I have the issue of trying to figure out if it's possible to adequately quiet my car and install a cat in the belly. There's about 24" to work with.

After talking to the guy at PowerEffects for ideas, he only said that my plan to head-port with a bigger cam -- combined with longtube headers and the loss of front cats will produce a loud car. He said "there's no way around it".

I cannot accept that -- at least if enough room is available. I've heard a new Z06 drive down the street with the manners of a gentleman!

Now, I don't mind if it's louder than stock. In fact, that's preferrable as long as the sound is not over-powering. If you can reasonably crank a stereo (aftermarket) and cover the sound of the exhaust, it's acceptable.

When I had only a main hiflo CAT and my sidepipes, it was too loud at 1500ish rpms. Really it was a little too loud in all ranges. When I sucked it up and re-installed new front CATs, life was good. The sound went slightly too quiet but better than the alternative. It sounds like a factory car with Magnaflows.

With longtubes, I have to remove the front CATS. There's no room. I might be able to install a single-chamber muffler and bullet cat (3 or 3.5") back to back in the middle, but I'm not sure that would be quieter than what I have now.

As the guy at Powereffects agreed, adding power is like blowing on a trumpet harder. More powerful engines blow out the pipes harder. It's just louder.

Had I known how long these long-tubes were, I would have picked another option -- either 4-1 design or shorties. At least shorties would remove the restriction of factory L98 manifolds. Of course, I could return or resell these mufflers, but I'm excited about their potential.

So, do you think I can get a modded motor reasonably quiet or would the resulting restriction kill too much of the long-tube flow improvement to make it all worth it.

Do these new factory HIPO cars sacrifice lots of power to get them streetable?

gp

 

I guess the main question would be, as I don't fully understand your question, is: Do you still want to keep the sidepipes?

If you don't, a dual exhaust with main cats would quiet it down a lot. An X-pipe would lower the resonance but give a higher pitch growl at upper RPMs. Also, if you go with chambered mufflers from Corsa, Borla, Flowmaster, etc., they eliminate the resonance from exhaust by echoing it into an empty chamber as the exhaust only exits from one tip. There's lots of ways to muffle the exhaust, I wish I had this problem - I want mine louder!

If you do, then you may have to modify the hell out of the sidepipes and therefore lose their potential for power anyway. Lots of 90 and 180 degree bends will baffle and restrict air flow.

 

I want to keep the sidepipes.

 

I"m not familar with how much room there is under your car but sidepipes doesnt leave alot of room for exhaust options / muffler choices.

BUT the fact remains you can have a quiet car that makes mad power. its all in how well the system flows. You can have sound reduction and good flow, with good power production

Its all in the design. There are ways to similar open header performance with a full muffled exhaust. Just have to spec the pipe size to the engine requirements as well as spec'ing the muffler flow and design to the motor as well.

For instance, for a true dual setup in your case.... a true dual 2.5 inch system of straight pipe will flow approximately 564 cfm each pipe. 3inch pipe will flow 812 cfm.

Rule of thumb is 115 cfm per square inch of pipe cross-sectional area. Also, there is approximately 2.2 cfm required for every single horsepower you make. So a 500hp motor (at the crank) will require 1100 cfm total flow for exhaust. that means 550 cfm per bank. A 2.5 inch true dual system will handle up to 500 hp safely. 3inch will handle theoretically 730 hp or so, n/a power that is.

Most real world testing seems to show 3inch duals are great for 500-600 hp motors while 2.5 is good to around 450 hp but can be used for 500.

But when you add a muffler you have to be careful that the muffler doesnt restrict flow, or else it doesnt matter how much pipe size you have, the muffler is the restriction and you'lll only flow as much as the muffler. 500 hp motor requiring 550cfm per bank will require a muffler that will flow atleast 550 cfm each side. Easy to figure. Most straight thru mufflers like magnaflow will do that. Boxed chambered mufflers like flowmaster most likely will NOT flow that much. typically 300-450 cfm for those mufflers.


Now sound reduction comes into play here. Usually restriction in flow can reduce sound...as evident in stock mufflers. but there are some high flowing mufflers making less noise than the Flowmaster type mufflers in some cases. There have been some tests floating around on the internet concerning flownumbers of popular mufflers and the DB ratings in sound.

A good straight thru muffler like magnaflow or dynomax ultraflo is a good choice to start with. Tube type mufflers in most cases that can fit well under cars, but dont over look the box/oval mufflers they make too. Can work out well, just take measurements

These mufflers have large area of fiberglass packing with a perforated core. Look for mufflers with large enough flow for your application and then look at the design for sound reduction. Lots of fiberglass packing is a good thing. Cats will help too but they tend to restrict flow alittle. Their screened/catacomb like core can flow ok in high flo cats, but really do reduce sound by deflection/canceling of sound waves

Superturbo from dynomax can flow over 400 cfm which is good for 360-375 ish hp. Whats your goals?

 

O.K., let me try this again....

I probably didn't make it clear in my first post. I currently have stock manifolds, MagnaFlow front cats, 2.5" front "Y", hi-flo main cat, and SideEffects SidePipes.

The sound at lower rpms was too loud/resonating w/o the front cats in this config. I know cause that's what I tried first. Putting front cats back in was like adding pretty decent performing mufflers. IOW, they removed all the low resonance.

I wanted to keep the same setup but add long-tube headers. Now that they've arrived, there's no room for the front cats. Since headers and engine mods will increase volume, I know it would be too loud.

I want the performance increases w/o the window-vibrating sound. And, I'm fine with healthy growls w/o nasty resonance.

I'd also prefer to keep a CAT in there. Maybe I'll have to go with a normal flow cat to get it quieter???

The point is to avoid quashing the gain of the headers. Otherwise, what's the point.

Edit:
Oh yeah... As I said in my first post, I only have 24" in the mid-section for setting up my CAT/Sound deadening. That's it.