The BS On Backpressure 

I hear this back pressure myth over and over. Many people don’t have the ability to learn. Thanks for posting.

I knew this instinctively because, back in WWII, all of the very high performance aircraft had nothing but 4" stacks coming off each cylinder. No manifold, no mufflers, no long exhaust pipe, and those were designed to get every ounce of H.P. from those engines that could be had. Also, top fuel motors that make 10,000 H.P. have "zoomies" coming off each cylinder with No BACK PRESSURE. I have argued with people about this for years. Thanks Tony.

ROLL UP YOUR WINDOWS!!! Automotive knuckleheads have be spewing the “Back Pressure” myth for decades. ...The only channel I don’t skip the ads.

All I heard is cut your mufflers off with a sawzall 👍

And proper jetting. Part of the old myth was guys putting on better flowing exhaust and not jetting the carb accordingly. Too lean, runs like crud and down on power, followed by “I guess this engine likes back pressure”.

Engine masters had a show on this subject and proved that exhaust systems with less back pressure did pick up hp.

Uncle Tony is just a fountain of knowledge that never runs out. I actually had this discussion with someone not to recently who was dead set you needed back pressure.

For an old car guy you sure are brilliant, I never really thought of it but you explained it perfectly.

Ive been trying to argue/explain this to people for eons... The relationship of overlap, exhaust velocity and vacuum...But you did it soooo much better. Im just gonna show this vid next time it comes up!!! Thanks Uncle!!!👍☺

There is ONE time when you do want a set, specific amount of back pressure in your exhaust to make the best power... . . . . . . Two strokes.

A friends dad explained this to us before we could drive. An exhaust manifold will eventually gather gases to go in a particular direction , but headers create a vacuum to pull ( scavange)exhaust out of a cylinder. Making the engine more efficient and slightly more power. He worked at Garrett air research. To us teens we got the concept, but my first car had tuned exhaust header from factory , `Oh My' (65 Ford Cortina GT) and so did my 84 Escort. Even though it was 4 cyl, it did make the engine run better than stock cast iron exhaust manifold, both had a cam profile to make more HP. Good lesson to learn at a young age. This vid continues the lesson.

Uncle Tony can you do a video on how to properly size the right exhaust.

Finally! I'm glad someone with an audience is covering this, the funny thing is if you have back pressure at idle where your volume of exhaust is at its lowest then you are only going to get more as the throttle opens and the revs climb. Any machine setup like that will fall down hard shortly after leaving the idle position.

A guy in my high school auto shop class was building a 400 Pontiac. I remember when they tried to start it up with no exhaust, just open ports. Now I understand why it ran so badly.

So glad I found your page. I was an avid fan back from the first issue of Mopar Muscle. Your 12.99 for $1299 still rings in my head, and I was there during the Mustang days, and you were right. It was yhe closest thing to a modern day Road Runner. Anyway, its great to see your vids. Man, just imagine if we'd had this in the '80's.

THANK YOU! I'm so glad you wen't over this! I can't tell you how many older guys will argue about this with me. I'm 25 and I get older men in their 50's trying to tell me Back Pressure Back Pressure etc.

Great video. Finally someone made a video explaining overlap and the truth about exhaust.

Always know I'm smarter after watching your videos , never left scratching my head , what took 3 years of school you explain in 3 minutes

Just another installment of 'Why Uncle Tony Is The Man'. I've been planning a rebuild and a whole new exhaust system for my old '71 F-100. I was planning to go 3" exhaust since the header collector would be 3", but the muff shop told me 2-1/2" would be plenty for a 360 (5.9). Now I understand the basis for determining what diameter to use. Thanks Uncle Tony! You da man!

Excellent video Uncle Tony! I heard of a guy that builds AMC V8 engines that swears by 1 5/8" header tubing and your video explains exactly why he's getting the performance numbers he claims. Scavenging and vacuum drawing the fuel and air during valve overlap is something I never would have thought about!

Yes! I couldn’t even tell you how many dipsticks have told me “engines need back pressure”! I knew it was bs as a little kid 😎🤓

YES !!! Finally a video that saves me 5 min of my life explaining all this. Tuning your exhaust size and length to the cam shaft is the secret squirrel of great engine builder/ tuners. Try playing with different size exhaust venturi at collector, another secret squirrel.

Hi Uncle Tony, When I was in my late teens I told my old mechanic car guy that I put headers on my 1981 C-10 with a 305 with a huge factory carb and he told me I should not have wasted my money there and left the original exhaust manifolds on and increase my pipe size 1/2" to a real dual instead of the combined catalytic convertor, because the heat generated with the cooling effect of the exhaust piping would help pull the exhaust out. I might have lost a bit in translation being 18/19 years old without much mechanical car knowledge. Sure did spend time back then burning through header exhaust gaskets, leaking every 10/15K and changing them out. Could smoke the tires on that truck with the old style speed control after hitting 25/30 mph on speedometer with it set at 70mph, no foot on the pedal when it engaged, blew my friends minds when I did it! Jam '63

Yup hit the nail on the head, the only thing that really matters is balancing the pressure waves so you get better volumetric efficiency and power. I got a book off my uncle a while ago called “Performance Tuning in Theory and Practice” by A. Graham Bell. It goes into detail about optimal exhaust manifold design for different engines. Pretty interesting read to get a general understanding of engine tuning. 👍

Doug Thorley understood this, hence Tri-Y headers. Shelby's 180 degree bundle of snakes on the GT-40, and the 5,7 cylinder swap on Chevy small blocks, the more you can even out the spacing and consistency of the exhaust pulse, the more efficient the engine and more you can optimize the horsepower.

Great video! The next person that tells me.. “You need some back pressure”, I’m gonna say.. So I guess all the racers with open headers have been doing it all wrong all these years! lol

This is essentially the same thing Engineering Explained described in his video about backpressure myths a few years ago (according to my memory). I'm happy to see so much accessible content out nowadays by knowledgeable people who have done the testing and are busting these myths and exposing them for what they are.

Always a learning experience. Thanks Uncle Tony. I wish I knew 10% of your knowledge and I'm probably older than you. Been wrenching on my own junk for 46 years since I was 16. Thanks again. See you tomorrow.

Best explanation I've heard on this.

Great job on this video,,,,I have tryed to explain this to a few younger guys I work with and they just dont get it they want a 3 an half inch straight pipe on there oldschool 350 Chevrolet that's stock and I tryed telling them it's the speed/velocity of the exhaust that's most important... you do a better job explaining I'm sharing this video right now

So I own an 04 yukon xl, it's my first truck. This channel and your videos have been a huge help to me.

The back pressure myth is not just an internal combustion engine problem. It is also a problem on steam engines that exhaust to the atmosphere. The Lempor exhaust was developed to create a partial vacuum in the cylinders during valve events to fully expunge the remaining steam before cutoff of the exhaust event. The valve events are not at top dead center but rather about 15 degrees either side thus it is called lap and lead. The Lempor exhaust helped prevent compression during the lapped time of the valve events. The compression issues are greater at full speed when, for efficiency sake, the engineers would change the timing of the valves to use the steam expansively. This is known as cutoff or hooking it up and was accomplished with the use of the reverser. The point is that most headers and modern exhaust are built on the concept of the Lempor exhaust. Using the inertia of the exhaust event to pull a vacuum on the other tubes of the exhaust. Thank you Uncle Tony for an excellent account of exhaust engineering.

@Uncle Tony’s Garage the thing that I remember hearing that helped this make sense to me is the explanation of why the vacuum forms behind the exhaust valve. It’s because the previous pulse from the last cylinder firing into the manifold caused the column of gas all the way to the tailpipe to move at a high velocity and have a high level of inertia. That gas wants to keep moving and therefore creates a vacuum behind the closed exhaust valves, one of which is about to open. Exhaust in pipes that are two big for the engine’s ability to move air doesn’t move out as fast and therefore doesn’t build that inertia to really make a vacuum. This is out of the idea that kinetic energy is 1/2 mass x velocity squared. The velocity is all important here - and velocity is high in a smaller pipe, so long as it’s not so small that it actually contributes significantly to back pressure. Exhausts need to be right sized for the power band you’re looking for and that the engine is capable of producing. Great video as usual. Just looking to add a slightly different explanation of the same concept that may click better for some.

Or you can be a weekend expert and install an aftermarket carburetor and headers, with 4-inch exhaust and complain it idles weird, stalls constantly and has very little low-end power.

Been watching your videos and I'm not a newbie I've been turning wrenches since I was 8 yrs old my dad was a mechanic and always had me in the garage what I wanted to say is for someone just learning you have found a way for anyone to understand what your explaining without using the "technical" terms bit also without making someone feel stupid. You would make a great teacher keep up the good work and the videos coming 👍

great video.. myself being 61 yrs old, always heard this when I was younger... but thanks to the internet, I studied this pretty extensively and you are correct.. it was interesting checking on tube diameters / length, etc affecting the powerband .....

I love this series It dumbs it down enough for me lol

Like # 98.. Just saying... Hey Uncle Tony, I love listening to you because you remind me of my grandfather. Who was always in his garage working on something or some car. So much knowledge!

I love to learn from Uncle Tony. Not that I plan to ever do any real exhaust work, but it's good to learn from an expert with decades of experience!! Thanks Uncle!

Preach brother! This comes up so often-even with EXHAUST EXPERTS-good stuff as usual T!

I really wish people like you could put everything yall know, like everything, even stuff yall forgot yall knew into like a holocron basically for future generations who are interested in building hot rods and muscle cars

For years I believed the backpressure myth. So, my question is, are you going to do a vid on how to determine the proper pipes and length, or just a simple formula? Miss ya on a Sunday UT, my son got hit by a car and has been in the hospital since Sunday afternoon. First chance I’ve had to chill for a minute and watch a vid.

Great explanation! One point to remember: Technically, vacuum does not suck or pull. Atmospheric 'pressure' always moves/rushes/pushes INTO a vacuum in order to balance the pressure. Nature abhors a vacuum. Also, exhaust noise usually equals horsepower. You don't get noise with back-pressure.

Thank you, Uncle Tony! It's all about the flow. Get the exhaust out as efficiently as possible to draw the charge in. You explained it wonderfully. This video is my new go-to for explaining this to folks.

I remember back when I was a kid, arguing with a guy I was working on his derby car with, he wanted to put a plate over his exhaust ports to "increase back pressure." and I told him he needed to put headers on the engine so it could breathe easier. As I got older I understood even more that back pressure is a myth, and having a set of headers that balance the exhaust pulses and allow a good balance between exhaust velocity and flow is the key to performance, and what that is may depend on your engines demands, its kind of like the guys who argue there is no replacement for displacement, and though I am a sucker for a big cube V8, the engine that creates the most performance, is the engine that most effectively uses as much air fuel mixture as you can cram into it. AKA Turbochargers and multiple cams, and that's why a lot of V6 engines can keep up with old V8s and a lot of I4 Engines can keep up with old V6 engines.

I never understood why people though backpressure was necessary... Headers are designed to maintain velocity and scavenging, some of the headers on racecars in the 70s actually cross over from bank to bank to perfectly match the pulse of the opposing cylinders.

THANKYOU for dispelling this BS. Drives me up a wall Keep making fantastic content and teaching us. You’re my favorite uncle!!

Perfect explanation. Love your channel and everything you teach!

Have you watched engine masters header bashing... I was amazed of how much you can smash a header in till it effect horse power...

Explains equal length pipe headers and "X" pipes in dual exhaust systems.

I’ve known this for more years than I care to say, but I’ve never heard it explained so clearly. Nice job Uncle Tony!

Thanks for clearing this us Tony. Very good to know. See you tomorrow.

Thank you I can finally tell my buddies that I was right!

Tony , you need a doctor's coat for these engineering lessons.

You explained that so well..I never realized the exhaust pressure helped draw in intake mixture..thank u for clearing that up

THANK YOU FOR TALKING ABOUT THIS! I have been in heated arguments with guys about this. You don't want back pressure, you want velocity where you want power; back pressure is an unfortunate side effect of velocity at lower RPM. I use air compressors as an example...if back pressure is good for a pump, why does an air compressor build pressure more slowly as tank pressure goes up? According to the "you need back pressure" camp's theory, it should build pressure faster with more tank pressure. That normally makes people think it out and at least stop saying "you need back pressure", even if they don't understand why you need velocity.

brb gunna go cutt off my cat and muffler from the SBC

Says no views so guess I’m the first hope your having a good day uncles

I love the way you explain things. It’s like anything else, once you understand what is happening, it’s just obvious! Thank you.

Sorry about the backpressure. Still hungover from yesterday's Chicken Fiesta, Baby. Those chicks grew fast and look good and healthy. Viva la Chickens!

old people would tell me the back pressure thing and i never understood it. i knew there was SOMEthing about it but could never quite understand. this all make sense now. its about flow and the flow of the exhaust out the pipe draws in the next breathduring that crytical overlap point. Mind blown. understanding of the machine enhanced. Today i had to start blasting a 50's ford angelia that looked like it had once been some kind of lil micro drag car. i noticed that it had 1 air shock at the passenger side rear corner and thought of you. i knew what it was for.Thank you for your library of knowledge .

Love the way you explained very simple and straight to the point

So, a friend of mine and I have found that his Honda engine, with 12.5:1 compression and a ported VTEC head, absolutely loves the 4 into 1 header he has installed, along with a 2.5 inch exhaust from the collector back, and a 70mm (or roughly 3 inch) throttle body. I have no doubts we have maxed out the stock cams, and there is absolutely more torque from just off idle to 7,250rpm which we currently have it limited to. I have no doubts that zinging the engine to 8,000 RPM would net is about 200hp at the crank, and right now I'm guessing I makes between 180-190hp. Doesn't sound like a lot, but it's a 1.8 liter engine.

When I used to build Nextel cup engines, we did everything we could to eliminate back pressure.

I've been trying to tell people this for a while. I'm sharing your video with a few people for sure. I love your welth of knowledge and I definitely learn so much from your videos. I was always a GM guy till I just got my first Mopar. Definitely hooked now

Thanks for bringing up the tuning aspects of header selection Uncle Tony

Wonderful explanation Uncle Tony. Just came across your channel a week ago and have been soaking in knowledge thank you! Never understood this

Hey Tony, Butch Dix here... I just want to say I love your show .I'm new to your Channel, fuel fire timing and Air it's what my father used to tell me you need to make a motor run.. we also had many discussions on back pressure. I agree with you it's wall overstated and it is pretty much a myth. Just want to say thanks for your wealth of knowledge and you sure did bring back a lot of great memories of my father and I and our garage..may he rest in peace.. I'm sure he would have loved your channel as well.Take care and I and my brother look forward to your next videos.. keep up the good work Buddy PS I'm from Rochester New York .home on the Rochester Quadrajet LOL

Another gem by Uncle Tony. Thank you and much respect from Canada Sir!

Excellent explanation Uncle Tony. I’ve heard it explained before but you did a better job, with a lot more enthusiasm I might add! Cheers😊

Im glad you addressed this.. I’ve been telling people this for years

Before reading this comment understand I am not an arrogant all knowing car guy. I know for a fact I am just beginning to scratch the surface of things car related. Another great video, I have countless bench racing sessions about "Back Pressure myth". I don't even bother discussing the subject anymore. Keep up the great work. A video idea could be "Things that people say and I roll my eyes". I have two, while working at a parts store I had a guy say "This Holley carb gave my engine too much power and broke my transmission" and another guy say "I used to keep a glove box full of holley power valves because when they would go I'd have fuel spraying all over my engine."

Great video tony, always learning something from you’re videos

You fully nailed that explanation. That's exactly what I have been saying for years and years :)

Thank you for the video. I've been arguing against back pressure for over 50 years. We called this scavenging even though the term is used more for 2 strokes. You give an excellent explanation for this process. That's why we disconnect the exhaust system from the headers when at the race track.

Glad to know .I was told years ago that you needed some back pressure to raise torque .thanks for dispelling that rumor

Thanks Tony! I am one of those trying to learn it right and these videos are pure gold. :)

Sound 100% Uncle Tony ! Glad your feeling better.

Cam timing, scavenging, anti reversion, afr. It's all about matching components. You're the best I've seen at describing this. Subscribed. Btw a D shaped exhaust port ain't begging for a die grinder. Learned that the hard way many years ago

I figured myself, it's not back pressure, it's the tune of the exhaust pulse causing scavaging at certain engine speeds, so you are saying it much better!

Great explanation Uncle Tony! Thank you.

Thanks so much for actually explaining this 👍🏻 I had a mechanic debating me about this just last week he was a big back pressure believer .

Damn Tony, I probably know more about how engines work than most of my friends but you consistently blow my mind with your knowledge.

Really appreciate your knowledge and how you explain things !! Thank you !!

Uncle Tony explains this topic so well 👍🏻

Thank you so much, I always argue with trolls saying "that engine is designed to have backpressure".

Thank you for explaining this properly!

Beautifully described! Loved it!

This was really interesting. Thanks for posting it.

Finally some clarity! For years I've heard all kinds of myths about this. I especially appreciate the information about too large of exhaust causing loss of torque or low end. I think this is the myth I most often experienced. So many guys I knew took modestly modified engines and slapped on huge duals . Bigger the better, then lost all low end power used in daily driving. For years the explanation was Loss of back pressure. Thanks for setting the record straight in a concise manner.

Great info Tony . Filled in some gaps I had regarding Valvetrain events .

Theres an old motorcycle tuners trick where you spray a line of paint on the exhaust pipe from after the manifold on, and where the paint stops burning off - you cut the pipe. I've only tried this on inline engines , but it NEVER fails to make the BEST power for an exhaust system on any engine

YES. Love this video. I'm really tired of people saying "engines need back pressure" omg... lol As an afterthought to this, I will add that the distance from the port to the collector (length of the primary tubes) is another factor to establish optimal scavenging RPM of the header. I've never actually tried this, because I haven't used cross-over pipes in my systems before,(I like straight pipes, "scavenger style" that run straight under the axle with zero bends and zero losses) BUT, from what I understand, the best way to find the ideal place to install a crossover pipe is by using a wax crayon(junkyard marker) and run lines down each pipe, from the collector, back a few feet. Run the car hard around the block, bring it back and find the spot in the wax lines where it just begins to burn off the pipe. That, supposedly, is the ideal crossover spot to optimize the exhaust system(and quiet it down) I wish I'd tested it to see if it really works like that. Have you ever used this method, Tony? Your videos are fantastic! Simple, to the point, and factual.

been working on cars for 55 years. you teach me something almost every time i watch you! oh, by the way, SAY HI TO THE POOCHES!

Great presentation. Passion about the topic without any hype. Rare to find on RU-vid.

Thanks, Tony. As always, the clearest explanations of ICE fundamentals to be found.

I haven't heard "backpressure" in a lot of years. It was something people barfed up all the time back in the early 90's, back when newer cars and trucks had first gen OBD engine control systems and fuel injection on them. It was, of course, a horrible oversimplification of what was happening to the guys that were taking their 302 powered F150's and putting straight pipes(not even a H) on and removing cats etc. First, the sparten OBD/EFI control simply didn;t know what to do with the extra air, sometimes it would run so bad because the engine actually went into "safe mode" because the A/F was so botched. Then a little later, removing cats had a terrible effect because they added O2 sensors before/after cats to help determine fuel rates, and the OBD went haywire without a cat in place. All of that could have been tuned away, but in most rural areas no one knew how to mess with the EFI computers and aftermarket even for a Ford 302(truck or otherwise) EFI was still pretty sparse and expensive. Like all that stuff, people figured it out and either bolted on aftermarket or replaced problem parts by the late 90's with those vehicles and good times were had again. But it has been decade+ since I have heard anyone barf up that term. I do recall Borla going out of their way in their ads to try to educate people about exaust scavenging in the late 90's, i think by then it was starting to sink in what the actual problems were though.

Thank you thank you thank you!!! Very helpful information Sir. Keep the videos coming.

Thank you for explaining that. Makes perfect sense!

Great video I’ve never thought of the exhaust system being used as a vacuum port but I guess that’s why drag cars put their PCV hoses on the headers👍