Does the 300 hp conversion with a continental IO 550 always require an auxiliary tank when used on a Cessna 182? I see some of these conversions indicate an auxiliary tank is present under pilot seat but I would like to try to avoid this.
This article gives a slightly dated (2002) overview of the engine upgrade options for a 182. There's no mention of auxiliary tanks being required, but it does note that for at least one option they're typically a good idea:
The IO-550 engine is a guzzler and if you don't have long-range tanks, you'll need them. Indeed, you'll probably want to consider Flint tip tanks for your Skylane.
It's possible that you've seen conversions where, for whatever reason, a cabin tank was added instead of - or even in addition to - tip tanks.
If you're looking into an upgrade then if you aren't already a member you should consider joining the Cessna Pilots Association and search through their forums (members only) for whatever you can find. I found one thread on upgrading the C182 to an IO-550 but it didn't mention the fuel tanks at all, although I didn't search thoroughly.
I don't think so. The modification is done in accordance with AIR PLAINS STC SA00152WI, which has no mention of any auxiliary fuel tank in the 'parts installed' section.
Also, the document says,
Fuel flows by gravity from the wing tanks to a four position selector valve..
There is no mention of auxiliary fuel tank anywhere.