the older rusty's had the 15 side exhaust, but the newer ones have the rear exhaust 2.5 .... the 2.5 is a really good motor, the guy across the alley had a rusty and i have seen him run the same motor for 4 years now and i've seen him submerge it at least 4 time totally under water and in the lake once and it still will not keep the front end on the ground.