[nicholastanguma]

I've read from several sources that a longer intake plenum length is better for an engine's performance than a pod filter simply bolted directly to the carb, as the carb's incoming air is less turbulent with a longer plenum. If true, this seems to me like it would be especially beneficial on an engine that isn't completely high rpm focused, a dual sport thumper for instance.

Can anyone confirm a longer plenum is better?


    pod filter on long plenum

   


    vs


    pod filter bolted directly to the carb

   

[Bullet Whisperer]

Having worked on a few modern 4 stroke dirt bikes, it seems that the carb [or injector] seems to be placed as close to the inlet valve as possible, and, based on that, I ditched longer intake manifolds for shorter ones on both our RE racers, with no ill effects.

[AzCal Retred]

That's a great question. Here's some basics. Understanding the PV=nRT equation is very helpful if one wishes to actually analyze the effects and predict behaviour. The PipeMax software has some free shareware that will help. In the "olden days" it was all "cut 'n try" and seat of the pants dyno, but cheap computing power has changed that. Have fun, report back and tell us all you've learned when you've got the "big picture". All this flow analysis also works on the exhaust side too, it gets really fun when building variable conical sectioned 2-stroke exhausts. - ACR -

https://www.enginebasics.com/Advanced%20Engine%20Tuning/Intake%20Runner%20Length.html
>>> "The intake system on a four-stroke car engine has one main goal, to get as much air-fuel mixture into the cylinder as possible. One way to help the intake is by tuning the lengths of the pipes.

When the intake valve is open on the engine, air is being sucked into the engine, so the air in the intake runner is moving rapidly toward the cylinder. When the intake valve closes suddenly, this air slams to a stop and stacks up on itself, forming an area of high pressure. This high-pressure wave makes its way up the intake runner away from the cylinder. When it reaches the end of the intake runner, where the runner connects to the intake manifold, the pressure wave bounces back down the intake runner.

If the intake runner is just the right length, that pressure wave will arrive back at the intake valve just as it opens for the next cycle. This extra pressure helps cram more air-fuel mix into the cylinder -- effectively acting like a turbocharger. The problem with this technique is that it only provides a benefit in a fairly narrow speed range. The pressure wave travels at the speed of sound (which depends on the density of the air) down the intake runner. The speed will vary a little bit depending on the temperature of the air and the speed it is moving, but a good guess for the speed of sound would be 1,300 feet per second (fps). Let's try to get an idea how long the intake runner would have to be to take advantage of this effect.

Let's say the engine is running at 5,000 rpm. The intake valve opens once every two revolutions (720 degrees), but let's say they stay open for 250 degrees. That means that there are 470 degrees between when the intake valve closes and when it opens again. At 5,000 rpm it will take the engine 0.012 seconds to turn one revolution, and 470 degrees is about 1.31 revolutions, so it takes 0.0156 seconds between when the valve closes and when it opens again. At 1,300 fps multiplied by 0.0156 seconds, the pressure wave would travel about 20 feet. But, since must go up the intake runner and then come back, the intake runner would only have to be half this length or about 10 feet.

Two things become apparent after doing this calculation:

1. The tuning of the intake runner will only have an effect in a fairly narrow RPM range. If we redo the calculation at 3,000 rpm, the length calculated would be completely different.

2. Ten feet is too long. You can't fit pipes that long under the hood of a car very easily. There is not too much that can be done about the first problem.

A tuned intake has its main benefit in a very narrow speed range. But there is a way to shorten the intake runners and still get some benefit from the pressure wave. If we shorten the intake runner length by a factor of four, making it 2.5 feet, the pressure wave will travel up and down the pipe four times before the intake valve opens again. But it still arrives at the valve at the right time.

There are a lot of intricacies and tricks to intake systems. For instance, it is beneficial to have the intake air moving as fast as possible into the cylinders. This increases the turbulence and mixes the fuel with the air better. One way to increase the air velocity is to use a smaller diameter intake runner. Since roughly the same volume of air enters the cylinder each cycle, if you pump that air through a smaller diameter pipe it will have to go faster.

The downside to using smaller diameter intake runners is that at high engine speeds when lots of air is going through the pipes, the restriction from the smaller diameter may inhibit airflow. So for the large airflows at higher speeds it is better to have large diameter pipes. Some carmakers attempt to get the best of both worlds by using dual intake runners for each cylinder -- one with a small diameter and one with a large diameter. They use a butterfly valve to close off the large diameter runner at lower engine speeds where the narrow runner can help performance. Then the valve opens up at higher engine speeds to reduce the intake restriction, increasing the top end power output. " <<<

https://thumpertalk.com/forums/topic/1267209-intake-length/
>>> "There are two wave events created in the intake with each intake cycle. The first is the suction pulse created near peak piston velocity. The second is a compression pulse created near the valve close event. The first is timed to provide a small increase in pressure just as the valve closes during a limited RPM target range in the same cycle it was created. The second is timed to create an increase in intake pressure just as the intake valve opens on the next intake cycle. It is also timed to a limited RPM range. Each pressure wave has multiple reflections of decreasing magnitude available depending on current RPM. They are there, helping and hurting at various rpm regardless of what you do. In general, longer intake lengths will increase the inertia force of the wave events and lower the RPM of the wave event arrival. The question is; Can you select a tuned length that is works better than what you have now by chance for the target application. It requires a fair amount of math skill to build the tables. <<<

http://maxracesoftware.com/PipeMaxPro400.htm
PipeMax Professional Engine and Header Design Simulation Models :
•  2 separate Torque and HP Models converge to predict very accurate Torque and HP Data
•  Header Design Model
•  Filling and Emptying Model
•  Intake and Exhaust System Acoustic Wave Effects Model
•  Exhaust Header material Model ( with EGT temperature User input or AutoCalc options )
•  Camshaft Design Specification Model
•  Piston and Cylinder Head Intake Flow Demand Model each 0.5 degree of Crankshaft rotation
•  Crankcase Vacuum , Oil Pan , and Oil Weight Models
•  Weather Conditions and Dyno HP Correction Model ( highest accuracy Weather equations used )
•  Engine Friction and Pumping Losses Model


https://en.wikipedia.org/wiki/Ideal_gas_law
PV=nRT
The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stated by Benoît Paul Émile Clapeyron in 1834 as a combination of the empirical Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law.[1] The ideal gas law is often written in an empirical form: PV=nRT
where P, V and T are the pressure, volume and temperature; n is the amount of substance; and R is the ideal gas constant. It is the same for all gases. It can also be derived from the microscopic kinetic theory, as was achieved (apparently independently) by August Krönig in 1856[2] and Rudolf Clausius in 1857.[3]

[cyrusb]

Sooo, that would be a "yes"?

[Bilgemaster]

Now and then I'm totally flabbergasted how some folks in this Forum REALLY know their shit. Now is one of these times.

Frankly, it'd make me happy to saddle up and drive all day and night to some gathering or other just to maybe soak in the factoids by osmosis with a cold one in my mitt and kebabs sizzling on the disposable grill once these Covfefe-19 woes pass over.

[ace.cafe]

To simplify, place the carburetor close to the cylinder head for best signal to the jet. Make the stack an appropriate length to reach a practical wave result like 3rd harmonic if possible, or as best you can fit into the structural limitations.
Use a still airbox volume at least 1.25 x displacement, and let the stack extend into the airbox away from the walls as best you can.

[cyrusb]

I believe all the original poster asked was is it better to locate the air cleaner farther away from the carb, or right on the carb mouth. Not how far from the head should the carb/injector be located. Read the post carefully before you present your P.H.D dissertation.

[AzCal Retred]

Soak...soak...soak...yuppers, it's REALLY NICE we have some real, hands-on Engineers able to distill out in a few concise words the hidden wisdom of resonant gas-flow physics for our benefit. That's what makes a great forum. Kudo's to Ace!  - ACR -

[AzCal Retred]

" Use a still airbox volume at least 1.25 x displacement, and let the stack extend into the airbox away from the walls as best you can. " covers that nicely. The column ahead of the slide will have different velocities, temperatures and resonances as the slide affects wave propagation beyond it to the air cleaner. My earlier "basics" post was intended to stimulate an appreciation for the complexity and maybe some self-study. Ace distilled that great wad-o-stuff down to some handy rules of thumb, and THAT requires a pretty solid grasp of the topic to do that. This is fascinating stuff, and all the components if it are highly interactive. If someone is involved in engine building and tuning, they need to learn this stuff. - ACR -

[ace.cafe]

Yes, AZR pointed out the airbox volume part of my post, which is almost never met with the pod filters on the carb.
That was intended to be the answer to the original question.

I quaintly call it "gulp volume", meaning the air volume inside the filter mesh barrier which the engine can aspirate without needing to pull air thru the filter to satisfy its "gulp". Then the airbox can fill again during the next engine cycle to be ready for the next "gulp". This helps keep the airbox as close to atmospheric pressure as possible, which helps because the force driving the air into the engine is the pressure differential between the airbox and the cylinder depression. If the airbox runs at a depression below atmospheric pressure, the result is counterproductive.  The filter should flow sufficiently to satisfy that requirement of airbox refill time.

[nicholastanguma]

That's a great question. Here's some basics. Understanding the PV=nRT equation is very helpful if one wishes to actually analyze the effects and predict behaviour. The PipeMax software has some free shareware that will help. In the "olden days" it was all "cut 'n try" and seat of the pants dyno, but cheap computing power has changed that. Have fun, report back and tell us all you've learned when you've got the "big picture". All this flow analysis also works on the exhaust side too, it gets really fun when building variable conical sectioned 2-stroke exhausts. - ACR -

https://www.enginebasics.com/Advanced%20Engine%20Tuning/Intake%20Runner%20Length.html
>>> "The intake system on a four-stroke car engine has one main goal, to get as much air-fuel mixture into the cylinder as possible. One way to help the intake is by tuning the lengths of the pipes.

When the intake valve is open on the engine, air is being sucked into the engine, so the air in the intake runner is moving rapidly toward the cylinder. When the intake valve closes suddenly, this air slams to a stop and stacks up on itself, forming an area of high pressure. This high-pressure wave makes its way up the intake runner away from the cylinder. When it reaches the end of the intake runner, where the runner connects to the intake manifold, the pressure wave bounces back down the intake runner.

If the intake runner is just the right length, that pressure wave will arrive back at the intake valve just as it opens for the next cycle. This extra pressure helps cram more air-fuel mix into the cylinder -- effectively acting like a turbocharger. The problem with this technique is that it only provides a benefit in a fairly narrow speed range. The pressure wave travels at the speed of sound (which depends on the density of the air) down the intake runner. The speed will vary a little bit depending on the temperature of the air and the speed it is moving, but a good guess for the speed of sound would be 1,300 feet per second (fps). Let's try to get an idea how long the intake runner would have to be to take advantage of this effect.

Let's say the engine is running at 5,000 rpm. The intake valve opens once every two revolutions (720 degrees), but let's say they stay open for 250 degrees. That means that there are 470 degrees between when the intake valve closes and when it opens again. At 5,000 rpm it will take the engine 0.012 seconds to turn one revolution, and 470 degrees is about 1.31 revolutions, so it takes 0.0156 seconds between when the valve closes and when it opens again. At 1,300 fps multiplied by 0.0156 seconds, the pressure wave would travel about 20 feet. But, since must go up the intake runner and then come back, the intake runner would only have to be half this length or about 10 feet.

Two things become apparent after doing this calculation:

1. The tuning of the intake runner will only have an effect in a fairly narrow RPM range. If we redo the calculation at 3,000 rpm, the length calculated would be completely different.

2. Ten feet is too long. You can't fit pipes that long under the hood of a car very easily. There is not too much that can be done about the first problem.

A tuned intake has its main benefit in a very narrow speed range. But there is a way to shorten the intake runners and still get some benefit from the pressure wave. If we shorten the intake runner length by a factor of four, making it 2.5 feet, the pressure wave will travel up and down the pipe four times before the intake valve opens again. But it still arrives at the valve at the right time.

There are a lot of intricacies and tricks to intake systems. For instance, it is beneficial to have the intake air moving as fast as possible into the cylinders. This increases the turbulence and mixes the fuel with the air better. One way to increase the air velocity is to use a smaller diameter intake runner. Since roughly the same volume of air enters the cylinder each cycle, if you pump that air through a smaller diameter pipe it will have to go faster.

The downside to using smaller diameter intake runners is that at high engine speeds when lots of air is going through the pipes, the restriction from the smaller diameter may inhibit airflow. So for the large airflows at higher speeds it is better to have large diameter pipes. Some carmakers attempt to get the best of both worlds by using dual intake runners for each cylinder -- one with a small diameter and one with a large diameter. They use a butterfly valve to close off the large diameter runner at lower engine speeds where the narrow runner can help performance. Then the valve opens up at higher engine speeds to reduce the intake restriction, increasing the top end power output. " <<<

https://thumpertalk.com/forums/topic/1267209-intake-length/
>>> "There are two wave events created in the intake with each intake cycle. The first is the suction pulse created near peak piston velocity. The second is a compression pulse created near the valve close event. The first is timed to provide a small increase in pressure just as the valve closes during a limited RPM target range in the same cycle it was created. The second is timed to create an increase in intake pressure just as the intake valve opens on the next intake cycle. It is also timed to a limited RPM range. Each pressure wave has multiple reflections of decreasing magnitude available depending on current RPM. They are there, helping and hurting at various rpm regardless of what you do. In general, longer intake lengths will increase the inertia force of the wave events and lower the RPM of the wave event arrival. The question is; Can you select a tuned length that is works better than what you have now by chance for the target application. It requires a fair amount of math skill to build the tables. <<<

http://maxracesoftware.com/PipeMaxPro400.htm
PipeMax Professional Engine and Header Design Simulation Models :
•  2 separate Torque and HP Models converge to predict very accurate Torque and HP Data
•  Header Design Model
•  Filling and Emptying Model
•  Intake and Exhaust System Acoustic Wave Effects Model
•  Exhaust Header material Model ( with EGT temperature User input or AutoCalc options )
•  Camshaft Design Specification Model
•  Piston and Cylinder Head Intake Flow Demand Model each 0.5 degree of Crankshaft rotation
•  Crankcase Vacuum , Oil Pan , and Oil Weight Models
•  Weather Conditions and Dyno HP Correction Model ( highest accuracy Weather equations used )
•  Engine Friction and Pumping Losses Model


https://en.wikipedia.org/wiki/Ideal_gas_law
PV=nRT
The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stated by Benoît Paul Émile Clapeyron in 1834 as a combination of the empirical Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law.[1] The ideal gas law is often written in an empirical form: PV=nRT
where P, V and T are the pressure, volume and temperature; n is the amount of substance; and R is the ideal gas constant. It is the same for all gases. It can also be derived from the microscopic kinetic theory, as was achieved (apparently independently) by August Krönig in 1856[2] and Rudolf Clausius in 1857.[3]

Good read, thanks for the info.

[nicholastanguma]

I believe all the original poster asked was is it better to locate the air cleaner farther away from the carb, or right on the carb mouth. Not how far from the head should the carb/injector be located. Read the post carefully before you present your P.H.D dissertation.

Yes!

[nicholastanguma]

Yes, AZR pointed out the airbox volume part of my post, which is almost never met with the pod filters on the carb.
That was intended to be the answer to the original question.

I quaintly call it "gulp volume", meaning the air volume inside the filter mesh barrier which the engine can aspirate without needing to pull air thru the filter to satisfy its "gulp". Then the airbox can fill again during the next engine cycle to be ready for the next "gulp". This helps keep the airbox as close to atmospheric pressure as possible, which helps because the force driving the air into the engine is the pressure differential between the airbox and the cylinder depression. If the airbox runs at a depression below atmospheric pressure, the result is counterproductive.  The filter should flow sufficiently to satisfy that requirement of airbox refill time.

Thank you.

[Paul W]

I have always thought that some small “pod” type filters are possibly as equally restrictive compared a standard air box (without access to a flow bench an owner has no real way of knowing).

When I bought my 350 I had plans to fit a high level exhaust which required removal of the air box and the rubber connecting hoses were all perished in any case, so instead of buying new I used a pod filter on the Mikcarb VM24.

I “up-jetted” the bike to compensate. Not having ridden the bike with the standard air box I can’t say if it would have made any difference.

However, due to a fuel feed problem with the VM24 I bought and fitted a “Wassell” 26mm concentric carb. It came with a short, screw on intake bellmouth. Rather than remove that in order to fit a pod filter, I retained it and fitted a larger, oiled foam “pit bike” filter I had in stock (originally purchased for a three cylinder Suzuki G10A engine in my trials car but not used as I fitted a supercharger rather than throttle bodies).

(These days I will never run an engine without an air filter, so I wasn’t happy to run with just an open bellmouth).

The filter sits right over the bellmouth so it doesn’t interfere with intake flow. The theory is that the end clearance between the intake and the inside face of the filter should be at least equal to the intake diameter and in this case that parameter is satisfied.

All I can say is that the improvement in performance was better than expected. I therefore believe the bellmouth has been very beneficial.

[Adrian II]

Let's see if I have this right, for those of us lacking AzCal's technical background. If so, ideally you would want:

1. Carb mounted as close to head as practical (allowing for heat insulating spacer, presumably);

2. Bellmouth on carb;

3. A good-sized chunk of hose or alloy pipe to put a bit of space between the carb and the bellmouth (if there's room);

4. A generously-sized filter at the far end of the whole thing.

All of which is pretty much what the first picture is showing.

A.