M4A1 Low FPS After Upgrades

[Bar1975]

Hey guys.. i am pretty sure i know what the issue is here but not 100% sure on the best approach for the solution.

A little back ground information first:

I just recently overhauled my M4A1 CQB AEG and stuck in a pile of new upgrades:

1. - 6.03 Prometheus tight bore barrel (407mm)
2. - Guarder full compression tune up Kit (Polycarb Piston, Piston Head w/Bearings, Ported Cylinder, Cylinder head, air nozzle with O-ring)
3. - G&P M120 Motor
4. - Madbull sp110
5. - Bearing Spring Guide
6. - 8mm Bearings
7. - New wiring w/Deans (16 gauge low resistance wire)
8. - 11.1V Lipo

Out of the box this gun shot around 395FPS (.20g) but after all the upgrades i am shooting 307FPS (.20g)

I am trying to set up a decent ROF (nothing crazy but it's shooting 28RPS now which is great for my needs)

Now.. I've added a Sorbo pad with a Neo Pad as well and corrected the AOE (removed two piston teeth). I've glued all of this directly on top of the new cylinder head rubber (didn't remove the original). I have read that with the decrease in cylinder volume (due to the extra space from the sorbo pads) that you should expect a drop in FPS...especially with a ported cylinder (1 port in cylinder)

A quick solution would be to upgrade the spring but I'm hesitant going up to a sp120 spring because I'd like to keep it lower to put less stress on the gearbox at this ROF.

I've read that swiss cheesing the Piston can add up to 20FPS so I'm leaning towards this right now.

I've also read that switching to a full cylinder (no ports TYPE 0) will be a good fix for the Sorbo upgrade but with this i'm concerned with the length of my inner barrel (407mm) causing issues. (recommended cylinder for my barrel length is a ported cylinder)

So what do you guys think?

Options i can think of are:

- New Spring (sp120)
- New Cylinder (Type 0)
- Swiss Cheese the Piston

I want to get this gun back up to around 390FPS (my field limit is 400fps with a .20g) with a .20g BB and preferably around 350FPS with the .25g BB that i normally use in the woodland games i play.

[L473ncy]

Test your compression.

Isolate the cylinder/piston area first and take out the whole compression unit including air nozzle. If that's sealing well and got perfect compression it might be the way the nozzle mates with your hop up assy.

Don't just toss more things into your gun when fundamentally it's not working. Thats like tossing in an M150 spring into a gun that has bad compression to make it 330 FPS for CQB. There's serious air leak issues there and tossing in a harder spring won't really help at the fundamental level.

[suzenonest]

I would have used a non ported cylinder for your 407mm barrel, but not sure what effect that'll have on a high rof setup. Try taping the port shut so that there's full cylinder volume and chrony it, maybe that'll answer some questions

[Stealth]

Originally Posted by Bar1975

Now.. I've added a Sorbo pad with a Neo Pad as well and corrected the AOE (removed two piston teeth). I've glued all of this directly on top of the new cylinder head rubber (didn't remove the original).

Your original pad should be removed. There's such a thing as too MUCH padding - 1/4" additional is far too much. Do you know how far back your piston is now that your stroke begins that much farther back?

Originally Posted by Bar1975

I have read that with the decrease in cylinder volume (due to the extra space from the sorbo pads) that you should expect a drop in FPS...especially with a ported cylinder (1 port in cylinder)

For a 407mm barrel and spring combo, you should be using a standard M4-hole cylinder.

Originally Posted by Bar1975

A quick solution would be to upgrade the spring but I'm hesitant going up to a sp120 spring because I'd like to keep it lower to put less stress on the gearbox at this ROF.

400+FPS @ 28rps is not that extreme of a setup.

I've read that swiss cheesing the Piston can add up to 20FPS so I'm leaning towards this right now.

Swiss cheesing will reduce mass of the piston, leading to increased ROF, not FPS.

Originally Posted by Bar1975

I've also read that switching to a full cylinder (no ports TYPE 0) will be a good fix for the Sorbo upgrade but with this i'm concerned with the length of my inner barrel (407mm) causing issues. (recommended cylinder for my barrel length is a ported cylinder)

See previous comment re: sorbo pad.


Ultimately though, the source of your FPS loss lies in either your piston head o-ring, cylinder head o-ring, and/or the interface between your new nozzle and hop-up rubber. All of those places need to be checked and verified for good seal.

You mentioned that you are reading a lot of things, where are you reading these things?

[coach]

Originally Posted by Bar1975

Now.. I've added a Sorbo pad with a Neo Pad as well and corrected the AOE (removed two piston teeth). I've glued all of this directly on top of the new cylinder head rubber (didn't remove the original). I have read that with the decrease in cylinder volume (due to the extra space from the sorbo pads) that you should expect a drop in FPS...especially with a ported cylinder (1 port in cylinder)

Wait, you needed to remove 2 teeth? You should only need to remove one!

Take off the original rubber pads and glue the Sorbo pad directly to the piston head and cylinder head.

[Bar1975]

I am pretty sure it's not a compression issue.. everything is brand new and when i put my finger over the air nozzle i can't move the piston at all. It's pretty air tight.

I think i over did it with the Sorbo Pad + Neo Pad all on top of the original cylinder head rubber.

I will try removing the original rubber on the cylinder head and attach the sorbo pad directly to the cylinder head.

i just picked up a new xcortech 3200 chrony as well so this should be easy to isolate and resolve.

I had to remove two piston teeth because the sector gear was hitting the second tooth once the sorbo pads were installed. If i reduce the space taken up by the sorbo pads though have i now ruined my piston?

[SuperCriollo]

It could be your hop up rubber, you should check it out.

[MaciekA]

A few things:

Firstly, a loss of 90fps cannot be explained only by adding a sorbo pad. You're losing volume to the sorbothane layer, but not THAT much volume. You've added a bunch of stuff at once and there are other issues going on at the same time. That said, for a barrel of the length you're using, absolutely go with a type 0. Make sure that when you swap the cylinder to a type 0 you ONLY change that one variable and then re-test. You'll get a much better idea of the effect of each component in your system.

Secondly, swiss cheesing a complete waste of time unless you're planning to go well north of 40rps and don't plan on using a very strong spring. Don't bother swiss cheesing. If you care about pistons then pick up the SHS blue piston with the full metal rack. You'll want it if you're planning on comfortable with running a 3 cell LiPo every week.

Thirdly, pay close attention to the interaction of your air seal nozzle with the bucking in your hopup chamber. When you mate your hopup to the gearbox in the tappet plate's most relaxed state (i.e. the state it is in after you reassemble your gearbox and it is the most forward it can possibly go), that's the position it's in when air is fired out of the cylinder and into the chamber. In this state it is important that the nozzle and bucking lips "kiss". You can test this by having your entire barrel/hopup assembly out of the gun and pressed hard against the gearbox. By varying the pressure that you're pushing with, you can detect whether the nozzle contacts the rubber of the bucking. If not, you may have used a nozzle that's either too short or has the wrong taper shape / cross section. This can make a huge difference in compression, far bigger than playing with the o-ring seal inside of the main cylinder. If you're not sure whether the two are contacting firmly enough, do everything and anything you can to ensure the hopup and barrel are slammed up against the gearbox when you put your gun back together. This stage can get fairly tricky and if you get stuck and can't figure out how to restore your compression, you may need to resort to tricks like sealing your bucking with Teflon tape, switching back to your original nozzle, etc.

Fourthly, sell your G&P M120 motor and get a high torque motor with neodymium magnets. I hate to be the barer of this news, but you have bought what is universally considered to be G&P's crappiest motor and is easily one of the weakest motors on the market. It may sound like it goes against the grain of the marketing name ("G&P M120"), but if you plan to be driving that thing in the M120/400fps range, this motor is going to be pure suck. The SHS high torque, G&P M160, G&P M170, G&P M180, Guarder Infinite Torque Up, Echo1 High Torque, JG Blue, Lonex High Torque, etc are all fantastic alternatives and in some cases cheaper than your M120. Dump this motor now. Do it.

Fifth, get a MOSFET. It's a must for 11.1V batteries. Make sure whatever MOSFET you get is configured in the manner where you have a minimum amount of current going through the trigger contacts. (I only mention this because Stealth is selling a MOSFET these days that can also be configured in simpler ways that don't protect your contacts as much, if you get one of those you'll want to use it in the proper mode). I generally use the Extreme-Fire MOSFETs.

Anyway, I suspect that in the heat of excitement, you have changed too many things at once and have obscured your view of the real culprit here, but that's part of the fun. Please make sure to take that to heart when debugging your system.. Upgrading so many parts at once can often result in multiple issues which interact in weird ways and can confuse you for many many sessions in your workshop... To dig yourself out, change one thing at a time and you should land on your feet.

[MaciekA]

By the way, I should add that you should resist all urges to upgrade your spring. Your torque-less motor is really not ready for it and I think we can all agree you're going to be able to solve this and get to your target velocity (and probably higher) without a stronger spring.

[XZIVR]

Actually, I believe swiss cheesing the piston will increase fps slightly, as well as ROF. The reason being that since the piston has less mass, the spring is able to accelerate it forward faster, which pushes the air our of the cylinder faster. On my 40 BPS build I had to switch from an untouched Modify Quantum piston to a swiss cheesed SHS piston because the sector gear kept engaging the Modify piston teeth before the thing was even CLOSE to the end of its stroke. It was hitting like the 3rd or 4th tooth (so not an AOE issue if that's what you're thinking)

[Bar1975]

Well this is definitely a lot of great advise guys.. thanks.

Few things..

1. I'm surprised about G&P M120 motor.. I've read so many reviews on this thing and it was always one of the better motors out there. I only found out about it because i was searching for good quality motors to support a higher rate of fire. I can tell you that I'll never run anything higher then my sp110 and want to keep my FPS around 350. I feel like im running a pretty safe, low stress set up so I'll burn this motor out for now i guess but don't think i'll be replacing it until then. It's literally 2 weeks old! But you think a high torque motor would be best suited for my weaker spring and <30RPS set up? I always assumed the Torque motors were geared more towards higher FPS set ups with springs in the range of 120++

2. I do plan to upgrade to a MOSFET. Just have not researched much on those yet.. it's coming though. I wired my own trigger assembly so pretty comfortable with that whole thing

3. Also surprised about moving to a Type 0 Cylinder.. all research indicates that a 407mm inner barrel is best suited with a Type 1 cylinder.. (one port) Would i not suffer from the 'suction' issue people talk about when dealing with cylinder volumes to barrel length?

4. I fiddled around with my hop up after and the rubber must have been off a little as now im back up in the 370-375FPS (.20g BB) but still only sitting at a little over 300FPS using .25g. Does the difference in BB weight make that much of a difference to the FPS? My Goal is to stay below 400FPS on .20 but get my FPS up to 350 with the .25's that i use on the field.

god i love this stuff!!!

[lurkingknight]

just because parts are new doesn't mean they will seal correctly.

Not all nozzles are the same length. For example a lonex nozzle vs a modify nozzle are not the same length, even if they are both for m4s.

I have also experienced a loss of 30- 40 fps with a sorbo install type 1 cylinder and shs 120 spring and 407 barrel. My next step is to possibly try a type 0 among a couple other things. I am exactly at 370fps with .2s. It's essentially the same as your setup, but the shs spring is rated on the packaging for 380-410.

I can't say enough about the lonex a1. I put one in my p90 with a 13:1 gearset and now I really want to put the same in my g36... DAT TRIGGER RESPONSE. Compression issues first however.

BB weight drastically lowers fps the higher you go in weight, but the tradeoff is consistency in accuracy. Chrono with .2s unless you know what it converts to.. the math isn't always right best to visually confirm it through the chrono.

[L473ncy]

Originally Posted by Bar1975

I fiddled around with my hop up after and the rubber must have been off a little as now im back up in the 370-375FPS (.20g BB) but still only sitting at a little over 300FPS using .25g. Does the difference in BB weight make that much of a difference to the FPS? My Goal is to stay below 400FPS on .20 but get my FPS up to 350 with the .25's that i use on the field

That's about right. IIRC it's about a 40 FPS loss going from .20 to .25. For 400 FPS it's 358 FPS (that number I know for sure based on a handy card with the BB weights and energy calculations that I made) on .25's. So at 370 it should be about 330 FPS not 300.

[Bar1975]

Well i have an update.

I've removed the Sorbo and put a new one back on. I removed the stock cylinder head rubber so the new Sorbo is directly onto the cylinder now.

I'm in the ball park of 320-330FPS using .25g and upwards of 370-380FPS for .2g (still maintaining a 26-28RPS ROF)

i guess that's everything i can do for now. Would still like to have this gun back up to it's original FPS of 390ish (.2g) though

[Rabbit]

Question in regards to cylinders -

Say your inner barrel is 300mm - do you then use the type 2 (200mm to 300mm) or the type 1 (300mm - 450mm)?

The confusion is with the 300mm exact length assuming I would use Modify brand ratio/matching.