Cats vs. Presilencers: Threat or Menace?
 

People:

Yeah, I know; this one's been done to death but I'd like a few opinions anyway from someone who's actually done this. My 85's custom exhaust is dead (again) an I just can't stand the screaming and yelling (not to mention the ringing in the ears...). Specs: Monza header welded onto Edelbrock (Race!) RPM muffler adapted to short section of stock center pipe and Racing Beat rear muffler ($30 in a junkyard; best bargain ever!).

Problem is the Edelbrock is dead after about one year (and this is number TWO!); packing gone and preforations rattling around inside. You've got to give it to street-ported rotarys; NOTHING beats them when it comes to muffler testing! Anyhow would like to replace it with either of the following options;

1. Racing Beat presilencer and thick wall pipe. Quietest option but... $160 plus pipe plus welding... $200 and I have another baby on the way! Yikes!

2. My engine builder suggests replacing the Edelbrock with a Dynomax "bullet" race muffler; he has run this with a Dynomax SuperTurbo on the rear and says it lasts forever and is not "very loud". Hmmm, cheapest option; $40 for the bullet, $20 for the pipe but how loud is "not very"? Not sure about this one but my RB rear muffler should be quieter than the SuperTurbo. I think...?

3. Catalyst. I'm kind of tired of the SMELL of the thing also, not to mention the typical near raw gas output at low rpm's of the rotary. Trying to pollute the atmosphere a little less for my children, too. Would make it alot easyer to pass inspection around here also. Q's; I remember that someone recently posted that NO cats lasted behind a rotary very long; does anyone have any success stories? I hate to keep cutting and welding on new cats even if it's still under warranty. Also how effective are they at muffling? The Edelbrock is thin-wall stainless and practically beats like a heart at idle; pulses go right through them. Can't see the cats as much better. Jegs has a new cat out; Dynatech Powercat, $149 but looks like it's very streamlined flowwise. Just curious...

4. Any more suggestions? Has anybody tried the Moroso Spin thingie yet?

I'm leaning toward a cat for the ease of inspections and environmental (INSIDE and BEHIND the car, not just for the EPA!!) aspects but am open for suggestions. Send me to a big thread on this if nothing else...

Thanks fellow rotor fanatics!

Sanspistons for cleaner air and less screaming (or at least less ringing in the ears...!)

I have headers to straight pipe to PP muffler on a streeported engine. Is not that loud.

Thanks cdrad51 for your quick reply but, again, HOW loud is not very? Your setup is basically what I have now and it's ear-splitting inside with several resonance periods that rattle my brains out. Not to mention I can't go anywhere with out several dirty looks... Maybe it's just me or I'm getting too old...

Thanks!

Well there are cats that last a good while on rotaries, they are the ones specifically made for our cars. They are, however, few and far between. Basically you'd be looking at a stock replacement (yeah right) or a Bonez superflow. Got one on the FC, works great. Pricey though, that would make it your most expensive option unless you find a good one used. (cat is around $300)

The only info I can provide you with is that my '83 widebody, that I recently traded, had a Racing Beat header into 2 inline Racing Beat presilencers (one behind the other), up over the axle into a (7" wide, 4" outlet) aftermarket muffler. It was surprisingly quiet and had no raspiness whatsoever in the exhaust throughout the entire rpm range. At idle, it could barely be heard, but was pretty loud when getting into it. It was quite bearable though for daily use. This setup was on a 12A with stock porting though. I'd have to say that it was an excellent setup!! Some friends of mine swore that it sounded like an airplane though on the highway while driving!!
As for the DYNOMAX "BULLET" race muffler, the one that looks like a presilencer?? Well, my brother is currently using one on his '79 widebody RX7 with a 13B Turbo, along with a high-flow Bonez catalytic converter. As for sound? I'm not too sure yet because the muffler he's using is a dual canister type that produces a shitload of bass sound in the exhaust note, but the exhaust setup produces no raspiness whatsoever. So far, after like 2 years, neither the Dynomax muffler (presilencer) or BONEZ high-flow catalytic converter have failed. In my opinion, I find the DYNOMAX muffler/presilencer to be excellent, and it's cheap!!! If your looking for a high-flow catalytic converter that will last and that is especially suited for rotary engines, go to www.rx7.com and you will find it there.
Hope this helps!!

Brian

I've had some luck with <$20 glass packs. If you have the dual outlet header, you can run 2 glass packs and and stagger them to keep the 2 runs paralell all the way back to the axle, then collect into 2-2 1/4" pipe

The trick with glass packs is to heat them up on first use, then shut down and let them cool completely. This allows the fiber to solidify into glass.

Keep the pipe diameterar 2" if single, 1 3/4-7/8" if dual. The smaller diameter will have a quieter resonance. Get as heavy a walled pipe and glass pack as you can.

This won't do anything for the smell but will quiet it down some.

I have headers and a glass pack on my 12a and at the end of the pipe, just before it turns up to go over the axle, i have one of the little precats that is partially drilled out. 6-8 3/8" diameter holes. I was suprised how much that little cat quieted the exhaust note down.

I have a friend with a GSL-SE with headers and a 2 1/4" glass pack and pipe, with no cats and his car is much louder.

Funny thing is with my stock tires and mikuni, I'm .4 of a sec quicker in the 1/8 mi. Drives him nuts, and I built both systems.

As a side note both cars have the oem muffler.

well, i've got the rb headers to presilencers(2 of them) to a straight through muffler. had the presilencers on for about 3 years my self and bought them pretty used. i also have the cat that bolts right in were one presilencer goes, never had to use it but i got it. all and all it's not load at all. nice medium tone. was alot quieter with rb muffler on though.

Take a look here:
http://www.rx7.com/store/rx7/saexhaust.html
BONEZ cats range from $175 for 2" inlet/outlet, $200 for 2.25" & 2.5" inlet/outlet, and $250 for 3" inlet/outlet.

A good replacment cat is going to cost $$$. I was quoted $700 so since there is no emissions test here I just had them all cut out.

Thanks everybody for your tips, they're really helpful.

I'm sort of leaning toward the Bonez 2" cat but the $175 is offputting this close to zero hour (the baby!). The Dynomax SuperCats are alot cheaper but there's that durability question, again. If not that then the Dynomax Race Bullet is my strong third choice; never heard anything bad about them. The price is right, too...

Was looking at the Moroso Spiral-Flow -neat idea and cheap but I'm not sure how long it would last in the face of Rotary Rage, maybe if the spirals were welded to the outside of the shell also.

Nice to belong to an organization (it THAT'S what you would call it!) with people that respond to reasonable questions so quickly and well.

Thanks again!

Sanspistons; Suck, Squeeze, Bang, Blow; it's not just for pistons anymore!

Well, after much soul searching (actually WALLET searching...) I've chosen the Dynomax Bullet along with some RB pipe to connect it. Cheapest alternative now than I found that I need brakes also.....

Thanks again for the advice!

Sanspistons; the ONLY way to fly!!!!