More pips in Engines give faster top speed, as well as a faster Boost recharge time. This charge is equivalent to your Power Distributor's "Engines Charge" value. For example; a Engines Charge value of 2.5 would mean, with full pips in Engines, 2.5 energy bars would be made available per second.
Feel free to correct me if I'm wrong, but this is how it seems to work for me.