7-8 tanks seem like an awful short glow plug life. That would be 2 dead plugs or more at a single club race. I would check your tune....or buy higher quality plugs (if you're not running them already). Everyone is different, but if your average for a single plug is 7-8 tanks....that's a bit short. If it's an occasional plug dying at 7-8 tanks them your on par with me at least. I typically blow a plug every 2-3 races.