True. A fast enrgy release/low weight density wood like spruce, versus a slow energy release wood like your hard/dense hardwood does take a "different hand" to operate with well.
The experienced rule of thumb is to make the "fast" low density wood chunks twice the size of a "slow" energy release hardwood chunk sizing.
So . . . beginning with a designed tuned hardwood chunked system the internal dimensions will probably be too small for best use with a larger chunked conifer softwood usage.
Why KristijanL some of our softwood systems seem so physically large to a mini-guy.
Exclusively softwood using/designing then the opposite learning curve to climb. Have to dramatically down size the chunk size using a hard wood to get the increased surface areas for the same instantaneous energy releases as had with softwoods.
This chunk sizing range, and the resulting physical space "problems" incurred separates out the limited fuel range type designs from the flexible fuel type use designs.
No criticism of your system intended saying this KistijanL.
Your challenge that you have accepted is making a small unnoticeable system to be able to culturally fit in.
Every thing you do must fit into this tight shoe.
Ha! Ha! Having to use a special "premium" fuel is just one of the use/design compromises you must choose to achieve your results.
My old Shihl chainsaws specified the use of 87 octane gasoline. These newer, lighter higher powered ones specify the use of 89-92 octane gasoline. Matters not for the costs. They do not use much. Engine heating and power it matters a lot!
J-I-C Steve Unruh