They don't. It's a 45 degree vee with a common crank pin. The front cylinder fires, the crank rotates 315 degrees before the rear cylinder fires, then the crank

They don't. It's a 45 degree vee with a common crank pin. The front cylinder fires, the crank rotates 315 degrees before the rear cylinder fires, then the crank rotates 405 degrees before the first cylinder fires again. The 90 degree difference between firing pulses is what makes the engine shake and sound the way it does.