Flub's General Discussion - CR600W - Mediocre power after common mods

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CR600W - Mediocre power after common mods

Printed From: Kiwi Airgunners
Category: General
Forum Name: Flub's General Discussion
Forum Description: General discussion goes in here.
URL: www.kiwiairgunners.co.nz/forum_posts.asp?TID=1160
Printed Date: 05 Feb 2025 at 10:40pm
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Topic: CR600W - Mediocre power after common mods

Posted By: Loud_Whisper
Subject: CR600W - Mediocre power after common mods
Date Posted: 18 Jan 2025 at 4:39pm

Hi all,

As the title says - I've gone done a bunch of the most common power mods on my CR600W and am getting less power than I'd expected, at around 10-11fpe.

Mods are after having read I think every CR600W thread on the internet, but mostly based around the great work documented by Mintie and Billbobnz.

Mods are:
#Transfer ports drilled to 4.5mm from valve to barrel and sanded with P600
#Increased depth of valve internals by maybe 4mm for slight (maybe 15%) increase in valve volume. Not widened.
# Polished hammer and tube with P400 > P600 >P1000 to mirror finish.
# Very sparing sprinkle of graphite powder in hammer tube. Slides like an ice cube on a frypan.
# Have played with packing out hammer spring even by as much as 5mm, with as little as 0-5fps difference. Reverted this to standard as Co2 cooling / wastage was very noticeable.
#Deburred Magazine > breech > barrel pathway (now grouping better than I'd ever hoped)

I saw posts of 18+fpe on these with a decent tune - roughly 50% more than what I'm still getting. Albeit those were with a larger valve volume, but not a lot else that I can see.

My average of 10-11fpe is across 6 or so different pellets (12-18gr) and is tested with an actual Crhono rather than phone app. I had understood these were meant to be around 12fpe from factory, so had expected more.

Has anyone done the same combo of mods of these and put it across a proper chrono? interested to hear thoughts!

Replies:

Posted By: kruzaroad
Date Posted: 18 Jan 2025 at 10:54pm

Might be of interest.
https://youtu.be/cSGtfOKn64Q?si=bp5_C9BHsqSCFCAq

Posted By: Pauly5
Date Posted: 19 Jan 2025 at 10:06am

Hmm, I have modded one, but didn't go for any great power increase. Looks like you've done a good job so far.

Did you soften the valve spring so it stays open a bit longer? I see Mintie lengthened the valve spring seat so the result was it being softer. Having more volume and the opened transfer port would be my main focus and I think using a heavier pellet is key for CO2.

Also make sure all the O rings around the TP are good too, as any leakage there will kill power.

Posted By: Loud_Whisper
Date Posted: 19 Jan 2025 at 10:08pm

Thanks guys.
Forgot to mention that deepening the valve did indeed decrease its spring tension, so some impact there as well.

Chrono again today a few times throughout the day and was around 560-570fps in the warmest part of the day. To be honest I probably need to forget trying to get too much power out of this one as I'll only ever use it for birds and have bigger badder things for other pests (Hatsan 135QE and Gamo G Magnum for possums & wallabies etc)

Next step will be to see if I can get it a bit quieter without loosing "too much" velocity, and to try some heavier pellets still just to see what happens. Next stop will be seeing if I can balance out the extra cooling - possibly backing off the gas volume in favour of some cleverer porting into the barrel.

Posted By: kruzaroad
Date Posted: 19 Jan 2025 at 10:45pm

My 1322 is only about 6fpe and snots out starling and mynar quite happily to 30m. Has taken a starling at 50m approx. Dropped a possum at about 2ft so I imagine your going to be getting a lot of pass thru on birds at those fpe. Should do the job fine.
You've obviously had a bit of fun working on it, so a win all round id say

Posted By: Loud_Whisper
Date Posted: 20 Jan 2025 at 2:24pm

Wow that is some low power and very reasonable distances. Big respect! Its So counter intuitive to want to go down, but makes so much sense when I already have bigbois for other jobs. Yep lots of fun indeed, and a few more non power mode lined up too. Just dropped two mynas this morning and about to head down and check the damage!

Thanks all.

Posted By: RangerPete
Date Posted: 20 Jan 2025 at 7:07pm

Sometimes you got to go backwards to go forwards....

Case in point is I wanted to lower the FPE of one of my pcp rifles a few days ago, so I lowered the reg pressure, thinking if the rifle is being fed less air (lower pressure) the fps will drop and the FPE will go down, right?

Lowered the reg pressure and the fps and FPE went UP!!!
Left me scratching my head.
Tried it again and lowered the reg further, and the fps and FPE went UP, again!!!

Contacted Pauly for some brain storming and sound advise and we figured out that the lower pressure behind the valve was allowing it to stay open for longer (before pushing it closed from behind). So the lower pressure, longer shot of air was causing higher velocitys.

So I opened the reg back up to where I had started, then opened it up another 1/16th of a turn, so it was now running 9bar higher then when I started (60bar up to 69bar) and guess what... velocitys and FPE went down!!!

With more pressure behind the valve, it was closing faster, letting out less air (although at a slightly higher pressure) and my FPE dropped and with a little more reg pressure, I got it to where I wanted it

Fuzzy logic, but once you understand whats happening inside the gun, it all makes sense.

-------------
Walk quietly, but carry a big stick.

Posted By: jwabfrog
Date Posted: 22 Jan 2025 at 10:41am

My understanding of tuning a PCP after reading and watching many youtube videos is to start with max hammer spring tension and then steadily increase reg pressure to get slightly more than your target fps. Then reduce the hammer tension to drop to your target. This results in a high efficiency tune on the shoulder of the fps vs hammer tension curve i.e. minimises wasted air and helps reduce fps variation. This is a good summary:

https://www.airgunnation.com/threads/how-to-do-a-basic-regulated-pcp-tune-the-95-tune-plus-m3-tuning-simplified.1242157/

Posted By: Pauly5
Date Posted: 22 Jan 2025 at 12:25pm

From my own experience, you do need to be prepared to waste quite a few pellets to achieve what I would call the "two stages" of tuning.

1, to get hammer and reg balancing for efficiency and accuracy, and then 2, to get harmonic tuning for accuracy.

The second part is harder depending on your gun and this is what attracted me to the impact as it had a valve adjuster.

Typically you have reg pressure and hammer tension, but then you have the dwell time of how long the valve stays open. This can be finely tuned by increasing or decreasing valve return spring tension.

Once you have that reg/hammer balance shoot a couple of groups at 30 and 50 m. If they are good and tight leave it.

You can tighten the groups by subtly changing the valve and this is where the impact is good in that you can do this externally. Adjust valve a touch, shoot groups, and so on until your groups tighten more.

But of course being fixated on a 6 mm hole at 100m is chasing your tail. I stick to minute of rabbit head personally.

Posted By: Loud_Whisper
Date Posted: 22 Jan 2025 at 9:54pm

Interesting!
I'm almost relieved I don't have a PCP simply for the fact that I'd go broke because I'd enjoy tuning it so much! Keeping in mind this will be used mostly only for mynas and pinking, I think I'll return the valve spring to standard tension, find some cheap ammo that works, and move on to getting the next one in the fleet dialled (Hatsan 135 springer)

Posted By: jwabfrog
Date Posted: 23 Jan 2025 at 9:38am

Lol Pauly, I think we should all be adopting / dialing minute of rabbit. Many a slug / pellet wasted chasing the 6mm hole at 100yds!

Posted By: Mintie
Date Posted: 29 Jan 2025 at 3:26pm

Sounds like you were on the right track for good power, just to check though when you opened up the TP hole you also matched the valve, breech and barrel to the same diameter?

Pauly is on the right track to bring up pellet weight too, my testing at 18 fpe was always with heavy pellets and warm weather. The .25 cal one I built for Zooboy ended up about 22fpe from memory.

Dropping the power back if necessary is just a matter of less HST or a lighter hammer, doesn't matter if the valve is still hogged out for max volume.