I think the general idea is that two cats is more mass, which means the exhaust can pass through the restriction on each side, thus making less of a boltneck of gasses. One cat would be one direct obstruction, since there's then twice the exhaust flowing through it.

 

You're confused.... no one is proposing to put 2 cats "back to back". That would be two cats in one pipe, and would be more restrictive. And while locating the cat "downstream" in the exhaust would produce less pressure loss, because the exhaust would be cooler, that would have a negative effect on the cat's emissions efficiency, because the cat has to run hot to work.

You have to look at how much pressure loss (aka "back-pressure") each component in the exhaust system contributes. If you reduce the flow volume through the single cat, by running two of them in parallel, you reduce the pressure loss contributed by the cat by 75%. That's an improvement. Has nothing to do with what happens next in the exhaust system.

What happens after the two pipes of the Y-pipe combine may present yet another "restriction", but you address each restriction individually to improve the total system. To solve that problem, you simply make the intermediate pipe larger in diameter than the individual branches of the Y-pipe.... i.e. two 3" "Y" branches feeding a 4" intermediate pipe. Now you have reduced the pressure loss in the Y-pipe. Now move on to the next component in the exhaust system and work on that.

Improving each part of the system adds up to better performance for the total system.

 

It basically comes down to this equation:

Q=AV

where Q=flow, A=cross sectional area, V=velocity

If you increase the area, the potential for flow will increase, the velocity will decrease and result in less frictional (head) loss. This is why a 3" cat will flow more than a 2" cat. By running a header collector pipe into a cat on each side of the engine, you are increasing the cross sectional area and decreasing the head loss. At low rpms where the engine isn't flowing that much, it won't make that much of a difference, with the exception of forced induction.

After I installed the flp header kit w/2 cats, my engine ran cleaner than stock.

The cats become more of a restriction at higher rpms.

 

I know nobody was talking about putting cats back to back. I wasn't confused there. I was confused how you figure two bottlenecks higher up on a stream where bottlenecks hurt more is more efficent then one further down the stream where bottlenecks of the same size hurt less. I found it hard to put down in writing my question so it was able to be understood.

For simplicity sake what do you propose the HP difference is between having a lets say 330 rwhp LT1 car having 2 3inch catco cats at the base of a 3 inch longtube colelctor vs having a single catco cat at the base of the Y.

Most y pipe designs are two 3 inchs into one 3 inch. I agree that is not very efficient.

Another question: What size I pipe should a car in the full bolt on/mild cam area be using? I have Jet Hot Longtubes that go into a Mufflex ORY since the collector on the Y is 3 1/2 inch vs the normal 3in for an ORY. That Y feeds into 3 inch pipe. Is 3 inch pipe a restriction at that point in the system? Should I have a 4 inch collector on the y pipe and 4 inches back for awhile then reduced down to 3 inch several feet back?

I have also heard of another solution to the cat problem is running 3 1/2 inch cats because supposedly a 3 and a 1/2 in cat flows better then just a regular 3 in. pipe.

 

$64...thats what i'm talking about. these guys at "Lou's custom exaust" in waltham mass wanted to charge me 270 for those same cats...watch out for those mark ups guys. Thanks for all the info

 

I do belive I purchased mine off ebay for cheaper then that. Def don't go to exhaust shops and expect a deal!

http://www.installuniversity.com/ins...sity/index.htm

Check that out, go to tech articles , then Flow Bench Test - Catalytic Converters: Ready Set Flow!

 

Let's play with some numbers.....

Let's assume you have a single cat, at the base of the Y-pipe, and the temperature there is 200degF cooler than at the collectors. Let's compare that to two cats, same size as the single one, one located on each collector, where the exhaust is 200degF warmer.

Pressure loss is proportional to the square of the velocity (you could argue that the exponent should be something different, based on wetted perimeter of two pipes rather than one, but we're not changing the pipe diameter). So, if we assume a velocity of "1.000" at the single cat in the Y-pipe, we can prorate the velocity to the two cats, and adjust for the difference in volume due to temperature, using the perfect gas law.

Since we have 1/2 the exhaust going through each of the "dual" cats, the velocity, without correcting for temperature is "0.500". To correct for temperature, lets assume 1,000degF at the collector and 800degF at the end of the Y-pipe. The effect of temperature is proportional to the absolute temperature......

1000 + 460 = 1460

800 + 460 = 1260

The effect of temperature is 1460/1260 = 1.159

Velocity at the dual cats = 0.500 x 1.159 = 0.579

So.... if pressure loss is proportional to the square of the velocity

Dual cats:

0.579^2 = 0.336

Single cat:

1.000^2 = 1.000

The pressure lost in the dual cats is only 33.6% of the pressure lost in the single cat.

You can do a sensitivity analysis and verify that no matter what you do with temperature, within reason, the dual cats always have less pressure drop than a single one. Even if you assumed a 400degF loss between the collectors and the end of the Y-pipe, the pressure loss in the dual cats would still be less than 1/2 of the pressure loss in the single cat.