same brewing pressure, basket, same coffee and amount of coffee grind, why different machine need different grind setting?

and often make different quality of espresso? what is the main factor?

recently I was able to use bigger 2 group head machine to make espresso, the bigger machine seems comes out better result.

Some machines have more push. Right now, I'm assessing a machine that requires 18 grams of the same coffee and grind in the same basket that will work with 14 on my regular machine.

You get the most similar taste by keeping the grind and brew ratio the same, by adjusting the relative doses to do this. If, using these identical settings, you prefer one machine to the other, it is probably more to your taste. But if you change grind settings and brew ratios when comparing machines, you are comparing different brews, not different machines.

Matt,
it doesn't take much difference in parameters to make a difference in the grind needed and the taste received. A little change in pressure that the gauge does not display and a slight difference in brew temperature can all combine to make two machines that seem similar create two very different tastes. It would be interesting to hear how you arrived a the conclusion that all things were the same (measuring temperature, dose, pressure, etc.). Maybe one has a different pre-infusion or something like that of which you are not aware...

I think that pressure and water debit are probably the main differences among machines. Machines have different water restrictions towards the head and also probably different water patterns in the head and basket. Small differences make big changes in extraction when you are doing concentrate coffee at fast speeds.

Cheers

Hard to say what makes the difference.

Don't have too much confidence that pump and boiler gauges on two machines giving identical readings means the machines are both producing the same pressures.

What machines are we talking about? What are their respective conditions? Is one kept cleaner than the other? Different pfs? Shots pulled at different times during the day? Banging shot after shot out of one, and using the other one only now and then? Any other information you can give?

When you say "better," what do you mean?

It begs the point of your "why" question, but the first take-away is that given almost any imperfection, grind is the first thing to tweak. The second is that better machines make better coffee with greater consistency, and are more forgiving of small barista technique flaws (those may be the same thing). It's what makes them better, right?

Setting aside the obvious fact that your wife is unreasonable about the expense and sheer space needed to trade in your second-hand Rocky for an Aurelia two group, what about grind? Can you tweak it so the one group produces espresso as good (or nearly as good) as the two group?

Frequent fooling around with grind is a simple fact of good pulls. There are lots of reasons why, and some of them aren't well understood either. A big advantage of the professional barista has over the home persuasion is a willingness to keep tasting and sinking until the grind is right (for the day).

Uneducated Guess Time: Since we're talking about a two group (big machine) vs a one group (small machine), my first guess is a hazy stab in the direction of temp, temp stability and temp consistency. But it could be any of the things the other things already mentioned in this thread, it could be plenty of things not mentioned whether obvious or not obvious (head space for instance), and it could be a big fat combo.

Good question, by the way.

BDL

Thanks for all answers. very interesting.

I agree that consistent tamp pressure is another key. maybe a good grinder is the first thing to make a good espresso.

I was able to make quite good shots in bigger machine, home type smaller machine is more challenging to me, more "skill" required.