When I recently Bombed my heap, I planned to include means of keeping things quiet. I didn't want any low rumble at all. A sleeper's sound if you will.
With the hot-rod turbo, manifold, injectors, 4" pipes, etc, I didn't want to restrict things.
So I did a little homework:
[QUOTE=BC847]
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CFM = [u]Engine Displ (cu in) x Full Load RPM x Eff x (Exh Temp F + 460)[/u]
C x 941760
CFM = exhaust flow rate in cubic feet per minute
Efficiency = .85 for naturally aspirated engines
Efficiency = 1.4 for turbo-charged engines
Efficiency = 1.2 for engines with scavenging blower
C = 1 for two-cycle engine C = 2 for four-cycle engine
If exhaust temperature is not available, use:
T = 1200F for gasoline engine
T = 900F for diesel engine
With the above, we're moving right at 1332 CFM through the exhaust system at 3000rpm.
In a perfect world, we'd be able to move all that through the plumbing with no restriction. The closest to that is the straight-pipe system.
As far as noise control goes, (note, I have no first hand experience with any of the brand-name systems), straight-through/glass-pack type mufflers (resonators) attenuate high frequency noise. They let the low rumble through. The baffled type mufflers attenuate the low frequency noise. While they do mute the high frequencies some, they also tend to contribute some with the baffles (it's weird science, do a Google). That's why you see both devices on the really quiet vehicles.
OK.
I've found that (outside of the brand-name packaged deals), many kits offer something like this;
Muffler 4" In/Out 8.5" Diameter X 34" Body Length (Link).
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Rated Flow LR (CFM): 1800
Rated Flow MR (CFM): 2546
Rated Flow HR (CFM): 3119
Back Press LR (In Hg): 1
Back Press MR (In Hg): 2
Back Press HR (In Hg): 3
Attenuation (dBA): 8-12
Note: The rated flow shown,
Low, Medium, and High, is at a given back-pressure (resistance to flow). That back-pressure is presented in Inches of Mercury (In Hg). 1 In HG = 0.4911541 psi, or just shy of 1/2 pound pressure.
In the Donaldson product line, that muffler is one of the highest flowing in a four inch pipe system. But it's also one of the loudest at only 8 to 12 db attenuation.
OK
Using the
Attribute Search (Link) feature of the Donaldson website, I found one muffler to be the quietest while still maintaining a high flow with the least back-pressure.
This one:
M110189 (Link). I call it the BHM (Big Honk'in Muffler)
The specs are:
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A - Inlet Diameter (Inches): 4
B - Body Diameter (Inches): 11
D - Overall Length (Inches): 41.25
E - Outlet Diameter (Inches): 4
Rated Flow LR (CFM): 1050
Rated Flow MR (CFM): 1485
Rated Flow HR (CFM): 1819
Back Press LR (In Hg): 1
Back Press MR (In Hg): 2
Back Press HR (In Hg): 3
Attenuation (dBA): 19-25
Product Type Description: Muffler, Round, Style 2
Primary Application: FORD MEDIUM DUTY TRUCK
Gross Weight in Pounds 35.5
Gross Length in Inches 45.5
Gross Width in Inches 11.8
Gross Height in Inches 11.8
Country of Origin US
As you can see, it flows the required volume with under 1 pound pack-pressure. AND, it attenuates the low frequency noise 19 to 25 db. A BIG drop in noise.
Now, it IS a Big Honk'in Muffler!
With a gross length of 45.5", and a diameter of just under 12", it's gonna be a close fit under my 1st gen (250 CC long-bed / Your mileage may vary). It's also a Round, Style 2 muffler, meaning it's inlet and outlet are not centered. With that, you'll most likely have to move your pipes a bit to get them to plug in.