The BS On Backpressure
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- Опубліковано 6 лип 2024
- It's the original internal combustion old-wives-tale, but there's more to the way your exhaust system functions than meets the eye.
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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.
Pirate Labs well those aircrafts also don’t stop at red lights, Go ahead and drive your car around with 4” stacks.
@@ze_german2921 I already do, thanks. I get a lot of tickets, ha ha.
Hook up your headers run 3" pipe where the flame stops in the pipe is where you cut the pipe anything more is a waste of pipe
Well that may be a bit different. If I'm correct almost all those engines were supercharged. Supercharged engines don't need to depend on exhaust scavenging to get a good fill of air/fuel charge into the cylinders. They just pump the air right in, and during overlap the spend charge would be blown right out.
@@Poop-nu1so you're partially correct. Nearly all WW2 aircraft were supercharged, certainly all the high performance aircraft. It wasn't long before WW2 many high performance aircraft were not supercharged. The short exhaust stacks actually served 2 purposes other than simply directing the exhaust away from the motor. First, the airflow past the tips provided vacuum to the cylinder. Second, at power the exhaust flow itself provided some bit of thrust increasing the performance. A supercharged engine doesn't "need" scavenging, but it certainly helps more effectively clear the cylinder of spent exhaust gasses, resulting in an increase in power and efficiency.
All I heard is cut your mufflers off with a sawzall 👍
Lmfaooooooo
Use a portaband, much easier.
@@jerrytee2688 Uncle Tony’s new tagline!
My dodge truck i deleted the muffler. But it was rusty and needed to go
you don't need to tell me twice brb.
ROLL UP YOUR WINDOWS!!!
Automotive knuckleheads have be spewing the “Back Pressure” myth for decades.
...The only channel I don’t skip the ads.
I do the same, it's not much. But it's the least we can do. Seriously if I'm watching something else I just open another window and mute it. By the time I'm ready its ready to go and I do the same if there's adds at the end.
I dont skip ads here either. Just another squirrel tryin to get a nut...in other words just another guy who lives paycheck to paycheck and supports my 3yr old grandson.
Cledus Snow.....are you Hank Snows brother?
Ok scottie I seen what you done there.😂😂😂😂😂😂😂
@john beaver no you even beat in your header pipe quite a bit and will lose minimal horsepower. At least based on the engine masters header bashed test
I hear this back pressure myth over and over. Many people don’t have the ability to learn. Thanks for posting.
Rob B that is a great insight. I appreciate that. 👍🏻
@@bjr2379 Too true. Nobody learns anything without asking questions.
@@bjr2379 Riced out honda morons, cough cough
Yeah but he's the only one to have ever answered what I've had in my head since 15 yrs ago when I saw someone try to start a motor without manifolds on and I saw how it didn't even want to start and would immediately die if it did and like he said I was 18 and at the time thought huh guess back pressure does exist lol
Its from 2-stroke engines. 4 stroke engines don't need back pressure as Uncle Tony correctly states, but the 2 strokes do. And then when you get to nitro RC 2 strokes, the exhaust is actually tuned to recycle unburnt fuel. The honda tweens won't watch this video unfortunately, so the myth may continue to spread. Thank you uncle tony for explaining why manifolds and headers are actually needed though-- much appreciated & I learned something today!
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”.
I can definitely see this in play. I've had coworkers tell me that I shouldn't touch the factory exhaust because it's designed specifically for the car. Or it'll run like crap.
I've noticed people don't want to tune after letting it breathe better.
@@jamesharris5232 the "stock is best" people rarely know what they're talking about. EFI cars can often compensate through fuel trim though some are better than others at doing that than others.
@@lb9gta307 some efi vehicles are designed to runs a little rich from the factory. So by opening the exhaust up it actually leans it out closer to where it should be
@@konnerkramer329 if they're in closed loop they normally just try to run their target air/fuel ratio. Short term and long term fuel trim only has so much room though
@@lb9gta307 right but most stock tunes only has so much room for adjustment. I know on my 6.0 Chevy it ran 11-11.5 to 1 stock. Then with long tubes a catless x pipe and flowmaster supper 44s it ran 12.5-13 to 1 when it went into closed Lope which is when the coolant was over 180 degrees
Engine masters had a show on this subject and proved that exhaust systems with less back pressure did pick up hp.
There is ONE time when you do want a set, specific amount of back pressure in your exhaust to make the best power...
.
.
.
.
.
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Two strokes.
And even more mystical... the two stroke turbo, which uses a tuned pipe followed by a turbo which acts as a variable restriction after the tuned pipe, maintaining good back pressure in the pipe, even as air pressure in the intake tract is increasing...
check out this one! V8 two stroke! ua-cam.com/video/JVfqqSbB3Po/v-deo.html
2 stroke Detroit v8s run best with smaller pipes as opposed to those goofy 10 or 8 inch pipes!
@@gallupcustomknives2293 ah, I see you beat me to the punch :)
Well diesel 2-strokes are another beast altogether. I don’t know much about them, I was referring to gas power sports 2 strokes
For an old car guy you sure are brilliant, I never really thought of it but you explained it perfectly.
Uncle Tony can you do a video on how to properly size the right exhaust.
That would be very helpful.
And also for inline 6's.
Jason Thunders my 67 Mustang with the stock 200 6 would benefit greatly from this video! Haha preferably before I waste some money on the wrong setup!
I was gonna say that. He should cover how cam shaft overlap and duration as well as primary pipe length and diameter and how they affect engine performance. It would be an in-depth and pretty long video cause there is a lot to cover and explain there
Thrill Hill Racing same thing with me except someone swapped in a 250 and got rid of the 200
lucas swearingen interested in a 200? I have an extra complete engine. I’d be interested in that 250 for sure.
Great video. Finally someone made a video explaining overlap and the truth about exhaust.
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.
Not only do they run poorly with no exhaust, you can warp the exhaust valves that way. When the valves heat up, then you shut it off, the cooler air is right there, the valve is right inside the port, so the cool air touches the valve and it cools unevenly, causing it to warp.
@@livewire2759 I've heard this several times but have never seen any proof of it. I've ran several engines open manifolds and open headers. Even ran a 283 right off the head. I've never wrapped a valve or seen anyone warp a valve from a lack of exhaust
@@konnerkramer329 I've seen it. It won't happen right away, so a test run with no manifolds probably won't hurt, but repeated heat cycles with open manifolds can and will warp the valves eventually... especially in cold weather.
When I was young, a friend of mine was wanting his truck to be as loud as possible, so he took his pipes off right at the manifolds (mid 90s f-250 with a 460) He ran it that way all summer with no problem, then about halfway thru the winter he suddenly stopped driving it. I asked him why, he said it was running like crap, took it to a mechanic and they told him the valves were warped. they did a valve job and told him to put the pipes back on it.
Some years later I had a customer (I'm a GM tech) who brought in a truck (early 2000s 1/2 ton with an LS) that was running rough and had the exhaust pipe broken off on the drivers side. He said he'd been driving it that way for several months. All the misfires were on the drivers side, all had low compression, all the exhaust valves were leaking. We sent both heads to the machine shop for a valve job and replaced his Y pipe, complete with converters.
It may not happen very often, but it CAN.
@@livewire2759 I'm sure on a hot engine with high egts it's possible. but I ran my 327 for almost 5,000 miles open manifolds until I got a set of heddmans for it. I ran it for probably another 10,000 open headers before I put an x pipe and 3 feet of tubing. I ran it like that for about 2 years until I tore the engine down.The engine was ran hard for a long and other than 10lbs oil pressure It seemed to run fine. It cruised 65mph all the way to my shop with with basically no oil pressure. I tore the engine down and found 1 broken piston ring and 3 cracked rocket arm nuts but everything else was fine.
@@livewire2759 More likely the drivers side exhaust pipe broke, because of a dead o2 sensor commanding max fuel on that bank, rotting out the exhaust and fowling the plugs, leading to intermediate missfires.. (based on load)
Back fires from this are known to blow the baffles out of mufflers and glass out of glass packs too.
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.
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.
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 😎🤓
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!
Yup. However, if he swears by those 1 5/8" headers on every engine he builds, they are ALL of similar displacement to each other, and that displacement is not very large. Otherwise, he is leaving power on the table for the larger engines.
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
That's what I always say, "Back pressure? So how does a top fuel engine work?"
@@livewire2759 Those engines work on "Pressure to Win"
@@livewire2759 A supercharged engine doesn't need the help of the exhaust to fill the cylinder.
@@mcplutt Yes, obviously, but the point is that most people who believe in back pressure think that every engine needs it.
The stacks even add downforce for traction. Flipin cool
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!!!👍☺
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.
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
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.
Best explanation I've heard on this.
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.
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.
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
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
Josh
Not to mention it would sound like ass.
If your head was literally inside the ass.
Extra large mid pipe size doesn't really hurt power. Engine Masters proved that. If it's a dual system the crossover won't be as effective but the scavenging is mostly done with the manifolds/headers. That's where proper diameter is most important. Overly large pipe is pointless weight though.
I love this series It dumbs it down enough for me lol
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.
Preach brother! This comes up so often-even with EXHAUST EXPERTS-good stuff as usual T!
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.
Oh shit! Send our best along to him.
There are some commonly used calculations. The problem is that everything from port shape, primary tube lengths, bends, collector style, collector diameter, cats, mufflers, tailpipe, even the tip, need to be taken into consideration when trying to optimize a setup. That's why stock vehicles always leave some on the table, because everything in a street vehicle is a battle of compromises. Also, electronically variable intakes lengths or tunable headers will greatly expand your powerband.
You let a small thing like an injured child stop you from watching UTG? Priorities, man, priorities.
@@rcnelson yeah, Tony is my uncle, but that’s my baby boy....
@@UncleTonysGarage thank you! He’s doing much better today....might get to go home Thursday
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
This really makes me wonder about the future of the hobby. I see dino powered stuff being around reliably for about 30-40 years and likely becoming a niche thing.
I absolutely love this stuff, but they are likely going to go the way of the reciprocating steam engine, and this knowledge will largely be lost outside of engineering textbooks of our day.
It's kinda hard to predict when, but it will happen. Sad since there really is nothing quite like the idle of a nasty cam with a single plane. Also nothing quite like hearing and feeling the bark of a steam engine.
Too bad they will be illegal, very soon...
@@herpnderpn2484
What's "dino powered stuff"?
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.
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.
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.
Theres a company that sells a naturally aspirated 2.4 liter i4 engine that makes over 300 whp. They sell an upgraded 2.7 liter version that makes over 500 crank. Engine tech has come a looong way.
Edit: based on the Honda k24 series of engines
@Michael Madden
Because a larger displacement engine is capable of flowing a larger amount of air and fuel because it has more displacement to do so.
@@snakeballs8965 At what, 12000 rpm?
@@ghj1502 www.4pistonracing.com/k24-480-2-7l-complete-engine
Not quite
@@snakeballs8965 not hating but u kinda agreed that more displacement makes more power i.e. the 2.7 makes more then the 2.4. Small engines dont make as much power bc they are " better" they just displace air and fuel mixture faster then a larger motor usually will.
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!
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 👍
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!
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.
An Auto-Zone fartcan muffler will add 100 horsepower! Ask any high schooler!
did the header [s] come with a sticker? has to be visible, back window preferred. scavenging effect.
@@dreadpenguinlord340 leave my cherry bomb Honda Civic out of this. Sure it may sound like a angry cloud of mosquitoes but it has character with the wing off a Boeing 747 and a hood scoop off a dragster
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.
180° headers
You explained that so well..I never realized the exhaust pressure helped draw in intake mixture..thank u for clearing that up
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. 👍
@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.
Explains equal length pipe headers and "X" pipes in dual exhaust systems.
Thanks, Tony. As always, the clearest explanations of ICE fundamentals to be found.
Glad to know .I was told years ago that you needed some back pressure to raise torque .thanks for dispelling that rumor
Tony , you need a doctor's coat for these engineering lessons.
He needs a pipe and big glasses as well like he teaches at a collage up north.
Prentice Chafin to be fair, that jacket is the mechanics lab coat
And some Buddy Holly glasses with a pocket protector. 🤣
no no don't do that doctors are liars and hypocrites you make great videos with common sense
Needs a professor jacket, a glass of bourbon lol
brb gunna go cutt off my cat and muffler from the SBC
I've said for years that you can do more by removing mufflers than by installing headers.
Live Wire except sound better, manifolds sound like tractors and shit. You can do more with headers.
@@Shade_tree_garage01 Manifolds sound just fine to me, but it depends on what you bolt to them. The point I was making is that putting a set of headers on and then reattaching the stock pipes and mufflers (like people tend to do with shorty headers ) makes little to no difference, where just cutting off your stock muffler does.
@@livewire2759
I did that to my 1990 f-150 4.9L straight six when the muffler fell off, just left catalytic convertor and straight pipe behind tire. Had a decent sound.
So anyway, I started cuttin'.
Thanks for clearing this us Tony. Very good to know. See you tomorrow.
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!
Have you watched engine masters header bashing... I was amazed of how much you can smash a header in till it effect horse power...
I once hit a HUGE pothole on a mountain road and smashed three header primary tubes on the driver’s side halfway flat on my 70 Dart. The car ran fine on those same headers for the few years they stayed on the car. Still, I was very surprised at their dyno test. Very interesting.
“On this episode, we beat a couple Hookers with hammers to see if they still work.”
Engine Masters proving PimpTheory on the dyno.
Thank you I can finally tell my buddies that I was right!
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.
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.
Says no views so guess I’m the first hope your having a good day uncles
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!
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!
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.
Damn Tony, I probably know more about how engines work than most of my friends but you consistently blow my mind with your knowledge.
Thank you, I get so tired of explaining this all the time.
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.
I think this idea about backpressure might come from the two-stroke world where the return exhaust pulse is used to tune the pipe to that specific engine. Maybe someone probably believed that it could be applied to four-stroke engines as well. In a two-stroke engine a tuned pipe is a very big part of getting the most out of the engine. Thanks for an awesome channel!
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!
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
Yup, been doing that on collector extensions since the 70's
@@UncleTonysGarage Heard the same thing about using a grease pencil on the collector on drag cars....where ever the grease doesn’t melt, cut it there and that’s the optimum position for torque
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.
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.
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.
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 .
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👍
Uncle Tony out here teaching these kids what they’re doing wrong with their Civic 4” straight pipe to fart cannon exhausts 😂 YOU GOTTA SCAVENGE!! We were all young and dumb at one time, gotta learn somehow 🤷🏻♂️
if you drive a civic you dont have a hope in life anyways , dont race trade , buy american
THANKYOU for dispelling this BS. Drives me up a wall
Keep making fantastic content and teaching us. You’re my favorite uncle!!
my brain is soaking up your knowledge...i need more, give me more...thank you sir.
100% correct........ GREAT JOB of explaining it to the new guys/gals!
That’s why I’m going to be using 180° headers onto two turbos the scavenging will actually help accelerate the turbos. It’s like you said before one thing may not directly impact the build but it’s the larger picture of all the little things that help produce more power
When I used to build Nextel cup engines, we did everything we could to eliminate back pressure.
It can be easy to connect dots that aren’t actually there. Thanks for explaining!!
Sorry about the backpressure. Still hungover from yesterday's Chicken Fiesta, Baby. Those chicks grew fast and look good and healthy. Viva la Chickens!
Thank you. Been an argument I've had for years. People never believe me.
Thanks Tony! I am one of those trying to learn it right and these videos are pure gold. :)
Another gem by Uncle Tony. Thank you and much respect from Canada Sir!
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 .
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 .....
Glad you covered this so many miths .When i got my first car always hear if you run open headers in cold the cold air will go back in headers and warp the valves always something I think about.
Long tube headers usually won't, but shorty headers absolutely will.
Pile of work to get done and then a new UTG video comes out.........work is now on hold!!!!
So I own an 04 yukon xl, it's my first truck. This channel and your videos have been a huge help to me.
I love the way you explain things. It’s like anything else, once you understand what is happening, it’s just obvious! Thank you.
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."
I was always curious as to the whole backpressure theory, figured as long as you have decent headers it shouldn't matter much if your piping is oversized. This video cleared up suspicions, thanks a lot.
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😊
I remember in highschool racing a guy I had beaten several times before. But this time I had no mufflers on the car I was in the process of redoing the exhaust and thought I will really blow his doors off. I LOST 3 times in a row. Ran home put my mufflers on and found him the next day and beat him. Lol
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
Thanks for the knowledge I've never heard it explaned this well .
You fully nailed that explanation. That's exactly what I have been saying for years and years :)
Thanks for bringing up the tuning aspects of header selection Uncle Tony
I'm 65 and until I was 28-30 yrs old I fell for it. Even an exhaust shop owner told me it was needed for scavenging. So Tony you are one of the few on the internet to put this myth on the table, explain it expertly then blow the myth up. Thanks for the videos
In that case it could be they meant the right thing, but used the wrong terms.
Depending on what rpm range you optimize the engine for, the diameter, surface and pipe length will tune the scavenging effect for a certain rpm range (frequency). If you want an efficient 30hp engine at 1500rpm, you will need a smaller pipe than a 700hp at 6500rpm to get the same scavenging result.
A smaller pipe will mean higher backpressure above the rpm range for the particular engine. A bigger pipe will always mean less back pressure, but in extreme cases can lead to gas reversion, which dilutes the charge with spent gases.
The inside surface of exhaust pipes on a street car will get rusty and soot deposits, which leads to back pressure due to the rougher surface, so optimizing the exact pipe diameter/length and surface would soon be inaccurate, so you are better off with slightly larger pipes than theroretically optimal. any sharp turns, any flangeconnectors along the pipe - will increase the back pressure. When I designed and built simple pneumatic systems in a small industry, it became obvious how much every connector and evert bend killed off the pneumatic signal. The harder the pipe the less the pressure loss (back pressure).
My favorite comment I got was when I changed from a stock briefcase muffler to a glasspack. Guy says I'm going to crack an exhaust valve from cold air rushing back up the pipe since it'll flow more than stock.
That's a good one!
An engine needs to be tuned with its exhaust as it will be run. The myth comes from people merely deleting exhaust components and losing power because they don't retune. When I worked in a muffler shop 30 years ago cars would come in with rotten tailpipes and wouldn't idle or stay running. Putting the tailpipes back made them purr like kittens. Fart cans on imports are a great example too. Exhaust has a huge effect on efficiency. "Backpressure" is an oversimplification. Exhaust system timing and designing headers to take advantage of the complimentary effects of other cylinder's gas pulses is a science. So you can't just remove restriction and expect an engine to breathe better if it disrupts the tuning of the exhaust.
This makes so much more sense when explaining savaging.
I love it! Your exactly right and it’s nice to hear someone that uses their head. The flow of exhaust gases creates a vacuum just like a Venturi nozzle. And with sizing it’s pretty simple, as velocity increases, pressure decreases. The smaller your exhaust the higher velocity of the gasses, but less pressure! A 2” exhaust would have less pressure but the gases would be going faster than a 4” pipe. It sounds backwards but that’s how it works. Don’t believe it look into it, it’s called Bernoulli’s equation!
Thanks for explaining that very well. I’m old enough to have heard from the shade tree mechanics in the 60s and 70s about back pressure being important. It never made sense to me though. They claimed it increased compression ratios of the pistons so gave more power but to me that meant more exhaust gas in the fuel mixture possibly leading to preignition problems. It always seemed you’d want as much exhaust scavenging as possible.
Exhaust in the mixture is exactly what the EGR system does and its purpose is to cool the combustion chamber.
Nicholas Rodrigues you are correct but the EGR valve is a smog reduction device. In effect it reduces power created in the power stroke by lowering combustion temp. What UT and I are taking about is whether or not back pressure in the exhaust aids or inhibits power. A totally different area from smog control like the PCV valve recirculates crankcase gases for pollution purposes.
@@lehampton1 my point is that it doesn't result in preignition problems like you said it possibly might
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.
One of the best explanations I have heard in my 45 years as a mechanic tuned length extractors always work better than stock manifolds . The only time this is not correct as far as I know is with forced induction ie super or turbo . The myth came about from when engine pipes burnt out lots of noise and no go only because the scavenging effect was no longer there . Correct me if I am wrong. I think you have forgotten more than I know as far as performance work goes
Superchargers just don't like a lot of overlap in the cam profile. A good header will make more power at a lower boost pressure. Turbo engines like a lot of the same things NA engines do.