Topic: Carb FAQ

[TwoTired]

Ilust. 015
Here is the inlet coupler mounted in the air plenum and onto the carb inlet.   Notice how well the inner diameters match without any stepped edges in the wall connection?


Ilust. 016
Here is a POD filter.  While I think it is an old K&N, I can find no markings on it to verify.
Note two important design errors.
1 – There is no molded step to match the inner diameter to carb throat inner diameter.
This is certain to cause turbulence, with eddies reaching well into the carb throat.

2 – The straight wall section ends abrubtly with a sharp corner and irregular features.
This is also certain to cause turbulence, with eddies reaching well into the carb throat.

IMO it may look cool on the outside, but is certainly inferior on the inside.

A third point about this filter is that the filter media certainly does nothing to keep the air flow laminar.  In fact, it is a near certainty to cause turbulence right at the entrance to the carb and reach well into the throat.

[TwoTired]

Ilust. 017
I would much rather see something more akin to this.

Air filters by nature present an obstruction to laminar flow, be it paper or foam or whatever.  If air has to move around fibers or through tiny tunnels, the obstructive and redirecting properties need, at least change direction of air movement and induce turbulence.


Ilust. 018
From K&N website.

Ilust. 019
Also from K&N website.

[TwoTired]

What did Honda do about this filter turbulence?

 
Ilust. 020
The 550 Induction diagram.

Ilust. 021
The 750 Induction diagram

[TwoTired]

Ilust. 022
This is the induction system of a CB550.  It has the same elements found in other SOHC4s, but in a slightly different arrangement.  (I didn’t have any dismantled CB750 inductions to photograph.)
 
Ilust. 023
The inlet air horn located under the seat.  Note the ramped entry.
 

[TwoTired]

Ilust. 024
Here you can see the air horn feed the filter chamber on the left.  Air flows through the filter, and is certainly turbulent behind and near the big oval section of the box.  The clean air quiets a bit as it enter the oval of the air plenum on the right, where negative pressure directs the air to the next demanding inlet tube where the flow is straightened out even more.
 
 
Ilust. 025
There’s that nice molded inlet funnel again.

[TwoTired]

Ilust. 026
Here are the bits of the carb that you do NOT want to have turbulent eddies of higher or lower pressure to reach.
The slide needle jet exit in the center of the venturi.
The mains emulsion tube air inlet.
The Pilot system air jet.


In the 550’s, the distance from air filter and duct turbulence is about 7 to 8 inches.

No POD or filter placed at the carb inlet can offer that isolation or buffering from turbulent air flow.

I estimate the 750 path is a bit shorter , around 4-5 inches.  But it has a much larger filter, and still has those beauty air inlet bells to straighten out flow to the carb inlets.

[TwoTired]

Finally, a comment about K&N claims.

Ilust. 027
This is also from their web site.  An artists representation of what K&N wishes you to believe.  First 4 points I have no argument against.  The 5th, however says: “Bonded cotton/wire mesh straightens airflow reducing turbulence.”
Sorry, saying this doesn’t make it so, even if you pay an artist to represent a fictional desire.  Based on all the physics and science of air flow principles, anything you put in an airflow where each fiber or obstruction isn’t aerodynamically shaped to smooth airflow transition, you WILL get turbulence.  Round wire mesh and bonded cotton are by no means aerodynamic.
Brag 6 from the above advert states: “Straightened and filtered high-volume air enters the intake system.”
Huh?  The exits are all wavy or folded with mesh and fiber that induces turbulence.  Yet the air is somehow straightened?  How can people accept this baseless brag as fact?

I use K&N air filters on my 750’s and like them in the stock air box, where the turbulence they undoubtedly induce is quieted well before getting to the carb throat.  But, sticking any filter at the carb entrance for all around performance reasons, doesn’t have any science or engineering to support the practice.

Well, that's my report.  I hope you find it of some value.

Cheers,