what are u guys seeing in the cat pieces that shows its plugged? I see some mangled bits, but I can stilll see honeycomb and most of it looks fairly clean.

also, needing backpressure is a myth conjured by ricers who couldnt understand why they lost power when they added a 6" exhaust system to their 4cyl. You don't need backpressure, but pushing exhaust gasses into a cavern will slow the entire system down. Think of what happens at the bottom of a water slide. a good header is the tubing of the slide. but what happens when you get to the pool at the bottom? everything stops up because you have fast moving exhaust hitting non-moving air.

I always herd it was the backpressure that helped to cool the manifolds to create the vacuum that assists in the scavenging which helps increase exhaust flow. It was not the backpressure directly but the cooling of the manifolds/headers that the backpressure provides. Too large of a space will remove the vacuum effects or not create enough vacuum thus hurting the scavenging of your exhaust gasses.

That is what I have always read as the reason for backpresure and why it is needed. Things like headers and other performance exhaust systems create the vacuum with out the need of backpressure.

Massey

backpressure would slow down exhaust gasses and I cant really see any reason why this would affect cooling. as exhaust gasses cool they will contract. this causes the negative pressure (vacuum) that produces scavenging. there is a reason top level drag cars run open headers. that said, the headers arent just randomly bent pieces of pipe, they are precision built.

OK so it is not the backpressure that causes the cooling that causes the scavenging it is simply the gasses cooling creating the vacuum. So if you have too big of piping you wont get the vacuum needed to pull the gasses out of the cylinder fast enough and that causes the loss of power.

Massey

not exactly. the vacuum will happen regardless. the issue with scavenging is making sure you get a vacuum at the proper times so the gasses dont all stop in their tracks.

Specter:

All of what you said actually makes sense. Now I know why headers and exhaust are so important. So now my question is, which do you think will perform better:

BBK Shorties, Aftermarket Catted Y, and GMMG?

or

BBK Shorties, true duals with 2 high flow cats on either bank, and some kind of muffler?

Thanks for all the info!


x01660

true duals will typically work better, but I dont now how difficult they will be to install. if ur car came with a y then I would keep it that way. you probably wont notice a difference in the two

Those pieces would be fine, if they were all connected as one and not broken up going every which way inside the can. That's what I figured he had, as I've had a couple cats do that very same thing. I replaced the back portion of the exhaust on my Vette last summer. When I cut off the pipes, I had a big chunk of cat-a-combs wedged right before one of the mufflers, and some more bits inside the muffler. And....the cat was almost empty!!!! Somebody behind me must have been getting sprayed with chunks. ;)

^^^ That reminds me of my old 86 Cutlass. It did the same thing. I was having trouble getting the car to keep freeway speeds and it would top out WOT at 65. I brought the car into my friendly local exhaust shop and they found 1/2 my cat in the muffler and the other 1/2 kinda melted and kinda rattling around inside the can. They started the car and revved the engine (with the back 1/2 cut off) and a piece shot out and hit a car across the parking lot, and almost hit a guy waiting at the bus stop with another piece. Needless to say they shut the door while they finished coring my cat. It was a "temporary" fix till I could get in to get my cat replaced.

Massey

^ lol. I was expecting you to say that a chunk flew and hit a cat.