Author Topic: Benjamin discovery tune/mod  (Read 273 times)

Disco2.0

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Benjamin discovery tune/mod
« on: March 19, 2020, 08:23:17 AM »
Thx in advance for your help.

I have a stock benjamin discovery, the only not stock part is the gauge port has been replaced with a high flow one. I'm planning on taking it apart and drilling out the ports but I don't know what I need to do to get the wanted results.
First of, if I drill out all 3 ports, can I go back to the original specs or close to it by just replacing the transfer port?
I posted two images of some charts I found in articles you wrote to give an example of the results I want. To get the power mod you had on the Beeman, what would I have to do, and could I get it on the discovery? And the end plan would be to be able to switch out the transfer port and maybe adjust the hammer spring and get to teal colored string on the other chart. If that is not possible without a bigger hassle, my primary goal would be to get the teal colored string or as close to it as I can. Thanks again👍


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Disco2.0

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Re: Benjamin discovery tune/mod
« Reply #1 on: March 19, 2020, 08:30:15 AM »
Images didn't post in the first post
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rsterne

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Re: Benjamin discovery tune/mod
« Reply #2 on: March 19, 2020, 09:14:07 AM »
My first PCP was a Discovery.... Here are the stock results....



This is with some relatively simple port mods, still running with a 2000 psi fill....



This is with more extensive mods, including upgraded valve screws, running a 2400 psi fill....



This is the above with a bored out gauge port.... Now I would substitute a PRod gauge port instead....



The curve marked "Heavy -4T" is about what that gun would do with a 2000 psi fill.... roughly 14 shots averaging 34 FPE.... This represents about the best performance that most guys manage to get with extensive mods on a Disco platform.... Higher performance can be obtained, but the amount of work goes up and the shot count drops significantly....

Within reason, you can return to the original tune by reinstalling a stock, or smaller than stock, transfer port....

Bob
« Last Edit: March 19, 2020, 09:18:07 AM by rsterne »
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Disco2.0

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Re: Benjamin discovery tune/mod
« Reply #3 on: March 19, 2020, 09:43:35 AM »
For now I want to stay relatively simple. I'll probably make a power adjuster in the near future, and for now I plan to shorten the spring and maybe make it heavier for a simple SSS. What are the simple port mods you did? And what size would you recommend per port?

Thx
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rsterne

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Re: Benjamin discovery tune/mod
« Reply #4 on: March 19, 2020, 04:46:47 PM »
Throat drilled to 7/32" and valve stem ground down to 1/8" in that area, exhaust port milled on a 20 deg. angle to 0.166" and the transfer and barrel ports drilled to 0.166".... You already have the PRod gauge port right?.... You probably don't need a stronger hammer spring if you stay with the 2000 psi fill.... The RVA (power adjuster) is a necessity for tuning....

Rather than boring out the inside of the valve to clear the poppet and grinding down the valve stem.... a far better solution is to use an MRod poppet and reduce the valve stem hole in the valve to 1/8" by gluing in a piece of 5/43" OD x 1/8" ID brass K&S Hobby tubing.... Use the poppet to keep it straight while the glue sets, but make sure you don't get any on the stem.... Shorten the MRod poppet stem to the same length as the Disco one, or even 1/16" shorter to increase the hammer stroke.... You can drill the inlet side of the valve out slightly to match the ID of the valve spring and improve flow into the valve....

Bob
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Disco2.0

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Re: Benjamin discovery tune/mod
« Reply #5 on: March 19, 2020, 06:48:52 PM »
I do have a enlarged gauge port, not sure if it's a prod. I guess ill have to get my hand on a mrod valve stem, although on the alterative I'm not too sure what you mean by clearing the poppet.
Unless that's pretty complicated or compromises the valve for future upgrades it sounds bit simpler to me.
How do you recommend I drill out the exhaust port on the valve at a 20 degree angle?
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Disco2.0

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Re: Benjamin discovery tune/mod
« Reply #6 on: March 19, 2020, 06:53:24 PM »
Also do you have a diagram of the disco valve I could look at? And what did you add in the more extensive mods other than valve screws and a heavier spring.
Thx
« Last Edit: March 19, 2020, 07:29:35 PM by Disco2.0 »
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rsterne

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Re: Benjamin discovery tune/mod
« Reply #7 on: March 19, 2020, 08:40:52 PM »
Sorry, I don't have any drawing of a Disco valve.... I used a milling machine with a 5/32" end mill set at a 20 deg. angle to enlarge and angle the exhaust port.... Then I rounded the bottom corner with a small spherical burr on a Dremel....

I would imagine you don't have access to a mill, so you would have to work on changing the diameter of the port by hand.... You can either drill straight down with a 5/32" drill, then number drills until you get to 0.166".... OR you can start with a 1/8" spherical burr in a Dremel and grind away only the front of the port until you get to the 0.166" diameter, angling it as you go.... and then round the bottom corner into the throat area....

The Disco poppet is quite large on the OD, and the purpose of boring out the inside of the valve is to increase the clearance around the poppet.... The MRod poppet is smaller on the OD and made from a stronger material, and is a good replacement for the Disco poppet, except that it has a smaller stem.... This again is good for flow through the throat, but to use it requires you bush up the hole in the valve for the smaller stem.... I used the stock Disco valve spring....

Bob
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Disco2.0

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Re: Benjamin discovery tune/mod
« Reply #8 on: March 20, 2020, 07:05:29 AM »
Will the original transfer port still fit the valve?

And how big a difference will it make if I go straight rather than at an angle?

Any chance I could clamp the valve in a vise at a 20 degree angle and then just drill the hole with a drill press?
« Last Edit: March 20, 2020, 07:43:27 AM by Disco2.0 »
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rsterne

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Re: Benjamin discovery tune/mod
« Reply #9 on: March 20, 2020, 09:12:46 AM »
You can drill out the transfer port carefully one number drill at a time.... Holding it without a lathe chuck is tricky, but you can probably use one drill chuck to hold it and another for the drill....

If you don't want to grind the hole with a Dremel, just drill it straight.... I would not try a 20 deg. angle, you will either ruin the valve or break the drill, most likely....

Bob
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