Large FPS drop when adding 1/8" sorbo

[crossfire876]

AEG:
King Arms Colt M4

Goal:
1/8 in sorbo

Bought 12" x 12" 1/8" sorbo 70d and used a hobby knife to cut out a circle. Then twisted the cylinder head through it to punch out a perfect sized hole for the center. Loctite and 30 seconds and it's secured to the cylinder head. Just glued it on top of the rubber "bumper" instead of removing it and flipping it around.

Pulled a G&G piston off the shelf (second tooth already removed) and filed down the third to correct AoE. Dropped a JG head on top of the piston. Meshes great, pretty excited at this point. Labored over the air seal for a good hour messing with this and that to get what I feel is rather good.

Put it all together, time to chrono.

Specifics:

Test1
-1/8" sorbo
-Type 3 Cylinder
-363mm 6.03mm Madbull V2 Black Python
-Modify M120 spring

~360 w/ 0.20
~320 w/ 0.25

Hmm, a little low. Maybe cylinder volume isn't enough with sorbo space.

Test2
-Same as test1, except
-Full Cylinder

~360 w/ 0.20
~320 w/ 0.25

Little disappointed at this point, so lets rule out the spring. Tore everything down, again, and dropped in a no sorbo'd cylinder head, same full cylinder, changed to KA CH and PH and piston, and put humpty dumpty back together again (twice, left out the tappet plate spring, didn't notice til chrono. So many cuss words...)

Test3
-Same as test2, except
-no sorbo

~400 w/ 0.20
~360 w/ 0.25

Okay, full cylinder, lets drop in...

Test4
-Same as test3, except
-455mm 6.03mm Madbull V2 Black Python

~400 w/ 0.20
~360 w/ 0.25

First time I've ever had the 455 not increase FPS at least slightly. Any ideas about what I'm seeing? Rather confused at this point.


Star_Folder mentioned I may have an air leak. Compression was rather good when I put it together, put it's always possible. Wondering if there is something mathematical that I'm missing or what. That's where you guys come in

[TokenTech89]

Sounds like an air leak around the air nozzle.

[crossfire876]

As long as there is a cylinder nozzle sized hole in the sorbo it wouldn't cause FPS issues right? Hole is exactly the diameter of the nozzle.

The air nozzle has an o-ring and gets good compression even when extended. Possible, but I don't think it's that since the same nozzle and spring got +40 fps when I removed the sorbo cylinder head. I did however change the CH PH and piston from the sorbo'd set. Not sure if that will turn out to be the issue.

It would be a relief if the problem is in the compression, that's easy enough to troubleshoot. I'm more looking to make sure I didn't mess up with the sorbo installation. If you guys can tell me that I can chase down compression issues from there.

If I get time this evening, I'll upload a picture or two.

[Star_folder]

Why did you switch out the piston and piston head?

That might be making your difference, both with compression and piston weight.

[crossfire876]

Why did you switch out the piston and piston head?

That might be making your difference, both with compression and piston weight.

Since this is my first go at sorbo I didn't want to file down the second and maybe third teeth on all my pistons. KA and JG piston both have full teeth and wanted to have something to test against or go back to if this didn't work.

G&G piston was easy since the second tooth is already removed.

Can virtually the same polycarb piston and head design/weight really throw off numbers this far? Assuming compression isn't the issue (may be, have to make am assumption or two)

[Dominum]

Two nights ago I put a G&G piston+piston head in a JG cylinder and it had utterly horrible compression with the stock Oring still on it. Fortunately, my tune up includes an aftermarket Oring that is slightly larger and thicker and less susceptible to corrosion, so once I replaced the stock Oring with that it was excellent. Just a case in point the answer to your questions is "Yes."

[crossfire876]

Two nights ago I put a G&G piston+piston head in a JG cylinder and it had utterly horrible compression with the stock Oring still on it. Fortunately, my tune up includes an aftermarket Oring that is slightly larger and thicker and less susceptible to corrosion, so once I replaced the stock Oring with that it was excellent. Just a case in point the answer to your questions is "Yes."

I had the same problem at first. Dropped in a Danco #14 O-ring on the piston head and it still didn't give me the compression I wanted. So I took the G&G piston head off and dropped the JG head on the G&G piston. With the Danco O-ring, instant compression magic.

Likely going to overhaul the entire build. Drop in an M130, flat hop, maybe say screw the sorbo and just fork out $35 for the STS system.

Looks like we're settled with the seal being the issue. Just to be clear, 1/8" sorbo hasn't dropped any of your FPS builds by the 40fps margin I'm seeing?

[SteevoLS]

With a 130 why are you worried about sorbo or STSs in the first place?

[crossfire876]

With a 130 why are you worried about sorbo or STSs in the first place?

I'm looking to get the supporting modifications before upgrading power, as I do plan on going up further. I'm trying to find that balance of "softness and strength" and the only way I've found to reliably do that is to fiield the AEG at that power level and decide for myself. So I'll continue to jump up spring strengths until I find a happy medium.

I had heard of the benefits of sorbo on high power setups, but didn't want to "cut my teeth" there since it may decide to chew up it's internals if I didn't do something correctly. The goal was to run it a while to check for excess wear etc. An M120 I didn't think could cause much damage in the event of a mistake.

I came across the mention of the STS system on a post you made likely a year or two ago, thought it was a cool idea and want to put it to the test. Eventually around the M150 level.

[SteevoLS]

I still have that STS in my SPR. It is not the perfect solution for every gun, but for those of us with one piece hopip chambers it is a perfect insurance policy for your gearbox shell.

[Dominum]

If you don't use the sorbo to correct AoE, you'll have to use spacers between the piston and piston head. No, I've never seen sorbo have anything to do with compression, unless your cylinder volume is barely enough for the barrel as is, and the sorbo removes just enough to cause barrel suck. It's more likely user error frankly. Got any pics of your sorbo job?

[crossfire876]

As promised.