# Bar pressure & the effect on timing of shots.



## Jumbo Ratty (Jan 12, 2015)

My question is,

I have a Gaggia Classic that's running at factory set bar pressure, so it should be 15 bar but imagine it is around 14 bar in reality.

If I follow the recipe of 14 grams in, 28 grams out in 30 seconds am I actually grinding finer that someone with an OPV modded machine which is set at 9 bar to achieve this ?

My thinking is because my machine is set at a higher bar pressure shouldnt my shots actually not be taking so long

*Scenario,* You are well accomplished at preparing the aforementioned ration and can nail it prep wise time and time again, you prep the portafilter exactly as you would every time but instead of putting it in your 9 bar modded machine you put it in my 15 bar machine, because of the extra pressure you would surely reach the 28 grams in less than 30 seconds ?


----------



## froggystyle (Oct 30, 2013)

Just lower the pressure....


----------



## garydyke1 (Mar 9, 2011)

Pressure and flow are inverse with vibe pumps


----------



## Jumbo Ratty (Jan 12, 2015)

froggystyle said:


> Just lower the pressure....


Ive read on here its not recommended. But dont know why it isnt recommended.

I know a lot of people do the OPV mod though



garydyke1 said:


> Pressure and flow are inverse with vibe pumps


Thats a bit cryptic for me, are you saying it should take longer ?


----------



## nufc1 (May 11, 2015)

I believe (correct me if i'm wrong) that the higher pressure actually forces the fines to the basket quicker, thus slowing down the flow (which is the opposite to what is expected)

Therefore, unless you have a side-by-side comparison, its not really possible to determine whether you're grinding finer than for a 9bar machine, or whether there are just more (unwanted) fines in your grind.


----------



## Dylan (Dec 5, 2011)

i.e. The higher the pressure the slower the pump can pump.

It makes a lot of sense if you just think of how hard the pump has to work against a pressure, the more there is the more the pump will struggle


----------



## MWJB (Feb 28, 2012)

The seconds aren't really a critical part of the recipe, the brew ratio is as is the flavour.

Pressure quoted, as in 15BAR, is usually blind pressure rather than actual pressure seen by the puck (which will be less than blind pressure).


----------



## risky (May 11, 2015)

Jumbo Ratty said:


> Ive read on here its not recommended. But dont know why it isnt recommended.


AFAIK the only person who doesn't recommend it is gaggiamanualservice because he feels there is a chance of it going wrong?

Nearly everyone who has a Classic does it and I've never heard of any horror stories.


----------

