I know some of you are going to cringe when you read the title of this post, so I want to preface it with this warning and disclaimer:

The following post describes experiments in which I used the bypass valve on the rotary pump of my La Marzocco GS/3 to vary pressure during a shot. The bypass valve and pump were not designed for this sort of operation, and I have no idea what the consequences may be, including the possibility of damage to the pump and/or personal injury. This post is not a recommendation or suggestion that anyone else should try to vary pressure during a shot by using the bypass valve on their espresso machine pump. Until more is known about it, do so at your own risk

While reading What is the Optimum Brew Pressure for Espresso, I wished that I could taste the difference between shots brewed at constant pressure and shots brewed with declining pressure to keep the flow rate constant, as suggested by Jim Schulman.

Then it dawned on me that in my setup, which is somewhat unique, there's a simple way to do this: adjust the bypass valve on the rotary pump.

This is possible because several years ago I outboarded the motor and pump to eliminate the noise and vibration that plagued earlier models of the GS/3 (LM has since redesigned the placement and mounting of the motor/pump to address this problem, but the new design can't be retrofitted into earlier models.) I thought I had posted photos of the mod, but evidently I didn't get around to it. A couple of pics are provided below.

There's nothing new about outboarding a pump and motor: The Speedster is designed that way, as are many commercial machines. What's different about my installation is that rather than locating the motor and pump in a cabinet under the machine, which is the normal location, constraints in my kitchen required that I mount them in an "appliance barn" right next to the GS/3:



This put the motor and pump at arm level, with the bypass valve facing the cabinet opening, which means I'm able to reach the bypass valve quite easily while pulling a shot:



The bypass screw is fairly small, about 10mm, and it takes several turns to span the pressure range of interest. So I fashioned a "knob" out of a 10mm socket and a finger ratchet:



The action isn't as smooth as I'd like, but it's probably easier to make small adjustments than it is with the MP modification that's kicking around. My understanding is the paddle's pressure adjustment range is very small with that mod. I have the opposite problem: I must turn the knob quickly and several times around to make it through the entire adjustment range while keeping the flow rate constant.

Note that with this setup I'm not able to read the pressure at the puck. But if Jim is right about the key being to maintain a constant flow rate, then the exact pressure reading isn't important. The relative pressure reading provided by the brew boiler pressure gauge is sufficient. Besides, I know that the brew boiler gauge reads 1-1.5 BAR higher than the actual pressure at the puck. Or at least it does at 9 BAR. I'll have to take more measurements to determine the offset at lower pressures.

So far, I've pulled two coffees with pressure profiling: Compass Roasters Yin-Yang, which is a blend of Ethiopian Konga and Indonesian Sulawesi roasted for either brew or espresso, and Terroir Ethiopian Yukro Co-op, which is roasted for espresso. Both are outstanding coffees (especially the Yin Yang, and I almost never drink blends.)

I haven't done blind tests, but the resulting shots certainly tasted good. On the other hand, so did the shots I pulled with the same dose and grind at constant pressure. I think at this early stage for me to say that there's a definite improvement would be, uh, confirmation bias . I can say, however, that the shots aren't bad. I enjoyed all of them.

The best shots were those I pulled with the pump starting out at 8.5 BAR. I waited until the flow started, then gradually backed off the pressure to keep the flow rate constant, usually ending around 5-6 BAR, sometimes as low as 4 BAR. The coffee had been dialed in for a pull of about 30 seconds with constant pressure, so the pressure reduction resulted in shots that lasted between 35-45 seconds. At constant pressure those shots would have been over extracted and bitter, but with the gradually reduced pressure they were not.

I tried two variations. In one set the pump to start at 8.5 BAR, but preinfused at 3 BAR with the motor off and the 3-way open (I installed a motor cutoff switch when I outboarded the motor.) When I saw the first drops of espresso, I flipped the motor on to kick the pressure up to 8.5 BAR, then I dialed it back down with the bypass valve to around 5-6 BAR. Another variation was to set the bypass to a little over line pressure, 4-5 BAR, preinfuse at 3 BAR, then flip the motor on to bring the pressure up to 4-5 BAR. Then I quickly but smoothly raised the pressure to 8.5 BAR, then gradually lowered it to keep the flow rate constant. Unfortunately, all of the shots pulled with preinfusion were over extracted and bitter. Coarsening the grind or lowering the dose might have corrected problem, but I didn't try that.

And that brings me to the last thought, which is that, as I said, the brew parameters had been dialed in for these coffees at constant pressure, 8.5 BAR. It may be that to truly experience what pressure profiling has to offer I'll have to dial them in with variable pressure. I'm curious if that's what the pressure profiling crowd does.

[NOTE: To those inspired by this post to outboard their GS/3 pump and motor, I do plan to post more photos showing how I did it -- eventually. In the meantime, be advised that while the GS/3 pump is easy to remove from the machine, the motor is not. It requires removing one or both boilers. I didn't do that. I purchased a spare motor for $169 and used that instead. Also note that the mod requires some rewiring in the brain box, which should only be done by a qualified person.]

Thanks for trying this. It's hard to get blind tasting confirmation.

So far, my assertions are based on having had three or four very bright coffees that pulled delicious with declining pressure/constant flow on the Strega or Cremina; but were undrinkable with the Elektra Semi. While this is good enough for me and suggestive; it's not really proof. In addition, there are now several people who think constant flow is a good way to get balanced tasting shots with variable pressure machines. Given the difficulties of doing an actual proper blind tasting experiment; it may be the weight of anecdotal experience that will be the decider.

So how is it working out?

If you're outboarding, why not go all the way and spring for a gear pump with electronic control?
'Pressure Profiling' With The Fluid-O-Tech TMFR Pump - Or, Wholesale Copying Greg Scace's Ideas

I agree with your assessment of the GS/3 MP Strada modification. The range of adjustment is far too narrow for really consistent pressure profiling. I am really hoping LM will consider producing a home machine like the GS/3 with Strada EP control (I believe James Hoffmann has a shot brewer with this setup). That would undoubtedly give me the final push to upgrade for the last time.

Yes, Chris - It is shot brewer EP that I really want! One video in here

https://vimeo.com/52786091

I own gs/3 mp with strada tuned, but SB EP looks so shiny woman -
Somedays, LM will make that model for home user, just hope :

pocojoe wrote:So how is it working out?

Pretty well. I've been brewing with the bypass valve for several days. I'm still trying to get the hang of keeping the flow rate constant while decreasing pressure as smoothly as possible. Like everything espresso, it takes practice.

The shots continue to be good. I need to refine my technique and try it with a wider variety of coffees to come to any solid conclusions.

One "downside" to pressure profiling that may not be obvious is that you can't steam and profile at the same time -- not enough hands! That's not a big deal for me. The effect of pressure profiling are subtle enough that I can simply pull espresso for milk drinks at 9 BAR. Or, I can pressure profile and let the espresso sit for a few seconds while I steam.

Bob_McBob wrote:If you're outboarding, why not go all the way and spring for a gear pump with electronic control?

Well, in part, my little experiment is intended to evaluate whether it's worth the trouble and expense.

Nicholas's solution is expensive, bulky (probably won't fit in the appliance barn with the appliances) and requires surgery to defeat the bypass valve.

I've only loosely followed the gear pump part of the thread, but at one point I thought I read that the available models may not have enough ooomph for tasks like filling the boilers. In other words, my impression was that this mod hasn't been perfected. I need to go through thread more carefully to assess whether it's actually a viable GS/3 solution for variable-speed pressure profiling.

I should add that I'm not terribly interested in being able to store and retrieve pressure profiles. My sense is that manual control will always be superior. That should simplify the project.

Bob_McBob wrote:That would undoubtedly give me the final push to upgrade for the last time.

Hah!

I knew that would get a chuckle or two when I wrote it

I think the way LM does profiling control with the Strada EP is amazing. The display gives you good manual control with the paddle compared to the MP, and you also get fully automated control for milk drinks and quick shots. I occasionally think about selling my current gear and buying a GS/3 to modify like Nicholas (who has since switched to a shot brewer with the same setup), but the thing that keeps me from doing it is the hope we might see the Strada EP technology in the GS/3 one day.

As far as "last upgrades" go, I have to wonder what the next "big thing" will be after precise temperature and pressure profiling control. Temperature profiling certainly never went anywhere.

Peppersass wrote: I've only loosely followed the gear pump part of the thread, but at one point I thought I read that the available models may not have enough ooomph for tasks like filling the boilers.

The lowest flow gear pump can do 1L/min @ 1.5 bar, about half as much as a rotary but double a vibe. How long is a boiler topup on the GS3?