Author Topic: When your making calculations of efficiency what ranges do you expect  (Read 66 times)

Mrshosted

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I've often seen where you will set your efficiency at 1.00 FPE/CI in some of your calculations.  What would consider the ranges of poor, good and excellent.  I understand when reading around the forums there many claims of very high efficiency tunes on guns.  One that come to mind was a circumstance of a tune stating a efficiency of 1.49 fpe/ci.  I personally have never seen a gun that good in my experience of tunning or calculating for efficiency.  In your best judgement what is the really capable efficiency scale?


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rsterne

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I often use 1.00 FPE/CI when doing estimates or calculations, because that is a number I expect to achieve or beat when tuning a PCP.... A few years ago it was the norm, but these days we are paying a lot more attention to getting better efficiency, so 1.20 is more like the average today, with 1.40 being well above average.... I have broken 2.00 FPE/CI, but only on a low power application.... The more power you extract, the lower the efficiency will be....

The baseline efficiency depends on where the pellet is in the barrel when the valve closes.... The closer it is to the muzzle, the less the air released by the valve can add to the FPE by expanding after the valve closes.... That expansion increases the efficiency.... The shorter the valve dwell, the less distance the pellet has travelled down the barrel when it closes.... Therefore, efficiency and short dwell go hand in hand.... In addition, the more barrel length available for the air released to expand into, the higher the efficiency.... which means that longer barrels tend towards greater efficiency....

If the valve closes just as the pellet exits the muzzle, you will probably be seeing around 0.5-0.6 FPE/CI.... (Incidently, if the valve stays open after that, like in some Big Bores, you can see efficiency down to 0.3-0.4 FPE/CI)…. If the valve closes when the pellet is halfway to the muzzle, the velocity drops only a few fps, but you use half the air, so the efficiency is more in the 1.00 FPE/CI range (these numbers assuming a rifle, not a pistol barrel).... Close the valve when the pellet has gone only 25-35% of the barrel length, and you will be in the 1.20 FPE/CI range, but the power is dropping below maximum.... Get down into the 20% range, and efficiency will be up around 1.40 FPE/CI, but the FPE will be quite a bit below what the gun can achieve.... By the time you get down to about 10%, if all the stars align, you might be seeing even better efficiency, but at much lower FPE.... At some point there isn't enough air released from the valve to overcome the barrel friction through expansion, and the pellet starts to slow before it gets to the muzzle, and the efficiency tanks.... 

HTHs you understand what is going on....

Bob
« Last Edit: March 07, 2020, 09:15:21 AM by rsterne »
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Mrshosted

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Thank you for your explanation.  I was struggling with reading about people making efficiency claims, when my gut filling was saying there's no way.  Your explanation has debunked many claims for me.  Further more you've also helped me understand that high power gun will suffer with efficiency.  Thank you, Mont
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