Precihole VX100: An Alternative State of Tune!
Posted: Mon Jun 18, 2018 5:33 pm
Precihole’s air rifles, as of some other local manufacturers’, have evoked great response and appreciation among enthusiasts; their springers range being no different! And yet, having owned and used one for quite some time now, my efforts to verify some technical aspects and attributes on various forums over these past months hardly yielded anything concrete, apart from some purely subjective feedback. Either most of us are unsure, or lack the needed info, or are simply reluctant to share. None, however, bodes good for either the sport or our fraternity! I, for one, have grown up with the belief that airgunning (as a sport and hobby) needs free sharing as much as plentiful practice and, of course, lots of tinkering!
[Pls check the add-on post made on 26 June 2018 for some additional info and clarification...]
Shoots great out-of-box...but with perceptible surge!!
[PS: For more on why I call it "Surge" (and not "recoil"), please check the post dated 05 July 2018]
So why didn’t I open up my VX100 all this while, like I do with all my other ARs in my continuous effort at altering their states of tune and checking behaviour and performance, you would ask. And the answer to this is that the VX behaves and shoots so mannerly out-of-box that one feels like waiting for it to perhaps go awry at some point, so you might justify the decision to open it up! To most people, it would seldom oblige though, and which really is a vital attribute of a good AR indeed ….Even in my case, but for two things, namely the low ME and the irritating surge on each shot, I may have postponed this tune up! (None of my ARs, including those shooting in the 10 to 12+flb range, have this kind of secondary recoil that is so predictable of the VX. All, by the way, are of similar weights. I must also admit that I was looking definitely also to explore enhancing its ME output.
Having said that, forums have been abuzz about the actual ME of PH guns being way too lower than what is claimed. Just to confirm, my VX, in its factory tune, and post over 2000 shot cycles, was performing between 6.96 to 8.57flb with various weight and fit pellets…all with comparable accuracy at short ranges. The highest figures surprisingly come not from one of Precihole’s own pellets but a US one (The Super Pointed which weighs the same as the PH Sting). This yet again confirmed what I have been saying about the low start pressure attribute of the new lot US pellets, by the way. [Please refer to some photographs attached to this post]
Sub-optimal Energy Efficiency
Low ME output is just one aspect though. Knowing PH springs as the back of my hand (I use and regularly swap several of them), and the bore and stroke of my VX, it was easy to calculate - much disturbingly though- that its energy efficiency in the stock set up was below 30% (29.96 to be precise). With just five-sixth of the spring energy, and with pretty similar PL Force to Force at Release ratios, my tuned SDB27 in .177 dishes out almost the same ME (8.52flb) with a decent 35% efficiency….and an almost recoilless cycle - evident in the better groupings it gives than the VX.
What did the re-tune achieve?
The above note tells you the mind set with which I approached this retune on the VX. The results first: I managed ME increase between 123 and 131% across three different pellets of varying profile and weights, with the highest ME at 10.52flb once again recorded by the US Super Pointed 9.6grn. The recoil behaiour was much evened out, albeit a wee bit snappier, than before. Accuracy as you can see in the photos attached, is at par with that in the previous state of tune.
However, that is just one part of it. The more pertinent aspect is that the efficiency trudged only just over two percentage points to 32.27%. Even for a .177, I would strive for more….and hence the quest will continue with other states of tune....wherein I will play around with other aspects! In any case, I stick to the basic tuning mantra the best I can: If you alter more than one aspect of a set up at a time, you will never be able to know which contributed to the resultant behaviour of your gun. So, do your pre calculations prior to tuning, and just alter one at a time.
A note for the more serious tuners: The stock spring at 267mm FL offered 58mm preload in the stock set up. I replaced this 29.84lb/in with a 278mm FL aftermarket one of comparable SR 30.24lb/in but with coil OD and no. of active coils at 19.3mm and 35 as against the 19mm and 38 active coils of the stock spring. As a result, even though with matching stiffness, the 70mm preload offered by the latter increased the spring energy from 35.08 to 42.16lbf, and preload energy from 6.48 to 9.56lbf. The resultant piston energy shot by 4lbf to 32.60. Overall, the PL Force as a percentage of the Force@Release went way up to 47.62%. Too bad for my taste! I like to keep it in the 40% bracket to achieve optimum peak compression ratio and pressure (And bravo! that's how it was,too, in the factory tune!!).
But WHAT WHEN YOU HAVE THIS MONSTER PISTON to tame!! To ensure a decent pellet driving pressure pulse before piston bounce, the force over the piston must be enough to keep in near or in the vicinity of the TP as long as possible. And you sure will need a lot of preload to keep this heavy piston there!
Not everyone would be interested in these finer aspects of tuning, so lets get back to issues of common interest….and look at some key aspects that stand out on this gun (most likely as well for the SX series, too).
1. Stroke: AT 77mm, it’s just shy of a typical short stroke AR. Why so short, you may ask! My reading is that PH has an evident leaning towards competition target shooting sport (as against promoting solely sporting air rifles).. where accuracy and low range performance are key. ME gets ruled out rather squarely so long as you are in the 6.5-8flb range, which the PH guns dish out any day. But for accuracy, short strokes are very forgiving. In competitions, they wouldn't allow a high ME gun anyway. So, a short stroke seems to work out good.
2. Bore:Stroke ratio: At below 1:3, I believe it’s sub optimal, (but I believe this would be more out of commercial considerations than anything…..so expect PH to come out with a longer stroke gun …in the same power plant!! No alterations other than a different piston as far as I can see. So, you have a winner commercial viability! That's also how it is on most popular Air Arms or Weihrauch models. For the PH guns, it is very much DOABLE from what I see in the comp tube and the action overall!!)
3. A typical energy lean set up: The soft and long spring (with SR in the range of 29.84lb/in with a full set FL of 260mm), the seal fit attribute and the final preload (with full set) working out to two inches….you can’t ask for anything more spot-on. Kudos to the designers at PH for that!
4. Heavy piston: At 284gms (piston, tophat and seal), I find the assembly supremely heavy and out-of-this-world! I am not aware of any pistons in popular adult air rifles in the world being anywhere near this range (not claiming though that I know many of them). Most have been found to work best in a much lower range....the SDB air rifles, to just cite a local example, have got that pretty right there!
[Pls refer to the post dated 6 Jun 2018 for further elaboration on this point...]
5. Limited scope piston: With its ID in the range of 19.4mm, it seriously limits your spring options. Very few aftermarket springs offer an OD to fit into this piston. Please note that it is the final OD at coil bound stage that you should consider here (and not just the ID at free length). Also, the rather longish (15mm) bottom bearing area not only adds to the weight but more importantly, negates scope for adding preload washers over guide.
6. Spring Too soft and LONG: I believe the PH spring is one of the most consistent. Not one of the several that I have played around with have budged beyond 260mm as their full set FL; none weighed anything over or under 115gms; and all except one of these 3mm wire springs had 19mm OD and 38 active coils. Now, if you compare this with a similar spec spring, say a G Smith 27, for example, you will notice that the stiffer latter spring (SR close to 33lb/in) with just 34 active coils perhaps has a higher modulus of rigidity. The final set FL of this 34 coil spring is 15mm more than the 38 coils PH spring. That’s what shorter length wires (read greater pitch) can do to your energy calculations. So, please do insist on getting these right before you attempt any tune up!
7. Limited spring room and preload room: DO not ever think of adding any more than 3mm of slip washers over the guide lest your piston bottom out even before it engages the sear. That is a serious limitation. The alternative lies in utilizing some 10mm of room available inside the piston…but that will further add to your piston weight, so it is a questionable option. Also, it will likely push the stock spring to coil bound state, as well as squeeze the rear guide too close to the tophat on cocking (both undesirable!!).
8. A soft set up: A great attribute on its own. No surprise, the gun feels smooth in its handling and cocking. Here’s a testimonial to this: Coming out of the action the stock spring was 267mm FL, and that’s after 2000+ shots. I had a bet with a friend that it would take a final set at 260, and that he wouldn’t be able to budge it beyond that ever! He just wouldn’t believe….and was game for the bet. After all, 2000+ shots are no joke …They can tame the stiffest springs, he believed! Just over 24hrs of scragging, and the spring was at its sweet 260. Now, I know that if it’s a quality spring, it will not budge beyond say 2mm (yes, two mm) even when kept coil bound for 1000 hours (you read it right, One Thousand hours!). An archaic, but a sureshot, way of verifying spring quality.
9. And finally, the SCR: To be honest, I have a serious hunch that this gun originally was designed to be a longer stroke one, and that the current 77mm models are only an interim offering….say, to get users into the groove first so they can be ready for more later! Aka, a Di engine as the first offering and a common rail direct injection follows some years later! Why I say this is that the TP of the VX is rather classical at 3X11mm. Now, vis-à-vis the current swept vol of 47.42cc, simple calculations will give you a whopping SCR of 609; and you will damn the designers for having committed a blunder. But that is where most of get it wrong. Just add the various grooves, nooks and corners atop the piston head and the cylinder wall, and it drops down to a respectable 450 (Notice those depressions in the cylinder end wall in the attached photo...all of that adds to the lost vol.).
With this TP dimension, I believe 500 should be the ballpark figure. Now, just increase the stroke length to say 85mm to get a swept vol of 52.34cc, and lo and behold, you are right there at the magic SCR of 523!! More reason, why the gun was originally designed for a longer stroke!!
So what holds the gun from delivering outstanding efficiency??
Well, if you notice closely, we have just stated the reasons above! And so, without making it long, I will just state my opinion in points:
1. An exceptionally heavy piston. [While it is able to use less spring energy to create a desired momentum and generate piston energy, the same soft spring even with a respectable preload is unable to tame the bounce securely….and hence you get the heavy surge in every shot]
2. The longish spring. At 120mm coil bound length, I wonder if it bodes well for a powerplant with just 131mm of spring room at lock up.
3. Though pretty much an industry standard TP, its dimensions seems much LESS suited to the current stroke.
4. And finally, the stroke. Take this in at least the 1:3 league, and you could put your money on the gun any day.
What options to try?
If you are into back garden plinking or target shooting, my advice would be to leave it alone. If you can manage good groups with this high surge gun, you can shoot any in the world. So, just master your hold technique!
But if you are into other forms of the sport, then, here goes…
1. The easiest option is what I did: install a spring that offers greater natural preload, and yet has smaller coil bound length. If its WD and CD are comparable to the PH one, it’s bound to be stiffer.
2. For the more seasoned DIY buffs, attempt two things: remove coils (say three, to begin with) from you stock spring to take its stiffness to some 5.6N/mm. But here you will need some 20mm of extra preload to increase the force at release. Manage this by removing the steel tophat, installing a light weight polymer tophat and adding slip washers (preferably derlin/polymer ones, to minimize weight). This will additionally work by lessening the sectional density of the piston…and thereby enhancing piston velocity during the compression stroke, and a lighter piston would be easier to be tamed by the stiffer spring during bounce. You may just have to watch out for the second landing velocity though (It can be a bit harsh in this case).
3. Reduce piston weight. Looks intimidating…but can be pretty easy going if you know how. Will do a separate post on this later.
4. Increase the stroke. Ditto as point #3….but very much doable. Do you have access to a mini lathe?? Great stuff for my next project. 85mm looks achievable….and should be a good case study for all of us! Let me see how soon I get to go at it!
Hope this note helps us make inputs, contest, qualify, refine or detail out any of the points above, and eventually enrich our understanding of this popular Indian air rifle.
Cheers!
Prashant
[Pls check the add-on post made on 26 June 2018 for some additional info and clarification...]
Shoots great out-of-box...but with perceptible surge!!
[PS: For more on why I call it "Surge" (and not "recoil"), please check the post dated 05 July 2018]
So why didn’t I open up my VX100 all this while, like I do with all my other ARs in my continuous effort at altering their states of tune and checking behaviour and performance, you would ask. And the answer to this is that the VX behaves and shoots so mannerly out-of-box that one feels like waiting for it to perhaps go awry at some point, so you might justify the decision to open it up! To most people, it would seldom oblige though, and which really is a vital attribute of a good AR indeed ….Even in my case, but for two things, namely the low ME and the irritating surge on each shot, I may have postponed this tune up! (None of my ARs, including those shooting in the 10 to 12+flb range, have this kind of secondary recoil that is so predictable of the VX. All, by the way, are of similar weights. I must also admit that I was looking definitely also to explore enhancing its ME output.
Having said that, forums have been abuzz about the actual ME of PH guns being way too lower than what is claimed. Just to confirm, my VX, in its factory tune, and post over 2000 shot cycles, was performing between 6.96 to 8.57flb with various weight and fit pellets…all with comparable accuracy at short ranges. The highest figures surprisingly come not from one of Precihole’s own pellets but a US one (The Super Pointed which weighs the same as the PH Sting). This yet again confirmed what I have been saying about the low start pressure attribute of the new lot US pellets, by the way. [Please refer to some photographs attached to this post]
Sub-optimal Energy Efficiency
Low ME output is just one aspect though. Knowing PH springs as the back of my hand (I use and regularly swap several of them), and the bore and stroke of my VX, it was easy to calculate - much disturbingly though- that its energy efficiency in the stock set up was below 30% (29.96 to be precise). With just five-sixth of the spring energy, and with pretty similar PL Force to Force at Release ratios, my tuned SDB27 in .177 dishes out almost the same ME (8.52flb) with a decent 35% efficiency….and an almost recoilless cycle - evident in the better groupings it gives than the VX.
What did the re-tune achieve?
The above note tells you the mind set with which I approached this retune on the VX. The results first: I managed ME increase between 123 and 131% across three different pellets of varying profile and weights, with the highest ME at 10.52flb once again recorded by the US Super Pointed 9.6grn. The recoil behaiour was much evened out, albeit a wee bit snappier, than before. Accuracy as you can see in the photos attached, is at par with that in the previous state of tune.
However, that is just one part of it. The more pertinent aspect is that the efficiency trudged only just over two percentage points to 32.27%. Even for a .177, I would strive for more….and hence the quest will continue with other states of tune....wherein I will play around with other aspects! In any case, I stick to the basic tuning mantra the best I can: If you alter more than one aspect of a set up at a time, you will never be able to know which contributed to the resultant behaviour of your gun. So, do your pre calculations prior to tuning, and just alter one at a time.
A note for the more serious tuners: The stock spring at 267mm FL offered 58mm preload in the stock set up. I replaced this 29.84lb/in with a 278mm FL aftermarket one of comparable SR 30.24lb/in but with coil OD and no. of active coils at 19.3mm and 35 as against the 19mm and 38 active coils of the stock spring. As a result, even though with matching stiffness, the 70mm preload offered by the latter increased the spring energy from 35.08 to 42.16lbf, and preload energy from 6.48 to 9.56lbf. The resultant piston energy shot by 4lbf to 32.60. Overall, the PL Force as a percentage of the Force@Release went way up to 47.62%. Too bad for my taste! I like to keep it in the 40% bracket to achieve optimum peak compression ratio and pressure (And bravo! that's how it was,too, in the factory tune!!).
But WHAT WHEN YOU HAVE THIS MONSTER PISTON to tame!! To ensure a decent pellet driving pressure pulse before piston bounce, the force over the piston must be enough to keep in near or in the vicinity of the TP as long as possible. And you sure will need a lot of preload to keep this heavy piston there!
Not everyone would be interested in these finer aspects of tuning, so lets get back to issues of common interest….and look at some key aspects that stand out on this gun (most likely as well for the SX series, too).
1. Stroke: AT 77mm, it’s just shy of a typical short stroke AR. Why so short, you may ask! My reading is that PH has an evident leaning towards competition target shooting sport (as against promoting solely sporting air rifles).. where accuracy and low range performance are key. ME gets ruled out rather squarely so long as you are in the 6.5-8flb range, which the PH guns dish out any day. But for accuracy, short strokes are very forgiving. In competitions, they wouldn't allow a high ME gun anyway. So, a short stroke seems to work out good.
2. Bore:Stroke ratio: At below 1:3, I believe it’s sub optimal, (but I believe this would be more out of commercial considerations than anything…..so expect PH to come out with a longer stroke gun …in the same power plant!! No alterations other than a different piston as far as I can see. So, you have a winner commercial viability! That's also how it is on most popular Air Arms or Weihrauch models. For the PH guns, it is very much DOABLE from what I see in the comp tube and the action overall!!)
3. A typical energy lean set up: The soft and long spring (with SR in the range of 29.84lb/in with a full set FL of 260mm), the seal fit attribute and the final preload (with full set) working out to two inches….you can’t ask for anything more spot-on. Kudos to the designers at PH for that!
4. Heavy piston: At 284gms (piston, tophat and seal), I find the assembly supremely heavy and out-of-this-world! I am not aware of any pistons in popular adult air rifles in the world being anywhere near this range (not claiming though that I know many of them). Most have been found to work best in a much lower range....the SDB air rifles, to just cite a local example, have got that pretty right there!
[Pls refer to the post dated 6 Jun 2018 for further elaboration on this point...]
5. Limited scope piston: With its ID in the range of 19.4mm, it seriously limits your spring options. Very few aftermarket springs offer an OD to fit into this piston. Please note that it is the final OD at coil bound stage that you should consider here (and not just the ID at free length). Also, the rather longish (15mm) bottom bearing area not only adds to the weight but more importantly, negates scope for adding preload washers over guide.
6. Spring Too soft and LONG: I believe the PH spring is one of the most consistent. Not one of the several that I have played around with have budged beyond 260mm as their full set FL; none weighed anything over or under 115gms; and all except one of these 3mm wire springs had 19mm OD and 38 active coils. Now, if you compare this with a similar spec spring, say a G Smith 27, for example, you will notice that the stiffer latter spring (SR close to 33lb/in) with just 34 active coils perhaps has a higher modulus of rigidity. The final set FL of this 34 coil spring is 15mm more than the 38 coils PH spring. That’s what shorter length wires (read greater pitch) can do to your energy calculations. So, please do insist on getting these right before you attempt any tune up!
7. Limited spring room and preload room: DO not ever think of adding any more than 3mm of slip washers over the guide lest your piston bottom out even before it engages the sear. That is a serious limitation. The alternative lies in utilizing some 10mm of room available inside the piston…but that will further add to your piston weight, so it is a questionable option. Also, it will likely push the stock spring to coil bound state, as well as squeeze the rear guide too close to the tophat on cocking (both undesirable!!).
8. A soft set up: A great attribute on its own. No surprise, the gun feels smooth in its handling and cocking. Here’s a testimonial to this: Coming out of the action the stock spring was 267mm FL, and that’s after 2000+ shots. I had a bet with a friend that it would take a final set at 260, and that he wouldn’t be able to budge it beyond that ever! He just wouldn’t believe….and was game for the bet. After all, 2000+ shots are no joke …They can tame the stiffest springs, he believed! Just over 24hrs of scragging, and the spring was at its sweet 260. Now, I know that if it’s a quality spring, it will not budge beyond say 2mm (yes, two mm) even when kept coil bound for 1000 hours (you read it right, One Thousand hours!). An archaic, but a sureshot, way of verifying spring quality.
9. And finally, the SCR: To be honest, I have a serious hunch that this gun originally was designed to be a longer stroke one, and that the current 77mm models are only an interim offering….say, to get users into the groove first so they can be ready for more later! Aka, a Di engine as the first offering and a common rail direct injection follows some years later! Why I say this is that the TP of the VX is rather classical at 3X11mm. Now, vis-à-vis the current swept vol of 47.42cc, simple calculations will give you a whopping SCR of 609; and you will damn the designers for having committed a blunder. But that is where most of get it wrong. Just add the various grooves, nooks and corners atop the piston head and the cylinder wall, and it drops down to a respectable 450 (Notice those depressions in the cylinder end wall in the attached photo...all of that adds to the lost vol.).
With this TP dimension, I believe 500 should be the ballpark figure. Now, just increase the stroke length to say 85mm to get a swept vol of 52.34cc, and lo and behold, you are right there at the magic SCR of 523!! More reason, why the gun was originally designed for a longer stroke!!
So what holds the gun from delivering outstanding efficiency??
Well, if you notice closely, we have just stated the reasons above! And so, without making it long, I will just state my opinion in points:
1. An exceptionally heavy piston. [While it is able to use less spring energy to create a desired momentum and generate piston energy, the same soft spring even with a respectable preload is unable to tame the bounce securely….and hence you get the heavy surge in every shot]
2. The longish spring. At 120mm coil bound length, I wonder if it bodes well for a powerplant with just 131mm of spring room at lock up.
3. Though pretty much an industry standard TP, its dimensions seems much LESS suited to the current stroke.
4. And finally, the stroke. Take this in at least the 1:3 league, and you could put your money on the gun any day.
What options to try?
If you are into back garden plinking or target shooting, my advice would be to leave it alone. If you can manage good groups with this high surge gun, you can shoot any in the world. So, just master your hold technique!
But if you are into other forms of the sport, then, here goes…
1. The easiest option is what I did: install a spring that offers greater natural preload, and yet has smaller coil bound length. If its WD and CD are comparable to the PH one, it’s bound to be stiffer.
2. For the more seasoned DIY buffs, attempt two things: remove coils (say three, to begin with) from you stock spring to take its stiffness to some 5.6N/mm. But here you will need some 20mm of extra preload to increase the force at release. Manage this by removing the steel tophat, installing a light weight polymer tophat and adding slip washers (preferably derlin/polymer ones, to minimize weight). This will additionally work by lessening the sectional density of the piston…and thereby enhancing piston velocity during the compression stroke, and a lighter piston would be easier to be tamed by the stiffer spring during bounce. You may just have to watch out for the second landing velocity though (It can be a bit harsh in this case).
3. Reduce piston weight. Looks intimidating…but can be pretty easy going if you know how. Will do a separate post on this later.
4. Increase the stroke. Ditto as point #3….but very much doable. Do you have access to a mini lathe?? Great stuff for my next project. 85mm looks achievable….and should be a good case study for all of us! Let me see how soon I get to go at it!
Hope this note helps us make inputs, contest, qualify, refine or detail out any of the points above, and eventually enrich our understanding of this popular Indian air rifle.
Cheers!
Prashant
