Air Machinegun Forum
General Air Machinegun Discussion Category => Drozd and Blackbird Discussion => Topic started by: East Milford on May 09, 2017, 04:07:49 PM
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It took a while to figure out a way to get velocity readings under full-auto fire but, i did, and the data was not surprising, as I suspected. It seemed clear to me that in single shot the FPS is significantly faster than what comes out under full-auto strings at 4k-6k. So after confirming this, a drop off from 1100 fps to around 700-800 fps during long automatic strings, I started to investigate.
The major restriction lies in the air source inlet. The steel adapter that feeds the intake manifold tube.
The air port that transfers the air from your hpa source into the air tube is literally a pin hole!
Now this isn't necessarily a problem for single shot or sliwer cyclic rates but once the RPM of fire starts quadrupling the factory deaign, the Drozd suffers considerable starvation of air during Full-auto.
If you do this mod the gains will be a night and day difference at full auto. You will pick up several hundred FPS at 4k-6k speeds. You will even gain a couple hundred FPS at 2k rpm cyclic rates.
The Mod is self explanatory and essentially free. What needs to be done is a Porting of the air intake adapter. Think of it like porting a throttle body or intake manifold on a car. The same reasoning applies
To increase the volumetric efficiency you will want to allign the intake tube inlet hole with the pin size air adapter outlet hole. here are some pictures of how i did this.
I used a 5mm drill bit to enlarge the outlet hole and create a straight through port into the manifold tube.
Next i Tig welded the two pieces together to permanently seal the connection.
Its basically a free mod with huge gains for those running 2k-6k boards.
Hope this may help someone
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Pics
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More pics of the complete assembly. Barrel to mag face mod, BB feed tube mod,
Ported and sealed air adapter to air feed tube.
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I did all that, too. But you know what I thought was the biggest issue is that the threads on these are machine cut, so no two are the same. When you screw the head on tight, the hole in the stem can end up anywhere in relation, not in line with the hole in the breech block. Their solution was to channel out a groove, all the way around the hole, between the seals, so that the air can get around to where ever the hole happens to be. I drilled a new hole in the right place that does line up with the hole in the block. And I drilled out that port in the block, too. I can see straight down the inlet port into the inside of my stem, now.