Topic: Building A Better Airbox - CB550

[RickThomas]

Introduction:
I have decided to begin sharing my process in designing a simpler reproduction CB550 airbox. The current ones on the market work as a compromise between pods/stacks and the stock airbox. Taking some inspiration from the CB750, I'm attempting to create a smaller single boxed system for the CB550. This thread will chart this progress.

Background:
Using the time left available from COVID-19, I have gone through the bike by completing all of the tune-up maintenance tasks. Prior to this process, the bike ran excellent with an even idle at 1000 rpms and an approximate 45 mpg from mixed driving.
I am using a 1974 CB550K. It has a '74 engine with approximately 10,000 miles and 022A carburetors. The bike has a Hondaman transistorized ignition (highly recommend), a 1974 CB750k gas tank with a right-hand side petcock, Hondaman's air tube modification, a CB650 camshaft, clipped advancer springs, matched intake runners, Delkevik 4into1 exhaust, CB650 Primary Chain Tensioner, a Uni foam filter, and a CB650 oil cooler. The carbs have stock jets (#40 Pilot #100 Main) with the needle clip in the middle position. The carb float height is at 22mm.

Preliminary Research:
Before I began, I wanted to understand the CB550 carburetors better. I spent copious amounts of time reading on the forums for insight on the function of these carburetors. I came away with two key takeaways for any airbox replacement.

1. The shape of the carb boot matters.
2. The air intake restriction is not the same across the CB500/550K/550F.

Hondaman notes in this forum the physics and the function of the air horn within the SOHC carburetors. Their function and design by Honda have been thoroughly discussed in this forum. Without devoting more time than necessary to rehash this issue, it is important to know that the carb boot shape matters.

The second point was something I had not seen documented in the forum. I found that there are differences between the CB550K and CB550F when it comes to the area of the intake opening. With any airbox system, the volume of air and the following vacuum pressure is dictated by the intake opening. With this in mind, I decided to measure the openings on the CB550K and CB550F.
The measurements are:
CB550k Tool Tray - 5.25in x 1.25in = 6.5625sqin
Cb550f/k Plenum Intake Hole - 3in x 2.5in = 7.5sqin
Cb550f Airbox Cover - 3.75in x 1.25in = 4.68750sqin

This difference may make sense in light of the model changes, in the sense that changes in intake restriction may have been necessary. The areas are quite different and may come into play as I develop the design further.

Also, the blow by gas scavenging system of the latter 550's (sans '76 550f) can have a significant impact on the air quality impacts of these vehicles. TwoTired has spent many posts dedicated to this notion. With this in mind, any design I develop will need to consider how to incorporate this system.

Another important modification that I have found useful is the air opening tube into the airbox as noted by Hondaman. He explains how routing tubes from the air opening tubes (topside of the carburetors between #2 and #3 - NOT the overflow tubes) into the airbox on the CB750 can have beneficial smoothing in the mid range. While he lists this for the CB750, I use this on my CB550 due to the CB750 tank and have found it, along with clipping the advanced springs, minimized the throttle flat spot.

[RickThomas]

Update #1:
Starting with some spare airbox parts, I began to develop a prototype. Ultimately, I want to design a 3D print, but a proper prototype will reduce waste in the printing process.

My first attempt is to take the plenum and remove the bulge on the back, effectively making it a slant down the back of the plenum. Then using some spare epoxy and acrylic, I affixed acrylic to cover the back of the modified plenum. Next, I decided to use a cutout of the air outlet from the first stage airbox. From my first post, this is the same surface area on all the mid four models. Then I cut a portion out of the acrylic to fit the cutout and affixed it to the acrylic with epoxy. Lastly, I cut some wire mesh and foam filter material to complete the rudimentary prototype. Note: I won't worry about the air tube holes or the blowby scavenging until I'm closer to finished.
After taking some pictures (attached), I easily installed it on the bike. Next I went for a spirited ride with some speeds up to 80 mph. During the ride I found that the midrange response seemed to big at times. There was also some inconsistency when riding at a steady speed. As a quick test, I removed the air filter and found that this improved the throttle response.

I pulled the plugs from #3 and #4 (attached). The bike seems to be doing fairly well if slightly on the more lean side.
Next, I'm going to test changing the inlet size.

[RickThomas]

Update #2:
I attempted several more runs with varying levels of coverage of the air intake hole. The optimal point appeared to be around a 1/2 to a  1/3 reduction in the area of the opening. This puts the intake more inline with that of the CB550F cover inlet opening. Driving wise, the bike still has moderate power loss in the mid-range and inconsistent power while maintaining speed. My next attempt is to reduce the intake area and add a cowling on the air intake hole. A cowling should serve to smooth the airflow entering the airbox. I believe the unevenness I described is a result of turbulent air being drawn into the airbox. As a bonus, a cowling could also mitigate some of the noise increase associated with the removal of the stock airbox.

[RickThomas]

Update #3:
I went back to my parts and found the cover from the CB550F air filter. I then reshaped the hole in the back of my custom plenum to fit the filter (attached). Then I reinstalled the airbox and went for a test ride. Immediately, it was apparent that the mid-range performance issue was gone. The bike went strong to redline in every gear. The bike also idled very well, so I was overall very satisfied. The one remaining issue that did not get fixed is a fluctuation in the power when maintaining speed. Upon inspection of the airbox, I found that the inlet of the CB550F cover I attached had a portion that was front facing. I thought that this may allow flowing air to have a greater impact in the air turbulence. To test, I taped cardboard around the inlet to block the air. Sure enough, the unevenness was reduced (attached).

I think this design is pretty close and with some smoothing in the angles in a 3D design, I believe I will have a working prototype. The next step is to begin a design.

[rb550four]

Interested....is there a plan for a filter? Just wondering.

[RickThomas]

Yes, I want to work a removable filter into the design. Perhaps it will rely on some cut-to-fit Uni foam which I have had success with in the past.

[Scott S]

 So, basically the K&N plenum style filter?

http://forums.sohc4.net/index.php?topic=108563.25

[RickThomas]

I have seen the K&N style filter mod in the last. I have a couple issues with the design. First, as I found out when I modified the plenum, the air intake dimensions and thus the final restriction is not only dictated by the plenum opening. Second, for the price of the K&N kit, one may as well search eBay for a replacement. Third, the design with the K&N does nothing for the bike visually or functionally. With my hopeful design, I want a box that can work on an open frame design, which the K&N will suffer with due to the crosswind and debris from the tire (if one opts to remove the inner rear fender). For me, I use my bike for commuting and store a variety of items in every nook and cranny, and if I was able to open up some more space under the seat, that would be a great help.

[Sano]

I am very interested in your progress since your engine configuration almost exactly matches my rebuild. 
  My understanding of the design of the intake tract is that the intake length is a major factor.  The long column of air provides more inertia and laminar flow than is possible when bike jewelry (stacks) is are expected to do the job.  I respect your efforts but you might need a more rigorous method to evaluate your results (not sure exactly what that is ?)
  If your reconfigured intake does nor compromise tractability and allows the carbs to be removed more easily it could be a real improvement.  Good luck.

[kenjataimu]

I've been interested in this as well. I saw bits and pieces of the thread posted by Scott, and found that you can get a 3D printed adapter from plenum to air intake (https://www.shapeways.com/product/N495XB3HL/honda-cb550-cone-filter-adapter)--but they dont have the file posted.

I also saw that a company CNC's this adapter, but...at $175 its a rough kick in the nards (https://www.broguemotorcycles.com/honda-cb550-parts/cb550-airbox).

Looking forward to your progress!

[wolf550]

not sure how much those adapters cost to make on a 3d machine but what I liked about the old metal adapters was the upper rocker evac tube port thingy that these don't have. hope you add that into the final piece. who knew that four pieces of metal welded together would make something like this, and at like 5 bucks worth of metal.

[XC204]

Just ordered a stock air box today and will be following your lead and maybe have a 3d printed front . I already have the rubbers and was going to weld up an aluminum one but the square sides could bring turbulence so your way seems to be the best way to go. I Just wish I hadn't thrown mine out a year ago before reading about pods and running poorly!
Thanks for your work on this.
Gary

[wolf550]

Hey Rick was thinking your airbox idea looked similar and remembered I saw turbo guzzi working on a 3d print design. check it out in hi performance section.

[Gurp]

*following

[XC204]

Spent today getting this far on my air cleaner Rick. Any progress on yours?
Gary

[RickThomas]

Sorry for the lack of updates. I haven't forgot about this one! I moved and started a new position. It took a while to get everything organized and my bike needed the head and front brakes rebuilt. With all of that behind me, hopefully I can get back to this project at some point. I will need to find a new 3-D printing resource since I moved, but I'm sure I can find something.

[XC204]

A move and a new position that can't be stressful! If want are going to use the carb side of the air box you can do it without a 3d printer. I used 1" thick off from lumber for the filter holder just a jig saw and router were used it snaps into the box held in with screws in the top and bottom could be made to fit your snorkel and hold the filter foam.