Drop In Power Mod for 880, 901 etc...

[Ronno6]

I'm pretty sure that you will fin=d that the piston contacts the abutment just before the end
of the handle stroke.
That is the sole reason for the bumper.......it provides the give so that the handle can latch after contact is made.

[pblawler]

So I got the saw out and went to town LOL... I notched an 880 compression tube, grabbed an 880 valvebody assembly, an 880 piston assembly, a 1977 piston assembly, an 880 frame, and on old style plastic pump handle and started making a test bed.  I cut a slot in the compression tube so we can see the piston to abutment interface, I cut the handle off the pump arm also so we can see the interface.  I assembled the valvebody with sloted compression tube, pump handle, piston assembly etc..... using the 880 piston assembly.







The pic below is of the 880 piston assembly with the pump arm fully closed.  It contacts the abutment and pushes it down as best I can measure 0.038" which is the amount of movement in the abutment to valvebody. 



Next I removed the 880 piston from the 880 piston assembly and installed a 1977 piston on the 880 piston rod.



In the next pic you can see the 1977 piston reach the valvebody just before the pump arm is fully closed.  You can see two spring coils and the bottom of a third coil.



The next pic is with the 1977 piston and the pump arm fully closed.  Now we can clearly see the third coil.  So the 1977 piston is fully engaged and loading the spring also but the spring is not fully compressed. 



Sooo the 1977 on the factory 880 rod is compressing the spring 0.050" more than the elastomer and factory piston.  The mod makes it's best power at higher pump counts (i.e. 15 to 20).  It must be that the spring is providing more force against the abutment such that the pressure in the compression chamber is not able to push the abutment the 0.038" out, where with the elastomer or less preload on the 1977 spring the compression chamber pressure pushes the piston head back more.  The reason for it building better pressure at higher pump counts is due to the decrease in swept volume from extending the air intake slot.  It takes more pumping to compensate.  Also the dual o-ring 1977 piston likely seals better.  Moving the pivot back could be reducing preload and allowing the abutment to push the piston spring in more decreasing the pressure to some extent.  I have not tested it yet but I'm betting the 1977 spring also provides more force against the abutment than the elastomer.

I have been wanting to see the piston to abutment interaction, now we know......

[Ronno6]

Great experiment!
I am quite surprised that there is that much travel available in the abutment.
It can only be a result of slop in the retaining pin to valve body and pin to abutment.
That is quite a buildup of tolerances......

If .038" of abutment movement makes a difference, maybe a slightly shorter abutment would be a benefit.
With .050" of spring travel being available, moving the retaining pin hole that distance closer to the head might help.
Of course that would require a new abutment.....drilling another cross pin hole would significantly weaken that part...
However, that would still require a lengthening of the intake slot.
My goal is to make this a simple part swap mod.

Not having empirical data on the spring and the elastomer makes calculations impossible...
It would be really interesting to know what chamber pressures are attained with this mechanism..

Be all this the case, maybe RobM's original solid spacer replacement for the spring or elastomer has merit.
With .038" of abutment travel available after contact, that may be doable.

As for the assertion that a reduction of swept volume results in higher pressure, I believe that Mr. Spock may have a comment on that.....
 

[pblawler]

Not the reduction causing higher pressure, but the reason the mod doesn't make as big of a difference at lower pump counts, i.e. it takes more pumps to get to the higher pressure.  With an upgraded abutment pin (i.e. 1/8" dowel pin) the movement on the 880 is removed.  The 1977 abutment is slotted where the pin goes through like it is designed to move in the tube.

[Ronno6]

Not the reduction causing higher pressure, but the reason the mod doesn't make as big of a difference at lower pump counts, i.e. it takes more pumps to get to the higher pressure.  With an upgraded abutment pin (i.e. 1/8" dowel pin) the movement on the 880 is removed.  The 1977 abutment is slotted where the pin goes through like it is designed to move in the tube.

Which abutment that allowed .038" did you use in your test?

[pblawler]

Not the reduction causing higher pressure, but the reason the mod doesn't make as big of a difference at lower pump counts, i.e. it takes more pumps to get to the higher pressure.  With an upgraded abutment pin (i.e. 1/8" dowel pin) the movement on the 880 is removed.  The 1977 abutment is slotted where the pin goes through like it is designed to move in the tube.

Which abutment that allowed .038" did you use in your test?

I used a new 880 valve body with all original components.  The piston assemblies were straight out the bag new also. The pump handle and frame were the only used parts.