https://www.hasbean.co.uk/products/Celebes-Toraja-Kalosi.html
https://www.hasbean.co.uk/products/The-Breakfast-Bomb.html
Or
any Robusta. If you really want caffeine (and it sounds like you're drinking pure drek already), you can't beat Robusta.
http://en.wikipedia.org/wiki/Coffea_canephora
People who give a shit about taste usually avoid Robusta like the plague, as virtually every bean has undertones of burnt rubber and sewage. However, scum mix it into rancid blends in order to add punch and an
appearance of a good extraction (it boosts crema, meaning even the most skill-bereft twunt can get a shot that
looks good, even if it tastes like febrile horse's piss).
Incidentally, it seems worth adding that many manufacturers / sellers use roast as a proxy for 'strength'. And often mean flavour 'strength' instead of caffeine strength - there isn't
that much variance in caffeine levels between varieties of Arabica (the stuff that doesn't taste like sewage and burnt rubber).
A lighter roast will
generally taste lighter. I believe the caffeine
may also be slightly less available / diffusable, but don't quote me on that.
Whereas a strong / dark / Italian / heading-towards-charcoal roast will taste of pure fucking roast coffee. It'll blitz all the origin characteristics out of the window, but will taste like 'strong coffee'.
Without meaning to state the absolutely fucking obvious, brewing beans for longer will add to the 'strength' of the brew. As will brewing a greater quantity of grounds.
If it's genuinely the caffeine you're after, you could probably get a decent *punch* of caffeine by brewing something on the 'low strength' end of the scale. But brewing lots of it, for longer. tbh, I'd imagine you'd benefit from doing the same with 'strength five' beans (whatever the fuck that's meant to mean). If you want more caffeine.
Hint: if your coffee tastes vile and bitter beyond belief, that's what you're after. Caffeine is
bitter. A higher percentage of caffeine in the cup will make a
bitter shot / cup.
Most people with any sense whatsoever would hope to stop the extraction somewhat earlier - at the point at which the best oils / solids have been extracted, and the flavour is decently balanced. Ideally, neither sharp (underextracted / cold) nor bitter (overextracted / hot).
A 'good' extraction won't be bitter. Genuinely. It won't be bitter.
A good extraction also won't be loaded with ridiculous levels of caffeine. I can drink two or three double shots of my home brew, but drinking a piss-poor cup in a shit cafe will often leave me wired.
Then again, it sounds like you honestly don't give a shit about flavour or a decent extraction. And, hey, it sounds like you're buying pretty awful beans atm anyhows (genuine life after roast = approx 3 weeks; average 'best before' on supermarket beans = 18 months). If so, more power to you.
I hope the above gives you some idea about how to wrangle more caffeine out of your cup.
