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The Quest to 90% Bore Porting.

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ackuric

ackuric

Member
Mar 10, 2017
1,232
61
Colorado, United States
I do a lot of modifications to PCP rifles. And the following I will be presenting to you should NOT BE ATTEMPTED unless you have thorough knowledge of how pcp's work, inside and out, but mostly inside. The following was done with VERY limited set of tools : drill, dremel, clamps, peek/delrin rod, brass/copper washer, hss 1/8 drill blank, jb weld, sand paper.



What separates my arrangement from most is the micro valve stem and reduced seat, thanks to it, I am allowed a smaller poppet OD, the latter 2 GREATLY reduce the requirements to open the valve. A stock valve stem would require a much larger valve seat/throat + poppet OD which in turn would make the valve harder to open. The last major change that reduces forces required to open the valve is higher quality material over stock with less young modulus ie: peek poppet and brass or copper seat. I'd love to try something MUCH more harder for a valve seat in the future, maybe if/when this valve ever fails.


Rifle:

.25 Cal Marauder 

Valve specs:

.2275" (.234" x .221") Valve exit port and Transfer port (91% of bore)

.225" Barrel port (.212" x .238") (90% of bore)

.237" Valve seat (over 5% greater cross sectional area than forward porting to compensate for flow loss across seat)

.275" Peek Poppet (.19" sealing margin)

.04" HSS Micro Valve Stem (ran one for a year to test, now testing further, may increase 10-20% if one fails in the future..)



Gen 1 style bolt probe modified for the above porting.

Valve hogged .5" down to valve seat (stock = .42")

Old valve seat sanded down/removed 

Stock hammer travel: .75"~ 

Modded hammer travel .88"~



47.5 Gr MDS hammer + 7.7 lb spring



As you see from the above specs, I stretched just about every single one of them to their max. Other rifles may be different but the Marauder is mostly limited to the flat spot on the barrel being .285" which would allow a transfer port of .285" with .02" walls and .245" porting which equates roughly to .245 x .21 = .2275"....going further risks damage to pellets, you can go to .23" but that is really about it without creating a new bigger flat spot and that I am not equipped to do, and even if I were I find the methods/techniques and my outcome acceptable enough.



Pic1: Here I admittedly messed up and drilled out the valve exit before the throat, which reduced a ton of metal at the top of the valve and caused my drill bit to wander when drilling the throat...very important to always drill out the throat first or use a really good vice + drill press but I do all by hand so, the original idea was to leave the original seat intact and attempt the crazy over 90% porting, but when life gives you lemons you make lemon aid. Next to the valve are the copper washers I picked up to 'Repair' the seat, I use this method to move the seat forward on my other valve anyhow..I really wanted to do over .225" porting without having to move the seat forward, which I know is possible but requires a 90 degree valve exit hole opposed to 45-65 degree, on top of quite a few other tricks and snags to avoid...maybe another day I will try again knowing what I know.







Pic2: Is just a picture showing the width of the valve exit port, as I stated earlier it is .234" wide (side to side) by .221" (front to back). Prior to breaking through the stock seat I planned on .24x.21.







Pic3: Peek poppet, micro valve stem and valve. Lots of markings on the valve as I just vice gripped it when hogging out and various other things and occasionally it would catch and spin..no functional damage only cosmetic that no one ever sees. Can also note the valve was shortened .1" in this pic. I also reduce the poppet sticking out of the stem over factory, which is .25" from factory and .185" in mine...which increases the hammer travel ever so slightly on top of the .1 I remove from back of valve.







Pic4: custom transfer port that matches both valve exit + barrel port. I hand make this starting with .3125" delrin taking it to .285"~ OD which allows up to .245"~ porting. I carefully match the porting and index the TP..definitely the most time consuming part of the whole job since matching oblong ports on TWO sides is a bit challenging, especially when striving for as near as perfect as one can do. The tp required me to drill through my air tube and receiver to allow the .285" Tp to slide in/out easily. The barrel port flat spot is .285"~ od and the valves is .3"~, while the receiver+air tube holes are closer to .26-.27.







Pic5: here is what it can do on just 7.7 lbs of hammer spring...at .6 pre-load which is not quite coil bound so there is more...and no this is NOT the valves PLATEAU. That would be roughly 900 FPS @ the regulated 2000 psi its currently at...this was with a 33.95 MK ll (56 FPE)











Pic7+8: Here is how I make my micro valve stem, chucked up in a drill with a dremel and cut off wheel I carefully remove area between the tape marks until I am roughly to .005" within spec where I then chuck the valve stem into the dremel and use sand paper to get a fine polished finish











Pic9: A graph of my tunes + cocking forces for said tunes..









That about sums up how to achieve over 90% bore porting with a modified OEM valve...by hand! The margins are small, and going even larger in ports without extensive modifications they become even smaller. Without creating new flat spots on valve + barrel, the safest I could personally take a .25 cal marauder is roughly .23"...which I came close enough to, when doing such by hand I like to leave a little extra margin for any mistakes I may make, .2275" valve and .225" barrel are those margins for me. Unfortunately I didn't snap pics of my poppet head making process but as you see from its pic, I think it came out well for being done by hand! I use the same method as the valve stem, for the most part, with a few techniques added to keep everything concentric.



And yes I have a marauder, with a 19.5" barrel, that makes 60 FPE (900 FPS 33.95 JSB) at just 138 bar...2000 psi. The valve seat is currently adhered using JB weld, where I ideally would use a 1 part epoxy that retains a LITTLE flex..and may move to such in the future if and when the current jb weld fails. I stress tested BOTH using a 98 gram hammer and fully preloaded 7.7 spring to wack the heck out of the valve, and it survived multiple. Also the micro stem reduced to .04 was only reduced by a factor equal to the force reduction required to unseat/open the valve PLUS the reduced energy required to open it...its quite the balance to make sure failure isn't inevitable.


FWIW there are people with 'balanced' valves that use heavier hammers and springs...so to achieve the above using a conventional valve is quite impressive...as you can see this project didn't quite go as planned, but failure or mistakes shouldn't keep anyone from pressing forward and striving for success! My marauder when stock came with a 8.5-9lb spring shooting 38-40 fpe @ 3k psi with a 78 gram hammer. I have since regulated and modified the valve so it does 60 fpe at 1000 less psi (2k) with 40% less hammer and 20% less spring...using the SAME valve with different specs.



Cost of modifications:

Peek + Delrin rod = 15$ (enough to attempt nearly 20 tp's and 20 poppet heads)

Copper washer = 75 cents (brass much cheaper, the #12 size I would need for brass isn't available locally)

Hss drill blank = 5$ ( I got 3 for 5 which in turn makes 6 stems)

Time invloved: Lots, so if you over value your time when tinkering with a hobby, then leave it to the pros ;)



-Matt


 
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I shoot various pellet weights at different tunes, I have 5 pre built ssg's (spring stop guides) that retain the tunes that I just swap out and don't have to re-chronograph for every time I swap tunes. They are they are (18.5, 25, 42, 56, 60+) fpe tunes. My 18.5 + 25 fpe tunes use 19.91 gr h&n, my 42~ fpe tune uses 25.4. The 56 + 60 fpe tune use 33.95 mk ll

Swapping tunes is just 3 simple screws that take 30 seconds to remove and reinstall. I can't really detune on the fly but not many can while retaining pin point accuracy which this method does for me, I tune all the ssg's once and they're good.

pic of my ssg's:







If you're asking for a comparison from stock to what I am now, I really can't provide great details because its been a year + since I was stock but its well documented that stock marauder's generally get between 16-24 shots stock @ 40 fpe. When regulating or running at lower pressure, efficiency of the AIR itself goes down, because low pressure requires longer dwells and wasted air, but VERY large ports require very SHORT dwell due to the nature of their size, so that is how you balance that act, when reducing pressure significantly one should INCREASE port sizing significantly, in my case stock was .14" and I am currently .2275".



On my .213" ported valve my sht counts were as such:



104 shots 18.5 FPE (13 mags)

72 Shots 25 FPE (9 mags) 
48 Shots @ 41 FPE (6 mags)
32 Shots 53 FPE (4 mags)
16 Shots 60 FPE (2 mags)



I have yet to fully test the .2275" ports. I don't expect any major increases compared to the above over stock, mostly I can squeeze a few more fps using the same air thanks to a shorter dwell provided by the larger porting. If I I kept it at the same tunes as .213" ports I'd imagine efficiency boost is fairly marginal, to where I wouldn't get a full extra clip, hence why I'll likely use the gain to boost power over shot count.



-Matt
 
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Depends on the flow area around the poppet but in my case it was more for a tad extra volume, imo more immediately available air volume = better, because air does have to travel, and if you're using 4x the amount of air than your valve holds, that means your valve has to refill 3x during the shot, albeit very quickly it still occurs, opposed to maybe only twice.



Also accuracy remains unchanged entirely due to carefully deburring the barrel after porting it to the .212 x .238. I had to do a few other modifications to allow the pellet to seat further which were relatively simple. (I shaved the cocking lug down in the direction facing the pellet roughly .05" or so which allows the pellet to seat that much deeper.)
 
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Pellet pushed into with bolt @ seating depth and then pushed back out with rod to check for any odd markings...flawless...I noticed so many odd markings on countless pellets as I was trying to pull out a really good one to test which just goes to show pellets still shoot really well if with a scratch or two, none of which my barrel is producing, generally manufacturers produce tons of tiny markings on our pellets between production and distribution ;)




 
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I have done a ton of porting to my Mrod as well. I thought about doing the same thing to the poppet stem but my fear was that it would weaken it to much.
Click to expand...



You can safely do it, its all about the balance of forces against that thinned part...That is why I use a HSS drill bit blank, for extra strength, as well as many reduced forces over stock. What I have done is on the extreme end and you can still reap similar benefits from running stems thinned down from flush to poppet head to back of throat @ .06"-.08"...I think its silly NOT to, even at 3k psi I'd run a .08"...its very important to just use superior materials and know some of the math behind the forces in the air gun. 
 
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Also FWIW, my 'plenum extension' on my rifle which increases my plenum volume from 21~ cc's to 54~ has an outlet of .238" which out flows the porting ahead of it, as well as the entry into the valve is roughly .255".

A VERY important factor when fine tuning your porting is you want the intakes to have 5-10% more flow than the exhaust, so the tp port would ideally be 5% LESS than the seats flow, and the barrel port will be 5% LESS than the tp port. Those 5% transitions are for FLOW loss across obstructions (seat / poppet / valve stem through transfer plenum) and then for the sharp turn into your transfer port, and then again for the airs last turn into the barrel. You absolutely DON'T want the opposite effect where porting increases downstream, as that would incur more loses due to wasted volume. You CAN get away with 100% equal porting but again, you'd encounter loses due to flow impediment and turns..


Just a few tips if anyone decides to endure a similar project on their own, I can't give the absolute best %'s to go by, but I know at least 5% and upwards of 10% is a good range to stick within, IMO. As you see in my current setup I go from (.237 - .04) = .233" which is roughly 7% more than the porting forward of it, which is .2275", which is roughly 3-4% more than the porting ahead of it, which is .225".
 
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Comparison of my old port tune to my new port tune. .215" Port vs .225" Port. As you see the primary changes are less dwell and a few more FPS gained over the same air usage. Efficiency went up a bit... The only other change was a SLIGHT reduction in regulation from 2030~ psi to roughly 2015 psi when changing valve/porting, so the average pressure at valve seat drops on the larger port slightly as well. Also the valve closes sooner due in this particular scenario MAINLY due to the same amount of air being released, BUT the volume from seat to tp is increased, therefore the pellet goes further in the smaller volume valve because air starts entering the barrel itself sooner than the larger ported one..its downside is increased wasted volume, intentionally increasing this volume without providing equal flow to said volume isn't a good idea. 



As you'll see from what I've stated earlier, bigger ports allow less dwell, while ejecting the same air mass, providing more FPS/FPE.



Also note the lift is near identical but the port equivalence is higher on the larger port due to the obvious reason, a bigger throat! 










 
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Interesting notes from this evening. I had to pull the 1.3" 6# hill valve spring that I ran as my hammer spring with my 18.5 tune cause its tolerance was too low with my .85" throw...compressed length is around .3 so .85+.3 = 1.15 leaving me .15" to play with... Ordering new springs for very low 18/25 tunes. They happen to be rated 5#'s and have a max load of 7 which is essentially leaving off where my minimum load on my 7.7 is (9.5*.85=8 lb) (note I have to cut down a 7.7 lb spring to achieve negative preload which makes it a 9.5~ lb)...The slightest turn on the preload with the 1.3" length spring either way could tank fps from 700 to 300...it was bonkers...but thats what happens with little tolerance.

So I tossed in my LOW 25 FPE tuned SSG which was a cut down 7.7 spring @ 9.5 lb/in with negative preload(8 lb cocked), and the sucker shot out at 881 FPS opposed to the 751 it was tuned on the other valve using 19.91 gr O.O. I basically had to use this spring with a tad more preload to tune for my new 25.4 gr / 41-42 FPE tune...verified no wonky fps fluctuations on this spring and called it good..

Hows that possible with a larger throat + larger poppet OD with more force holding the valve shut? Well simple.

-Removing .1 from the back of the valve increases throw which increases hammer energy (ke)

-Increasing hammer weight 10%, which increased momentum 

-Higher quality material (Peek poppet + copper seat)

-Decreased set point 15-25 psi, 1% is 1%..

The odd thing is all my math predicted this outcome yet I didn't wanna believe the math would be verified empirically with this configuration...I thought to myself no way! Well, yes way...
 
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Click to expand...

Click to expand...

Click to expand...

I shoot various pellet weights at different tunes, I have 5 pre built ssg's (spring stop guides) that retain the tunes that I just swap out and don't have to re-chronograph for every time I swap tunes. They are they are (18.5, 25, 42, 56, 60+) fpe tunes. My 18.5 + 25 fpe tunes use 19.91 gr h&n, my 42~ fpe tune uses 25.4. The 56 + 60 fpe tune use 33.95 mk ll

Swapping tunes is just 3 simple screws that take 30 seconds to remove and reinstall. I can't really detune on the fly but not many can while retaining pin point accuracy which this method does for me, I tune all the ssg's once and they're good.

pic of my ssg's:







If you're asking for a comparison from stock to what I am now, I really can't provide great details because its been a year + since I was stock but its well documented that stock marauder's generally get between 16-24 shots stock @ 40 fpe. When regulating or running at lower pressure, efficiency of the AIR itself goes down, because low pressure requires longer dwells and wasted air, but VERY large ports require very SHORT dwell due to the nature of their size, so that is how you balance that act, when reducing pressure significantly one should INCREASE port sizing significantly, in my case stock was .14" and I am currently .2275".



On my .213" ported valve my sht counts were as such:



104 shots 18.5 FPE (13 mags)

72 Shots 25 FPE (9 mags) 
48 Shots @ 41 FPE (6 mags)
32 Shots 53 FPE (4 mags)
16 Shots 60 FPE (2 mags)



I have yet to fully test the .2275" ports. I don't expect any major increases compared to the above over stock, mostly I can squeeze a few more fps using the same air thanks to a shorter dwell provided by the larger porting. If I I kept it at the same tunes as .213" ports I'd imagine efficiency boost is fairly marginal, to where I wouldn't get a full extra clip, hence why I'll likely use the gain to boost power over shot count.



-Matt
Click to expand...




Impressive results Matt, is a bigger port between plenum and valve chamber always a good thing? Just a generic question, are there any downsides?
Click to expand...

Hello,

can you post a pic of how your hammer mounts to these ssg's?

i thought the hammer rides on the bolt itself.

thanks,
 
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Here is a video demonstrating outside of the rifle how it works.















https://youtu.be/Y81ikD3gRK0

















Although that was with a 11 lb spring and stock hammer and you see how easily I cock it with index + thumb? Well now I run a 5lb and a 7.7 lb spring depending on my tune and a 47.5 gram hammer...which is nearly a 40% reduction in force required to pull the spring back into the cocked position as well as 40%~ less hammer weight.



Many versions of the SSG have the hammer ride on the bolt, but there are LOTS of down sides to this version. You have 2 collisions, one with the bolt head and the other with the valve stem, both reducing hammer KE, which requires more hammer weight or spring to counter act. The other pitfall of this design is it adds a ton of unnecessary weight to the hammer being the spring + bolt/guide ect all gets added to the hammer weight. My version shown above has the spring/guide DETACHED from the hammer which reduces the hammer weight and removes most the losses from the initial collision with the guides head, thus requiring slightly less spring force or hammer weight. 



The above style SSG requires modification to work within most rifles and a shorter spring due to lack of space. The hammer pocket has to be deep enough to allow full cocking length, in my case my hammer pocket depth is .955~ and my cocking distance max is .878~ (with .3 gap), the head of my ssg that fits into the hammer is .072" which leaves .878+.072 = .95...very tight fitments...on a stock setup however I believe you would only need around .8" pocket depth, where as I have increased my hammer travel greatly over stock.



Cost to build above ssg is very cheap, 2" bolt, nylon bushings and washer + some modifications to rifle to allow adequate space for fitment. You can only run modified 2" springs or 1.75" springs on this setup but as you see it works just fine and cocks very easily even with short springs. 
 
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Correct, SSG rests on hammer rear face, and when cocking the rifle the hammer pushes back on a washer that compresses the spring until hammer reaches sear. The problem running this is the required hammer pocket depth and the space for it all to move back, very simple modifications required but worth it if you sit down and calculate them all and fabricate a few of these.



Here is an updated graph of my cocking forces that I just got done running tests on. Took a bit to compile the data...







An equally ported valve with a stock .125" stem and .265" throat would require roughly 20% more spring energy to open the valve roughly the same.



With this current setup, I can run low power equal to a strong .177 @ 18.5 fpe (can detune down to 3 fpe..), or run it equal to a moderately powered .22 cal @ 27 fpe (may re-tune to 28~), all the way to a commonly powered .25 cal @ 42-56 fpe, as well as covering very low powered 30 cals @ 63 fpe (need 43gr to hit this). Who needs 4 rifles/calibers when one properly set up and tuned does it all? The ES even on my very low FPE tune is nominal @ 4% or better, for being such a low power tune on a highly ported valve that is quite impressive, where as the ES on my 42+ fpe tunes will be 2% or less.



Lots of effort went into reducing the cocking effort to make this valve operate nominally, from increased travel, increased young modulus of materials, increased hammer weight, reduced valve stem diameter within the throat allowing a smaller seat / poppet od...so please don't attempt similar without having lots of inner working knowledge of a pcp expecting the same or better results as I have.



The 5# spring I ordered ended working for me for up to 29 Fpe~.



Also note 2 interesting facts: A lighter valve stem when collided upon (hammer opening valve) will move FASTER (albeit small amount) due to less mass...just like shooting a lighter pellet...as WELL as less hammer KE goes into opening a valve stem with less mass, due to the law of conservation of momentum...the reduction in hammer energy lost while opening the valve is significant enough to consider beneficial. FWIW



-Matt
 
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In case others attempt similar ssg's here is a pic of my gap + preload adjustment and how I accomplish. The slot is an index for gap, as well as access to the nut slots that will keep the nut locked so you can add/remove preload when initially tuning this style SSG, I built 4 of these and each tune remains statically the same on its SSG. I use a small allen/paper clip inserted into the slots, and a flat head to adjust preload initially, once tuned I generally don't have to revisit the tunes for quite some time.







The gap adjuster has a small .1 - .15~" pocket, which is always greater than the typical SSG gap ran (.03-.05") so when firing the rifle the bolt/guide always stays inside the pocket not falling out, as well as the head of my SSG guide/bolt is roughly .1"~ to avoid the same. In some cases pocket size may need to be increased if you run larger gaps or have a setup that creates tons of lift.






 
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Here is a slow motion (1/8th speed) video that can be further slowed on youtube showing the lack of hammer bounce in my current setup. As you see the gap adjuster in the rear moves forward once and thats it, if hammer bounce were prevalent the hammer would force this adjuster back and it would launch the hammer forward and the process would repeat until residual hammer energy was lost.

One reason I exhibit less hammer bounce is because the huge reduction in force required to open the valve, with less total sum of energy there is less energy to diminish after opening the valve. If I needed a 10+ lb spring the the bounce would likely be more violent than what is seen in this video...



The gap in video is approximately .04" between hammer and valve stem when left uncocked.



https://youtu.be/Egsu9dIW0Jo
 
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