my car is 1990, originally 5.0 TPI, now 5.7 TPI, magnaflow sells a y pipe, and cat back that fits this car (Summit Racing) they also have a new CA legal Cat converter, my question is there a reason that I would have to use the twin Cat (stock style) configuration? or can I switch to a simpler single cat without any issues?

 

Dual cats makes more power. Thats why the G92 cars came with them. Look at it like this. You have 4cylinders going into dual cats vs. 8 cylinders into 1cat.

 

It's not that simple.

1 big cat or two smaller cats?

Guess it will also depend if the cat is big enough to avoid being the point of restriction.

 

You have less restriction if you have two 2 1/2" cats than one 3" cat. 8 cylinders is fighting to get into a 3" cat.

 

Being from CA, don't you have to use whatever system that came with the car or motor?
Me, I would use 2 cats if I had 2 use them.

If I remember correctly, when the N10 came out, the press releases said the cats flowed 275 cfm each and the single was less than just one of the dual cats or just barely more than one. I'll try and find that info.

 

@TTOP350 - that's a good point, I'll have to check the CFM ratings of the various cats.

One factor to consider is that cat technology has improved a lot over the past 20 years, and my impression was that going to a single new high flow 3" cat would work better than a pair of old 2.5" cats.

If I can find the CFM specs it should be straight forward to validate this.

I guess the next thing is to estimate what CFM is required to stop the cat becoming a restriction?

 

Doing some reading, it appears the stock cat flows 250 cfm, each of the dual cats flows 275 cfm and a good quality after market 3" cat would flow something like 575 cfm.

It looks like things have changed and the dual cat setup is probably not longer the greatest option.

 

If I have 2 have cat(s), I would use two 3" cats.
The original cats were only 2.5" but the pipe leading into them is barely 2". (I think they came from a ZR1?)
It also depends on what kind of HP your making or want to make. I tend to error on the side of huge by large.. lol

 

wow, great input and fast replies, I have none of the original system, so I was thinking single 3" cat would accept the flow from the magnaflow Y tube, the stock pipes that lead to the twin cats had more and tighter bends, the cats exit into a y in a less than graceful merge, so I need to check the flow for the 3" magnaflow 3000 (the CA legal cat may suffer a lesser flow rate than their high flow 3") the alternative is probably costly the oem is one piece (p/n 10136824 in my old dealer parts book) doubt that is available anyway, or custom work that I really don't want to pay for unless I have to.
I certainly appreciate your inputs, thanks to all

 

Honestly, the idea was good, but the execution was **** poor at best.

A quality (like Magnaflow) 3" high flow cat is going to outflow both the 3" factory and probably the dual cat system due to design of the cat (you are talking 1986 technology with OEM).

The real issue with the dual cats is availability - 15+ years ago when the cars were very popular, you had a choice of headers and cat-hsystems. Today you are really limited. And considering how well new cats flow, a 3" aftermarket cat ($150) with a Magnaflow TPI y-pipe ($165) will equal or exceed a factory dual cat system for a lot less. Don't get me wrong, an aftermarket dual cat system will flow great, but not $900 worth (or whatever they are now).

 

The reason I'm interested in this topic is I need a new exhaust for the L98 in my 1988 IROC.

My plans are to buy a Magnaflow 93441 converter and Magnaflow 16829 Stainless Steel 3" Dual Cat-Back Exhaust System.

The complication is that the car is Right Hand Drive, so options for headers and Y-pipes are difficult. Flowtech headers seem to have the best clearance when the steering shaft is on the right-hand side.

The water flow figures I've seen for modern cats are:

5” cat with 3” ends - 706cfm
5” cat with 3.5” ends - 885cfm
5” cat with 4” ends - 1015cfm

My understanding is that you need about 2cfm per 1 hp, so I assume a single Magnaflow 93441 converter is good for upto 350hp?

 

No cats, problem solved.
more power
free cost
no work
/thread

 

thanks paul, that's kinda what I am thinking now, that oem system did not look flow worthy at all, thanks peter for the flow info for reference, and evgax please convince the CA government to get out of our business and I'll gladly jump on the no cats wagon

 

Now my car is back from the paint shop I need to sort out stuff like the exhaust and this thread has been a good chance to do that.

I'm hoping for 350hp from my L98 and guess I need to find a complete exhaust system that will flow at least 700cfm to support that.

 

Dual cats make your ride purr nicely.

 

Not as good as no cats.

 

Two 2.5" pipes have the same cross-sectional area as one 3.5" pipe. I'd rather have two pipes for my exhaust than just one, even if it costs more. Why is dual-exhaust so popular?

And cats are definitely worth it. They clean up the exhaust and help keep the fumes out of your passenger compartment. Eventually every state is going to have CA emissions so you might as well plan for it now. We have cars in CA passing the emissions test and making over 400 RWHP.

 

True on the dual pipes, problem is the packaging to fit under a 3rd gen, which had a really compromised exhaust routing from GM. Also keep in mind that dual exhaust usually weighs more in most cases.

Dual exhaust is popular as a lot of V8 cars old & new have enough space to route 2 pipes under a car without a lot of problems.

I'll agree with the cats being worth it - definitely can clean up the tailpipe emissions on older cars with less sophisticated engine control systems. In theory with today's engine management systems, new cars should be able to run clean without cats, but they are required by our government.

I'm not so sure about all states moving to the CA emissions level. Here in NJ, vehicle inspection has been stripped down to only emissions testing and from what I understand very few cars fail the test (they probably loosened the levels ). Keep in mind that about 14 years ago, the state had an emissions program so rediculus that it make CA look like they were stuck in the 60's - most brand new cars would fail and they gave it up.

 

There is another thread someplace that has the CFM rating of factory cats and the dual set up is 275 cfm each.
I can't remember the single cat flow but its lower than that..