Ok I've read up, and re-read. I have a few clarifications:

1.) So with the stiffer spring, I will be able to mount more pressure on the puck immediately, because filling of the lower pre-infusion chamber (cavity) will be delayed. By delaying opening access to this cavity, we increase puck pressure sooner. But with the softer stock spring, the puck gets relief at about 1.5-2 bars for a few seconds as the pre-infusion chamber/cavity fills (because this gives water room, thus delaying the build up of pressure by a few seconds) and then, after it fills, we get a more rapid increase from 1.5/2 bars up to whatever pressure is dictated by the flow (so, at stock flow, around ~8/9 bar depending on our settings).

In contrast, with a stiffer spring, the puck will rise to ~4 bar pressure much sooner (since the stiffer spring is preventing pressure from escaping into that lower pre-infusion chamber/cavity for longer due to its higher resistance), but then at 4-bar, the stiffer spring gives way and allows some water to expand into the lower pre-infusion chamber/cavity. This expanse of water will cause a very slight lag at up to 4-bar pressure on the puck as water fills that little chamber, before pressure begins to rise. However, this lag will be more minimal than it is at the lower 1.5 bar pressure, since 4-bar pressure will naturally fill up that lower pre-infusion chamber more rapidly (perhaps twice as fast), so that this "blip" is hardly noticeable.

Now, with the softer stock spring this means our we will be capped at 1.5/2 bar pressure for a few seconds at the start of the brew, because a softer stock spring essentially means the machine has to fill an overall larger cavity before it can produce more than this 1.5/2 bar of pressure (dictated by said spring). But with a stiffer spring, we are capped in this initial phase at up to 4 bar pressure, because the stiffer spring effectively temporarily reduces the size of the water cavity by adding more resistance until access to the cavity is allowed, thus meaning we can get to 4 bar pressure on the puck much quicker (well, a few seconds quicker). But this does not mean we are necessarily doing a more aggressive initial puck infusion, because we now have the ability to control, via the FCD itself, how quickly we reach that 4 bar temporary pressure cap. We do this by reducing flow via the FCD, thus, we could cause the puck to very slowly, or very quickly, reach 4 bar pressure as we desire. But, if we ever just wanted to run the machine normal (that is, by setting the FCD dial to the "stock flow" position as we measured), we cannot, because automatic pre-infusion is cancelled out via the stiffer spring.

Correct?

2.) I had confused automatic preinfusion with pre-wetting (which is engaging the brew lever part way to allow line pressure to flow to the puck). To clarify, it sounds like this pre-wetting/partial brew lever engagement is NOT impacted at all, whatsoever, by either the softer or stiffer spring? This functionality remains regardless.

Correct?

3.) In actual practice, when trying to replicate a profile, if we keep the stock spring in, this simply means that we are forced to do a few seconds of gentle 1.5/2 bar pre-infusion each and every time, dictated by the softer spring and subsequent larger early cavity to fill before higher pressures can mount. This may add a few seconds to the start of any profile. Once sufficient pressure builds to fill the cavity (that this softer spring exposes sooner), then the sky is the limit, and we can dictate our pressure ramp up via the FCD - to 3 bar, 4 bar, 6 bar, 8 bar, and beyond.

Compare this to the softer stock spring, which gives us a larger cavity to fill right from the start, means with the stiffer spring we can either (a) achieve 4 bar much quicker, or (b) choke the flow to replicate the softer stock spring and hold things at a lower pressure for a few seconds more if we wish, simply by reducing flow.

Thus, control is greater with the stiffer spring. But it might be harder on the cam lever, and it precludes the ability to merely leave the FCD at the normal stock flow position and let the machine function as it always has with its default automatic pre-infusion.

That's it, in a nutshell.

Correct?

I read somewhere that the infusion chamber spring was set around 2bar for the original E61, but I believe the stock springs for modern E61's are set to open at higher pressure, but not as high as the stiffer FCD replacement spring. With those corrections to the spring opening pressures in mind, I think your take on what's happening during extraction is accurate, as are your other comments.

Good to know, if that's all accurate, I think I have a better take on it then. Thanks for the help!

So I ran into a strange issue. After calibrating my FCD as listed in a previous post, I let the machine sit until smart plug powered it off. If it matters, I kept the FCD in the "1 full turn" position so it was equal to stock flow rate position while idle. Then it powered it back on in the AM and it sat until around 4pm (I made drip coffee in the AM, and came to pull my first flow control shot in the afternoon).

But the flow seemed off, and sure enough, it was. I re-measured and the "1 full turn" position which was giving me stock flow yesterday (11g/sec), for some reason, was now giving me a much more restricted flow @ 5g/sec - about half! I had to loosen the knob further and then flow really picked up like a torrent, then I turned it back down, and now it seems to have all reset. 1 turn once again operates at stock equivalent of 11g/sec.

Why, how? Don't understand how this could happen.

Pressino wrote:I read somewhere that the infusion chamber spring was set around 2bar for the original E61, but I believe the stock springs for modern E61's are set to open at higher pressure, but not as high as the stiffer FCD replacement spring. With those corrections to the spring opening pressures in mind, I think your take on what's happening during extraction is accurate, as are your other comments.

That's my understanding too. I read that they are now about 4 bar and this is more or less what is see on my brew pressure gauge if I set the FC to low flow (1/4 to 1/2)...slow rise to about 4-bar then more rapid after that.

Wait, so the stock spring is 4 bar these days? Then what's the stiff spring?

I don't know. But, my understanding is that with the stiff spring installed the valve does not open at all. So, it must be 9-bar or more.

I found my FC spring and installed it. There's definitely no e61 automatic preinfusion at all with it installed on my Synk. No matter where the FC is adjusted, the pressure gauge goes up in a more or less linear manner. With the stock spring, it goes up very slowly (with a small FC setting such as 1/4) and when it gets to about 4-bar it goes up more rapidly. Incidentally my stock spring weighs 6 grams while my FC kit spring weighs 11 grams. It's way stiffer. According to the destruction sheet it totally inhibits the e61's inbuilt preinfusion. Also, when dropping the lever from mid to off, there's way more resistance, lending credence to the argument that it will increase wear on the cam and cam follower.

I tried to measure how much force it took to depress each spring 1", but the damned thing flew up and hit me in the eye . I'm back to the stock spring and liking it.

Thanks for checking into this, Nunas, and for measuring timing on your blank disk the other day! Super helpful to know. And I am sorry about your eye!! Glad you're ok.

So in practice then, this is essentially just means that there with the stock spring, there is always a soft pre-infusion (up to 4 bar) for the first few seconds, and thus an initial lag before the FCD responds directly to user input. VS Stiff spring, you can either blast the buck right away, or control in a very direct and linear matter the first few seconds of pre-infusion.

Has it also been your experience with the soft spring and FCD that you can hold the puck around 2-3 bar (below 4 bar) by sufficiently reducing flow?

dsc106 wrote:Thanks for checking into this, Nunas, and for measuring timing on your blank disk the other day! Super helpful to know. And I am sorry about your eye!! Glad you're ok. So in practice then, this is essentially just means that there with the stock spring, there is always a soft pre-infusion (up to 4 bar) for the first few seconds, and thus an initial lag before the FCD responds directly to user input. VS Stiff spring, you can either blast the buck right away, or control in a very direct and linear matter the first few seconds of pre-infusion.

That's about it, except that the e61 preinfusion can be anything from about a minute to a few seconds, depending where the FC is set.

Has it also been your experience with the soft spring and FCD that you can hold the puck around 2-3 bar (below 4 bar) by sufficiently reducing flow?

Yes, I can. I typically set the FC to about 1/2 to 3/4 and leave it there. The pressure mounts very slowly, then when it passes about 4 bars it mounts a bit more quickly. At any time, I can cut the pressure back or increase it with the FC control to whatever pressure I want. Usually, though, I look for a setting that starts dripping coffee just about the time the pressure hits 4 bar. If I get it just right, it rises to pump pressure, then gradually the pressure drops a bar or so as the solids and oils leach out of the puck. That kind of duplicates a spring lever pull, if you know what I mean. If I set it a bit higher, then the pressure stays at pump pressure.

So, I'm having an issue - and this is the second time now in a week since install. Anyone else, or any idea why?

Tonight after the machine sat and heated all day, with the flow control set to stock flow position about one turn, the flow control wouldn't close all the way, only to about 1/2 turn position and full stop. The lever also feels stiff and difficult to move like it's dragging or overly tight.

Turned off machine and once it cooled down, the lever seems looser again. All I can think is the heat is causing it to expand and become difficult to rotate and stop it from closing fully.

Any ideas what I need to do to fix, or if i installed it too tightly or something?