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 Post subject: Disco velocity curve
PostPosted: Fri Sep 16, 2016 5:33 am 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
I just got a disco and right out of the box the shot string using 14.3gr pellets filled to 2000psi looks like this,

1-906fps 2-900 3-896 4-888 5-898 6-865 7-863 8-848 9-839 10-849

11-823 12-811 13-785 14-770 15-750 16=746 17-722 18-690 19-655 20-636 ending at <1000psi.

That's a whopping 207fps spread!
I'd like to lower the initial power to get a flatter velocity curve without using a regulator. I have a heavy spring and a RVA on order but I guess that's a joke now since they can only increase the hammer spring tension.
Do I need a weaker hammer spring? Smaller transfer port?
Help.

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 Post subject: Re: Disco velocity curve
PostPosted: Fri Sep 16, 2016 6:14 am 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
It's becoming a pattern that when I post a question I then start searching how Bob would've handle it. I guess I should search his solutions first but there may be others with other ideas I'd love to hear also. Below is Bob's post that I found .

The simplest and best mod IMO is to replace the hammer spring with one made from 0.040" wire (stock is 0.035").... That will allow the Disco to dump 2000 psi in the same manner as it currently does 1600 psi (ie at the top of the bell-curve).... This means, of course, that in order to rebalance the shot string you have to fill to 2400-2500 psi (ie the top of the yellow zone) and refill at ~1400 psi instead of ~1100.... Crosman don't say in their instructions that this is unsafe, only that you will lose velocity.... which you won't with the heavier hammer spring, you will gain velocity.... Calculations done by many different people tend to show that the weak point in a Disco is the valve mounting screws which should, in theory, fail at ~6500 psi.... It is up to you to decide if you feel safe filling the Disco to more than 2000 psi....

How much difference does the stiffer hammer spring make?.... Basically it bumps the gun up to MRod power.... With NO other changes (not even an RVA) you will gain ~80-100 fps depending on pellet.... which puts the gun up to ~29 FPE.... If you fit an RVA (rear velocity adjuster / power adjuster / hammer spring tensioner) then you can increase the power even further.... up to about 34 FPE with the heavier pellets.... Here are some shot strings with the heavy hammer spring just shy of coil bind and NO other modifications....

Image

Note that I only included shots which were within 4% of the maximum velocity for each pellet.... Also note how flat the shot strings are.... If you start hogging out the ports you can get even more power.... HOWEVER, the shot strings will become more curved and therefore shorter.... You will notice that the lighter pellets are travelling too fast (transonic) and the shot strings are also limited by the 2400 psi filll chosen.... This state of tune is at its best using 18 gr. and heavier pellets.... in particular I like the JSB Exact Heavy and the Kodiak/Baracuda.... as both have great BCs and can really let the Disco stretch out its range...

Bob

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 Post subject: Re: Disco velocity curve
PostPosted: Fri Sep 16, 2016 3:59 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8974
Location: Coalmont BC
Crosman replaced the hammer spring a while back with a stronger one.... Velocity sells, but of course by doing that they spoiled what was a perfectly tuned PCP.... You need to reduce the preload on the spring, or shorten it, to get back to the way they built then originally.... or fit an RVA (power adjuster).... Here is the way a stock Disco shot when they first came out....

Image

PS, you can get RVAs that will reduce the preload as well as increase it.... or make your own....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Disco velocity curve
PostPosted: Fri Sep 16, 2016 4:15 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
That sure explains things. For the RVA to reduce the preload I'm assuming it has an extended end cap. Installing a weaker spring will be no problem now that I've started making my own using music wire.
Well that's a relief, thanks a lot Bob.

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 Post subject: Re: Disco velocity curve
PostPosted: Fri Sep 16, 2016 5:21 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
Yea the stock spring was 0.049" on my caliper. Replaced it with a 0.042".
Cheapo 15gr. pellets and a 2000 psi start.

1-795fps 2-801 3-795 4-807 5-808 6-807 7-800 8-817 9-812 10-816
11-813 12-805 13-815 14-822 15-812 16-819 17-801 18-811 19-807 20-798
21-791 / 22-777 23-765 24-760 25-753 1300psi

21 shots with a ES of only 31fps all with 4%

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 Post subject: Re: Disco velocity curve
PostPosted: Sat Sep 17, 2016 6:10 pm 
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Joined: Wed Sep 10, 2014 2:42 pm
Posts: 147
Location: Ontario
SSG is the latest greatest thing. I have a thread on it and went from 16 or so good shots to over 30 and much quieter and efficient. I have a thread on it somewhere. There are a few guys making them to sell now but I made my own. All started by Bob Sterne.


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 Post subject: Re: Disco velocity curve
PostPosted: Sat Sep 17, 2016 6:15 pm 
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Joined: Wed Sep 10, 2014 2:42 pm
Posts: 147
Location: Ontario
Here is the link to the thread.

topic70738-120.html


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 Post subject: Re: Disco velocity curve
PostPosted: Sat Sep 17, 2016 7:08 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
Posts: 462
Location: Tecumseh, Ontario in South Western Ontario
This why I bought the disco. It's so freakin tunable. Oh yes Bob's invention the SSG is in the parts collecting phase. I have a Challenger adjustable hammer coming. I don't know if I'll order an RVA or build one. Any spring diameter and length is still on the table. Double disco, valve enhancement it's all possible.
One thing I'm really happy about is the accuracy is crazy good even though I've only tested in my 15 yard basement range. I know these clichés get tiring but every shot through the same ragged hole. I was wondering if I forgot to load a pellet :lol:

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