[QUOTE] You don't for that particular operation, but it's very nice to have one for the subsequent mixture richening/egt cooling, and to gauge the overall improvement or otherwise at throttle settings. Of course you can do all of that aurally, but only after a lot of experience. Having said that, I would be a bit suspicious of an app for an iPhone, but at the price, I would be tempted to experiment.[/QUOTE] For tuning I find a pair of mercury tubes quite useful, and much more sensitive to minor adjustments. Theo