I know what the hp stands for. What about the x4? And what does the 'x' position refer to? (A detent at the handle end of the tube.)

I've got an old Zefal fpx4. The 'x' is the designation for a pump where you can lockout the spring. On mine, when the end of the handle is turned to 'fp' position, the spring is locked out and you don't have to compress the spring to put air in the tube. When it is turned to 'x', the spring is active and can be compressed to hold the pump in the frame. I think there are four sizes with '4' being the largest.

I also have one of these (an hpx3 and the lockout position is labeled "hp") and never saw the benefit of the "hp" position. It's not like the stroke is any longer when the spring is locked out, you just hit a solid stop instead if bouncing off the spring but at the same point in the stroke. What good does that do?

The "lock out' position makes the pump's stroke more straight forward without a secondary element right at the air compression point. And it does lessen the stroke length by the handle's frame fit spring loaded movement. Andy.

When you hit that solid stop (with the pump in the 'hp' position) you've already pushed the piston all the way to the end of the pump so no additional air can be pushed into the tire. If the pump is in the 'x' position you would be able to push the handle a little farther (and use up some of your arm muscle power to do it), but all that you'd be doing at that point is compressing the spring inside the handle without actually moving the pump piston. So the 'hp' position keeps you from wasting your energy repeatedly compressing the spring. It does make it considerably easier to pump up a tire to high pressures compared to the earlier Zefal HP pump (which I still have as well) which lacked this lockout mechanism.

I have one of the hp pumps too, and I find it to be practicfally useless.

It's a 2 stage pump.........low and high pressure......low pressure high volume and high pressure low volume.....1-2-3-4 refer to lenght for your frame size.

No, The HPX is a single stage pump. One cylinder, one piston. The handle is spring loaded only to allow compression for fitting to the frame, this sprung handle can be locked out without effecting the pump stroke. (And this is what I meant in my earlier post. The handle's spring/compression would be extra stroke length for the hands and add no additional piston travel.)

As mentioned the numbers are the size reference. HP is for high pressure. X is for the frame fit compressing handle feature.

The HPX was the second generation of the classic "Competition" pump from the 1970s, red/silver/blue with a head connect that didn't have any lever to compress the seal. Next came the same barrel and handle but with a lever/cam seal compression feature (the HP part). Then add the frame fit handle compression that could be locked out and you have the current HPX.

When i hear talk of "The HPX is useless" i think of all those riders who can't shift their bike well and then ride in one gear. Like so many things the use of a frame pume is a skill. How to hold the tire and pump so that no bending force is action on the valve and there's no air loss. How to stroke the pump to get full volume and compression. How to care for the pump so that when you really need it you can get full pressure levels. Andy.