The Art of Exhaust Systems: Understanding Flow and Scavenging

When it comes to exhaust systems, many people focus on power output and sound quality, but there's another important aspect to consider: flow and scavenging. In this article, we'll delve into the world of exhaust systems and explore how to increase the pulse velocity and create a better breathing cycle for your engine.

The Problem with Backpressure

When you're working on an exhaust system, one common mistake is to focus too much on backpressure. While it's true that backpressure can be beneficial in certain situations, such as with turbocharged engines, it's not always the best solution. In fact, excessive backpressure can actually decrease power output and overall engine performance.

So, how do you balance flow and scavenging? The answer lies in understanding the relationship between exhaust diameter and velocity. When you decrease the diameter of the exhaust pipe, you increase the speed at which the exhaust gases flow. This is because the same amount of air is being forced through a smaller space, resulting in a faster velocity.

The Scavenging Effect

Scavenging is an important process that occurs during the exhaust cycle. It's when the exhaust gases are pushed out of the engine and into the atmosphere, creating a vacuum behind each pulse. This vacuum is essential for creating power, as it allows the engine to draw in more air and fuel.

However, scavenging can be difficult to achieve, especially with larger diameter exhaust pipes. This is because the backpressure created by the larger pipe can actually counteract the scavenging effect, reducing its effectiveness.

Striking a Balance

So, how do you strike a balance between flow and scavenging? The answer lies in finding the perfect diameter for your exhaust system. If you're looking to increase velocity without sacrificing too much backpressure, you'll need to find a compromise.

In general, a smaller diameter exhaust pipe is better suited for creating high-speed flows, while a larger diameter is better suited for reducing backpressure and increasing scavenging effects. By finding the right balance between these two factors, you can create an exhaust system that works seamlessly with your engine.

The Role of Exhaust Diameter

When it comes to exhaust diameter, there are several key considerations to keep in mind. As mentioned earlier, a smaller diameter exhaust pipe is better suited for creating high-speed flows, while a larger diameter is better suited for reducing backpressure and increasing scavenging effects.

However, the relationship between exhaust diameter and velocity isn't always straightforward. In some cases, decreasing the diameter of the exhaust pipe can actually decrease power output, especially if it's done without proper adjustment.

So, how do you adjust your exhaust system to optimize performance? The answer lies in fine-tuning the fitment of the exhaust components, such as the muffler and downpipe. By ensuring that these components are properly fitted and adjusted, you can create a seamless flow of exhaust gases that enhances engine performance.

A Word of Caution

When working with exhaust systems, it's essential to be mindful of potential issues, such as hand prints and smudges on the pipes. These imperfections can create problems down the road, especially if they're heated by the exhaust gases.

To avoid this, make sure to clean your exhaust system thoroughly before installing any components. You should also take the time to lubricate all hardware and ensure that it's not over-tightened, as this can create backpressure and reduce performance.

Putting it All Together

In conclusion, creating an effective exhaust system requires a deep understanding of flow, scavenging, and fitment. By finding the perfect balance between velocity and backpressure, you can enhance engine performance and create a better breathing cycle for your vehicle.

So, remember to prioritize fitment and fine-tune your exhaust components to optimize performance. And always keep in mind the importance of lubrication and proper adjustment to ensure that your exhaust system is running at its best.

Let's Take a Listen

Now that our exhaust system is installed and tuned, it's time to take a listen and see how it sounds. (engine revving) Ah, that's music to my ears! The sound is smooth and refined, with just the right amount of bass tone to get your engine purring like a kitten.

But don't just take our word for it – let's put this exhaust system through its paces and see how it performs. (engine revving) Wow, that sounds pretty good! Not too bad, not too loud. It's exactly what we were looking for in an exhaust system.

And the best part? We didn't feel any significant power difference. But we know that with a turbocharger coming down the pipe, this exhaust system will make all the difference in creating more power and performance.

Thanks for watching, guys! If you enjoyed this article and want to stay up-to-date on the latest exhaust systems and engine modifications, be sure to subscribe to Donut Media and follow us on Instagram @zachjobe. And if you happen to like what you see, consider sharing this video with your friends and family – we appreciate it!

The Final Word

In conclusion, creating an effective exhaust system requires a deep understanding of flow, scavenging, and fitment. By finding the perfect balance between velocity and backpressure, you can enhance engine performance and create a better breathing cycle for your vehicle.

So, remember to prioritize fitment and fine-tune your exhaust components to optimize performance. And always keep in mind the importance of lubrication and proper adjustment to ensure that your exhaust system is running at its best.

Thanks for joining us on this journey into the world of exhaust systems! We'll catch you in the next article!

WEBVTTKind: captionsLanguage: en(car engine revving)- Oh, sounds like a BMW N55.(car engine revving)That one sounds like an RB25.(car engine revving)Oh, that's a Toyota 2JZ baby.(car engine revving)(Eddie speaking off mic)- Something drive by?I didn't hear anything.Now how did I know any of thatwithout seeing any of those cars?Well, for one, becauseI wrote this episodeand those are me and my roommates cars,but two, because I could hear 'em.The exhaust note of the caris basically its mating call.What your car sounds like isa big part of who your car is.It's like its signature.Having a cool andrecognizable exhaust soundis one of the main reasonsthat an aftermarket exhaustis one of the first upgradesa lot of people do to their car.They come in different shapes,sizes, and of course prices,but they all promisepretty much the same thing,slightly more horsepowerand a better and generallylouder overall sound.But will they always give you more power?And if so, how?And why does one pipe sounddifferent than another?Today we're gonna install a $600 MagnaFlowcat back exhaust onto the Miata.And along the way, I'm gonnaanswer all of those questionsand more and then we'll take it for a spinand I'll give you myopinion on whether or notit was worth it for theold Money Pit Miata.And hopefully that'llgive you enough contextto make the same decision for yourselves.I'm Zach and this is Money Pit.Let's do it.(thunder crashing)(car engine revving)Now today's challenge is tomake the Miata sound like that.All right, so we're cruising aroundwith our stock exhaust right now.Take a listen.(car engine revving)It doesn't really sound like much.So we wanna put an exhauston it that's gonna flowbetter than stock andsound better than stock.So let's go back to the garageand I'll show you what we got.All right, this is theexhaust that we got,it's a MagnaFlow.It costs about $590.It's made out of stainless steelso it shouldn't rust or corrode.Let's get the bubble wrap offand see what we're looking at.Woo!Shiny.So you can see we've got some welds here.They look nice.This thing is weldedtogether by robots I believe,robots tend to not make mistakes.So that's cool.So build quality seems pretty goodand really it should for600 bucks for a cat back,it's not the cheapest thing in the world.Let's get the rest of it out.Oh, what's this?A little plug for a bum.So this is our resonator.These are used to cancel outdrone and harsh frequencies.And you know a rasp.That's a big muffler.We got a big honk andresonator and we only havewhat is it?A 1.8 liter, four-cylinder.I think this should be pretty quiet,which honestly is what I want.I don't wanna make thatmuch noise with the Miata.Yeah,- Old man,- It's a 100 horsepower four-cylinder.We don't need to scream about that.We can just be quiet.We can just(revving)Like that.So the way a resonator works,basically exhaust comes flowing throughand you can see thisresonator straight through.But this outer case ispacked full of material.Sometimes like a fiberglassor something thatabsorbs sound and energy.So as the exhaust goes through,the packing materialabsorbs a lot of sound.It's really simple.This is a cat backexhaustfor a 1994 Miata.Now when I say cat back,what does that mean?It means what it says.The exhaust starts atthe end of your stock catand goes the rest of the way back.We're gonna be keeping the stock catbecause again, we're only makinga 100 horsepower right now.So we're okay with theemissions equipment.It might gain us a horsepower or two.It should gain us some sound at least.But, it's gonna take uslike 10 minutes to install.Ten minutes to install.- Want to make a bet?- No.Oops too far.So, my driveway is slanted downwardsand we're lifting up therear to put on an exhaust.That's why I put somethingin front of the front wheelso that it couldn't roll away.Always good to give her a little shake.Make sure your work is stable.All right, let's take thisold piece of (bleep) off.All right, so this isour catalytic converter.So we're going to be replacingeverything from here back.So, I'm gonna start hereand pop off these nuts.But the first thing you should dois use some penetrating oil,ideally the night beforeor even a few days before you get going.And, this will kind ofseep into those threadsand help break free any of that rustso that you not end upstripping your hardwareor rounding off any of your nuts.Okay, so now our nuts are loose.Now we need to pull the exhaustout of its rubber hanger.So we're gonna add a little bit of lubeand then we're just gonnaforce it with some pliersand screwdrivers and our hands.Okay, I think we have one more left.Okay, now(chuckles)Is there a ridiculous way to do this?Now, there's gotta be a way.Oh, so close.Think that will be enough?Eddie thinks so.- I know so.- Eddie knows so.Eddie's the smartest dude ever.Whoa (bleep)Okay.The old exhaust is out.That was pretty easy.Even with a couple of little hiccups.I think it's been 15 minutes.So removal, installation, thiswhole thing is not too bad.So, so far worth it.Alright, so let's talk aboutsome of the pros and consof an aftermarket exhaust tosee what all the fuss is aboutand see if this thingis actually worth it.The point of these is to letyour engine breathe better.The exhaling part by increasingthe diameter of the piping.They'll also make the muffler designa little more free flowingand they'll remove anyunnecessary bends or you know,sharp turns that you don't need to have.Just to try to decrease restrictionand improve flow overallin the exhaust system.Now improved flow anddecreased restriction,can mean you know, a fewextra horsepower up top.They also usually tend tomake more noise than stockdue to the decrease in restrictionand the change in muffler design.Now that part is obviously awesomeas long as the muffleris designed to sound goodand the engine that you're putting it onis capable of making good noises.That's a big part of this.Hit the YouTube and the Google,find examples of your carwith specific exhausts.Chances are if somebody makesan exhaust for your car,someone out there has bought itand someone has made a video of it.Now, in addition to beinglouder because we like that,being louder can also save your life.You may have heard thephrase loud pipes save livesand that can be true,especially on a motorcycleor in a tiny car like the Miata,making a little noise can make somebodywho didn't see you coming, hear you comingand not pull out in front of youor run you over all together.Okay, so generally aftermarket exhaustsare made out of aluminizedsteel or stainless steel,both materials that last a very long time,which is why a lot of aftermarket exhaustswill come with lifetimewarranties like this one'cause the materials are just good.Now, okay, let's talk about the cons.Some cons side of thing.Let's talk about cons, baby.Con artists, they can be expensive.This, just for the Miata was $600.It's not cheap, it's not nothing.Generally speaking, the exhaust priceand just aftermarket partsin general are commensuratewith the price of your platform.So, unexpensive cars,your exhaust is gonna be alittle bit more expensive.Other than that, you know,there are some people out therethat just don't wanna make more noise.I know that sounds ridiculous,but what I'm talking about is drone.(buzzing)That's drone.Automotive manufacturers, OEMsspend a lot of time and moneyand brain power making the stock partsand the truth is the stock partsare usually reallyimpressive in a lot of ways.They always tend to land pretty squarelyat the crossroads of comfort,reliability and power,which is a pretty impressive thing.Once you replace your exhaust,you move away from thatcrossroads of comfortand reliability and youhead down the road to power,which is a fun road, but it isdefinitely a lot more noisy.But how do you know ifyou need an exhaust?Honestly, it reallydepends on the platform.So you're gonna have to do some research.Some cars, like big old V8'sor cars that have areally restrictive exhaustfrom the factory can gain a lot of powerfrom just an exhaust where some carswon't gain much of anything.And then what kind of exhaust do you need?A cat back, an axle back,a header back, turbo back?Literally the AL of thosejust mean that the exhauststarts at the end of that thing and goesall the rest of the way backto the end of the exhaust.And what you need depends on your goals.The more of the exhaustsystem you replacedwith free or flowing parts,the more power you can potentially make.So what size exhaust do you need?Well, the answer is, there's a lot of mathto figure that out, butpractically speaking,there are a lot ofcalculators and charts onlinethat will help you determinewhat diameter pipe to usefor a given horsepower leveland engine displacement.So in that sense, it's pretty simple.Just look up exhaust diameter calculatorand you'll find everything you need.Let's put this thing on.We're just gonna install this pipe firstto the catalytic converter.I think I should be able to do it this wayand then we'll put the back half onsince the MagnaFlow comes in two pieces.Now, this brace on its way back up.Isn't aligning that quite perfect.So, just giving her the old persuasion,sometimes we're gonnahave a car that sucks.Like right now. (chuckles)Before we finish installing this exhaust,let's take a minute totalk about one of the mostenduring myths in allof automotive history.Back pressure.The myth is that you need back pressureto make power or morespecifically low end torqueand back pressure is theresistance your exhaust fightson its way out of your exhaust pipes.Back pressure is actuallycounterproductive to making power.You don't want anyrestriction in your exhaust.Think about a Top Fueldragster running open headers.There's no back pressure there.Okay, but then why do people actually losesome of that precious low end torqueby increasing their exhaust diameter?Well, first we have tounderstand that the exhaustflowing through your pipesisn't a steady flow of exhaust.It's pulse.Every time a cylindercompletes an exhaust stroke,a pulse, a pulse, a pulse,pulse, pulse, pulse.And each time one ofthose little baby pulsesshoots down your exhaust,it creates a little pocketof vacuum behind it.And just like drafting in NASCAR,if you can get up on the guyin front of you's bumper,you can take advantage ofhis little vacuum pocket.Now we want lots of vacuumbehind each of our exhaust pulsesso that the vacuum created can help pullthe next exhaust pulse outof the combustion chamber.So this little vacuum pocket,NASCAR drafting stuff,it's called scavenging.And it's most effective at low RPMsbecause once you get into higher RPMs,your exhaust does start to flow,more like a constant flowrather than the pulse.Okay, so how do we increasethe exhaust pulse velocity?Well, think of yourexhaust like a garden hose.All right, so increasing the speedof your exhaust gases flow.Dang, I gotta- You got a weak flow, man.- Yeah, I know.Well, I have, I have.Oh God.(laughter)Now when the hose is wide open,we're using all the diameter of that hose.We've got really good flow.There's very little back pressureand this thing is cruising right out.We've got a lot of flow here,but what we don't have is very much speed.This is coming out pretty slowly.Now, when you decreasethe diameter of the hosewithout changing anything else,you pretty clearly change the speedthat the fluid is traveling in there.Same thing with your exhaust,decrease the diameterand the exhaust gasesare forced to travel fasterthan when the exhaust pipe is wide open.Great flow, terrible speed.All right, so wait a minute.We want a small diameterexhaust to increase the velocityto increase the vacuum behind each pulse.So we get the scavenging effect,but we want a big diameter, sowe can have no back pressure?Exactly.And the problem is that obviouslyyou can't really have both.So you gotta strike a compromise,like so many things in the car world.So this has been a really long way to say,that you don't want backpressure, but you do want velocityin order to get some scavenging effectsand you happen to get each ofthose things in opposite ways.Okay, the end.Let's put this on.Before we install this,we're gonna make surethat it's really cleanbecause if we have any big old hand printsor smudges on here, and thenwe get hot for the first time,that stuff will burn in to the finish.So like if I put a thumbprint thereand we went and drove for the first time,that thumbprint would bethere pretty much forever.So you don't want that.Just give it a quick wipedown, before you get hot.So, put a little,a little lube in our hangers.Always helps.So, anytime we're doing an exhaust installor something wherethere's multiple pieces,you should kind of leave all the hardwarethat you're working withnot completely tightso that you can get alittle bit of adjustment.So now, we've got all the parts in placeand now all we gotta do is tighten upour exhaust fittings.So, this hardware and that hardware there.And then it's just timeto make some noise.Okay, so, let's take a listenand see if this has all been worth it.(car engine revving)Oh, that sounds pretty good.(car engine revving)Not too bad.It's not obnoxious, it's not super loud.It compliments the intake sound.I can still hear the intake.(engine revving)That sounds pretty good.Let's take it for a drive.(engine revving)You know what?That's exactly what Iwanted out of an exhaust.It's understated, it's not god-awful loud.Oh, it's got a nice base tone to it.(engine revving)It makes this thing sound good.(loud engine revving)(soft engine revving)Now, I don't feel any power difference.I'll tell you that.But, I didn't expect to.The point of this exhaust, in our case,isn't to give us more power now,it's to allow us to makemore power later when we adda turbo and whatever the hellelse we do to this thing.Well, sweet.And on that note guys,I wanna say thank you somuch for watching the show.I hope you're learning.We're learning a lot making it.Make sure you subscribe to Donutand you have the notification bell rungso you know what's coming.We're releasing stuff every single day.And I appreciate you guys,being so involved in the commentsand talking about what's going onand telling me what youguys are working on.Keep it up.Let me know what you're doing,how you're passing yourtime in the quarantine.Throw in the comments, I'llbe in there commenting.I'm always in there.And, if you guys happento like this super sicktotally awesome shirt, youcan buy it at shop.donut.com.Join our gang with the boost creepsand you can be one of us.Follow me on Instagram @zachjobeFollow Donut @donutmedia andI'll see you in the comments.Goodbye.