How it works Fuel injection
Here is the standard fuel injection throttle body. The throttle plate, which some people refer to as a butterfly valve, is the largest restriction in airflow and openings less than wide open. That should be obvious, because when it is closed, no air goes by and the motor doesn't run. The movement of the throttle plate, at the lower openings, which is where you almost always are driving at, is where the largest problems occur. This is because the plate pivots in the middle. As you can see, the top half of the plate pivots or moves in the same direction that the air is flowing, so this is not a problem. It is the bottom half, that is pivoting into the air stream that causes the problem. This is because it's going against the flow. This creates a turbulent ball in front of the plate and keeps air from getting by as quicly. If you slow down the air moving around the plate, you slow down the amount of air that gets past the intake valve and into the cylinder. This, of course, means you not building power as quickly as possible when you move the throttle.
Now you can see how the power now helps. By dividing the throat into an upper and lower section, you keep the lower half of the throttle plate from interfering with the good air stream. This increases the throttle response, the torque every time you twist the throttle, and fuel mileage.

Fits

PN-HCV - This fits the Harley 40mm CV Kehien Carb
found on '98-2006 88"Evo engines, '88-'06 Sportsters, '90-'05 big twins, '98-'04 buell
 
PN-HFI - This fits the Harley fuel injected 88" engines using the
Delphi injection system (OEM on Harleys).

PN-HRS-42 - This is for the Mikuni HRS 42 carb.
 
PN-HRS45 - for the Mikuni HRS45

PN-HRS48 - for the Mikuni HRS48

Why it Works!

 CV carb

The PowerNow™, when installed in a CV type of carburetor, seems to work better than any other application. The reason why, is because of the way a CV type of carb works in the first place.

A constant velocity (CV) carb really works on air pressure, or the difference in air pressure, which is normally just called vacuum. When you open the throttle, you are opening a throttle plate, or butterfly valve. The plate however, is located between the engine and carb slide, so the slide still needs to open to let the air from the atmosphere past the throttle plate and into the engine.

The difference in air pressure between the atmosphere and the lack of air pressure in the engine, cause the slide to open then, and really feed the air and fuel to the engine. This is why the PowerNow™ is so effective on these carbs. It works for the same reason that it works on any slide carb, as seen in the “how it works” description for the slide carbs, but also because it needs less of a vacuum to open the slide.

The difference in air pressure is greater with the PowerNow™ installed, which means there is more of a vacuum and the throttle doesn’t need to open as far to get th slide to respond. And the slide movement responds much quicker to changes in throttle position.

This isn’t just theory. It is evidenced and proven with the Superflow 110 flowbench. With the stock Harley CV carb set open at ½ throttle and the incoming air pressure set at 7” of water, the flow rate measures at 50%. Leave the flowbench running and just place the PowerNow into the carb and the air pressure automatically drops to 6.5”. This means it takes this much less pressure to flow though the carb with the PowerNow™ in place. Turn the pressure back up to 7”, and the flow increases to 54%.

There is a 9.5% increase in flow with the PowerNow™. More importantly, since the vacuum required is less with the PowerNow™, the engine makes more power at a lower throttle opening and responds much quicker to changes in throttle position.