Both of my stock exhaust manifolds are cracked and Im replacing them and now I want to know if I should leave the cat or cats or go for a straight through single of perhaps even duals?

Answer 1

If this is on a vehicle that needs to stay emissions legal, then yes, you need to keep your cats. Otherwise, you can remove your cats and it will remove some of the restriction in your exhaust system. This would translate to more horsepower, torque, and a little better mileage. You'll also have a smellier exhaust.

As for single vs duals, that will depend on your engine. If you let your exhaust piping get too big in diameter, your exhaust gas velocity at low RPMs will drop, and you will feel a loss in low end torque. You might see an increase in peak horsepower. For a large engine (making 400+ HP), you could still be fine with a single 3 inch exhaust. If you really want the dual exhaust, at 400 HP you probably should go with 2 inch piping. Maybe 2.25. Any larger and you risk losing low end torque. If this is a 4X4 going mudding, or going to be an exclusive track car, you could go larger, because you'll be keeping that engine at higher RPMs.

If you want the dual exhaust look, but want to keep the low end torque, there are mufflers that take a single inlet and have two outlets. This would allow you the best of both worlds.