Simple PCP Performance Calculator

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ackuric

ackuric

Member
Mar 10, 2017
1,232
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Colorado, United States
Some may be aware of my formulations in regards to pcp's...I have tons, many of which I keep to myself, because I worked hard on them. This is one of my pride and joys, I feel it's time to share, for anyone interested, for anyone who builds pcp's, for anyone who has specific goals and wants them met...here ya go! The FPE/FPS is approximated, as there are MANY variables at play that will alter results, its a good base line to go by and has never steered me wrong, from .177 caliber making 20 fpe to .50 caliber making 600+ FPE.




download.png
View attachment 1550645995_20201074555c6cfaeb27e5e4.70344290_PCP_Performance.ods





External link: http://s000.tinyupload.com/?file_id=60686858958191045898


*edited new version includes units for each field to minimize confusion.*

*added graphical data

*added volumetric efficiency calculator (fpe/ci) for regulated and unregulated rifles.

*added approximate valve closure distance / percentage. This distance is the pellets distance in the barrel upon valve closure.

*added fps to graphical data / added check box for FPS - cleaned up chart

*added more approximate calculation for TP waste, opposed to using 1 cc as standard.

*advanced statistics added

*modified formula for smaller plenums

*added pressure gradient graphs and caliber change

*added projectile friction modifier 

*added metric conversions and output option

*added option to allow regulator that feeds plenum during shot cycle, and one that does not (small intake vs large)

*added ***approximate*** velocity curve to pressure gradient







-Matt
 
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Hi Matt,

The values are all pretty clear except the plenum and the pressure. How do you calculate or estimated the plenum? What unit does that field expect (CCs)? Can I assume that you're entering your max (starting) fill pressure? Actually just this morning I opened up the transfer ports a bit (to 0.164 in) on my Kral Bigmax. 

Here are the values I used:

barrel = 22 inches
caliber = .22
pressure = 2900
plenum = 8
port size = 0.164
projectile weight = 18.3

954 fps
38 fpe

Now I "worked" the sheet by changing the plenum value to get the actual FPS I saw in testing. I also need to measure my barrel length because I see discrepancies in the specifications I see online for my rifle. It's either 22 inches or 19 inches. I will verify this tonight :)
 
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Pressure is whatever pressure the shot is taken at. The calculation is for peak/plateau performance. Works for unregulated and regulated. Ideally for unregulated you would pick the pressure right in the middle of your shot string. 

Plenum volume is in CC's. Its the volume of unrestricted (or restricted equally to forward porting) air capacity behind the valve seat to aid in keeping pressure up during the shot cycle.
 
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OK so when you refer to "air capacity behind the valve seat" that would be the volume inside my valve body in the photo below, correct? (Please ignore the caliper, I took this photo for another purpose.) You can see the very tip of the valve stem on the left and the threads where it screws into the bottle block on the right.

1548292127_9283563215c49101fcc9941.05557099_valve.jpg


And for some context, here is the same valve body screwed into the bottle block:

1548292417_20639756655c491141e05b07.96570771_valve2.jpg


And thank you for sharing this tool with us :) 
 
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I can't say for certain by just the pictures alone. But the volume from valve seat to any restriction well below the diameter of the forward porting would be considered 'shot plenum' volume. I would assume its any volume inside that body plus what it screws into until an orifice is restricted well below the diameter of your forward porting (valve exit/barrel port)

You have the right idea though...and if what is pictured IS your plenum volume, than 8 cc's seems fairly close. Just a hunch!



**Updated to include a graph that shows minor changes based on a selected parameter from the provided drop down menu. 
 
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Glad to help! I can add more data/features but I want to keep this version small compared to my other spreadsheet, so I likely won't go crazy. 

The current valve closure estimation does use the following data that is not visible, that I can add but likely won't because it requires temperature / altitude and then the sheet just grows and grows like my personal one, so it'll likely remain hidden! I figured the valve close distance was the most unique calculation out of all the below



CC ejected / MG ejected / Flow rate (mg/ms) / Air density (mg/cc) / Avg Throat Pressure (psi) / Pressure Drop (psi) / Valve dwell time (ms)



-Matt
 
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Added data on max power / plateau for comparison next to a known tune you may be running...also added a max energy decline graph so one can decide an optimal power level based on the energy drop, which is generally 3-5% reduced from Shot 1's plateau. As you see I am tuned for 56~ FPE, so once I drop off reg, I could easily obtain 6+ more shots, which is represented on the energy decline graph, and very much the case with real world testing, hence why I am tuned as such.



You can notice the HUGE difference in air usage between a pcp tuned near the plateau, and one tuned 3-5% below it. If this data doesn't express and explain that well enough, I don't know what will!







***do note that pcp's setup poorly with hammer bounce / multiple valve openings or even too much over dwell @ plateau when comparing real world to this will see a large variance in air usage ***




 
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Added *very* approximated dwell times and cleaned up a few things. Had a fluke in the pressure graph where I was using full bore porting so it gave much higher than expected readings at higher pressures. Valve dwell time is entirely based on assuming a 1 ms open/close time (poppet travel) at half the ports flow rate, and the remaining mass of air over the ports max flow rate...its very crude but good enough for an approximation, certainly the most crude of any on this sheet. Approximating dwell with as minimal data on a particular configuration more accurately than the above method would be trying. I think for all intended purposes the current value is close enough.




 
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*Small change made, using different formula to figure barrel volume now opposed to a static 45% to determine pressure drop over the approximate peak energy shot. This slightly increased the approximated energy output.

Its a negligible difference of less than 1% in FPS but I am fairly nit picky when it comes to having something done 'right', at least to my potential.



-Matt
 
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