Help! Going Dual Exhaust

[KANE]

Can you quote any law of physics that would support this?

Gasses tends to move from areas where they are concentrated to areas of less concentration. Some back pressure promotes scavenging- which is like the siphon effect but using gasses instead of liquid- although one could argue the principle is similar.

This article breaks it down nicely though. I don't think a PhD is required- Super Chevy takes it down in easy terms.

Assuming for a moment no flow losses, the piston traveling down the bore will pull in one-eighth of 350 cubic inches. That's 43.75 cubic-inch, or in metric, 717cc. If the compression ratio is say 11:1, the total combustion chamber volume above this 717cc will be 71.7cc. If a negative pressure wave sucks out the residual exhaust gases remaining in the combustion chamber at TDC, then the cylinder, when the piston reached BDC, will contain not just 717 cc but 717 + 71.7 cc = 788.7 cc. The result is that this engine now runs like a 385 cubic-inch motor instead of a 350. That scavenging process is, in effect, a fifth cycle contributing to total output.


But there are more exhaust-derived benefits than just chamber scavenging. Just as fish don't feel the weight of water, we don't readily appreciate the weight of air. Just to set the record straight, a cube of air 100 feet square will weigh 38 tons! If enough port velocity is put into the incoming charge by the exhaust scavenging action, it becomes possible to build a higher velocity throughout the rest of the piston-initiated induction cycle. The increased port velocity then drives the cylinder filling above atmospheric pressure just prior to the point of intake valve closure. Compared with intake, exhaust tuning is far more potent and can operate over ten times as wide an rpm band. When it comes to our discussion of exhaust pipe lengths it will be important to remember this.

More from Super Chevy...

 

[84Turbo]

Nope, like I said I read it somewhere. But if less back pressure is good, then why do bigger engines have larger headers than smaller ones do?

Because bigger engines have to expel a greater mass of exhaust gas than a small engine does, and it's frequently called upon to do this task in the same amount of time as the smaller engine. Trying to push a lot of stuff through a small hole (pipe) takes more effort/pressure at the pressurehead. This pressure has to come from the piston and crankshaft. Any work that the crank has to do to push out the exhaust results in less work/energy/hp available to accelerate the car.
But be careful, don't mistake header pipe sizing/tuning with generic backpressure issues. Putting a cork in the muffler or header collector will obviously increase the backpressure (as will small exhaust pipes and restrictive mufflers). Obviously this is not a smart thing to do if performance is the goal.

 

[73shark]

Well said.

 

[84Turbo]

Gasses tends to move from areas where they are concentrated to areas of less concentration. Yes, and they do that by two different methods. Diffusion (which is painfully slow), or by being pushed out. It is easier to push the gas out if it is not having to work against high pressures downstream. Some back pressure promotes scavenging- Backpressure inhibits scavenging, by preventing or slowing down the exhaust gas flow. Scavenging is only accomplished when the inertial flow of the exhaust gas can continue to flow, and not be slowed down by the restrictive path downstream. which is like the siphon effect but using gasses instead of liquid- although one could argue the principle is similar.

This article breaks it down nicely though. I don't think a PhD is required- Super Chevy takes it down in easy terms.



More from Super Chevy...

Your highlighted (bold) sentences support my position. You cannot have good scavenging if the exhaust stream slows down quickly due to the backpressure encountered when gas is forced to travel through a restrictive path.

 

[KANE]

Your highlighted (bold) sentences support my position. You cannot have good scavenging if the exhaust stream slows down quickly due to the backpressure encountered when gas is forced to travel through a restrictive path.

Funny how that physics stuff works... balance is the key.

 

[84Turbo]

Funny how that physics stuff works... balance is the key.

I'm not sure I understand your balance statement. I am in disagreement with some previous posters who state that backpressure is beneficial to engine performance. I still say it is not, and I am still waiting for any examples to show where it is beneficial.

 

[73shark]

Here's what I found and it pretty much supports your theory: http://www.bigcitythunder.com/pages/understanding_exhaust.pdf

However the flow vs velocity paragraph is along the line of what I remember reading. Sort of what Kane said about balance.

 

[KANE]

I'm not sure I understand your balance statement. I am in disagreement with some previous posters who state that backpressure is beneficial to engine performance. I still say it is not, and I am still waiting for any examples to show where it is beneficial.

Overly lean conditions due to the inability to meet a 14.7 to 1 ratio of fuel was common for carbureted vehicles as they were not able to meet the adjustments and maintain that ratio.

So yes- with the design of fuel delivery systems on some cars... back pressure is a silent partner to ensure as much usable power can be created as possible.

Back pressure is the silent partner to ensure the balance on many cars, maintaining the 14.7:1 ratio, so the right amount of air and fuel make the best flame kernel.

The truth about exhaust backpressure and torque

You won't make a lot of power when overly lean. You won't make a lot of power unless you get as much air and fuel in the right ratio into the chamber either.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

I think where people go wrong is attributing the science to what is happening. It's like the folks in the middle ages who could create matter... their recipes included throwing grain in the corner and that would "create" rats.

 

[84Turbo]

Overly lean conditions due to the inability to meet a 14.7 to 1 ratio of fuel was common for carbureted vehicles as they were not able to meet the adjustments and maintain that ratio. Lean issues are a problem no mattter what the fuel system is. You are straying off topic, which is backpressure.

So yes- with the design of fuel delivery systems on some cars... back pressure is a silent partner to ensure as much usable power can be created as possible. Nonsense. I repeat, NONSENSE.

Back pressure is the silent partner to ensure the balance on many cars, maintaining the 14.7:1 ratio, so the right amount of air and fuel make the best flame kernel. See above comment regarding nonsense.

The truth about exhaust backpressure and torque

You won't make a lot of power when overly lean. You won't make a lot of power unless you get as much air and fuel in the right ratio into the chamber either. That's true, but is not relavent to this topic.



I think where people go wrong is attributing the science to what is happening. It's like the folks in the middle ages who could create matter... their recipes included throwing grain in the corner and that would "create" rats.

We are in agreement that there are a lot of myths out there (needing backpressure, for one).

But, back to your statements. You are mistaking the effects of reversion (reverse flow into the intake manifold) and its effect on A/F ratio in carb'd engines, and backpressure. These are two different topics. No one with any credible automotive expertise would modify the exhaust restriction (and the accompanying backpressure) to change/bandaid his carburetor fuel metering. This is pure silliness.

An engine makes power by maximizing the amount of fresh air that gets into a cylinder. Backpressure reduces the amount of fresh air that gets into the cylinder (by competing for the available volume in the cylinder).

Again, show me an example where backpressure increases horsepower. We are not discussing ill tuned carburetors. We are discussing backpressure.

 

[Evolution1980]

An engine makes power by maximizing the amount of fresh air that gets into a cylinder. Backpressure reduces the amount of fresh air that gets into the cylinder (by competing for the available volume in the cylinder).

Jumping in here without really reading the previous posts, but... Your statement would only make sense in a 2-cycle (2-stroke) engine. Ours are 4 stroke. The intake and exhaust strokes are on different strokes. So it's the exhaust stroke where backpressure is scavenging gasses out of the combustion chamber. When the fuel/air mix comes in on the intake stroke, the exhaust valves are closed so there's no exhaust "competing" for space in the cylinder. And that is probably the answer right there: By helping ("scavenging") remaining exhaust gas out of the combustion chamber during the exhaust stroke, you'll have more volume for 'fresh air' and less exhaust remaining in the chamber during the intake stroke.

I suppose as an additional item to consider, we could mention "port velocity" which may very likely have a direct relation to 'backpressure'. This discussion may go there, but that's more for discussion of cylinder heads and not strictly exhaust setups (even though cylinder heads are part of the exhaust equation).

 

[73shark]

When the fuel/air mix comes in on the intake stroke, the exhaust valves are closed so there's no exhaust "competing" for space in the cylinder.

Technically you are ignoring valve overlap.

 

[73shark]

Here's the aforementioned paragraph (bigcitythunder) related to the discussion:
Flow Volume & Flow Velocity One of the biggest issues with exhaust systems, is the relationship between gas flow volume and gas flow velocity (which also applies to the intake track). An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust pipe will provide higher flow velocity than a larger diameter pipe. Unfortunately, the laws of physics will not allow that same small diameter pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband.

Don't know if this helps.

 

[Evolution1980]

Unfortunately, the laws of physics will not allow that same small diameter pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer.

Essentially what I was referring to in the last paragraph of Post #59

Looking back further, I didn't realize this was the same thread from a year ago...So 84Turbo you're still trying to battle this topic, eh? :L As long as you keep it civil, "Have at it!" Although looking back to the beginning of this thread, I don't think at this point that you're going to be satisfied with any answer presented to you, regardless of the source, unless of course they 100% back you up.

 

[Peer81]

Very nice topic to read!

But to go on the subject. I still didn't read any explination why backpressure would be something you wanted besides people that swap exhaust without rechecking there A/F ratio. Till so far I have to agree with 73shark and 84turbo.

Greetings Peter

 

[KANE]

An engine makes power by maximizing the amount of fresh air that gets into a cylinder.

Yes... and it needs fuel and spark. You can't talk hypotheticals unless you look at the car as a system. Fuel, air, and spark must be accounted for as they are are critical components to the formula.

Backpressure reduces the amount of fresh air that gets into the cylinder (by competing for the available volume in the cylinder).

Yes it does... but as a system, the ratio needs to be as close to 14.7:1 as possible. If back pressure gets the engine to that ratio because of the laws of physics- then so be it... it is the end product that counts- the level of power. So yes- it can be useful if the alternative is a lack of power.

And if you reduce that back pressure- you'll need to retune. And then you'll still have back pressure.

:L

If you can show me a streetable exhaust system that create zero back pressure- I'll give into the argument. Otherwise, it must be factored into the vehicle as a part of the overall system. Less back pressure is always better than more back pressure- but it's not realistic that anyone could get rid of all of it. So it is important to work around it- mitigate it- and deal with it... and if possible- use it to your advantage.


And, if it creates additional power that is the result of overcoming mechanical shortfalls- than how can it not be useful- and therefore important or... good for that motor?

 

[92BlackVette]

Turbo84 and I will never agree on spoilers/body kits on Corvettes (Greenwood), but I do like his argument, and I think it is pointless to argue with him. I think the closer you get back pressure to zero the better. Good luck with the side effects though.

 

[84Turbo]

Jumping in here without really reading the previous posts, but... Your statement would only make sense in a 2-cycle (2-stroke) engine. Ours are 4 stroke. Are you serious???? Do you actually think it's news to me that our engines are 4 cycles? C'mon, if you want to insult me, just go straight to making comments about my ancestry. Don't beat around the bush with this 4 cycle comment. The intake and exhaust strokes are on different strokes. So it's the exhaust stroke where backpressure is scavenging gasses out of the combustion chamber. Backpressure does not push the exhaust gas out of the cylinders. The piston does. Backpressure is residual exhaust mass and pressure in the exhaust pipes inhibiting the further removal and flow of gas mass out of the cylinders. If you're going to "contribute", at least understand the technical definitions of the things you're writing about. When the fuel/air mix comes in on the intake stroke, the exhaust valves are closed so there's no exhaust "competing" for space in the cylinder. Did the significant (and measurable) residual exhaust mass and pressure in the cylinder just magically disappear??? And that is probably thee answer right there: By helping ("scavenging") remaining exhaust gas out of the combustion chamber during the exhaust stroke, you'll have more volume for 'fresh air' and less exhaust remaining in the chamber during the intake stroke.

I suppose as an additional item to consider, we could mention "port velocity" which may very likely have a direct relation to 'backpressure'. I'm getting a headache reading this. This discussion may go there, but that's more for discussion of cylinder heads and not strictly exhaust setups (even though cylinder heads are part of the exhaust equation).

I'm not trying to be rude here, but you have a serious lack of understanding of how an engine works, not only in the mechanical details, but the gas flow dynamics. Please, for your good, and my sanity, get a textbook on internal combustion engines and do some reading.

 

[84Turbo]

Essentially what I was referring to in the last paragraph of Post #59

Looking back further, I didn't realize this was the same thread from a year ago...So 84Turbo you're still trying to battle this topic, eh? :LA professor I greatly respect frequently says his mission in life is to "Push back the frontiers of ignorance". I'm just trying to follow his example, but in the automotive world. As long as you keep it civil, "Have at it!" Although looking back to the beginning of this thread, I don't think at this point that you're going to be satisfied with any answer presented to you, regardless of the source, unless of course they 100% back you up.

On the contrary, I will be happy with any answer that obeys the laws of physics.

 

[84Turbo]

Yes... and it needs fuel and spark. You can't talk hypotheticals unless you look at the car as a system. Fuel, air, and spark must be accounted for as they are are critical components to the formula.

I didn't mention fuel and spark for two reasons. 1) Everybody knows you need fuel and spark, so it's a waste of time typing "fuel and spark". and 2) fuel and spark are not causes of backpressure.

Yes it does... but as a system, the ratio needs to be as close to 14.7:1 as possible. The A/F ratio is academic to the original question of backpressure. Put the A/F anywhere you want, and backpressure will still cause a loss of horsepower and performance. If back pressure gets the engine to that ratio because of the laws of physics- then so be it...No knowledgeable mechanic or race engineer would purposely cork up their exhaust to cause poor intake flow (yes, I said intake flow) which would cause erroneous carburetor metering, just to get to some desired A/F ratio. it is the end product that counts- the level of power. So yes- it can be useful if the alternative is a lack of power.

And if you reduce that back pressure- you'll need to retune. And then you'll still have back pressure. Uhm, if you reduce the backpressure, you'll have less backpresure. Less backpressure will cause an increase in VE, which will result in more horsepower. The point I have trying to get across for way too many posts.

:L

If you can show me a streetable exhaust system that create zero back pressure- I'll give into the argument. Otherwise, it must be factored into the vehicle as a part of the overall system. Less back pressure is always better than more back pressure-Thank you!!! That's what I've been trying to get across to people. but it's not realistic that anyone could get rid of all of it. So it is important to work around it- mitigate it- and deal with it... and if possible- use it to your advantage. Not possible. Backpressure is like paying taxes. You don't "use it to your advantage", you minimize it.


And, if it creates additional power that is the result of overcoming mechanical shortfalls- than how can it not be useful- and therefore important or... good for that motor?

Because it can't create additional power. An internal combustion engine is an air pump. You make power by injesting (and eventually expelling) the maximum amount of air (and yes, fuel, but fuel is not the difficult part to increase in an engine). Backpressure reduces the mount of fresh air that can be pumped into an engine. It's simple physics, and it's common sense.

 

[84Turbo]

Turbo84 and I will never agree on spoilers/body kits on Corvettes (Greenwood), but I do like his argument, and I think it is pointless to argue with him. I think the closer you get back pressure to zero the better. Good luck with the side effects though.

You're spot on!