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Joined: Jan 07 2016
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Aloha,
I have been messing about with a couple of spare compressors this afternoon and have noticed something possibly amiss.....
The exhaust valve makes sense with it's O ring seal and stiff backing plate, but the way both of the units I took apart were assembled, the
intake reed valve has a short piece under it which effectively prevents it from sealing on the flat surface of the head. This allows a lot of air to return to the intake side
on every compression stroke!
Thinking this was wrong, and as I have 2 on the bench, I assembled one as it was (and like I saw in several you tube videos) with the short intake reed below
and one with it in the logical place, above.....
Upon bench testing, the second method provided about 2x the airflow. To eliminate the variables of piston and motor, I swapped the order of assembly on both and re tested.
Same result. Short reed above and long reed on the bottom works much better......
Next, I will check the 2 pumps on the Borrego and Bolt.....
Thoughts?
Theories?
Cheers!
Tom

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I always assumed that the short leaf was under the reed to raise it up and keep it parallel with the body as the O ring that it bears on stands slightly proud of the body. Without the leaf under it the reed will be held at an angle by the edge of the O ring closest to it.

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That is the exhaust side It has guides in the casting to keep it aligned, I am talking the intake reed.......No O ring. just flaps on the flat surface

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As I've got one in bits here at the moment, just had a look. You're absolutely right, why does it have the leaf under the reed?

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Pics?

I've always seen them and rebuilt them with the reed valve closest to the surface, and then the metal strip on top of them - I was under the impression they were there as a hard stop so the reed part didn't get bent too far and damaged over time...

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It would seem to me the short piece would be on top to act as an progressive "Leaf" spring assist for the actual working reed.
Assembling them with the short bit underneath could cause a lot of bending moment against the shorter one, possibly leading to
fatigue and cracking??
I can not understand just why the designer would handicap the capacity so badly?
Got caught up with boat projects so will check the other 2 pumps in the morning.
I may even purge the system and time how long to full with both assembly methods.......

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Even I have a spare pump ready to rebuild.... watching with interest!

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Add myself to the list :-)

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This one, the inlet reed

enter image description here

Where the shorter leaf is under the long one that does the business

enter image description here

The outlet reed has a thicker, angled backstop plate and bears against an O ring but, as Bolt says, why is there a spacer when the reed bears against a simple hole in the body?

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Hmmm, Now I can see pics it makes more sense - I forgot the inlet one had the short spacer thing and the long reed was on top - my memory was fuzzy and thought it had a longer bit on top aswell...

Weird alright. I'd be interested in the longevity of the reed if it was fitted the other way around... Not that I've had need to mess around with a P38 compressor for ages since fitting the Viair to mine!

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I will try the "air up from empty" test today with both orientations, and report back.....
Thanks for posting photos, Gilbert.
Anyone got an exploded view diagram of this beast?

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Joined: Sep 02 2016
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As you indicate yourself in #6 Bolt the short strip should be added to act as a 'damper' to restict the oscillations of the longer strip (?)

Putting the short piece on top may increase the airflow but also reduce the working life of the long strip (?)

In short placed on top it should reduce the chances of metal fatigue in the longer strip... so why isn't it indeed ?.......

EDIT: Did some research Google Reed Valve Stop Plate for details..... as that seems essentially the basic function of the shorter piece here,
or it could/should be (?)

Or perhaps the short piece being on top could cause the longer strip to fail sooner at their junction (?)....
(Although now I am wondering why it is there at all if it is/was just a spacer..!)

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A reed valve stop plate is what the outlet valve has, a thicker solid plate above the reed to stop it going too far, but this isn't the case with the inlet reed. For the valve to close off fully it needs the reed to be flat against the body where the hole is but with the smaller leaf under it, it never can close off fully. If the smaller leaf was on the top it would act as a stop plate although, as you say, it would put all the bending strain at the point where it meets the smaller leaf, just the same as you would expect it to crack around the screw if there was nothing there.

Buggered if I know why it is there and what it is supposed to be doing but at the end of the day, if the reed breaks it's easy enough to make a new one from an old feeler gauge.

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That's what I find a little strange Richard, if they bothered to put a stop plate on the outlet then why not the inlet too ? (if there is space enough)

Guessing we won't know until someone reverses these strips and runs it for a while either; With the short strip on top we might get twice the output but the compressor fails in half the time ?? (and/but having half a long strip drop off into the cylinder could also rapidly do some damage within too...)

I am intrigued Bolt..... do let us know !

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Well, the results are in, and they are very conclusive, if counter intuitive......
Rather than doing this on the vehicle, I set up a test rig in the shop.
Filling an 9.5l tank from empty to 125 Psi.
Vbatt was 12.1v for the duration of both tests.

With the reeds in the "Logical" configuration with the long reed beneath the short spacer :
50 Psi- 5 Min
75 Psi- 10 Min
100 Psi- 17 Min
125 Psi- 21 Min

Annnnnnnd......In the "Normal" configuration with the short spacer beneath the long reed:
50 Psi- 2 Min 10 Sec
75 Psi- 3 Min 45 Sec
100 Psi- 5 Min 45 Sec
125 Psi- 7 Min 45 Sec

So, surprising but conclusive proof that the designers got it right, The intake reed is indeed not supposed to directly cover the hole..Go figure?
This would be something to look for if the compressor has been rebuilt by PO and seems a bit slow to pump up.
This was done with a used pump of unknown age. (Tag missing) But probably 99-02 vintage
As the motor and bearings sound very good, I will now go and rebuild it with a new kit, and re test. Just to see if it makes any
difference (and to know I have a good spare.)

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As you say, conclusive but counter intuitive. I can't explain it at all but the test results don't lie, the designer did get it right.

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Rebuilt with new cylinder, piston seal, and O rings.....
Of course "It will ride in with wear sir" however, I probably should have left it alone......
Here are the new numbers:
50 Psi- 2:20
75 Psi- 4:10
100 Psi- 6:35
125 Psi- 10:00

Really had to struggle from 100-125. I did not use any sealant on the reed valve O ring..You hear both use it or do not....
New seal obviously will need time to set, so I will run it a few more times to quantify that time.
May put a dab of silicone on the O ring...Or not?
Fun way to waste a morning though!
OK, so NOW I need to pull the pumps out of both 02s and test them.
Maybe do an O ring job on one of the spare valve blocks?
With luck, I can stretch that out to the afternoon, and avoid any yard work.....Hehe..
Cheers,
Tom

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Joined: Sep 02 2016
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Interesting results Bolt !

Maybe with the short srtip on top this then actually interferes (ie. reduces) with the flow through the inlet hole instead (?)

Also I think the long strip is flexible enough to block the hole (?)

It is possible that the resonant frequency of the reed plays a part in all this too

Just goes to show that 'counter-intuative sometimes really works (better)' too !?

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HA!
Counter intuitive? P-38? Hmmmmmmmm
There is certainly something very interesting going on with the inlet reed. Perhaps some Physics type will crop up.
I like the theory of the resonant reed....If the long reed is actually bending itself around the edge to fully block the hole,
I would expect to see noticeable wear, or fatigue cracks at the fulcrum point?
Off to test the other 2 compressors.
I want to establish just how good a "Good" pump is.

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Unfortunately I am a Physics type Bolt (!), my 'resonant' comment being based on the fact the air 'flow' is actually a series of high-frequency pulses...
The 'spring steel' the reed is made of must be particularly flexible though ! ...

Yes, it is quite easy to fall into the trap that P38s are 'the result of logical thoughts'.... (albeit tinged only by all the random cr@p within !) ?