Exhaust Header - Explained

What are exhaust headers? How does an exhaust header increase horsepower? What's the difference between a 4-2-1 header vs 4-1 header? This video explains how exhaust headers are used as a portion of the exhaust to increase horsepower, by using long piping and larger diameter piping, as well as a larger diameter collector.

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[kathrynck]

First, I like your channel, and i'm subscribing, so don't take this the wrong way :-) but... 

You missed the science behind the main reason for using an aftermarket header... it is *Not* the larger diameter header piping (although that *can* help... or hurt... "sometimes").

The main rationale for a header is to create an equal length of piping from start to finish for each of the individual pistons on the engine.

Why?

- The pistons fire in very rapid succession, and when each piston releases it's exhaust gases into the exhaust system, it represents a pressure spike in the exhaust system. Each of these pressure spikes seeks to equalize pressure by expanding down the exhaust pipe, eventually exiting the exhaust.

- At the rate of piston firing in typical engine use, any given piston's exhaust pressure spike has not yet dissipated before the next pressure spike from another cylinder arrives (more noticeably so at higher rpms). So the pressure spikes from the cylinders start to overlay each other, kind of like people waiting in line to go through a ticket booth. This generates exhaust gas pressure, and the higher the pressure, the more it creates back-force on the engine's pistons which push out the exhaust, robbing power.

- While larger diameter exhaust components are helpful for reducing gas pressures in the exhaust, thus reducing resistance to the engine's job of forcing exhaust out and in turn the amount of power it takes to pump the exhaust out... there is another factor at play which a header is specifically designed to address: *The spacing of each cylinder's exhaust pressure spikes in the exhaust system.*

- In a typical original equipment exhaust manifold, the goal is often to use as little metal as possible (cheaper/lighter), and take up as little space as possible, while getting exhaust to the catalytic converter as quickly as possible so that it heats rapidly, reducing emissions for legal reasons. This results in exhaust manifold piping where the pistons nearer the catalytic converter having a shorter distance to travel to enter the main exhaust, and pistons further away from the catalytic converter having a longer distance to travel ("unequal exhaust path lengths").

- The pistons fire in a cycle where each piston's exhaust has equal timing between it and it's neighbors, so at any given RPM, the exhaust pressure spikes are occurring at equal intervals. But when they travel through piping of different lengths, these exhaust pressure spikes can end up closer or further apart when they reach the catalytic converter and remainder of the exhaust. If you want to go back to our people waiting in line analogy, imagine two people side by side in line, both trying to get through a ticket booth which is only one person wide, at the same time.

- A header is designed *specifically* to reduce (short header) or eliminate (full header) that timing differential, by making the length of pipe for each cylinder going to the catalytic converter identical, or nearer to identical than stock. So that your people waiting in line to go through the ticket booth (spikes of gas pressure in the exhaust which must dissipate to reduce exhaust pressure) are in a nice single file line which moves through the ticket booth efficiently.

(( For reference, when you see an engine's power band (in both HP and Torque) following a curve where you get diminishing power returns at higher RPM, that downward curve (which seems to prevent an engine from continuing to ramp upwards in power at higher RPM's) is very heavily influenced by the resistance of gas movement. Friction, and oscillating weight such as pistons, valves, & rods also affect it, not to mention losses in the drive chain taking it to the wheels. But back to gas movement... If you tuned the *entire* breathing system of a car, including: filter, intake, throttle body, intake manifold, valves, valve timing (cam), exhaust ports, header, cat, and cat-back... you could shift the peak of an engine's power output several thousand RPM's higher, allowing it to climb to a significantly higher peak, at much higher RPM, before diminishing returns bring it back down, and also provide a broader power band which helps reduce time spent shifting gears. BUT most of these components *by themselves* offer very little benefit, because the stock components are like a whole set of bottlenecks, and power gains are minimal when you only remove some of the bottlenecks. A header is the glaring exception though, as it can help *any* exhaust system move air significantly more efficiently. Although from a labor standpoint, if you're installing a header, you probably should do the whole exhaust anyway. Similarly a turbo or supercharger has a very strong single-upgrade benefit because it can brute-force it's gains through a restrictive throttle body & intake manifold. ))

*** Header Advantages ***
Headers can reduce exhaust pressure *significantly*, or more accurately reduce variances in exhaust pressure, so that any given diameter of exhaust piping, catalytic converter, muffler, etc. can work most efficiently. At low RPM where exhaust pressure is minimal, there are little to no power gains, but at high RPM's a header will make any exhaust system have much more efficient gas movement, keeping exhaust back pressure to a minimum. A good header can help reduce the need for excessively oversized cat-back tubing, reducing weight and improving ground clearance. It also makes a smoother sound at the exhaust. While a un uneven rumble may sound cool, a smooth deep exhaust note is the sound of efficient air flow.

A header can add more power (by itself) by increasing the efficiency of even a stock exhaust system, than a cat-back can. It's also much more of a pain to install though.

*** Header Disadvantages ***
Headers add weight. All that extra piping is more metal for your car to carry around. For normal driving in the 500-2500 RPM range, headers will weigh more than they're worth. Which is one of two reasons they aren't factory equipped.

Headers put more travel distance between the engine and the catalytic converter (another minor help for reducing exhaust pressure), but that makes the Cat take longer to heat up after starting an engine cold... and that will make your pollution emissions significantly less favorable. Some schemes that auto makers have tried to overcome this issue is to put a mini-catalytic converter in each header pipe, or to electrically heat the Cat at startup. Some performance Cats are designed to somewhat alleviate this concern with more pure platinum coating, etc.

Another possible disadvantage is that some exhaust headers have oversized piping, but if the exhaust ports on the engine block are smaller, that creates a sudden change in exhaust diameter... while the explanation is quite a bit more in depth, in simplest possible terms, you can think of sudden squared off changes in exhaust diameter as being "very NOT aerodynamic" in terms of moving gas through a tube with minimal resistance. Conical changes in diameter are not problematic, but squared off changes in diameter are not good. Large diameter header tubing on standard diameter exhaust ports could potentially rob a bit of power even at low RPM. I'd recommend getting standard diameter piping on the header (matching the factory engine block ports) *OR* milling out the engine block to match the header diameter if you want really big pipes (milling the exhaust ports is probably only worth it if you're going to increase the engine's gas pressures with a turbo or supercharger). For similar reasons (avoiding abrupt changes in pipe diameter), I'd recommend a low-resistance performance catalytic converter with nice gentle funnel shapes fore and aft, a low resistance muffler, and to avoid using a resonator. Likewise, the design of a header's "collector" (where the pipes come together) is crucially important as well.


ALSO:
A 4to2to1 header will perform well *on a 4 cylinder engine* for piston ignition sequences of: 1-3-4-2 and 1-3-2-4, but less well with ignition firings of 1-2-4-3, 1-4-3-2, and 1-2-3-4. Meanwhile a 4to1 full header will perform equally well with any ignition sequence. This is all due to getting the people going through the ticket booth as equally spaced and spread out as possible. If two neighboring cylinders in the right or left pair of a 4to2to1 header fire one after the other, then that creates additional pressure on one side in the 2-pipe stage of the header. Personally I think a 4to2to1 header is the better choice, being generally lighter, and having staged collection (merging of pipes), but only *IF* the ignition sequence on your car is one of the two sequences with which it can perform to it's best potential.


In terms of parts & labor, it's almost always cheaper to get a car with a larger displacement engine than it is to tune a smaller engine to be more powerful. However, I get the hobbyist interest. And with gasoline at around $3/gallon, 100, 000 miles at 20mpg is $15, 000 in gas, while 100, 000 miles at 30mpg is $10, 000 in gas. I doubt you'd usually see that large of a gas mileage difference, but there can definitely be some back-loaded savings on tuning a smaller engine. Plus "the engine in the car you already have" is always priced very attractively.

[SUPRADUBB]

Your honestly a huge help man. I'm really into cars but I have literally no one to teach me anything about them. I thank you for these videos even tho learning from them is looked down upon by others. They still help me and inspire me.

[zeriouslobo]

Finally someone who doesn't swear by turbos. Engineering at it's finest indeed! I love the mathematical breakdowns, really helps me understand the concepts of these videos. Very good work, mate! Cheers!

[EngineeringExplained]

You have to understand the basics to understand it all. Some videos are simpler than others, ultimately (hopefully) it provides a good understanding of the overall system.

[EngineeringExplained]

Appreciate the suggestions! I do have videos on cam lift, multiple valvetrain videos, but many of your other suggestions I am limited on. Cheers

[PeterPhan]

I've been going through a lot of your videos. You're an excellent teacher. You teach like how I teach others.
I'm going to school to teach and while I don't know what topic specifically, but I do know that I will remember the style of your teaching for sure!

[EngineeringExplained]

Yes, sometimes curves are put in to keep the length of the pipes equal, could also be done for space constraints.

[joshuayoungs]

Thank you so much for all of your explanations. I've learned about cars and machinery in general with your videos more than with any other resources. Again, thank you so much, I look forward to more of your videos!

[EngineeringExplained]

Depends of course on the engine, number of cylinders, and size of the engine. If you check out my video on catbacks I give a fairly vague rule of thumb about how it may work. Likely need additional videos on this kind of topic in the future.

[EngineeringExplained]

Header comes directly from the engine head, it's the first tubing the exhaust goes into after leaving the cylinder head. The downpipe is directly after the turbo (exhaust side).

[lliaolsen]

You should also touch on aspects like
- Sequential vs non sequential pairing of 4-2-1. There was an Exup Valve on a header that would switch between the two pairings.
- Long tubed primaries vs short tubed primaries. Some headers will split the primary secondary length 50/50 but I've seen some primaries that are closer to 60% of the length perform like a 4-1, while still reaping the benefits of the 4-2-1.
- Then there are collectors that have venturies/choke points in them to alter their performance.
- Stepped headers and stepped port vs port matched head -> header

[EngineeringExplained]

Back pressure is not needed, velocity is what you want. Check out my video "how exhaust systems increase horsepower." Here I discuss the balance, you may not have enough power to warrant a larger exhaust, thus it may actually reduce performance slightly.

[SuMaWsUm]

You sound like Melvin from the movie Madagascar

[EngineeringExplained]

Actually, I am an engineer, working in the professional world..

[salcarceller]

Nice job on this video. You really should discuss the primary pipe length as it is the single biggest factor for the tuned header. A shorter pipe length move the peak torque higher up the RPM band while a longer pipe length moves this down the RPM scale.

[drake]

I love the quality you put in your videos. If i never saw your subscriber count, i would have thought you had a million subs

[EngineeringExplained]

Perhaps, if it's really heating up the engine bay a lot/the intake manifold. Also most exhaust manifolds have a heat shield, and this requires removing it so it could potentially help.

[EngineeringExplained]

Most of what I saw only differed by a few hundred RPM, and most of the time it seemed the 4-1 did a little bit better. Definitely didn't pool a very large sample of dynos though, so it'd take a bit more to reach more conclusive decisions. I hope to get more into it over time.

[jose]

Thank you for not having any video intros and just going straight to the point.

[happyhippr]

Just gotta say, i love your videos, Im a first year Freshy engineer on a Formula SAE team and these are such a great way to get introductory information. Keep up the good work!