Don't actually do this. If you are doing an APC rush, that usually means you were spotted, and there's a whole Engineering battalion hot on your heels with repair guns ready to disarm anything you put on the MCT, not to mention anyone that spawned at that building. Nine times out of ten, you won't be able to defend the timed C4's long enough for them to actually explode, and if you are particularly unlucky (meaning: the opposing team is semi-competent) you won't even have enough time to put more than one C4 of any kind before you get killed.
Best thing I think is to just go with remote C4's - place one, if the insides of the building aren't swarming with enemies, place another, and then detonate them. Now defend your fellow engies.
With APC rushes, or any kind of rushes the enemy is aware of really, you need to be quick. Move fast, strike hard, die and hope your fellow teammates managed to accomplish anything worthwhile.
Or simply backup and general area awareness. One engie got out of his medium tank to repair it, I sniped his head off with Sakura, ran towards it and stole it. One more tank for the team.