Topic: Split - What is wrong with the stock Honda induction system?

[Joel]

Yes, you definitely have lower vacuum with PODS because there is less restriction.  But, it doesn't matter.  At idle, the slides are closed, so the vacuum behind them is high regardless of filter.  At part to WOT, the vacuum is sufficient to pull the fuel through, and fuel is an incompressible liquid, so the same amount of fuel is pulled through the fixed-size jet.  As long as sufficient vacuum is present to pull fuel, the flow will always be the same, since more fuel cannot be pushed through the hole regardless of the pressure differential behind it.

Another analogy:  You have a giant aquarium.  You drill a 1/4" hole near the bottom of the aquarium.  The volume of water that flows will be exactly the same as if you had drilled a 1/4" hole near the bottom of a glass of water.  The pressure is higher near the bottom of the aquarium, but you still are not going to flow more gallons of water per second.

Air is compressible, so if you super-charge or turbo-charge, for example, you need to put more fuel into the same volume of air so that it will burn properly.

Sorry, but if the straw were "punched" (blocked you mean?) the vacuum would be HIGHER.  But either way, gasoline is a non-compressible fluid, so as long as there is enough vacuum to pull ANY gas out of the jets, then the volume will be the same.

Sorry yes, meant "pinched".  Edited previous post.

With an airbox, you have a pinched straw (restriction).  With pods, you have an open straw (no restriction).  So pods = less vacuum, which is what I said before.

mystic_1

Not true.  If incompressible is assumed, you can apply Bernoulli's.  Assuming the flow is along a constant elevation, v₂² = v₁² + 2 * Δp / ρ.  Assuming the flow is one-dimensional and the cross-section is constant, the flow rate is proportional to the fluid velocity which is proportional to the pressure differential.

[TwoTired]

The increased volume of air that pod filters flow clearly has NOT been addressed.  If pods did not flow more air, then there would be no need to re-jet with mains that flow more fuel.  Air/fuel ratio MUST be the same in order for the engine to run well.  Chemistry is chemistry.  Flow more air, you need more fuel.  Flow less air, you need less fuel.  Get more air/fuel mixture into engine, you get more horsepower.

The engine, particularly at lower speeds, flows the exact same amount of air to achieve idle whether there are pods or stock induction.  The engine volumetric efficiency does not change with a simple switch to pods.  What changes with pods is the vacuum pressure at the jet exit.  
The fuel metering jets flow at a rate determined by the pressure differential across them and the orifice size.  A deeper vacuum will draw more fuel through a given orifice than a lower one.  Witness that the jets flow no fuel when no vacuum is present.  Flow is proportional to the pressure differential across the jet orifice.
The pods also shorten the inlet runner, which percentagewise moves the the jet away from the vacuum source extreme and closer to the atmospheric pressure at the inlet.  The pressure at the entrance of the filter is zero, slightly closer to the engine valve pressure on the inside of the filter membrane and as you move closer to the vacuum source the it pressure level gets closer to the source of the vacuum.

This is why you must change jet size with pods. The original jets provide less fuel for the same amount air ingested by the engine.  To restore that a/f mixture you must increase the jet size so more fuel can flow at a lower pressure for the same volume of air demanded by the engine.

Re-jetting to larger jets does not mean that you give up fuel mileage, either.  Having more power might just mean that you don't need to open the throttle as much in order to generate the same acceleration or maintain the same velocity.

Agreed you don't have to give up fuel mileage if the fuel metering is correctly adjusted.
But, it is quite unlikely the engine has more power, which would be accurately demonstrated on a dyno test.
Again the engine has not been made more efficient.  Just the pressure levels in the carb have changed.  

When I re-jetted my bike I went from 90 mains to 105 mains.  That's a 16.67% increase in fuel requirement, which, based on balanced chemical reactions, indicates that approximately 16.67% more air is flowing, too.

I'm sorry, this is projection.  You are incorrectly inferring that your current jets are actually delivering 16% more fuel than the original ones.  And not taking into account the reduced flow from lower pressure differentials.  The engine is still normally aspirated, which means it can breath in only that which its displacement and valve train allow the air pump to ingest.

I tried to explain this to you once before and you rejected the explanation favoring your own.  I don't know what else to say to better your understanding.
Well, maybe this:
If you have a drinking straw, do you not get more fluid through the straw if you suck harder, then if you apply just enough suction to raise the level to your lips?
If you suck as hard as you can on a straw, do you not get more fluid through a straw with a larger diameter?
In both cases the straw is behaving the exact same way as a jet orifice.  The jet tube has one end immersed in the fuel and the other end is presented to the carb throat.

Perhaps you missed what I posted earlier:
For simplicity, lets compare two theoretical tubes (because we have no real data for the systems we use now.)

1 - A 12 inch runner, where exists on one end 30 inches of vacuum, and the other zero (as referenced to local atmospheric pressure.)
Lets locate a fuel metering jet exit port 1/3 the way from the vacuum source  (or Four inches).
 The inside of the tube will have a gradient difference of vacuum within it.  For the purpose of illustration, let's say that it is a strictly linear relationship.
One third away from the vacuum source would therefore be  2/3 of the full 30 inch difference, or 20 inches of mercury which the pressure difference pulls the fuel into the intake runner tube.

2. Cut the intake runner tube in half to 6 inches.  It has the same gradient differential end to end as the longer one (30 inches Hg at one end, 0 inches Hg at the air inlet), but over a shorter distance.  The fuel metering exit is not moved, it is still 4 inches from the source.  But, is now 2/3 along the length of the entire runner.  Assuming the same linear gradient relationship along the tube, the fuel metering jet exit port now sees only 10 inches of vacuum pulling on the fuel jet exit, as it is nearer to the pressure equalizing inlet.

Pods, in effect move both the venturi as well as the jet exit percentagewise along a tube (which is the entire intake duct) farther away from the vacuum source and closer to atmospheric pressure at the entrance to the duct.

Still not convinced?  Observe choke operation.  This device effectively moves the vacuum pressure at the piston, all the way to the choke plate.  Often the engine will not run with choke fully closed as it indeed blocks air flow as well.  But, it provide full enrichment even at kick start engine speeds, not only drawing more fuel from the pilot circuit, but the mains/midrange delivery circuit as well.   How often have we seen complaints that the engine will only run when the choke is partially applied?  It's often because the pilot circuit is plugged and the engine can only get fuel delivery from the mains/midrange circuit.  The choke move the vacuum pressure closer to the mains/midrange delivery so the engine gets enough fuel fire.  Of course it won't run at WOT, particularly under load as there is not enough fuel or air to support that power setting.

Still don't believe me?  Then check out Hondaman's earlier post in this thread about dyno testing with pods and stock induction.

Cheers,

[Markcb750]

Damn, I'm impressed.   Are you a Thermodynamics Professor?



I was taught a little carburetor theory in Thermodynamics III  "Internal Combustion Engines".


My paper for that class was on the effect of water injection on the performance of a single cylinder four stroke engine. I had to find a way to induce a little water into the combustion chamber via the carburetor.  It would have been great to have this explanation then.

[Syscrush]

As long as sufficient vacuum is present to pull fuel, the flow will always be the same, since more fuel cannot be pushed through the hole regardless of the pressure differential behind it.

Another analogy:  You have a giant aquarium.  You drill a 1/4" hole near the bottom of the aquarium.  The volume of water that flows will be exactly the same as if you had drilled a 1/4" hole near the bottom of a glass of water.  The pressure is higher near the bottom of the aquarium, but you still are not going to flow more gallons of water per second.

Wow, this is shockingly incorrect.

Incompressible liquids always flow through a given orifice at exactly the same rate regardless of pressure differential? Is that really the claim you are making?

If so, then how is it possible for me to adjust my shower by turning the tap?

[edbikerii]

The tap is the opening.  closing the tap makes a smaller opening.

As long as sufficient vacuum is present to pull fuel, the flow will always be the same, since more fuel cannot be pushed through the hole regardless of the pressure differential behind it.

Another analogy:  You have a giant aquarium.  You drill a 1/4" hole near the bottom of the aquarium.  The volume of water that flows will be exactly the same as if you had drilled a 1/4" hole near the bottom of a glass of water.  The pressure is higher near the bottom of the aquarium, but you still are not going to flow more gallons of water per second.

Wow, this is shockingly incorrect.

Incompressible liquids always flow through a given orifice at exactly the same rate regardless of pressure differential? Is that really the claim you are making?

If so, then how is it possible for me to adjust my shower by turning the tap?

[edbikerii]

TT, Obviously I'm not disputing that the choke makes the mixture richer.  However, it is not by increasing vacuum and therefore pulling more fuel from fixed-sized orifices.  It is much simpler than that.  It is by choking off most of the air, so the existing fuel flow makes a richer mixture.  Hence the name, "choke".

No, you cannot suck unlimited amounts of liquid through a straw simply by sucking harder and harder.  There is a limit imposed purely by the internal diameter of the straw.  You can NEVER EXCEED THE LIMIT, regardless of how much vacuum you apply.  A straw is a horrible analogy, as a straw is capable of flowing quite a bit more liquid than most people can consume at maximum flow, whereas a carburetor jet is designed to flow at maximum flow when sufficient vacuum is applied to it.

Pods made no impact on my jetting at idle and part-throttle openings where the mains were not contributing significantly to the fuel.  This makes perfect sense, since the airbox is capable of flowing more than enough air at idle and part throttle, but only becomes a restriction at larger throttle openings when more air is required than the airbox can flow.  That's why the main jets had to be changed when pods were installed.

[KeithB]

I have carefully read TT's posts and I have to admit that I am re-thinking the pods on my cafe.
They were not installed because I thought they worked better, or had any proof to that effect.
They do look good though and have a nice sound...
Soooo, I'm re-thinking the stock intake.

BTW... someone asked if Honda was still using this type of intake.
I have a 1999 ST1100 and it's basically the same setup. What looks like a gas tank is actually a large airbox and the carb intakes have short , tapered stacks on them. Big space to calm the air and stacks to smooth air flow.



Thanks TT for the info. I appreciate the time you take to defend your statements. 

[Syscrush]

The tap is the opening.  closing the tap makes a smaller opening.

The shower outlet is the opening, closing the tap decreases the pressure differential across the shower outlet.

[Markcb750]

There is a limit imposed purely by the internal diameter of the straw.  You can NEVER EXCEED THE LIMIT, regardless of how much vacuum you apply.

The ID of the tube (well its area anyway) establishes flow across any given  pressure differential. Using vacuum within our atmosphere  to establish the pressure differential limits it to a maximum of 14.7 lb/in^2 if you could establish an absolute vacuum at the low pressure end.  If you vary the vacuum you vary the flow Particularly if the flow is kept laminar/subsonic.

If I read your statement correctly a turbo charger would have zero effect on motor performance.   A turbo simply changes the system to a two stage pump increasing the flow through the intake manifold.

[Laminar]

TT, Obviously I'm not disputing that the choke makes the mixture richer.  However, it is not by increasing vacuum and therefore pulling more fuel from fixed-sized orifices.  It is much simpler than that.  It is by choking off most of the air, so the existing fuel flow makes a richer mixture.  Hence the name, "choke".

Wrong.

No, you cannot suck unlimited amounts of liquid through a straw simply by sucking harder and harder.  There is a limit imposed purely by the internal diameter of the straw.  You can NEVER EXCEED THE LIMIT, regardless of how much vacuum you apply.

Please post ANYTHING mathematical that supports this theory of yours.

A straw is a horrible analogy, as a straw is capable of flowing quite a bit more liquid than most people can consume at maximum flow, whereas a carburetor jet is designed to flow at maximum flow when sufficient vacuum is applied to it.

Let's describe a straw:

A cylindrical tube with the lower end immersed in a fluid (soda), and the other end being subject to a vacuum source (lips).

Now let's describe a main jet:

A cylindrical tube with the lower end immersed in a fluid (fuel), and the other end being subject to a vacuum source (venturi).

You're right, it's a terrible analogy, and there's no way the two could have anything in common.

Pods made no impact on my jetting at idle and part-throttle openings where the mains were not contributing significantly to the fuel.

Is this backed up with dyno testing and wideband O2 sensor readings? Or did it just "feel" the same?

This makes perfect sense, since the airbox is capable of flowing more than enough air at idle and part throttle, but only becomes a restriction at larger throttle openings when more air is required than the airbox can flow.

Show me the calculations you did to determine the flow requirements of the engine at different RPM levels.

That's why the main jets had to be changed when pods were installed.

No.

[mlinder]

Well, there is a limit to how much you can pull through a given tube, which is a funtion of diameter and length. But the volume of air or liquid you are pulling through is, obviously, variable.

However, ed, you are completely wrong about the choke. It is a vacuum-creating device, and the vacuum is what causes the enriching of the charge.

[kghost]

TT, Obviously I'm not disputing that the choke makes the mixture richer.  However, it is not by increasing vacuum and therefore pulling more fuel from fixed-sized orifices.  It is much simpler than that.  It is by choking off most of the air, so the existing fuel flow makes a richer mixture.  Hence the name, "choke".

No, you cannot suck unlimited amounts of liquid through a straw simply by sucking harder and harder.  There is a limit imposed purely by the internal diameter of the straw.  You can NEVER EXCEED THE LIMIT, regardless of how much vacuum you apply.  A straw is a horrible analogy, as a straw is capable of flowing quite a bit more liquid than most people can consume at maximum flow, whereas a carburetor jet is designed to flow at maximum flow when sufficient vacuum is applied to it.

Pods made no impact on my jetting at idle and part-throttle openings where the mains were not contributing significantly to the fuel.  This makes perfect sense, since the airbox is capable of flowing more than enough air at idle and part throttle, but only becomes a restriction at larger throttle openings when more air is required than the airbox can flow.  That's why the main jets had to be changed when pods were installed.

Boy....you've missed the boat on Quite a few levels.

You seem fixated on what the Main Jet is doing.

Bear in mind that the main jet does its job at Wide Open throttle.

What pulls the fuel thru the jets ....any jet or orifice is pressure differential.

Vacuum. The difference between atmospheric pressure on the fuel in the float bowls and the pressure thru the venturi.

Your knowlege seems rather limited. Maybe some reading or research is in order.

[DaveL]

My $.02 is that I didn't get my 1975 CB750 to be the fastest, baddest bike out there. Nothing I do to it will ever make it competitive with a current sport bike in any performance category whether it be handling, acceleration, deceleration, or whatever. Where it can compete with them is style and uniqueness. To that end I have no problem making changes to my bike that individualize it and (to my eye) make it better looking. If that means I have to run an alternate air filter arrangement to make room for other mods I'll do it even if it's not ideal from a performance standpoint.

That's not to say I don't enjoy and appreciate the research and ideas many here have - and I must say I really enjoy the aftermath       

[kghost]

Here you go.



You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Changing the main Jet only has an effect on fuel flow if the metering Force remain Constant.

If you change the Metering force (By for example adding pods as TT has explained) you must change the main jet.

[Laminar]

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

[TwoTired]

No, you cannot suck unlimited amounts of liquid through a straw simply by sucking harder and harder. 

Try sucking fluids through a 1/8 inch (or smaller) straw.  Then try sucking through a 1/4" straw.  Try not to drown.  
Don't just think about it, actually try it.  Prove it to yourself.

There is a limit imposed purely by the internal diameter of the straw.  You can NEVER EXCEED THE LIMIT, regardless of how much vacuum you apply.  A straw is a horrible analogy, as a straw is capable of flowing quite a bit more liquid than most people can consume at maximum flow, whereas a carburetor jet is designed to flow at maximum flow when sufficient vacuum is applied to it.

No sir, I'm afraid you have your fundamentals incorrect.
"An orifice meter is a device used for measuring the rate of fluid flow. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which says that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa."  See:
http://en.wikipedia.org/wiki/Orifice_plate

Also:
Since I can't seem to convince you.  Please do some research on pressure drop across filter media.  This is a common rating for filter media, especially for an air handler.  It won't take too long to see that filters that that block smaller particle sizes, generally create more pressure drop across them. (Given the same filter area size.)  One way to change that dynamic is to increases the area, which lowers the velocity over a fibers creating less turbulence and less pressure drop.  (This assumes the same filter media type, of course).

I have no doubt that the stock paper air filter with it's demanding small particle trapping capability creates more pressure drop across it than the more open, and possibly larger area pod filters.  This will make the carb throat pressure different, and that presents a change in pressure differential across the fuel metering orifices.

Pods not only reduce the pressure drop, they also shorten the intake tube, which moves the fuel orifice exit closer to the atmospheric reference pressure, which is the pressure pushing fuel through the jet orifice.  Per formula, lower the pressure differential, you lower the velocity, which equates to less volume of fuel flow through the orifice.  Open the orifice to restore the same volume as before the carb throat pressure change.

Cheers,

[kghost]

Simplified Bernoulli.....





And why less or more restriction changes fuel pressure metering......

[Laminar]

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

[kghost]

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

Because its the orifice that limits the flow of fuel.

Using the example of the straw that has been bandied about...........

A certain diameter straw will in fact flow only so much liquid. eventually you reach a point at which no more fluid can flow regardless of the suction.

However the diameter of the straw (or jet) is in relation to the suction.

So at wide open throttle....the diameter of the jet (or straw) AND the suction determine the flow.

Where ed went off the rails as it were was imagining the straw as the carb throat.

[mlinder]

I fuxin luv fizix!!!

TT, excellent write up. A copy should be made without the comments and made sticky and closed.

In my opinion, anyway.

[Laminar]

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

Because its the orifice that limits the flow of fuel.

Using the example of the straw that has been bandied about...........

A certain diameter straw will in fact flow only so much liquid. eventually you reach a point at which no more fluid can flow regardless of the suction.

However the diameter of the straw (or jet) is in relation to the suction.

So at wide open throttle....the diameter of the jet (or straw) AND the suction determine the flow.

Where ed went off the rails as it were was imagining the straw as the carb throat.

Why does it affect fuel flow only at WOT?

[TwoTired]

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

If I may clarify.
WOT represents the maximum pressure drop at the venturi.  And, therefore the most pressure differential applied to the jet tube.  The main jet acts to restrict maximum fuel flow at that maximum pressure differential to yield the correct air/fuel mixture.

In the mechanical slide carbs, the slide needle and needle jet further restrict the flow from the main jet at lower throttle settings.  Therefore, these become the limitation on the max fuel flow, which is less than the full flow from the main jet.

Obviously there aren't strict cut-offs for the two devices.  Still, it's the main jet that feeds the slide needle jet, and is the limiting factor at WOT.

A certain diameter straw will in fact flow only so much liquid. eventually you reach a point at which no more fluid can flow regardless of the suction.

Agree, but the limit is where the flow stops being laminar and turbulent forces steal energy during the transfer.

Cheers,

[kghost]

Thanks TT for the better explaination.

Correct on the laminar flowand transfer.


Laminar....if you'll look at the chart you posted you'll see why the main jet is the limiting factor at WOT.

Note the blue shaded areas corresponding to the different circuits.

Other than pilot the only area shaded at Full throttle is the main jet.

By posting the chart you in effect answered your own question.

[edbikerii]

OK, now we're talking.  AS I SAID BEFORE, NO REJETTING WAS NEEDED ON MY BIKE ANYWHERE BUT THE MAINS.  Yes, read the thread.  I tried, and things only got worse when I moved the needles, pilots are essentially irrelevant regardless of airbox/pods, unless your pilots are clogged.

Thank you for confirming what I said, what, 10 posts ago?  No matter how much vacuum you apply, once you have reached the maximum flow for a given orifice, NO MORE FUEL WILL FLOW.  Does everybody agree on that?

The smaller the orifice (the straw) is, the less liquid will flow.  Everybody agree on that?

The smaller the orifice is, the lower the vacuum (meaning less pressure differential) required to reach the maximum flow through the orifice.  Everyone agree on that?

At WOT, the main jet has reached its maximum flow.  Everyone agree on that?

This has nothing to do with turbocharging, as AIR IS COMPRESSIBLE and fuel is not.

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

Because its the orifice that limits the flow of fuel.

Using the example of the straw that has been bandied about...........

A certain diameter straw will in fact flow only so much liquid. eventually you reach a point at which no more fluid can flow regardless of the suction.

However the diameter of the straw (or jet) is in relation to the suction.

So at wide open throttle....the diameter of the jet (or straw) AND the suction determine the flow.

Where ed went off the rails as it were was imagining the straw as the carb throat.

[sangyo soichiro]

I fuxin luv fizix!!!

TT, excellent write up. A copy should be made without the comments and made sticky and closed.

In my opinion, anyway.

I say leave the comments in, but definitely sticky it.  Actually, why not just sticky this very thread...

I already have it bookmarked on my computer.