Don Cothran M-Rod Power House Valve

[rsterne]

HI Lloyd....

As I mentioned earlier in this thread, I now have a Cothran Powerhouse Valve for my Disco Double.... I am in the process of doing a set of tests like you did, in .250, .300, and .357 cal.... I have started with the .357 barrel which is 28" long, and I got 808 fps with a 154 gr. bullet (223 FPE) running on 2900 psi, so I'm very pleased with the power.... However, try as I might, I can't get any kind of unregulated shot string where the velocity doesn't drop with each shot.... I have skimmed through this thread, and looked at every chart you published, and most of the later stuff was regulated, so that doesn't apply to what I'm doing.... I only found a few places where you actually tested a sequence of shots where the pressure dropped from shot to shot, which was on page 7 in reply #131.... You used three different setups, and in every case, each shot was slower than the previous shot as the pressure dropped.... That is what I am finding as well, and so far I haven't been able to find an adjustment that will produce a "normal" bell-curve, or even two shots of equal velocity at the beginning of the string.... My question is.... did you ever manage to get a bell curve with the Cothran valve in an unregulated PCP?.... I can't seem to find a point where the Powerhouse valve will self-regulate....

BTW, I have also found the extreme sensitivity to hammer strike that you did.... Just 1/2 turn on the preload can cut the FPE in half.... I still have lots to learn about this valve.... Any tips you have would be greatly appreciated....

Bob

[QVTom]

Very interesting about the hammer strike sensitivity.  The Slayer valve has almost no reaction to hammer strike adjustments.  Maybe your hammer is way to heavy?  Just a WAG, I've never seen a PowerHouse other than in this thread.

Tom

[rsterne]

Tom, here's the thread on my Disco Double with the Cothran valve....  http://www.gatewaytoairguns.org/GTA/index.php?topic=117089.0

Bob

[oldpro]

HI Lloyd....

As I mentioned earlier in this thread, I now have a Cothran Powerhouse Valve for my Disco Double.... I am in the process of doing a set of tests like you did, in .250, .300, and .357 cal.... I have started with the .357 barrel which is 28" long, and I got 808 fps with a 154 gr. bullet (223 FPE) running on 2900 psi, so I'm very pleased with the power.... However, try as I might, I can't get any kind of unregulated shot string where the velocity doesn't drop with each shot.... I have skimmed through this thread, and looked at every chart you published, and most of the later stuff was regulated, so that doesn't apply to what I'm doing.... I only found a few places where you actually tested a sequence of shots where the pressure dropped from shot to shot, which was on page 7 in reply #131.... You used three different setups, and in every case, each shot was slower than the previous shot as the pressure dropped.... That is what I am finding as well, and so far I haven't been able to find an adjustment that will produce a "normal" bell-curve, or even two shots of equal velocity at the beginning of the string.... My question is.... did you ever manage to get a bell curve with the Cothran valve in an unregulated PCP?.... I can't seem to find a point where the Powerhouse valve will self-regulate....

BTW, I have also found the extreme sensitivity to hammer strike that you did.... Just 1/2 turn on the preload can cut the FPE in half.... I still have lots to learn about this valve.... Any tips you have would be greatly appreciated....

Bob

I asked this question myself before this valve became available. I donot believe you can make a curve with a balance valve not even a small curve. The amount of energy it takes to open the valve leaves no self regulation of the valve itself. You could have a slower first shot if the energy level was low enough to effect the crack opening pressure but thats it. I do not believe a balance valve will be any benefit to most air guns outside of big bores.

[QVTom]

HI Lloyd....

As I mentioned earlier in this thread, I now have a Cothran Powerhouse Valve for my Disco Double.... I am in the process of doing a set of tests like you did, in .250, .300, and .357 cal.... I have started with the .357 barrel which is 28" long, and I got 808 fps with a 154 gr. bullet (223 FPE) running on 2900 psi, so I'm very pleased with the power.... However, try as I might, I can't get any kind of unregulated shot string where the velocity doesn't drop with each shot.... I have skimmed through this thread, and looked at every chart you published, and most of the later stuff was regulated, so that doesn't apply to what I'm doing.... I only found a few places where you actually tested a sequence of shots where the pressure dropped from shot to shot, which was on page 7 in reply #131.... You used three different setups, and in every case, each shot was slower than the previous shot as the pressure dropped.... That is what I am finding as well, and so far I haven't been able to find an adjustment that will produce a "normal" bell-curve, or even two shots of equal velocity at the beginning of the string.... My question is.... did you ever manage to get a bell curve with the Cothran valve in an unregulated PCP?.... I can't seem to find a point where the Powerhouse valve will self-regulate....

BTW, I have also found the extreme sensitivity to hammer strike that you did.... Just 1/2 turn on the preload can cut the FPE in half.... I still have lots to learn about this valve.... Any tips you have would be greatly appreciated....

Bob

I asked this question myself before this valve became available. I donot believe you can make a curve with a balance valve not even a small curve. The amount of energy it takes to open the valve leaves no self regulation of the valve itself. You could have a slower first shot if the energy level was low enough to effect the crack opening pressure but thats it. I do not believe a balance valve will be any benefit to most air guns outside of big bores.

The Slayer valve provides a decent curve.  Generally steeper on the front than the back but a nice curve nonetheless.  I can't speak for the Powerhouse though.  It's all about balancing forces, the only difference is that the forces are much smaller.  Your are correct about them not being suited for a small bores.

Tom

I spent lot of hours figuring out the tuning on the balance valve.  Once a cle

[lloyd-ss]

This is a late comment to the question about being able to shoot a velocity curve with a balanced valve.  I agree with Tom, it definitely can be done, if the valve is designed to produce a curve.  There are a number of different ways to make a balanced valve and the various diameters and ports can be varied to produce whatever your desired result might be.  It can be anything from a tiny bit of opening assist, all the way to valve that won't even stay shut.  The balanced valve has to be viewed as a concept that can be adapted to its end use, not as a single specific design that functions within a single set of constraints.

Here are 2 different balanced valve concepts (and there are many more), and as you can see, varying the 3 different diameters can drastically alter the performance, and the ability to produce a decent shot string curve.  (Note that the little formulas with A, B, and C, refer to the forces acting on those diameters and are proportional to the square of each particular diameter.)
Lloyd

[Hobbyman2007]

I believe the Cothran valve was designed for big bore use. After trying to tune it in a .25 cal pellet shooter I gave up and installed the valve in a .357 . I now understand that the only way to get where I wanted (40 to 50 fpe ) would have been to lower the regulator pressure from the 1800psi I was using . Now if someone had the magic formula of port size ,pressure and caliber for desired FPE for this valve it would have made my build a lot easier.

[rsterne]

Thanks for those diagrams, Lloyd, it makes it obvious when you see them what is happening.... As you say, there are other versions, of which the Cothran is one.... I will point out one thing for those that missed it.... In the bottom example, if area "B" equals area "C", then the only force left is area "A", either opening or closing....

I don't know of anyone who has a "magic formula", if there was such a thing if would have to be developed, or at least tested the old fashioned way.... Install the valve in the caliber of your choice, with big enough ports to do the job you want (there is a guideline for that), pick your bullet to get the velocity range you want, and then try a few pressures (let's say 3) and draw yourself a graph of velocity vs. pressure for YOUR gun.... You should then be able to get REALLY close by setting your regulator to the corresponding pressure....

Sometimes there is no substitute for experimenting yourself.... and the added benefit is you will ALWAYS learn something....

Bob

[Hobbyman2007]

True enough Bob . I was trying to take the lazy / easy way. I'm pretty sure if I use one of the charts you posted previously regarding pressure vs. fpe for each caliber it would be very close. Looks like another trip through my best of Bob folder.

[QVTom]

Balance valves only make sense for 150 fpe and up.  The main advantage of the balance valve is the reduced hammer energy to open.  The disadvantages are more complexity and in most designs, dynamic seals that must work under very high pressure and create a measureable amount of seal drag. 

For smaller power outputs, the seal drag alone is a reason not to use one and consider the cocking forces are already manageable.  All the negatives can be accepted in a bigbore where the goal is is big flow and easy cocking. 

The valve in the Slayer 45 would need close to 800 lbs to crack at maximum fill pressure; I can't even imagine the hammer, spring and cocking lever with a conventional valve.

All balance valve designs have their the ups and downs.  The Powerhouse, from what I've seen here, seems to have tackled the complexity, has only one dynamic seal, and the advantage of easy retrofit into Crosman guns.  No small feat for Cothran.

Tom

[Hobbyman2007]

I knew the cheat sheet existed http://www.gatewaytoairguns.org/GTA/index.php?topic=46011.msg431654#msg431654

[rsterne]

Denis, I use the graphs in that post as a "lofty goal" for a 24" barrel.... In fact, it is rarely achieved without using a bit more pressure or a bit longer barrel or a heavier bullet than 50% of the FPE, even with extremely well designed and built PCPs.... in fact I have never quite made it, the best I have done is within about 10% of the "predictions"....

Bob

[Rattus58]

Brent at Extreme has been building and promoting the .40 cal for a while, he uses the .400/.408 barrel shooting heavy rifle bullets at 4500 psi.... No reason the lighter pistol bullets at 3000 psi wouldn't be worth a try.... I have designed some .408 cal BBTs for NOE in 204, 234, and 266 gr.... Even the heaviest should work in a 24" twist.... The 204 gr. FN should be a good choice in a 3000 psi gun.... It is 193 gr. as a HP....



My W.A.G is that with 1/4" ports, .357 cal may be as big as you want to go, to get decent velocity at 3000 psi....

Bob

Does CJ make a 40 caliber or maybe .410.. since my muzzleloaders are .410 using sharps bullets already.... and out of curiosity... how much machining if any would be required to fit one to a Marauder breech ? Thank you in advance for this...

Much Aloha,

Tom

[lloyd-ss]

Rattus58,
I've put copy of TJ's March 2014 price sheet (the prices might not be current) and it does include several .40 cal barrel liners. Any one of them could be made to fit in a .25 cal M-Rod breech block, but I would not recommend it.  Safety is the reason.   The M-Rod bolt is locked for firing with a single bolt lug and at .40 cal you could end up at 3 or 4 times the power level of the original factory gun.  Unless some important safety mods are done to handle the huge increase in power, you might have some serious problems.
Lloyd

[Rattus58]

Rattus58,
I've put copy of TJ's March 2014 price sheet (the prices might not be current) and it does include several .40 cal barrel liners. Any one of them could be made to fit in a .25 cal M-Rod breech block, but I would not recommend it.  Safety is the reason.   The M-Rod bolt is locked for firing with a single bolt lug and at .40 cal you could end up at 3 or 4 times the power level of the original factory gun.  Unless some important safety mods are done to handle the huge increase in power, you might have some serious problems.
Lloyd

Ok... then... are you suggesting that the Cothrain valve would be a waste of time for a stock Marauder... or conversely... at what caliber would be safe, if it is a footprint thing of the air blast... or lastly,  exactly what safety concerns are you referring that can be mitigated?

Much Aloha,

Tom

[rsterne]

The Cothran valve is supplied with a 1/4" exhaust port, but Lloyd has shown that can be increased to 9/32".... Going larger would require enlarging the throat to increase flow, which would alter the internal balancing of the valve, something I would not personally be willing to experiment with.... Don spent a long time getting it right.... It is typical to use porting about 75-80% of the bore diameter, going larger than that can be done, but requires machining an oblong port in the barrel.... The 9/32" port is 79% of a .357 bore, but only 69% of a .408 bore, so although you could get a bit more FPE with a .408 than a .357, you are starting to handicap the larger caliber, relative to what it is really capable of....

As Lloyd says, there are structural concerns with the MRod bolt lockup when you start pushing the caliber too large, there is only one small pin preventing it from being blown back into your face.... A .357 cal already doubles the force compared to a .25 cal, and puts 4 times the load on that pin as does the .177 cal, and all the MRods use the same single 8-32 bolt to take the load.... Even if the pin survived normal use, if you ever fired the gun with the bolt handle not locked down (bumped it and it popped up, for instance), I feel pretty confident that the bolt wouldn't stop at the end of the slot, and just imagine what a mess that bolt could make of your face....

The bigger you go, the higher the forces (they go up by the square of the caliber).... so to quote Dirty Harry.... "How lucky do you feel, Punk?".... no offense intended....

Bob

[lloyd-ss]

I just looked back over my notes and drawings on the M-Rod version of Cothran Powerhouse valve installed in an M-Rod.  As supplied by Don, the ex port is .257 dia, and the valve throat (max throat I.D. less the stem dia) is equivalent to a .301 port.  I managed to open the ex port up to .293 dia, but I had to offset it, and I almost ruined the valve by cutting into an internal o-ring groove.  Still the .293 ex-port mod did function properly.

On some M-Rod bolts, I have replaced the 8-32 bolt lug with a 10-32.  You have to use the correct grade of bolt, though, and make sure the bolt lug is pocketed properly into the bolt itself, and possible fracture points are minimized, etc.  Personally,.357 caliber is about as far as I would want to go in the M-Rod.  Plus, you can get a .357 barrel blank that has a 1/2" O.D. that will fit the .25 cal breech block.
Lloyd

[rsterne]

Good point, Lloyd, if you go larger than .357 cal you will have to bore out the .25 cal breech to accommodate.... Turning the barrel down to 1/2" is not an option, as even on the .357 cal barrel there is precious little wall thickness remaining.... Increasing the bolt lockup pin to 10-32.... properly installed in a counterbore in the bolt so that the shear plane is on the full shank diameter, not on the threads.... should be part of the process any time you go larger than .25 cal in an MRod breech, IMO....

If someone wanted to play around with anything larger than .357 cal in an MRod, the practical approach would be to design and machine a custom breech to incorporate a larger barrel diameter, and properly address the bolt lockup safety issue with a 1/4" or larger bolt lockup, IMO.... In anything that large, I would NOT use a bolt that can be extracted from the rear of the breech without two shear points as a backup in case the gun was fired with the bolt open.... A conventional side-bolt arrangement, with a safety plug behind the bolt in case of a lockup or bolt handle failure, would be an extremely good idea.... Brent's Extremes have this, and it has already proven it's worth....

Bob

[Rattus58]

The Cothran valve is supplied with a 1/4" exhaust port, but Lloyd has shown that can be increased to 9/32".... Going larger would require enlarging the throat to increase flow, which would alter the internal balancing of the valve, something I would not personally be willing to experiment with.... Don spent a long time getting it right.... It is typical to use porting about 75-80% of the bore diameter, going larger than that can be done, but requires machining an oblong port in the barrel.... The 9/32" port is 79% of a .357 bore, but only 69% of a .408 bore, so although you could get a bit more FPE with a .408 than a .357, you are starting to handicap the larger caliber, relative to what it is really capable of....

As Lloyd says, there are structural concerns with the MRod bolt lockup when you start pushing the caliber too large, there is only one small pin preventing it from being blown back into your face.... A .357 cal already doubles the force compared to a .25 cal, and puts 4 times the load on that pin as does the .177 cal, and all the MRods use the same single 8-32 bolt to take the load.... Even if the pin survived normal use, if you ever fired the gun with the bolt handle not locked down (bumped it and it popped up, for instance), I feel pretty confident that the bolt wouldn't stop at the end of the slot, and just imagine what a mess that bolt could make of your face....

The bigger you go, the higher the forces (they go up by the square of the caliber).... so to quote Dirty Harry.... "How lucky do you feel, Punk?".... no offense intended....

Bob

Ok... So in the case of a stock Marauder, what is the overall benefit increase of the valve in real numbers?

As to .357 and shear... is it really that difficult to provide more shear strength to the cap or behind the hammer?

What does " Increasing the bolt lockup pin to 10-32.... properly installed in a counterbore in the bolt so that the shear plane is on the full shank diameter, not on the threads.... should be part of the process any time you go larger than .25 cal in an MRod breech, IMO...." Actually mean?


Aloha,

Tom

[rsterne]

We aren't talking about any additional forces on the valve, hammer, or rear cap.... we are talking about the additional force on the bolt itself.... The bolt lockup in an MRod is achieved by the cocking pin under the bolt.... It pulls the hammer back to cock the gun, then when you close the bolt it slides forward and rotates left or right (depending on which way you have the bolt handle set up) into a slot in the receiver.... That pin in that slot takes ALL the force on the bolt when you fire the gun.... At 3000 psi, that's only 83 lbs. in .177 cal, but it's 167 lbs. in .25 cal, and 330 lbs. in .357.... It could be as much as 450 lbs. in .408 cal (depending on the O-ring and bolt size sealing the breech).... all trying to shear off that single 8-32 screw that is threaded into the bolt.... If that pin shears off, there is nothing to prevent the bolt from flying out of the back of the breech and across the room.... except whatever part of your body might be in the way, of course.... If you increase the caliber above Crosman's .25 cal, IMO it is prudent to change that 8-32 screw to a 10-32, set deep enough into the bolt that the shear plane is not through the threads of the screw....

Even worse is what could happen if you forgot to push the bolt handle down, or it got knocked back up, and then you fired the gun.... Instead of the cocking/lockup pinn only having to withstand a stationary force, now it gets a running start, giving the mass of the bolt energy and momentum, until the nose of the bolt pops out of the back of the barrel O-ring.... You would be astounded as to the destructive power that energy has.... Lloyd did an experiment a while back where he sheared a 1/4" bolt handle clean off under those circumstances.... Any increase in pressure above Crosman's 3000 psi fill increases the loads under all conditions, of course.... in direct proportion to the pressure....

Bob