It probably won't make too much difference spraying it pre-run, as you're suggesting, if the I/C is already heat soaked. However, by fitting the manual switch you'd be able to spray during your cool down drive (for what it's worth) at least there's still some airflow to help cooling. Pop the bonnet once you park up so the engine heat isn't trapped around the I/C. That'll get/keep the I/C as cool as possible before next run, if you're queuing at the drags leave the bonnet up & engine off as long as you can.

Probably best to just check in with Maximums about adding the manual switch, how they wired the auto-spray setup would likely affect how/where you connect the manual switch in. But I'm no autosparky.

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