better air seal = lower FPS ???

[L1ct0r]

So a while back I acquired a second hand Umarex sportline UMP45. A few external bumps and scratches, but mechanically felt solid and looked in good shape. I stripped it down, cleaned it, shimmed and regreased it and had it running at around 335fps with the hop set for .2s. This was without changing any internals and the previous owner assured me he had owned it from new and never been inside it. I have no reason to disbelieve him and I had to break the factory warranty seal to get in, so it all seems above board. While stripping and cleaning I found that despite hitting 335 on the chrono, the stock nozzle/cylinder/piston had no air-seal to speak of.

I had to leave it to go on holiday, but when I came back I swapped out the old piston for a nice new ZCI piston and head. even on the stock cylinder/nozzel this instantly created a good air seal when testing it outside of the gearbox. Everything went back together without a hitch and sounded like it was running smoothly, but all of a sudden the chrono is giving me 325 with the hop set for .2s. I put the old piston back in, and it's back up to 335.

One theory is that the good air seal on the ZCI piston is slowing the piston travel down so much that a badly sealed piston traveling faster is actually able to produce higher FPS. My other thought was that the stock set-up is an unported cylinder with a short barrel (around the 200mm mark). Is it possible that that should be over volume for the barrel length, but the terrible air seal is essentially doing the same job as a port. If that was the case, fixing the air seal would be removing the 'port'. Can over-voluming reduce FPS?

I'm pretty new to teching, so if anyone with more experience has any ideas then I'd love to hear them. I should add the stock piston head has 4 vents while the ZCI has 8 and a noticeably chunkier o-ring. Putting the ZCI o ring on the old piston head also provides a noticeably better air seal, but I am getting the best fps figures with the old piston and old o-ring. It also seems a tiny bit more accurate with the old piston set-up, but that could just be me imagining things.

What does the hive-mind think?

 

[Lozart]

I'd say that if your cylinder is unported then you're almost certainly WAY over volume if the new piston seals that much better. As you surmise, the shitty piston was doing such a bad job that you effectively have a ported cylinder. If your way over volume the BB will be leaving the barrel before the piston has completed its travel which is also a sure fire way to wreck your gearbox.

Swap the cylinder out for a ported one.

 

[EDcase]

Are you aware that the piston should not be tight in the cylinder?

The design relies on the air pressure  entering the ports and expanding the loose rubber seal to make it airtight without  much friction.

 

[Sneaky]

Try all the above. Also remove the rubber o ring from the standard piston head and replace the ZCI o ring. See what happens there. 
 

There should not be much friction, any spring is designed to take into account of any minor friction.

 

[L1ct0r]

Are you aware that the piston should not be tight in the cylinder?

The design relies on the air pressure  entering the ports and expanding the loose rubber seal to make it airtight without  much friction.

Yeah. The ZCI piston isn't tight in the cylinder normally. I'm talking about when I block the nozzle with my finger and then push the piston in as it would when under load from the spring.

I'd say that if your cylinder is unported then you're almost certainly WAY over volume if the new piston seals that much better. As you surmise, the shitty piston was doing such a bad job that you effectively have a ported cylinder. If your way over volume the BB will be leaving the barrel before the piston has completed its travel which is also a sure fire way to wreck your gearbox.

Swap the cylinder out for a ported one.

According to the previous owner, he skirmished the gun for about six years without ever servicing or upgrading it. The gearbox shell is steel and looks pretty bomb-proof. I've radiused the shell, but honestly I'm inclined to leave it as is. It's been doing regular skirmishes for that long with no signs of wear, and it's making 335 fps on the internals that are in there. I think the old adage of 'if it ain't broke don't fix it' applies here. If something starts playing up I'll revise - or if a ported cylinder happens to fall into my lap I might experiment further to see what happens.

Cheers for the input though. It sounds like my initial diagnoses wasn't a million miles off.

 

[EDcase]

Yeah, modding can be hit & miss even with good parts they don't always play well together.

Its obviously bedded in after about 6 years ?

 

[Lozart]

According to the previous owner, he skirmished the gun for about six years without ever servicing or upgrading it. The gearbox shell is steel and looks pretty bomb-proof. I've radiused the shell, but honestly I'm inclined to leave it as is. It's been doing regular skirmishes for that long with no signs of wear, and it's making 335 fps on the internals that are in there. I think the old adage of 'if it ain't broke don't fix it' applies here. If something starts playing up I'll revise - or if a ported cylinder happens to fall into my lap I might experiment further to see what happens.

Cheers for the input though. It sounds like my initial diagnoses wasn't a million miles off.

You're right, "as was" it'll be fine, over volumed it won't.