Originally Posted by zabeard
I want to see some more information on that. All I have ever read is that the ranger cats are high flow from the factory.
I replaced all mine with 2 magnaflow high flow cats, it didnt make that much difference that i can tell.
I don't have any proof for the 4.0L & I've never pulled one apart. I'm speaking in theory only on this one.
As far as reading.. I seldom pay attention to uninformed opinions.
I'll *write* this though.. Why would ford put high flow cats on a vehicle that has a very restrictive cat back system? That doesn't make sense unless they were using the cats from an off the shelf part. (ie.. an already produced car/truck)
Also, the whole point of cats are to superheat and chemically change the exhaust gasses. The lower the delta P across a cat.. the faster it'll move through it. They would have to make the cats longer and run the motor leaner to give the exhaust time to change.
As far as the bumps in the stock manifolds go. There are two things at work. And often times these things have been engineered and tested.
1) Air flow over bumps / dips often times stays laminar. Meaning that it's kind of like an airplane wing. Yes it's obstructive.. but it's also a smooth flow that is not objectionable. A hard step on the otherhand disrupts laminar flow. Picture the leading edge of a wing having a 2" flat face instead of the rounded shape. Keeping flow laminar is critical. Smooth bumps do that.
2) Crossectional area. If there is a bump protruding into the main flow path.. very seldom will they have another bump across from it. If they did it would reduce the crossectional area and choke the flow. I'd be very supprised to see an exhaust manifold (or intake manifold) designed that way from an OEM.
Understanding flow (of any media) within a closed chamber is not always a simple thing to see. It takes an in-depth study of the velocities up/down/around the chamber to say it's "good" or "bad". Basicly.. it just depends on many other things. Things like what's the vectors and velocities of the media entering the chamber? It may not be square to the mounting flange! So.. you can't just look at a bump or shape change and say it's restrictive or not restrictive. It's not that simple.
Headers almost always make more power. But.. you'll **pay** for that extra power. Porting a iron manifold and going with high flow cats is a solid bang for the buck way to gain the loins share of that power. I'm not running down headers... I'm just trying to point out that there are other ways of gaining the majority of the power a header would provide.