The time has come to replace the factory exhaust and make that stock 305 scream with a more aggressive and sporty tone.
Like many, I’ve watched Kentucky Tim’s Magnaflow 16829 video and love the end result.
However, some people on other forums think 3-inch systems—and really anything above 2.5 inches—can hurt performance by reducing back pressure and the exhaust’s ability to scavenge. Is this actually a noticeable power loss and a reason to look for a different cat-back kit with smaller tubing, or are others just being dramatic? My engine is a stock 305 TPI and by no means a beast, so I’m not looking to make any crazy power gains, but I also don’t want to sacrifice what little performance I already have just for a little extra sound.

I later want to install some headers for a bit more of a sporty feel and to help it breathe better, so maybe now is a good time to install some shorty or mid-length headers to regain a bit of low-end power that a 3-inch exhaust system might lose.

 

Answer to your question is no! Over the years engine builders have proven that back pressure is not of any concern.
May be more concern about header length. This info originated on "Engine Masters" tv show where they test all these setups to obtain max hp & torque.
These guys know their stuff!

 

anything gained, or lost, you wont feel in your car on the street.
On a dyno you will see hp gained or lost, but thats it.

Unless youre above 500hp and doing some type of race class, you wont know anything, but youll hear the sweet sound

 

NO engine needs "back-pressure". Speed or velocity maximizes performance, and if the pipe is too large, the exhaust gases can actually slow down.
These cars need all the help they can get in the exhaust department. A single 3-inch pipe is not going to reduce performance...

 

You won't regret a 3" cat-back. I replaced everything from the manifolds back with a 3" system on my 305 TPI 20 years ago (aftermarket Y-pipe for manifolds, 3" cat, 3" cat-back [not Magnaflow]). Improvements in the way it runs, and, especially, the way it sounds are very noticeable. I have video on the cat-back sticky thread, but it doesn't play anymore because the hosting site no longer exists, and I don't think TGO hosts videos. I'd have to post it on youtube, but my youtube page is for other interests, not cars, so I'm not going upload it there. That was 4 computers ago, so the video resolution looks kinda squirrely on my current laptop, but the audio still sounds good.

The only issue you might have to overcome could be the size of your cat. If the 305/auto has a 2.5" cat and exhaust, then you might have to get creative mating the cat to the new 3" I-pipe, or it might bolt right up, I couldn't say.

 

First off I suggest a real world dynojet back to back comparison. Probably some online. To actually answer the question.

Now the rest of this is theory
In theory, and I am a engineering PhD teaching at university level keep that in mind, in theory there is going to be some loss whenever the increase or decrease in exhaust pipe diameter inducts turbulence, disorganization, or adds friction. Generally, it is safe to assume exhaust flow is already turbulent flow. But there is a difference between turbulent flow and disorganization brought in by entropy of a fluid in a gas state - gasses, especially high temp gas, has an internal kinetic energy randomizing component vector which at microscopic level is a specific vector but at macroscopic level looks like random disorganized chaos. The more internal energy (heating) the gas has, the more it will want to spread out in its container faster, even while its moving in a turbulent flow through a tube. The faster that turbulent gas is moving, the less time entropy has to spread the gas out, so a high velocity exhaust gas is more organized than a low velocity exhaust gas, even when both are turbulent which its safe to assume they are in an exhaust system from a vehicle. Unless we are talking very modern high efficiency OEM exhaust systems which may produce laminar flow in some sections, a delicate balance but not applicable here probably.

SO yes velocity is a kind of key component of maintaining organization flow in a turbulent gas. But there is so much more to unpack here that we have not addressed.
The next thing to point out is that, as velocity increases by either cramming or fluids(air is a fluid, exhaust is a fluid) per unit volume (density matters too) or by making a pipe smaller, increased velocity also increases friction at the pipe walls. Increasing friction means higher energy losses for the flow, which slows it down, radiates that energy, and adds 'back pressure' so to speak by raising the pressure due to friction and lost velocity. In other words, high velocity gas has lower pressure than low velocity gas, and if we have a section of pipe that has initially a high velocity gas and on the end it has a low velocity gas, there will be a pressure gradient from low to high in the direction of flow. In other words, pressure rises as a gas flow loses energy and velocity along a tube, and pressure drops anywhere velocity speeds up such as where a tube necks down(this is how venturi works). In an exhaust system, the exit has to be atmospheric pressure. So how can atmospheric pressure be 'highest point' doesn't gas flow from high to low pressure? The reason these ideas are both correct and seem backwards is because non-engineering terminology in the hands of general mechanics and would be car enthusiasts almost always fails to identify pressure energy as a component of both pressure and velocity. In other words, velocity energy also known as macroscopic kinetic energy component vector of the gas flow, is part of the pressure energy that was converted from pressure to velocity for a time being. This can easily be illustrated by undergrad fluid mechanics equations such as energy equation and Bernoulli's (Bernoulli's is just a modifed form of energy equation). Bernoulli's shows us that the pressure + velocity (in automotive systems where z axis is negligible generally) can be exchanged. So for example by the engine where exhaust exits at high pressure and begins to flow down the tube as a hot high velocity gas - it will have the lowest measured scalar pressure compared to atmospheric but the highest driving scalar pressure as measured by the unmoving source of pressure from the combustion chamber. The gas doesn't drop in pressure measurement until it starts moving. Then gradually the velocity creates friction and energy is lost and the pressure begins to rise along the tube, as measured by a scalar pressure tool such as a map sensor.

And we have only begun to understand the scratched surface of exhaust flow character in performance apps. There is still harmony/frequency/Helmholtz which is important even in turbo apps that can make or break an exhaust performance. And must be timed to the engine's abilities and systems such as cam events. This is another reason not to go aftermarket with exhaust as vehicles become more modern they depend more and more on energy scavenging from Helmholtz. Many older vehicles as well just to different extents practically require it with OEM valve timing just to function adequately economy and torque wise. Many times the torque lost due to large systems is mainly due to the lack of proper Helmholtz rather than the velocity and scavenging influence of velocity I think.

In terms of velocity and 'back pressure' I think the term back pressure is a misnomer or incorrect statement. I don't think it really exists the way people imagine it maybe. "back pressure" what we really mean and find in exhaust systems is that friction is an energy loss and potentially disorganizing component, and increases with velocity and pipe wall roughness and other minute details. When searching for high velocity gas the 'back pressure' losses cannot be avoided, so it is incorrect to say 'you want velocity, not back pressure' since you can't really get more velocity without increases the friction-wall interactions which cause energy losses associated with a rising pressure or back pressure in any fluid flowing system. When friction takes the energy the pressure rises because velocity converts to pressure and some energy is lost to the pipe wall as heat, AND the fluid inducts more disorganization(just imagine rubbing your hand over a cheese grater(the wall is like a cheese grater roughness sometimes) how disorganized it would make your skin surface, and then imagine trying to reorganize all of that with a flowing fluid, it takes more energy).

Now lets talk practical application and experience , I also have 26 years tuning turbo daily drivers of all makes models sizes 4 6 8 cyl engines motec haltech aem big stuff megasquirt power fc holley alcohol nitrous drag roll road etc
Generally it is difficult or nearly impossible to create/fabricate and test a laminar exhaust system in a one-off performance installation so lets set that aside
When designing a performance 'aftermarket' system is calculate the exhaust gas velocity given its pressure and volume flow rate at various tube diameters and select a tube diameter based on the minimum and maximum expected flows that meets the application demands. For example daily drivers I size pipe a bit smaller than optimal for peak power because it will give gains in the mid and low range and sacrifice up top. This applies to turbocharged vehicles and natural aspirated. As many have said, velocity is a key. It is a key to keep velocity up, but way away from the speed of sound, both in the intake and exhaust systems. Exhaust flow seems to have a great organization and velocity until it hits roughly 220-250*F so we like to keep the temperature trapped inside the tube as much as possible using wrap, coatings, insulations, sheets, shields, paints, whatever it takes, and then cut the exhaust off (end it) wherever the temp finally drops below that range. The reason low temp exhaust gas is unwanted is the temperature is what keeps the flow velocity up and gives it energy and keeps it from becoming disorganized, but once it hits about 200*F it slows and starts to 'wander around' creating additional turbulence like an invisible cork at the end of the tube. An old trick is to cut the exhaust off at the point where it stops immediately boiling water. Since we enthusiasts have no way to test Helmholtz it can be a trial and error situation for some vehicles with tiny engines that depend heavily on that technique but for V8 engines especially turbo the best way to remove/add low and mid-range power while ignoring Helmholtz is to play with velocity using a larger cutout exhaust, which can be closed for high velocity at low speeds in a smaller pipe and open for high velocity at high output where volume flow rate is high enough to utilize the extra space of cutout's larger pipe diameter and reduced friction of an open exit near the engine. Making the cutout extend as long as possible to conserve heat is also desirable. In both cases you wish to keep the heat as long as possible along a tube as it acts like a siphon as long as its hot fast moving exhaust.

I hope this has been interesting at least

 

i have really missed your lectures over at hptuners

 

I knew if I left for a while, they would tear themselves apart. Now there his historical behavioral evidence, trends, I can point to, when dealing with the upper management.

 

Kingtal0n;
Excellent response!.
I do understand a lot of it. I would say that the normal car owner should use whatever exhaust fits is desire, sound & budget.