This is a crazy idea I'm about to try for my 6 port FC NA with a large street port. From my engineering weed-out classes last year I learned that two resistors in an electric circuit, if placed in parallel, have half the resistance of one by itself. Most presilencers (or highflow cats, in my case) are usually placed right after the header collector in NA rotary street exhausts. I started wondering if I could keep the sound reduction properties of a restrictive highflow cat but halve the restriction by adding another beside it with an X pipe allowing the header to collect, then re-split a few inches later. Presilencers or cats are just like resistors in electric circuits, they resist fluid flow which is like electron flow in a circuit.

This principle seems to work well with the FC stock dual mufflers, which are damn quiet even on a modded, ported, headered rotary but seem to flow pretty well.

What do you think? It should reduce the backpressure a lot compared to my current setup (stock main cat behind the RB header).

 

Since your running a collected header your exhaust tuning has already taken place so it shouldn't matter from that point of view. I considered trying what you suggested. Two 12A presilencers after the collector and some magnaflow cans out back on my mildly ported 2nd gen. Should work better but how much better than a traditional RB header/single collector? Not sure it would be worth it with the extra expense of a 2 smaller presilencers compared to one large one.

 

I've basically got the pair of highflow cats already so I thought I'd go for it. Getting a ported NA rotary to be streetable (not a cop magnet) and fast is the goal.

 

Hey, I had this idea LAST year!!! Great minds think alike....

Not a crazy idea at all; a proven concept in every H or X pipe V-8 ever made. The X (made from two RB collectors, I assuming) doubles the area of the exhaust system slowing the exhaust velocity and increasing it's dwell time in the silencers. Also each pulse has to run through four silencers instead of two. I'm sure there's no other possible way to make a nearly zero back-pressure exhaust still streetable.

Let us know how it works out!

Sanspistons; years ahead of his time!

 

We'll see, maybe I'll have it fabbed in a month from now. I'm trying not to inflict too much damage to my credit card but progress is valuable.

The plan is an RB streetable header modified for ~42" collected length, to tune to the second resonance at 6500 rpm and to the first at 3250 (kind of like a long collected setup tuned to the fourth resonance but not as extreme), then a car chemistry 2.5" three disc insert in the collector, the pair of highflow cats after an x pipe and a pair of dynomax super-turbo 2" inlet / outlet mufflers with 2" pipes.

 

Read thru this thread. I had a similiar question like you did but I had it last year.

http://ausrotary.dntinternet.com/for...741&highlight=

 

Interesting thread, Racing Beat's comment makes me wonder if it might be better to build a long collected system with the pair of cats positioned halfway down the primaries, then collect them and build some funky dual muffler system way in the back. I just don't know how the positive pressure wave produced by the cats would intereact with the negative pressure wave produced by the collector.

 

88IntegralLS I think you'd be better off collecting first and then dual cats; most cats open up internally and the change in volume/velocity would act like a collector and send a negative wave back up the pipe. The collector would dampen this effect somewhat. Also the pulses the cats would see would be much reduced which would help cat life, always a rotary problem!

As far as pulse tuning goes I think only the RB presilencers would have minimum negative effect because they are equal I.D. to the pipes.

Keep in touch..

Sanspistons for stronger pulses

 

I think you are right about that. I've pretty much decided to go with my original idea, now I'm just trying to figure out how far away from the collector to put the cats in order to time the positive pressure wave they create to enhance the negative pressure wave the collecter makes. I want to get these two waves 1/2 phase apart, obviously. Time to brush up on some of my old physics classes.