I'd be very wary of those. I think they're normally a resistor which tricks the ECU into thinking the engine is cold(er) and hence it puts a richer mix of fuel in. I'd think in all the years that manufacturers have been developing engines if there was such a great quick fix which gave +15bhp they'd have done it already. If you're serious about getting more out of the engine on a budget, have a look on www.msefi.com. Jon