I have a full 3 inch exhaust going into 2 1/2 mufflers. Actually po did. I am not sure if I should be running 3 inch mufflers or not. I have been reading about exhaust and there are a lot of different beliefs on muffler sizing. Motor is naturally aspirated but not stock. I am curious to hear your opinions and what you believe I should do.

Thanks.

 

Exhaust sizing is personal preference & space limitations along with budget limitations.

Most say 2-1/4" & 2-1/2" is just fine all the time on a C4.
They are likely correct too on the street driving most of the time cruising lightly.

Anything to me with 500 flywheel HP & 400 ci or more should have dual 3" exhaust headers back.

Just my opinion & own personal experience.
Also know from years earlier when a few of my engines were on Superflow engine dyno's,
Dinky headers & exhaust system killed HP numbers fast above 6K RPM's.
Lose 50 HP or more at times.

 

It really depends on the Muffler design. Here's a test that was done by Car Craft on different muffler flow, noise and horsepower. I found it very interesting. All mufflers were 2 1/2 inch

The Mufflers
MAKE MODEL Part No. COST
SUMMIT Turbo 630125 $14.75
THRUSH Magnum Glasspack 24214 $16.50
THRUSH Boss Turbo 17718 $23.95
HOOKER Competition 21006 $25.95
DYNOMAX Super Turbo 17733 $28.50
DYNOMAX Race Magnum 24215 $31.95
HOOKER Super Competition 21106 $35.95
SUMMIT Fully Welded 630325 $38.69
FLOWTECH Afterburner 50322 $39.95

MUFFLER FLOW TEST

MUFFLER Flow at 28-in H20
DynoMax Race Magnum 528.64 cfm
Thrush Magnum Glasspack 507.40 cfm
Summit Fully Welded 343.38 cfm
Flowtech Afterburner 342.20 cfm
DynoMax Super Turbo 333.94 cfm
Hooker Competition 232.46 cfm
Hooker Super Competition 320.96 cfm
Summit Turbo 331.16 cfm
Thrush Boss Turbo 297.36 cfm

MUFFLER Idle dB WOT dB
DynoMax Super Turbo 89 123
DynoMax Race Magnum 94 133
Flowtech Afterburner 92 124
Hooker Competion 92 122
Hooker Super Competion 90 125
Summit Turbo 89 124
Summit Fully Welded 92 125
Thrush Boss Turbo 90 123
Thrush Magnum Glasspack 92 128

DYNO TEST
All mufflers were dyno-tested on a 355-cube SBC with 10.0:1 compression, Air Flow Research 190 aluminum heads, a CompCams 292 hyd.
A Victor Jr. intake, a Holley 750-cfm double-pumper, and 1 5/8 Headman headers.


MUFFLER HP TORQUE 2,500-6,000rpmAverage
Hooker Competition 397.4 381.1 286.8hp/351.9 lb-ft
Thrush Boss Turbo 407.1 384.9 292.1 hp/357.5 lb-ft
DynoMax Race Magnum 409.5 394.3 298.8 hp/366.9 lb-ft
Flowtech Afterburner 409.7 391.2 294.8 hp/361.7 lb-ft
Thrush Glasspack 409.5 389.8 297.7 hp/365.3 lb-ft
Summit Turbo 411.5 386.3 291.5 hp/357.4 lb-ft
DynoMax Super Turbo 412.7 387.2 292.6 hp/358.6 lb-ft
Hooker Super Comp 413.8 387.2 292.8 hp/359.0 lb-ft
Summit Fully Welded 415.4 390.7 295.6 hp/362.4 lb-ft

 

I know damn well an old street racer like you wants 700 HP before the Nitrous Oxide is turned on for extra fun.



I can tell you for sure to forget about 2-1/2".

LOL

Kill 100 HP immediate you can use to win on the strip or street too.

BR

 

I appreciate the info.

 

I know you are asking about mufflers specifically but, it's funny the tv show "Gears" just covered exhaust size in the new episode and gave some general information:

I THINK it went something like this:

up to 250 hp: 2 inch is fine (either dual or single exhaust is fine)
250 to 350 hp: 2 1/2 inch (either dual or single exhaust)
350hp to 450hp: 2 1/2 inch (duals) or 3 inch single exhaust
500hp: 3 inch duals and larger if single exhaust

larger than 3 inch duals can create clearance problems on most cars I would think; especially a low profile car like a Vette.

 

Since I'm a fan of Vizzard, I'll mention some things I've learned from him....maybe add a couple of my own.

Normally, cats, mufflers, and resonators will be the restrictive part of an exhaust system. While the tubing itself, doesn't really present backpressure, the components installed in it, are. Smaller pipe encourages faster velocity and tunes to lower rpms. Larger pipe to higher rpms. Of course the same is true for intakes.

When you put other pieces into your exhaust pipe, you will introduce turbulence and often back pressure. For this reason, Vizzard often recommends the use of upsized mufflers in a system. For example, if you're running 2.5" pipe, install 3" mufflers. Since the larger mufflers have a bigger opening and exit, their presence is reduced in the system. Performance muffler designs are pretty good these days because they consider pulse vs flow.

Installing mufflers can still induce a restriction though. especially when everything's trying to "recompress" thru the exit hole of the muffler. Having a bigger exit hole helps. (If you weren't a student of geometry, know that modest changes in a circle's diameter produce big surface area changes. That's because PIE ARE SQUARE! ) In other words, reduce a muffler's outlet restriction.

One way to see how OEM designers took this into consideration is to look at the typicial catalytic converter. The opening and exit are funnel-shaped and smooth. That's good for flow. More importantly, their cross-section is much larger than the pipe in which they're mounted. Inside a converter, pressure is lowered, and fumes pass thru their restrictive cells more readily. (But, even those cells have laminar flow....which isn't bad.) That's why converters aren't as bad as people think.

That said, street mufflers have a different purpose. They are only there to make things quiet. (And, to reduce resonance or annoying sounds.) In a street car, gaining/losing 10-20 HP takes a back seat to everyday driving and comfort. Mufflers become more important.

Sometimes, owners install full 3" systems only to find the sound and resonance are overwhelming. After spending that kind of money, it sucks to consider a move back to smaller pipe. OTOH, a move to smaller mufflers can accomplish the same thing. Maybe that's what happened to your car?

If the mufflers are well-designed, performance mufflers, you're getting good flow through their bodies. The only restriction is when gases come back out....when they come back out that smaller hole.

One thing you find with restriction is the restriction often depends on its size AND length. That means shorter restrictions are less problematic. In some cases, they present very little problem. In the case of Kook's-style muffler collectors, a reduction in size can actually help -- since it speeds the escape of gas at the output area.

Bottom line: If you like your system as-is, don't mess with it. Getting a performance exhaust setup where it doesn't drone or make too much noise, is tough. Some spend lots and lots of money trying to get it right. While there are some guidelines to follow, there's no real science (yet) to predict the outcome of sound.

Maybe that's next on the horizon.

 

If you ever spend time reading in C4 ZR-1 tech Greg,

Those guys have available out of the box 2" inch primary tube headers with BIG COLLECTORS.

And they have true 3" dual Exhaust systems.

Drives me crazy us pushrod V8 C4 guys can not have the same without custom fabrication work.

Those "Z" guys could care less about exhaust drone down the road.
I don't ever recall any of them complaining.
Just winding out that DAUL OHC 32 valve Lotus designed/ Mercury marine V8 to 7K to 9K is all they care about.
Running with the latest C6 muscle & often beating them on the strip too.

We should have the same 2" inch primary header & true 3" daul ex power fun too!!!

 

Your 2 1/2" mufflers are fine.

 

My car has true duals with an H pipe, dual cats and 1990 convertible corvette mufflers. It has ported factory 1989 exhaust manifolds and the head pipes are 2 3/4". Coming out of the H pipe, the pipes narrow down to 2 1/2"
I high centered my car going in a driveway about a month ago. The cats took the hit.

Instantly, the exhaust was banging, so I knew the bricks in the cats were broken. I didn't hit them very hard either.
I lived with the noise for awhile, then had dual bullet cats installed. When they removed the cats, after driving with them banging around in the exhaust , the right cat had nothing inside except the air tube, and the left one had a section about the size of the 2 1/2 inch round pipe, only it was about 4 inches long, stuck on the air tube, but really loose.
When the new cats were installed, I couldn't tell any difference from when I drove on the muffler shop rack and when I drove away except for the noise. (engine still retained a lot of heat)
Now, I have a little rumble noise that seems to be under the cargo area, and the tailpipe sound is a little tamer, but basically unchanged from no bricks to bullet cats. The rumble noise seemed to gradually get to a level, and now it's no louder.
Soooooooo
I didn't try taking the mufflers off and running it that way, but the muffler guy took my mufflers off, and shook some of the brick material out, but he didn't get much. So, I think most of the material pounded itself into powder and exited the car.
I think my situation is a little unique because one tailpipe on each muffler is baffled, and one tailpipe on each muffler has a 3/4" hole in the blocked off pipe that gives it the 69 camaro big block sound. I think the brick material blew out of that hole, and the remaining brick material would have eventually bounced around until it was small enough to go through that hole also.
These mufflers have a heimholz chamber, and the added holes in the dummy pipes, that add additional airflow and noise.

I have a 383, and it is very torquey. What I have noticed is that when the engine is in open loop and the computer isn't fully regulating with all parameters, the cold engine has more torque than it does leaving a stop sign 2 miles away after the thermostat is open and the water temp is 180 degrees.....I don't know if it's because the new bullets are there, or I need a better tune.
How does this help?
My conclusion is:
2 1/2 inch pipes are ok for a 383 with open un restricted tubes that dump into factory mufflers, since cats or no cats, it seemed to run the same. So without cats, just the housing, I could feel no difference, indicating that the tubes were big enough.......Remember, I have an H pipe, so that might offset the pulses' pressure waves.

basically I had a restricted exhaust that became un restricted (to a point) and then was restricted again, and the only difference was that the tune was affected. The bullet cats have a smaller brick than the universal cats did, but I have noticed with the smaller cats, my trans tunnel isn't as hot. That's a plus because it affects my ice cold drinks in the cup holder.
So for low speed grunt, go smaller tubes, and for high engine speed horsepower, go with the big tubes.

 

You are a Veteran Piston small aircraft Mechanic & Jet Mechanic I recall from the past talking.

Those old late 1930's- 1940's Giant WW2 Radial Engines.
Stearman's TOO.

They have a true 5" to 8" inch exhaust outlets.....

They fly in & out of the Peru, Illinois & Peoria Airports.
Airshows too.

I have stuck my right closed fist down the exhaust pipes.

Plenty of room to spare.

LOL

When those planes are pulling under full throttle,
its like 2,000 Harley Davidon's stroker 110ci + being unleashed.

They are just BAD AZZZ those old monster radial airplane engines.

I love them too.

Bigger the exhaust..........the better.

Worry about the noise cops afterwards I say.



Brian R.

 

Yes, I am an aircraft mechanic. I work on just one aircraft anymore. It is a twin engine business jet.
Those old radials and mostly all aircraft with piston engines usually operate at 3 speeds.
Idle, wide open, and cruise power. Those with variable pitch props add more problems to the mix.
Autos operate at any speed at any time, and rarely at max throttle.
Aircraft exhaust systems are designed to be efficient at cruise, and adequate at max power.
Aircraft engines are not high speed engines, usually operating at 3500 -3750 RPM.
Pistons even for a 200 horse 4 cylinder engine are usually 5.5 or 6 inches diameter. those pistons speeding up and stopping at each end of the stroke of the crankshaft represents a lot of mass accelerating and stopping on each stroke, so the RPM is limited, plus the smaller engines are opposed, like the corvair engine, with no central intake manifold as part of the engine case, but rather like the TPI manifold with long runners for maximized torque.
Cylinders are replaced individually such as a volkswagen or a corvair, and each cylinder head is pinned onto it's cylinder.
Radial engine exhaust is large because the typical "header" is accumulative, and serves multiple cylinders, starting out with just one and may end up serving 7 or 9 cylinders before it snakes out of the cowl. No mufflers whatsoever. The engineers relied on the air blowing back from the propeller to muffle some sound, but replaced that muffled exhaust with the scream of the propeller smashing air molecules.
They are interesting to work on, and can be frustrating if you troubleshot a problem, it turned out to be something else, and you have to tear it down more than once.
Just bear in mind, each engine is designed by engineers, approved by the FAA and given a type certificate that prohibits any deviation except by FAA notice or manufacturer's service bulletin.
That means that a 200 horse engine that is certified to conform will always conform to the manufacturer's specifications and the only deviations will be to restore the engine to the original specifications. Any unauthorized changes will put the mechanic at risk of losing his license and there is jail time if it can be proved that the alteration caused loss of life or property damage or both.
Mechanics are prohibited from repairing a device or appliance without previous instruction on that part and supporting documentation for repair. That means that a mechanic cannot disassemble a part like a magneto or generator without having done it before under the instruction of an accomplished repairman and he has to have documentation for the return of the part to it's original dimensions, clearances, torques and tolerances and be able to test the part before returning it to service, then testing that part on the airplane before returning the airplane to service. It just keeps heavy metal objects from falling out of the sky.

 

See! I told you there's nothing wrong with a torquey TPI! He$$, you can down-right fly with one!

So really...It sounds like the OP needs to put his entire exhaust system back to stock until he obtains EXPRESSED WRITTEN CONSENT from the FAA. (Obviously, he is trying to fly by running a 406 with 3" exhaust!)

 

That is why I am not an Aviation Mechanic Greg........

I would want to Hop up / Hot Rod every single piston powered plane present.

The FFA would frown very much on a Guy like myself.

 

Those old Radial Airplane engines are an engineering marvel.
Heart of that Engine is the "Master Rod".
Then the special "MAGS".
They start easier if they have an impulse unit installed.

My late bud was one of the few in the USA certified by the FAA to service & rebuild all piston powered aircraft Mags.
He taught me how to.
Being a hotrodder, Vertex is what I prefer.
I am not certified by the FAA to repair & rebuild aircraft mags.
Though my late friend said I could easily do it with my skills & knowledge.

I admire the work you Aviation mechanics perform.
The paperwork is incredible & sign off sheets for legal documentation.
Fill up a filling cabinet with a complete plane & engine overhaul.

An old friend came by with his 1955 Chevy Belair this past week.
I call him "The Canadian".
He is a crazy hotrodder like me.
Loves his Tri- Chevy's just like you do too.

When he left.
He pulled his '55 on the highway & lined her up straight.
Wound the 383 sbc stroker up to 5K & launched her.
Rear tires just about caught on fire.
Then he was banging every gear of the Muncie 4-speed at 7K RPM's.
410 gears out back in the 9" Ford.
Holley 4- barrel, looked like 1 of those special old 900cfm Holley 3-barrel 427 Yenko units with vacuum secondary.
This guy is about 58 years old too.

He burned rubber in every gear.
Left a pair of Posi- traction tire marks for about 500 feet in the blacktop highway.
I was just laughing my azzz off.

If he would have let me drive his '55,
I could have managed a low 12 second ET 1/4 mile time for him I say.
Enough to blow the doors off most anything.
C6 included.
Hooking up is the key & perfect 1-2, 2-3, 3-4 shifts in the engines peak power band.
Muncie 4-speed set up right & good Hurst shifter,
I shift very clean, near an automatic car running down the strip.

Brian

 

Wow Brian, that brings back memories. When I came back from overseas in 1969 I bought a 440 Charger R/T brand new. I knew very little to nothing about engines. My first mod was to put a Holley 1050 CFM 3 bbl on my stock 440. I put the distributor back in out of time.

After a while of trying to start it and wearing out the accelerator pump it finally "burped"/blew up whatever. I guarantee that a ball of fire about the size of a weather balloon blew out of the carb, rolled under the hood and chased me half way down the block. When my eyebrows grew back I was good as new.

 

I have done that also to myself in the early 1980's.

Revving up old V8's grabbing the throttle and hitting around 7K floating the valves.
Running an air cleaner just was not coool or manly.

Intake valves floated, huge backfire through the carb,
Not as big of a Fireball........
But still burnt most of my eyebrows off & Mullet Haircut up front.

LOL

Good thing I had lots of hair, just brushed the hair forward to cover up the missing parts.



stupid chit you do at 14 years old in 1983.
Every one is street racing at night.
Including cops off duty.

 

Great insight guys, it is really appreciated. I am going to keep it the way it is, even though po was was an older man, he said it was just too loud with the 3" mufflers he had on them.

 

Yea, us older guys are innundated with common sense and reason. We are also tired of some stupid loud exhaust drowning out our radios and putting screws back in that have vibrated out from the drone.
also, something else to consider is that the exhaust is a lot cooler by the time it gets to the mufflers. Losing thermal energy will actually shrink the air from it's initial volume coming out of the engine. That far back, the mufflers would be more efficient than say, header mufflers that get a higher temp right out of the header.
Not to say that some header mufflers aren't efficient, but a really efficient header muffler is actually an air flow straightener and expansion chamber.