The International CBX Owners Association

A few years ago I made SuperJim's Ultimate Intake System for the Kawasaki H2 750 triple 2-stroke. It worked great. So I finally decided to make one for my CBX.

Preface: Combustion engines, especially ones with multiple carburetors, like intake runners instead of just a filter pod on the carburetor. This is especially true with CV (Constant Velocity) carbs. And one thing CV carbs really don't like it short filter pods with a flat metal back. Here's why. The sound waves coming out of the carb hit the flat metal back of the filter pod and bounce right back into the inlet of the carb. These sound waves are like pressure pulses that move in and out of the carb making the slide and needle make rapid and short movements up and down. There goes the Constant Velocity. Now it's varying velocity. That makes tuning difficult and causes flat spots in the power curve because the sound waves change with rpm.

I had the K&N pod filters on the CBX. You all know the ones. Each pod goes to a pair of carbs. Short filter with a large flat metal back to bounce those sound waves directly back into the carbs. Also the incoming air flow must make an immediate 90 degree turn to get into the carb which is never a good design. My solution was to purchase some 2.25" (57mm) polished aluminum intercooler piping bends and cut them to make my desired runners. The carbs inlets are 2" (51mm). I used silicone 2" x 2.25" (51MM X 57MM) reducer couplings to connect the runners to the carbs. This serves two purposes: the transition from 2.25" to 2 also servers as a little velocity stack. The larger 2.25" runner reduces flow restriction then increases velocity at the reducer.

I routed the four inner carbs directly under the seat, in front of the battery compartment. I then installed four individual Uni foam filters. There was not room to route the outside carbs into that same area, so I made the runners on the outside carbs turn down and installed K&N cone filters on those. The K&M cone filters look better since they are very visible and the cone shape fits perfectly along the frame. Sure they have a metal back but it's very small and the cone design allows most of the sound waves to exit the filter instead of bouncing back. Then the 90 degree runner takes care of the rest.

A test ride confirmed my expectations. The mid range was much smoother and pulls stronger down low as well as up top. Another plus is I no longer have to be concerned about water going straight into my carbs during a wash or rain storm like with the K&M's.

I"ll post a few pictures.

Here's a few pictures

I could tell Pit Girl Barbarilla was really excited about the new intake system although she didn't say much about them.

How did you end up with Paul Jasovsky's mannequin, she sure does get around and it looks like she has had some enhancements since I saw her last!

I thought she retired after the Marlinton rally?

OK - seriously, for all the trouble, why didn't you just keep the stock airbox, with it's built-in runners and enjoy a the smooth-pulling engine the way Honda intended? Couldn't agree more with all your comments on pod filters and CV carbs.

Dave

Just today I exchanged texts with another cbx owner who used a method I've never seen before. He removed the air box and used a single large pod filter and connected that to the plenum. He says it was jetted for pods and claims the single pod works great.

daves79x wrote:OK - seriously, for all the trouble, why didn't you just keep the stock airbox, with it's built-in runners and enjoy a the smooth-pulling engine the way Honda intended? Couldn't agree more with all your comments on pod filters and CV carbs.

Dave

Because anybody can do that. This is one of a kind, sounds better than stock and, in my opinion, looks much cooler. I would expect it also makes an extra pony or two than the stock air box with less restriction. Mainly just did it for fun and to be different.

I've ridden a bunch of CBXs with pods and frankly, can't stand the intake honk. If everyone could get their CBXs running as Honda intended, I guess we'd get pretty bored here.

Dave

daves79x wrote:I've ridden a bunch of CBXs with pods and frankly, can't stand the intake honk. If everyone could get their CBXs running as Honda intended, I guess we'd get pretty bored here.

Dave

When I was a kid, I would drive my parent's big Buick Lesabre with a 400 4 barrel. First thing I would do after I got around the corner was flip the lid on the air filter. Still love that sound.

I respectfully suggest that if this setup works better than the stock airbox, then something is/was wrong with your airbox or carbs.

Syscrush wrote:I respectfully suggest that if this setup works better than the stock airbox, then something is/was wrong with your airbox or carbs.

Interesting assumption. My experience in working on, and hot-rodding motorcycles over the past 45+ years is there is often some performance gains that can be made over the factory systems. The factory systems, including air boxes, were designed to do more that maximize performance. They needed to reduce induction sound, keep the filter as clean and dry as possible and look good while being cost effective. The CBX air box was well engineered to do well at all of these things but I won't go as far to say that is was the best it could be at maximizing performance over the other attributes. Often the air boxes of yesteryears did not maximize performance and modifications could be made to improve performance. It's been dyno proven many times. Today's air boxes are designed much better through computer dynamic design programs to enhance performance while also serving all the other purposes. Today modifying or removing a factory air box will usually result in a loss of performance.

I don't have any back to back dyno comparison of the stock air box vs. my ultimate air intake. I did a lot of dyno tuning with the K&N pod filters and my rwhp was a tad over 98 hp with a pretty nice torque curve. I did not have any dyno numbers from the stock air box. My claim was the new intake system performs better than the K&N pods I removed and it should given the reasons I mentioned above. I don't have any facts or data on how it compares to the stock air box so I can't make any claims on that. BUT from my experience and engineering background I would feel pretty confident it would provide better hp and torque numbers on a dyno over the stock air box. But no, I am not going to go through the time, effort and cost to prove it. That's not really why I did the project to start with. I wanted to do something unique and looked cool and different.

It has been pointed out many times: The CV carbs on the CBX are sensitive to the intake configuration. Proper performance depends on intake vacuum. If it is being changed, a lot of details need to be worked out.
The specific problem with the CBX is, that due to the inward angle of the outside carbs - which the Honda engineers facilitated to provide knee clearance for the rider - the intake runners between carbs and intake valves are of unequal length for the six units. To compensate for this, Honda configured the intake runners in the air box. If you eliminate the air box, you eliminate the balance and you have to find a different way to address this. I am sure there are ways to enhance the performance by taking out the stock airbox, but it may involve more than what most of us are able to do in our garages.

EMS wrote:
"The specific problem with the CBX is, that due to the inward angle of the outside carbs - which the Honda engineers facilitated to provide knee clearance for the rider - the intake runners between carbs and intake valves are of unequal length for the six units."

One theory as to why the CBX sounds like an F1 race car is that the unequal lengths between the carbs and intake valves create 3 different tones emanating from the exhaust system (three-part harmony!) making the CBX sound like a V12.

- Scott