I have read here that the reason a Corsa cat back does not drone because one of the dual outlets on each side is greatly restricted to something like a 1/4 inch hole and the other is straight through. If Corsa owners could verify that it would be helpful for the rest of my plan.

If this is true and part of the 'drone go away' solution I am thinking about doing the same to my Borla system so I can hear the radio and not get a headache on a 2 hour drive. For me the enjoyment fades after about 45 minutes and did one 5 hour round trip last fall and was exhausted when i got home from the racket.

Dave

 

The Borla mufflers that my car was wearing when I bought it had a terrible drone.
Those mufflers had a single inlet, and two outlets inside the square factory style tips.

I have since replaced the entire exhaust with a complete Stainless Works system.
Long-tubes, cat delete, x-pipe, 2.5" pipes, and chambered mufflers.
The drone is gone.
FWIW - The SW mufflers have a single inlet, and a single exit inside the tips.

Now... it's obviously not a direct comparison, but I have had others state they felt the single inlet / two exits contributed to their drone.

YMMV.

 

My thoughts exactly and the reason for the question. I have the same ones you just described. It seemed simple to make a hacksaw cut close to the muffler slip in a cut to shape stainless with a small hole in it like Corsa uses to basically let out condensation. I am betting Corsa figured this out during their development and have 2 tips as many like the look but hate the sound. In mine the drone is worse will low rpm and large load. If I put it in D instead of OD gain some rpm the drone dissapears so has to be cylinder pressure and frequency of pulses. My thought is blocking off one of the outlets will actually turn the unused side into a damper / resonator to equalize the pressure and help a bunch. If no one has done it maybe I just need to try it and see what happens. I am sure with a stock L98 one outlet is plenty as I have removed the cats in the Y pipe and have a high flow main cat.
Dave

 

My outlets are more rectangular with rounded ends so have to make something and have it welded in then. I can fab stuff but sold the tig welder a while back as the business taxes on it compared to how often I used it did not make sense. Even having another shop do the weld probably 50 to 75 bucks. I think much cheaper than a new pair of mufflers and then not know if I will like those and go to set #3..
Dave

 

Stuff a wad of steel wool in one outlet

 

Well if that works and does not get spit back out I guess it means 1 outlet is more than enough!
Dave

 

Or bronze wool...
Bronze wool won't rust.
You can get it at any boating supply store.

 

Be worth it to stuff something in there, temporary, just to see if it works.
Remember the old 'tater in the tailpipe prank?

 

Hi Dave,
Since I worked at Flowmaster for 2 1.2 years as lead tech advisor (back when L-98 C4's were the rage) I can answer your questions.

Most people don't know that the Flowmaster 425501-L & -R muffler outlets are reduced down (we used to say "doinked down") to 1 3/4". The doink is right at the inlet of the two tips and inside the case where you can't see it. This was done because they are "direct replacement" mufflers - meaning they were designed for a stock or slightly modified car. The stock dual 2 1/4" pipes running through a single 3" cat can support a max of 300-325 hp before the exhaust system starts to become a restriction. The quad tips on the stock mufflers was done for looks, which is why the stock mufflers only had 1 functional tip in the first place.

Placing the mufflers at the rear of the car is not optimal as the exhasut has shed most of it's heat and also reduced in velocity - both of which allow the exhasut to "stack up" in the system. This is one of the factors that can cause drone - faster pulses crash into the slower ones that are piling up in the exhaust system, especially if the physical volume of the system is too large. Reducing the diameter of the system helps to maintain the velocity and eliminate the drone.

You can try to tune some of it out with tuning chambers, but if the velocity drops too low even that won't do it. The bottom line is that a single outlet muffler will typically have less/no drone as long as the rest of the system is sized correctly as well.

If your making stock-range power (250-300 or so) I'd suggest the Flowmasters, although even I am not particularly fond of the tips that are offered. Of course, they can be changed, and the mufflers ceramic coated if you want a different look.

 

Thanks for the insight. I don't see myself doing much with the engine so mufflers that flow a little less but let me enjoy the car more would be be an improvement.
Dave

 

Right, but remember that mufflers (exhaust systems) don't flow a steady-state stream, so the CFM rating of a muffler is pointless. Every time an exhaust valve opens you get a pulse in the exhaust system. That's why lumpy cams lope, otherwise it would just sound like an open air hose.

 

Understood comepletly. I have been professionally racing cars and bikes for a living for over 40 years. I was hopeing the unused side whould act as a pulse damper at best and at worst the decreased area to the atmosphere would smooth out the pulses.
Dave

 

I have a magnaflow setup I made.

3 inch from SW headers into a dual in/dual Magnaflow resonator and then into their mufflers and the drone is killer! Hurt the ear a bit.

I stuffed wool in 1 pipe in each muffler and the drone was greatly reduced, however the restriction kept blowing the wool out.

The key to that experiment is the addional length of exhaust with no exit. It creates a path for the sound waves to travel and deflect back and I think that is cancelling out the drone.

There is 300page plus post in the G8 forums about an individual who added an J shaped pipe just before his muffler inlet and that trick eliminated his drone.

The corso does not have drone because it has chambers for the sound waves to travel and cancel out. The G8 guys figured out there drone was around 100-120 hz and determined the length of the sound wave was 26-29 inches. This gave them the length of the J pipe there were installing.

When I plugged my Magnaflows, that did the same thing, however in the muffler, but it restricted flow.

I was contemplating putting a Flowmaster 40 series in the spot of the resonator to use the chambered muffler to cancel some of the resonant sound waves out.

 

That is interesting as the factory LT1 system is 2.75" but it necks down to 2.25" before/after the resonator and before mufflers.

When I had my stock exhuast on interior noise was never a problem.
Now with my muffler eliminators/x-pipe in place of mufflers/resonator, it is a bit loud after 1.5Hr. Stock manifolds and cats in place and functional. I do get some interior resonance at certain RPM's.

I've thought of putting the stock mufflers back on and leaving the x-pipe in place.

Article from CAC on flowmaster.
http://www.corvetteactioncenter.com/...and...is+Quiet

 

I have the Corsa system and can confirm that two of the four muffler outlets flow far less than the others. I am not an engineer so I have no idea if that is the entire "secret" to eliminating drone and have no plans to hack my mufflers to verify. Good luck with your experiment, hope you are successful.