Hi guys!
I just completed and tested my latest mod, which is an inlet tract extension on my carburetor.
It essentially performs a similar function as a velocity stack, but perhaps a little less perfectly. However, it allows me to use my existing K&N air filter on the end, because it does not have a flared bellmouth on it.
So, I get the inlet tract length increase that I wanted, and still get to use my air filter.
I felt that I'd get enough benefit from the constant-diameter tube to suit my needs, and wanted to keep the filter on the end, so that was the basis for the decision making process.
There are a couple main targets for the inlet tract increase.
One is very simply to permit a larger amount of air volume on the "engine side" of the filter barrier. The stock position of the K&N filter is right on the carb mouth, and the only volume inside the filter barrier is basically the volume of the carb throat and the inlet port. This does not amount to even 500cc, which would be the minimum volume needed for "one big gulp" of air when opening the throttle quickly. Having more air volume inside the filter barrier can provide better throttle-response because it doesn't have to draw air thru the filter for the first "gulp". It's common practice to provide greater air volume than the engine displacement inside the filter barrier for this purpose.
So, it accomplshed that goal at the very least. The "pod type" filters such as the K&N are less than ideal in their normal mounting position on the carb, because of this factor.
Another target is to achieve a "ramcharging" effect, by the inertia of this moving air inside the inlet tract. Once this "cylinder of air" in the inlet tract gets moving, it continues to move from inertia, even when the piston stops drawing air in. This assists the filling of the engine cylinder by using all the available time that the inlet valve is open, to get the most air into the engine.
And lastly, it is for "resonance wave tuning", which uses the reflection of sound waves that reflect up and down the intake tract during running. By tuning the wave behavior in the inlet tract by using certain lengths, the waves can be used to assist the flow of the air into the cylinder, in a very simlar way that exhaust length tuning helps to extract the exhaust out of the engine. Just the other side of the same coin, so to speak. The longer tract can bring the reflection frequency down lower in the rpm band, where it can be better utilized by the Bullet's slow-revving engine design. The short inlet tract in the stock engine is not optimized in any way but for fitting on the motorcycle. It's too short to provide any real wave-tuning benefits in the low rpms that we can access. Short inlet tracts serve higher rpms better, and we can't even run our engines at rpms that would be good for an inlet tract length that is as short as the stock Bullet has.
Lengthening this inlet tract can work out as beneficial for us in the low rpms that our engine can use.
So, for a very cheap cost, some benefits can be achieved for the actual running improvements of the bike. It increases torque and hp at certain rpm points, and we strive to time this activity to coincide with the rpm of our engine's torque peak.
Actually, the length that I am using is still a little shorter than optimal, because I ran out of room to put the extension, without it hanging out too far off the side of the bike. So, I selected a 6" length, which would be good for rpms slightly higher than the torque peak, but not too much higher, and still useful, while fitting into the available space and visual appearance parameters.
For those interested, I used the stock rubber bellows that usually connects the carb throat to the stock air filter housing, to connect the stack to the carb. I used a rubber o-ring as the "clamp" to hold it on the carb.
The tube is nothing more than a 1.75" I.D. tube, cut to 6" length.
The K&N just fits on the end, because the tube is the same diameter as the carb throat body, and it just clamps right on.
I used a cable tie to secure the "flying end" to the frame, so that it didn't sag and fall off, or over-stress the intake rubber manifold part from the added weight and distance. Gives the whole thing more support and stops it from falling off due to vibrations. You could use a mounting clamp or bracket for this purpose too. I just used a cable tie, and it seems to work ok.
Now, I realize that this may not be a pretty as Chumma7's stack, nor probably quite as ideal in function. Chumma's stack is really nice. But I wanted to have the K&N on the end, so I adjusted the concept to fit my need.
Here's a photo of it on my bike.