As I wrote in the Beef Curry  recipe, I don’t make my own curry powder. Lomo asked in the comments  about the “secret” 15 to 20 spices that make up curry powder. After poking around a bit on Japanese web sites, I came up with this page  that describes what goes into S & B curry powders, the most popular brand by far in Japan. It’s an official S & B page, so should be accurate, though as you can see the percentages given have a pretty wide range. I guess it’s because the actual formulas are ‘secret’. In any case it gives a starting point for any experimentation I think.
I’ve also included a recipe for making garam masala. Note that I make no claims whatsoever that these are authentic mixes for Indian or other curries, but I’m talking here about Japanese curry.
The following is a rough summary/translation of the Japanese article.
These basic four spices make up 80 to 90% of the mix:
Then the following ‘hot’ spices make up about 5% of the blend. If you want to increase the amount of hot spices, decrease the turmeric accordingly.
The remaining 5-15% is taken up with aromatic spices. Adjusting these spices makes the powder distinctive.
All of the above are the basic spices (that go into all the powders, I assume).
Other spices, herbs and so on are added to give distinction to each blend, such as:
They say to limit the amount of ‘other’ ingredients to about 1-2% of the total.
To make up the curry powder, roast the spices (I think they assume you are starting out with ground spices) in a dry frying pan for about 2-3 minutes. Cool the spices, and if possible let them mature in a cool, dark place for about a month before using.
Note that a good garam masala mix will contain the aromatic spices like cloves and fennel too. Here’s a standard garam masala mixture from an old Japanese curry cookbook I have, if you’d like to make up your own, starting from whole spices:
Put all of the above onto a baking sheet, and roast in a 90°C / 195°F oven. Roast for about 25 minutes, stirring occasionally.
Take the cardamon out of the pods. Grind it all up in a mixer, dividing up if necessary, until ground to a fine powder. (Note: nowadays I would use an electric coffee mill reserved for spices.) Store in an airtight jar.