I've done extensive testing with the Cothran valve in many different applications/configurations. It is a great valve, but the tuning process is totally different than a traditional knock-open valve. The Cothran can be described as a reduced effort blow open valve... Here are a couple of pointers. First make certain you are using as light of a hammer as possible to get the dwell you need for your FPE goals and regulator set point/fill pressure range. For example, I use a 14gram all peek hammer on a Cothran equipped QB78 .30 pellet shooter. it is regulated at 1900psi and tuned for right at 90fpe. If I wanted more FPE, I would use a hammer that weighed more and bump up the reg pressure. I do not have a calculation for this, it is mostly testing, trial and error. Second, make certain you have just enough hammer strike to open the valve consistently---verify this by letting it sit overnight and taking a shot over the chronograph. you want to reduce hammer spring until this shot doesn't develop full velocity. Once you find this point, add a little hammer spring until this low velocity shot no longer occurs after sitting. Once you have a combination that opens consistently, but isn't excessive in hammer strike, you are running the stock Cothran Valve as efficiently as possible for your application.
There are modifications you can make to get the valve to close faster
1) Increase stem vent size. You can go up to about .060ish. If you go too far, you can order a replacement stem from Don. Relatively low risk.
2) Decrease the balance ratio by increasing valve throat area. I've found this to have a significant effect on how fast the Cothran will close. If you mess up the seat, you have to buy another valve.
There are likely different and better methods of tuning a Powerhouse valve. I cannot take credit for developing these methods---there are a handful of guys that published Cothran valve testing data in recent years, this is a combination of their data and my personal experience. I have spent a bunch of lead and nitrogen trying different things including running without the stem rod, peek sealing material, etc. Most of these tests yielded negative results. What I outlined above is very low risk, and works across multiple calibers, pressures, and fpe levels.
Good luck, please continue to share your results.
