The rest of the paragraph in the Illy book has a bit more to say on the subject:

....... If the height-to-diameter ratio of about 0.2 is lowered, very fine grounds are required for coffee to percolate within a standard time; this choice would degrade the reproducibility of percolation, because of the creation of localized channels by water. A larger height-to-diameter ratio would give the cake the shape of a column; in this case, optimal extraction would demand an excessively high pressure (as in liquid-phase chromatographic columns). Conversely, for normal pressure values the grind would be so coarse as not to offer a sufficient number of fractured cells, resulting in a low extraction. Consequently, only minor variations from the optimal form of the established filter can be recommended.

Endo wrote: I do miss my old ridgeless basket (not available on the Vivaldi).

Ridgeless double-basket? I have both a ridged and ridgeless for the Vivaldi, so they should be available somehow.

TruthBrew wrote:Ridgeless double-basket? I have both a ridged and ridgeless for the Vivaldi, so they should be available somehow.

I have seen someone remove the ridge (with a bit of time and good metal working skills), but I have never seen one for sale. If you have a source, please let me know.

Where did you find a ridgeless 53mm double basket? Inquiring minds...

Frost wrote:The rest of the paragraph in the Illy book has a bit more to say on the subject:

Yes, I read that part. The paragraph wording addresses the basket shape, not the diameter, and does not suggest that any testing was actually performed. There's also a paragraph on grinds expansion (with a suggestion of 5-6mm headspace), and a paragraph on the holes in the bottom of the basket. But there is nothing that compares the performance of different diameter filter baskets.

Frost wrote:The rest of the paragraph in the Illy book has a bit more to say on the subject:

Man, that Illy book is about as interesting as reading a Chemistry textbook (but without the reference notes).

His "magic number" is 20% (puck height 20% of puck width). Anyone know what the ratio numbers are for a double (14g) 58mm and for 53mm? Also, what's the ratio for a more typical 17g overdosed shot of both diameters?

Also, according to Illy, should I be using a pressure higher than 9 bar for a 53mm? I'm experimenting with different pressures on my Vivaldi right now, so that part is very interesting to me.

Yes, the Illy 'specs' a large headspace and a thin puck. The relation to basket diameter would imply that the smaller is more ideal for smaller doses. No mention of what is considered a small or large deviation either. One must assume an empirical method was used to arrive at the specs.

I'll try and dig up the study.
It's taking more time than I thought (though the search has resulted in some good things - like my finding the Illycaffe research abstract that talks about the perspex portafilter results; and the Nestle abstract talking about how both water pH and coffee type impact dynamic compaction of coffee during extraction).

Frost wrote:Yes, the Illy 'specs' a large headspace and a thin puck. The relation to basket diameter would imply that the smaller is more ideal for smaller doses. No mention of what is considered a small or large deviation either. One must assume an empirical method was used to arrive at the specs.

Or a WAG. As much respect as I have for Petracco, he needs to support his observations with evidence, just like everyone else.

But this is a good point. According to Illy/Petracco, the 53mm Spaziale basket really should be shorter, not taller. This is not the case:

From left to right: 57mm Marzocco double, 58mm Quick Mill double, 53mm Spaziale double, 58mm Rancilio (?) triple

The 53mm Spaziale basket is the same height as a 58mm triple basket. The 58mm Quick Mill double approximates the height:width ratio of 0.2 more closely than the other baskets, using the ridge as a rough guide to height.

The Petracco excerpt states that a taller puck requires a higher pump pressure. It would be easy enough to run La Spaz, DC and San Marco groups at 10 to 11 bar, if one feels the shots at 9 bar are underextracted or ground too coarsely.

However, this is hard to understand in context. He says taller percolation columns require higher pressure (and this is borne out in instant coffee plants), but he also states that the flow is maximal at 9 bar and actually slows down at higher pressures (and this has ben confirmed by people experimenting). The two statements together contradict each other, or imply that grind styles have to change if the puck depth changes a lot. Finally, most shots in Italy are singles, and single baskets have truncated cone cross sections, not a cylindrical one, and this either contradicts his statement about cylinders being optimal, or means that the Italians pull bad single shots.

The Illy book is wonderful; but it also has a lot of holes, non-sequiturs and contradictions, like this one, probably due to trade secrets.

Personally, I prefer the 58mm goups to the smaller ones I've tried; but I don't think the information in Illy invalidates the DC and LS machines. Also, my preference is based on very limited experience with small groups and a lot of experience with full sized ones; so even I consider it quite suspect.