valve overlapping, head grinding and piston rings changed. Thank god i saved my block-pistons.
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Old 25th February 2008, 11:29   #39
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Default back pressure a requirement?

Quote:
Originally Posted by amit_mechengg View Post
a perfect calibrated backpressure is always needed for proper functioning of the engine. backpressure causes scavenging of the exhaust gases, as well as it helps in low end torque. also the exhaust valves get the cushioning effect due to this backpressure. Extra backpressure will cause the intake charge to dilute and the power drop will be drastic. a small backpressure helps this intake charge for a better combustion as well as perfect valve timings.

a FFE header which is not designed to deliver this will always affect the engine life.

i have put a ffe on my Bike. i ended up screwing my exhaust valves completely within few months. the valves had got serious banging. also the valve seats had got damaged. now i am back on my OEM exhaust after changing the exhaust valves , valve overlapping, head grinding and piston rings changed. Thank god i saved my block-pistons.
So the backpressure is a requirement for a smooth valve operation ,isnt it.I had that impression when i was thinking of the total process and i strongly feel the same about a proper F.F.E. too.
ram
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Old 25th February 2008, 13:14   #40
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Quote:
Originally Posted by amit_mechengg View Post
a perfect calibrated backpressure is always needed for proper functioning of the engine.
Not needed for a 4 stroke engine.

Quote:
backpressure causes scavenging of the exhaust gases, as well as it helps in low end torque.
Back pressure does not help in exhaust gas scavenging. The negative pulse generated in a properly designed header helps in scavenging of the spent gases.

Quote:
also the exhaust valves get the cushioning effect due to this backpressure.
Lol... There is no 'cushion' effect needed for the exhaust valve seat. Heard about the hardened valve seat? And know why it was used in all the engines once the switch was made to Unleaded fuel from leaded??

The rising piston will have to fight the 'gas cushion' in order to expel the gas efficiently out of the chamber. Thus resulting in a power loss and inefficient engine.

Quote:
Extra backpressure will cause the intake charge to dilute and the power drop will be drastic.
Yes.

Quote:
a small backpressure helps this intake charge for a better combustion
The negative pulse ideally should help to suck in the fresh charge out through the intake valve during the period of valve overlap thus helping in aiding better combustion chamber filling.

Quote:
as well as perfect valve timings.
How the hell does a exhaust change affect valve timings???!!!

Valves timing is controlled by the camshaft.


In the case of two stroke engines the negative pulse generated in a properly designed expansion chamber would suck the burnt gases out through the port and alongwith it fresh charge from the crankcase into the exhaust port into the pipe. A correctly designed chamber then would create a reversion of gas flow which would cram the fresh charge sucked into the pipe back into the combustion chamber just before the piston closes the ex port.

This reversal of gases/backpressure is required in a two stroke engine. And it is not needed in a 4 stroke engine with valves to open and close the ports.

Don't confuse backpressure with negative and positive pulses generated in a exhaust system That is what is doing the job of scavenging.
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Old 25th February 2008, 22:49   #41
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How come backpressure is an indeed item in a 4-stroke engine? With all the backup of mech theory it's against it.

For a 2 stroke, where compression and exhaust stroke are same, its the sweeping of the cylinder by the fresh charge of air and fuel which is pushing the exhaust gas out. If theres no back pressure, the fresh air and fuel will not only sweep out the exhaust gasses, but they will also begin to exit with the exhaust gas, resulting in a loss of fuel and air which reduces efficiency. If the back pressure is too high, the exhaust gasses cant all be removed by the fresh charge. So for 2 strokes, there is an optimal back pressure to produce the most efficient fuel use while maximizing power. This optimal back pressure must be tuned to a certain RPM, because above and below that RPM, one of the two problems will surface. 2 strokes have a very dynamic gas flow which is difficult to optimize because of this. Most 2 stroke mfgs simply tune the engine at a given RPM and throttle setting which results in the best overall performance.

In the case of the 4 stroke, where we have 4 different strokes, each designated for its own purpose, and the only set of valves (intake or exhaust) work at a given stroke, the desire is to eliminate ALL the burned gasses from the cylinder when the piston gets to TDC - so the more backpressure, the higher the pressure will be inside the cylinder, and the less exhaust gas we will be able to expel. There should be no gasses inside the cylinder, so the backpressure on a 4 stroke should be as low as possible. This is a very basic common theory.

If there's a cushioniong effect on valves due to backpressure, it's bad, not good (in a 4 stroke)

If you want more authoritative answer on how a good header works and how bad a backpressure in 4-stroke engine read this (How stuff works)

Howstuffworks "How do exhaust headers work to improve engine performance?"

Also read this (Wikipedia)

Manifold (automotive engineering) - Wikipedia, the free encyclopedia
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Old 26th February 2008, 11:43   #42
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Wonderfull explanation guys.I was thinking of a 2 stroke,i am sorry.So headers are good if designed properly.And were do we get one which is properly designed.
ram
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Old 26th February 2008, 12:56   #43
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Well I did think the burnt valve problems went away post the leaded fuel days with hardened valve seats. Bigger problems that you would naturally face is immediate leaning of the engine as the restriction goes away and the engine revs freely but that too tends to get overcome by the closed loop function of the ECU unless you go too far beyond the capacity of your stock fuelling system.

As for backpressure... read this :

Exhaust Theory
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Old 26th February 2008, 15:11   #44
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Thanks phsyco on the enlightening words.I was expecting a reply from you gurus sooner or later.Very informative piece to read also.Thanks for that too.
ram
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Old 26th February 2008, 21:58   #45
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Quote:
Originally Posted by Torqueguru View Post
Please elaborate! i would love to know more!

well dude go see his garage and you will know i mean
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