Do glasspacks really add performance or are they just for sound?
Also do you get a better sound from a shorter one or from a longer one?
Glass packs and other mufflers do not âadd powerâ. Glass packs will reduce the power loss from a restrictive factory exhaust at the cost of a lot more noise. A glass pack muffler is basically a single perforated tube with fiberglass packing around the outside of the tube, and a larger non-perforated tube around that as a casing. Very simple, and almost as open as a straight pipe. But, very little sound suppression. You may free up a few HP with them, but the sound from a big-block V8 will be fairly uncomfortable for any period of time in the car. OK for a local cruise, but hard for any travel.
Depending where you live, this may be illegal. Local noise bylaws often forbid modified exhaust systems. I have personally been ticketed in the past for âunnecessary noiseâ just because the muffler sprung a leak.
On a 1956 Chevy a glass pack would likely add a noticeable amount of power and if not driver harder, it might even provide better mileage.
On a 2006 Chevy it likely would not do enough to be noticeable nor would it likely provide a measurable difference in mileage. Modern cars are far different from the cars that made glass packs popular. Also many areas have laws prohibiting them. If they interfere with the pollution control elements of the car they are illegal.
IMO you get the best sound from the original exhaust system. If noise is what you want, and it appears it is, why have anything at all? Frankly I hope you grow up and realized that quiet is good. Not aggravating others is also good.Im not looking for a really loud sound, Im looking for just a rumble that aint going to get me arrested, and if you live where I live almost everything has either fart cans or them big exhaust tips on the back of cars and trucks.
âhorsepowerâ is one word.
What kind of car is it and what model year? As Joseph said, there should not be much effect on newer cars.
Its a 1975 Ford F150 with a 390. I was wondering because I have a 24 inch cherry bomb on it right now with single exhaust dumping in front of the rear end. My buddy told me that you need a bigger one with single exhaust for a better sound. He has a 1992 Chevy Beretta with a 32 inch cherry bomb where his cat. converter used to be with a tailpipe out the bumper, and it sounds really nice.
I know its one word.
my chevelle had dual exhaust with glasspacks coming off a 283 small block, sounded just like a big block when revvin the engine.
man I miss that thing 
Glass packs are for small blocks that want to sound like a big block. The throaty rumble from a big block w/ a turbo muffler (for example) is much more pleasing and impressive than the rap youâre going to get from a glass pack.
Joseph gave an excellent answer regarding the functional benefit (or lack of) of glasspacks, but none of us can tell you whether youâll get a citation. That varies widely from locale to locale.
Try it. Save the old muffler in case you need to put it back on.
But hopefully youâre enjoying the house you were able to buy by selling the Chevelle.
Years ago, our neighbor put a gutted muffler on his 1946 Buick and claimed that it gave the engine more power. It sounded like the buses we used to take on tours. On one of the bus tours, I asked the driver about the engine in the bus. When we stopped, he took me around to the back of the bus and showed me the engine. It was a straight eight Buick engine just like the one in my neighborâs Roadmaster. The bus had an unmuffled exhaust system, so I suppose it was done for more power. This was before diesel engines were common in buses. Iâm certain that most of todayâs cars have exhaust systems that have much less back pressure than the exhaust systems of the 1940âs and 1950âs.
As to engine sound, this is a matter of personal taste. I prefer the sound of an old 2 cycle Lawnboy mower to that of a 4 cycle engine. The Lawnboy engine sound reminded me of an outboard boat motor and that I would rather be fishing than mowing lawn.
Iâm from way back in the fifties when noise was in. However, you do not gain horsepower, although it may appear that it does. All youâre doing is removing back pressure from the huge mufflers that are normal on new cars. The less back pressure, the quicker the exhaust leaves the engine.
Since itâs an old truck you may have success. Replace the old pipes with larger diameter pipes too; something like 3-inch diameter.
You will get a better sound from dual-exhaust. The reason is more seperation in the exhaust pulses. Glass-packs will âblow outâ and the sound will change to a harsh sound. Turbo ovals sound great on old Ford motors as do straight-through Thrush mufflers. With a more open exhaust, you will be gaining top-end flow at the expense of some bottom-end power. However, you have enough spare with the 390 not to have to worry about it. As for your friend with the cavalier, he is comparing two different engines. His engine will benefit more from a single exhaust because itâs a 4-cylinder and you need the other exhaust pulses to help with scavenging. A V-8 has the same issues but, each side (which is a 4-cylinder) has itâs own single exhaust. You can also put a crossover tube to further assist with scavenging as the natural firing order of a 390 doesnât do the best at it but, youâre after sound anyway so, it doesnât really matter. Just remember, loud doesnât mean fast or powerful and no 1992 cavalier sounds good!
On newer vehicles it does not have near as much of an affect as on older vehicles; e.g., the older carbureted big block engines.
A dual exhaust with glass packs can add some horsepower to them.
I have a very old manual here with specifications on a lot of the older 50s/60s era big block vehicles and on average, dual exhaust added about 10-15 HP as compared to the single exhaust version of the same car. This was with the factory mufflers and one could assume that a dual system with less restrictive glass packs should do a bit better than factory mufflers.
Using the word âfreeâ instead of gain Is ignorant yeas you gain horse power everyone knows that
There can be an exhaust sound improvement, but I doubt you will notice an hp gain.