so when the impluse engine becomes level 5 it replaces the combustion engine in light fighters does it do the same for light and heavy cargo ships?

It doesn't replace the engine, the Combustion Engine Research always upgrades the engines (speed) of small cargo's, large cargo's, light fighters, recyclers and espionage probes by 10%. The Impulse Engine Research always upgrades the engines (speed) of bombers, cruisers, heavy fighters, Elite cargo's and colony ships by 20%. Finally Hyperspace Engine Research upgrades the engine (speed) of battleships, battlecruisers, destroyers and deathstars by 30%.

[Ship refitted with Impulse Drives once reached level 5]
As soon as the hyperspace drive is developed to Level 8, the Bomber is retrofitted with the hyperspace engine and can fly at higher speeds.

Small Cargo Ship
Bombers
Technology

jayman wrote:so when the impluse engine becomes level 5 it replaces the combustion engine in light fighters does it do the same for light and heavy cargo ships?

to answer your question: no it does not replace the engines on the Large cargo or the Small fighter. the Small Cargo is the ship to get the upgrade to Impulse.