Omega Owners Forum
Chat Area => General Car Chat => Topic started by: Webby the Bear on 08 October 2014, 18:18:15
-
Hey y'all. Hope you're well.
Have a topic I'd like to discuss. Few questions and explanations if possible.....
Exhaust blocked = nowhere for inert gases to go other than backwards = inert gas in combustion chamber = lack of performance. Is this correct?
Exhaust back pressure = this is the pressure created when there is some resistance to the flow of exhaust gas i.e, a baffle, a bend in the pipe etc. = is back pressure a good thing or bad thing? Should there be some at all? And does it mean that exhaust gases are being forced back up the pipe, like with a blocked exhaust? Could this explain engine braking (as I don't really understand why theres engine braking).
Lastly exhaust leaks = if these are after the o2 sensor how is this a bad thing in relation to performance? Would have thought the less resistant flow would actually be a good thing. But articles I've read are adamant that exhaust leakage affects performance....?
Any info guys?
-
You can't have a discussion in here, that's general discussion. You can have a chat though. :y
-
Oh dear.
1: Yes
2: forced induction needs more than NA.
3: Leaks are bad - The exhaust is routed with a specific diameter and a specific volume of resonators to make the exhaust gasses flow in the most efficient manner whilst producing the least noise.
-
Thanks BM
Why particularly would exhaust leaks affect performance though?
-
Thanks BM
Why particularly would exhaust leaks affect performance though?
Unless it's a forced induction engine, it shouldn't affect performance. (Assuming the leak is beyond any lambda sensor readings)
-
Gas flow in an exhaust is not smooth and laminar .. it is a series of pulses caused by the opening/closing of the exhaust valves and the expulsion of the gases by the exhaust "stroke" (blow) of the cylinder .. a 6 cylinder has a much "faster" series of pulses than a 4 pot, but still has these pulses.
The exhaust is "tuned" to prevent these pulses either speeding up and catching each other up, as then then form a blockage, or slowing down too much and "backing" into the next pulse along, which also causes a blockage.
If these "blockages" occur too close to the valves then can actually stop the cylinder being emptied on the exhaust stroke, so on the subsequent induction (suck) too little fuel enters the cylinder, this gives a lack of power as fuel burning is what creates power.
A leaking exhaust simply upsets the balance/tuning within the exhaust pipe/manifold and may cause the blockages I refer to.
In many engines there is a degree of overlap of the exhaust and inlet valves, they are both partially open at the same time at the end of the exhaust/beginning of the induction strokes, the tuned amount of backpressure exists to ensure that the fuel/air mix stays in the cylinder and does not just exit straight through the open exhaust valve.. too much back pressure means too much exhaust remains behind, too little and the fuel/air mix escapes .. both lead to the "wrong" amount in the cylinder so giving reduced performance.
very much simplified .. but I hope it helps
-
Gas flow in an exhaust is not smooth and laminar .. it is a series of pulses caused by the opening/closing of the exhaust valves and the expulsion of the gases by the exhaust "stroke" (blow) of the cylinder .. a 6 cylinder has a much "faster" series of pulses than a 4 pot, but still has these pulses.
The exhaust is "tuned" to prevent these pulses either speeding up and catching each other up, as then then form a blockage, or slowing down too much and "backing" into the next pulse along, which also causes a blockage.
If these "blockages" occur too close to the valves then can actually stop the cylinder being emptied on the exhaust stroke, so on the subsequent induction (suck) too little fuel enters the cylinder, this gives a lack of power as fuel burning is what creates power.
A leaking exhaust simply upsets the balance/tuning within the exhaust pipe/manifold and may cause the blockages I refer to.
In many engines there is a degree of overlap of the exhaust and inlet valves, they are both partially open at the same time at the end of the exhaust/beginning of the induction strokes, the tuned amount of backpressure exists to ensure that the fuel/air mix stays in the cylinder and does not just exit straight through the open exhaust valve.. too much back pressure means too much exhaust remains behind, too little and the fuel/air mix escapes .. both lead to the "wrong" amount in the cylinder so giving reduced performance.
very much simplified .. but I hope it helps
Nige, that's an absolutely perfect explanation! Many thanks for that.
So, having read and re-read your explanation........ in short... exhaust leaks (around the cat to centre section join) are upsetting the precise back pressure that's needed to a.) scavenge the used exhaust gas out of the cylinder and/or to make sure the new ''live'' inlet air isn't forced out the exhaust valve.
So if ive got this right..... ANY exhaust leak can cause power problems
:y :y :y :y
-
Gas flow in an exhaust is not smooth and laminar .. it is a series of pulses caused by the opening/closing of the exhaust valves and the expulsion of the gases by the exhaust "stroke" (blow) of the cylinder .. a 6 cylinder has a much "faster" series of pulses than a 4 pot, but still has these pulses.
The exhaust is "tuned" to prevent these pulses either speeding up and catching each other up, as then then form a blockage, or slowing down too much and "backing" into the next pulse along, which also causes a blockage.
If these "blockages" occur too close to the valves then can actually stop the cylinder being emptied on the exhaust stroke, so on the subsequent induction (suck) too little fuel enters the cylinder, this gives a lack of power as fuel burning is what creates power.
A leaking exhaust simply upsets the balance/tuning within the exhaust pipe/manifold and may cause the blockages I refer to.
In many engines there is a degree of overlap of the exhaust and inlet valves, they are both partially open at the same time at the end of the exhaust/beginning of the induction strokes, the tuned amount of backpressure exists to ensure that the fuel/air mix stays in the cylinder and does not just exit straight through the open exhaust valve.. too much back pressure means too much exhaust remains behind, too little and the fuel/air mix escapes .. both lead to the "wrong" amount in the cylinder so giving reduced performance.
very much simplified .. but I hope it helps
Nige, that's an absolutely perfect explanation! Many thanks for that.
So, having read and re-read your explanation........ in short... exhaust leaks (around the cat to centre section join) are upsetting the precise back pressure that's needed to a.) scavenge the used exhaust gas out of the cylinder and/or to make sure the new ''live'' inlet air isn't forced out the exhaust valve.
So if ive got this right..... ANY exhaust leak can cause power problems
:y :y :y :y
Not all exhaust leaks are bad. If you get one just before the backbox, your car sounds well chavvy innit ;D
-
pressure test (http://www.youtube.com/watch?v=tM6A8OpqBpc)
::)
-
Gas flow in an exhaust is not smooth and laminar .. it is a series of pulses caused by the opening/closing of the exhaust valves and the expulsion of the gases by the exhaust "stroke" (blow) of the cylinder .. a 6 cylinder has a much "faster" series of pulses than a 4 pot, but still has these pulses.
The exhaust is "tuned" to prevent these pulses either speeding up and catching each other up, as then then form a blockage, or slowing down too much and "backing" into the next pulse along, which also causes a blockage.
If these "blockages" occur too close to the valves then can actually stop the cylinder being emptied on the exhaust stroke, so on the subsequent induction (suck) too little fuel enters the cylinder, this gives a lack of power as fuel burning is what creates power.
A leaking exhaust simply upsets the balance/tuning within the exhaust pipe/manifold and may cause the blockages I refer to.
In many engines there is a degree of overlap of the exhaust and inlet valves, they are both partially open at the same time at the end of the exhaust/beginning of the induction strokes, the tuned amount of backpressure exists to ensure that the fuel/air mix stays in the cylinder and does not just exit straight through the open exhaust valve.. too much back pressure means too much exhaust remains behind, too little and the fuel/air mix escapes .. both lead to the "wrong" amount in the cylinder so giving reduced performance.
very much simplified .. but I hope it helps
Nige, that's an absolutely perfect explanation! Many thanks for that.
So, having read and re-read your explanation........ in short... exhaust leaks (around the cat to centre section join) are upsetting the precise back pressure that's needed to a.) scavenge the used exhaust gas out of the cylinder and/or to make sure the new ''live'' inlet air isn't forced out the exhaust valve.
So if ive got this right..... ANY exhaust leak can cause power problems
:y :y :y :y
To a certain extent, but a small leak right by the back box will have a significantly different effect to a large leak right by the manifold.
Just to elaborate on the idea of "tuning" an exhaust, as a pipe carrying a gas flow narrows (converges) the gas flow speeds up but any pressure changes depend on the "back pressure" the gas is "pushing" against ..[pressure is nothing more than resistance to flow].. if the back pressure equals the forward pressure the flow stalls and a "blockage" occurs. In a widening pipe (divergent) the opposite happens, the speed slows but the pressure always drops at that point ... so much for empirical theory .. what does it mean ??
The exhaust consists not only of pipes, which are usually of common diameter, but of boxes as well, and at the entry / exit of each box the effect is either divergent (entry) or convergent (exit), also .. the baffle pipes inside each box have some weird shapes that cause all sorts of weird problems .. which is why high performance motors tend not to have silencers !! but placing the boxes as far from the engine/manifold as possible reduces the problems they cause.
Next problem, as gas flow is not linear the bends/joins in the pipework cause further effects as the curves can act as either converging or diverging.....
The speed/size of all these little pulses leaving the tailpipe are what actually makes the "noise" of an exhaust system, sound being changes in pressure waves that our ears detect
The designer/tuner of an exhaust system will use all the above effects to a) try and reduce any "stall" points (blockages) in the system, b) ensure an optimised back pressure at the exhaust valves c) create the sound he wants d) at the volume level he wants ....
A leak is ALWAYS a divergent duct as the universe is much bigger than the inside of the pipe, so at the leak the pressure will always drop (sounds obvious but it matters !!) but the AMOUNT it drops will depend on the pressure it was just before the leak.
So the effect of a leaking exhaust really depends where the leak is, a leak in a low pressure, low speed area will have very little effect on gas flow, whilst a leak in a high pressure/high flow area might have a major effect.
HTH :)
-
Again Nige thanks for that... bloody love theory! :) :) :)
So if we consider the Omega V6 exhaust.
We have a wide manifold thus pressure drop right from the off. BUT then a converging downpipe. So this suggests that some pressure has already built.
If there was a leak at the join of the cat to centre silencer... there's a bend. then a leak. then another bend. if I was to guess based on everything you've said I think we could be looking at a potential performance drop with a leak there as we have built up pressure followed by a bend (resistance) followed by an immediate drop (divergent) due to the leak.
what are your thoughts?
Sorry my explanation isntas good as yours :-[
-
I suspect a leak at that point will have minimal effect based on:
1) The leak is likely to be small given the arrangement of the joint
2) The gases have already passed a major restriction/chamber in the form of the Cat (or two cats on a 2.2/3.2
-
I suspect a leak at that point will have minimal effect based on:
1) The leak is likely to be small given the arrangement of the joint
2) The gases have already passed a major restriction/chamber in the form of the Cat (or two cats on a 2.2/3.2
Fair enough. But taking in to account the leak is near a previously constricted exit from the cat (restrictive in comparison with the centre section of the cat) I'd have thought the opposite. Obviously you know more about it than me but basing my opinion on Nige's theory
-
Its the Cat exit where the real restriction is.
The gases enter the cat, expand and consequently slow down.
Next they have to exit the cat where they are 'compressed' and hopefully speed up to enter the next section of exhaust.
Consequently, the joint becomes a very minimal influence and any leak here being small and minor (more annoyance than anything)
-
Its the Cat exit where the real restriction is.
The gases enter the cat, expand and consequently slow down.
Next they have to exit the cat where they are 'compressed' and hopefully speed up to enter the next section of exhaust.
Consequently, the joint becomes a very minimal influence and any leak here being small and minor (more annoyance than anything)
We're talking about the join After the cat, right? As opposed to the downpipe to fannymould joint?
-
Yep
If the cat to manifold joint was leaking you would get a whole host of issues including 02 readings which are off.
-
Yep
If the cat to manifold joint was leaking you would get a whole host of issues including 02 readings which are off.
:y
I'm struggling to understand this so apologies for banging on
but the pressure coming out of the cat is high (exit - converge). the join is just after it, like in entwoods post.... a leak in a high pressure section...........
-
In addition to my last comment.....
If the leak was around the manifold downpipe (before the O2 sensor) would that set a check engine light? And if not what am I looking for on the live data to tell me whether its not working properly (the cat that is)