Exhaust System Info! Choosing Your Exhaust!

Discussion in 'General' started by donki, Aug 14, 2012.

  1. donki

    donki Administrator Forum Administrator

    Heyy all

    I got this off MiragePerformance.com (with permission from the author, "Merrill" (who owns a custom supercharged turbo 4G15 lancer for those who dont know!)), but it can be related to any car.
    Plenty of information about exhaust and sizing and sound and everything else with it. Enjoy!

    Bit more info on DIY and that

    The Best Exhaust For Me

    Exhaust is really simple and there is no exact pipe size or right/wrong rule when it comes to figuring out exhaust pipe diameter. There are many things to consider and many formulas that I will summarize here.

    There are 3 major types of bends that have an effect on flow; Crushed, crinkle, and Mandrel.

    Crush bends are formed in a die where the center of the bend radius is literally crushed into a smaller diameter. There are dozens of reasons for doing this but its mostly due to the fact that somethings gotta give and to avoid a kink, this is what will give. A general rule of thumb is 1/4 less diameter than the OD will be its equivalent to flow (eg. 2.5" OD bend = 2.25" OD flow capable).

    Crinkle bends are pretty much the same thing as above but they are made with a die that puts multiple folds into the bend radius. This creates a turbulence in the exhaust and prevents a nice smooth flow. The better part of crinkle bends is that they do not affect the diameter of the pipe through the radius as much as the crush bend style.

    Mandrel bends are created on a different style die to create an unchanged pipe diameter. Throughout the bend radius, the diameter is always the same. This is excellent for performance in that there will always be the smoothest transition and flow possible compared to the other styles. Mandrel bends are also perfect for DIY fabrication because you can cut and mate virtually any angle without the problem of interrupted flow.

    In budget performance, a simple crush bend will do just fine and is what people use most often due to the fact that almost all exhaust shops have a crush style pipe bender. A good rule for having an exhaust with the crush style is to + the pipe diameter one size to make the flow capable of what you actually want. So if you think a 2.25" system is perfect flow for your car, use 2.5". When at all possible, use mandrel bends at the size diameter you want. Let me reassure you that there is nothing wrong with using crush bent pipe. If you are serious about performance but working on a tight budget, all you need to do is compensate.

    How to choose the right pipe size (general). Without getting into flow formulas and what not, think of the simple rules: diameter = current mods vs. future mods. If you have the current mods done to your engine to support having a larger exhaust system then all you have to do is follow a general guideline:

    Merrill Performance Exhaust Guide (click here)

    A chambered muffler has internal baffles and sections with bulkheads (chambers) filled with baffles and will create a slight back pressure on the system. The baffles also have an effect on tone changes in the exhaust system and often times decrease the decibel level. Internal diameters and flow capabilities are generally matched to their inlet and outlet diameter.

    Flow through mufflers do not commonly have any chambers internally and should be intended for higher supporting mods to justify little to no back pressure in the exhaust system. Their tones are created through a metal tube with perforations surrounded by a fiberglass packing material. Some manufacturers even use steel wool for their packing material. Flow through style mufflers do not change the exhaust note dramatically compared to straight pipe and do very little for decibel level decreases.

    Negative back pressure mufflers are designed to create a vacuum in the system and "suck" the exhaust gasses out. Negative back pressure styles are not common and are mostly seen in races where the engine is mostly running in the high RPM range. Common designs for negative back pressure style mufflers include a flared opening around the inlet port or use perforations in the bulkhead of the inlet pipe. When the vehicle is in motion at higher speeds, the air enters the opening and creating a Venturi effect which pulls the air from the exhaust. Engines using a negative back pressure style are ones that spend much of their time in the higher RPM range.

    Stainless steel tends to be thinner, and as such has a higher pitches resonance. Mild steel is thicker, and has a lower resonance. In laymans terms, the thicker your exhaust material, the deeper the sound. Higher quality brands tend to be thicker, for both pipe and mufflers, and also tend to use denser, higher quality materials.

    The Exhaust Note
    One thing that every automotive enthusiast is always after is the popular exhaust note. That mean sounding tone that screams dominance on the streets. Many people believe that this note is controlled by the muffler and the muffler only; WRONG! There are many contributing factors that make up a vehicles exhaust note. Engine displacement is a contributing factor as well. Obviously the smaller engines will have a higher pitched note because of the volume pumped through the engine is less. Imagine the sound as a bubble starting from the exhaust runner in the head. As it exist the engine, it is ready to make sudden changes in size and shape (expansion and contraction) resulting in a change in tone. The sound bubble is literally getting reformulated as it travels down the stream. The larger diameter pipe will create a larger boom inside the pipe when the pipe changes diameter from small to large creating a new tone. Keeping that tone locked is the tricky part. An entire book can be written about this but in the end, larger diameter pipe will create a lower tone.

    Attaching Sections
    There are a few common ways to attach an exhaust section together: they include but are not limited to welding, clamping, flanges with fasteners, V-band, and ID-OD overlapping. The 3 most common in the performance areas are welding, flange with fasteners, and V-band. Here is a simplified comparison:

    Welding 2 sections of pipe together is most commonly used but does make it difficult to remove exhaust sections without cutting it apart or removing many vehicle components such as control arms to remove the exhaust as a whole. Direct welding is not recommended if you EVER think you will remove it again for ANY reason.

    Flanges with fasteners are the most common method of mating two sections of exhaust. Each flange is welded to the pipe end and fasteners (nuts and bolts) are used to clamp the two sections together with a gasket between them to seal it. Most medium to larger pipe diameter flanges are triangular shaped with 3 holes drilled every 1/3rd of the pipe OD. The only downside to using flanges with fasteners are often having difficulty accessing fasteners for quick removal, and replacing gaskets.

    V-band flanges are a self sealing (no gasket required) quick release style mating system that decreases removal time of the exhaust section. The flange ends are round (sightly larger than the pipe diameter) and each flange is shaped in a half 'V'. When mated together, each end of the 'V' is forced together via a V-band clamp. The major benefit to using V-bands are never using a gasket, and being able to pull the exhaust conveniently and quickly due to only needing to loosen one nut on the V-band clamp.

    Choose your exhaust system components wisely and also base it off of future intentions. Do not spend lots of money on anything now if you intend on upgrading your engine capabilities in the end (such as turbo, nitrous, internals, etc.). Make sure you are certain about future plans (or as certain as you can be). The reason why is simple - How many times do you want to do the exhaust until it was done right?

    Last edited: Apr 17, 2019
    Res likes this.
  2. danx22

    danx22 New Member

    very good read!!
  3. hotwyr

    hotwyr Member

    Can I just say something? Not to be rude, but I feel this really needs to be addressed as it frustrates the living hell out of me.

    Exhaust systems do NOT have back pressure. If it did, the car would not run as the escaping exhausts would be met with pressure, keeping them in the cylinders preventing further combustion.

    Yes, piping diameter does effect performance, but it is called a scavaging effect. The exhaust is designed to remove the exhaust gasses so new ones have a place to escape to. It is also common for an extractor manifold to be designed to have a negative pressure to help draw the gasses out, but I feel that's getting a little complicated.

    Simply put, get two pieces of hose, same length same type, but different internal diameters.
    Pump the same amount of water (let's say 1L per min)

    Hose 1 (the smaller I.d hose): water flows fast through hose

    Hose 2 (bigger I.d): water flows slower

    However, increase that flow to 2L per min. Hose 1 would become a restriction, stopping you from flowing 2L and instead maybe 1.7L

    However hose 2 will flow 2L.

    This basic theory is why we increase piping diameter.

    Now, according to the above, if we increase our exhaust pipe to fulleh sick 5" pipe bro with a 7" cannon my 1.8l will make heaps of pooooooowaaaaaaah.


    Read above? For 1L per min, the bigger hose flowed SLOWER than the smaller one.

    Meaning exhaust gases escape slower, leaving less room for new gases, less scavaging effect, reducing performance.

    You match exhaust size for the SCAVAGING EFFECT, NOT BACK-PRESSURE!

    End rant. Hope that helps people too :)
  4. Res

    Res Well-Known Member Lifetime Premium Member

    exhaust velocity is what you are talking about ;)
  5. Liom the Chef

    Liom the Chef Member

    that was a lot of energy to say something that was really just a terminology issue, the OP had the appropriate description of the effect similar to yours, even had the term scavenging.
    The problem you want to remove is back-pressure, but as you said by throwing a massive diameter pipe in there you cut down velocity and the scavenging effect, hence why in laymans terms a lot of people say "you need a little backpressure" as it's the logical extension of what they're trying to achieve as opposed to the most appropriate term.
  6. mmc77

    mmc77 Active Member

    +1 to what Liom said plus reading that at 7am really hurt my head..... alot
  7. charliem

    charliem Member

    Fluid dynamics is something that escapes most people who have not taken at least a first year uni subject in physics....its super counter intuitive.
    Faster flow = lower internal pressure = higher external pressure = lower diameter (read the equality's as equivalents, they aren't exactly the same)

    With less surface area in the 'pipe', there is less surface tension for the body of fluid (given there is enough fluid to fill the pipe..) to hold onto and slow down. This causes the fluid as a whole to move faster, as there is less hydro-static drag.

    There is a limit as to how much fluid you can push through the pipe though, consider it analogous to terminal velocity on a falling object. Where the acceleration of gravity meets the frictional drag forces of the atmosphere, a falling body will reach a maximum speed. Likewise in fluid transfer, there is a maximal flow rate depending on the surface area of the inside of the pipe (consider this your drag in free-fall), and the opening size of the pipe / pressure of the flow into the pipe (consider this your acceleration under gravity).

    Given there are optimal 'pipe' sizes for fluid flow speeds, there are likewise non-optimal pipe sizes, where there is too much surface area, in turn creating more than optimal surface tension (drag!!) to slow the body of fluid down.

    ....aaand I think ill stop there. Moral of the story, massive pipes are bad, tiny pipes are bad...get a pipe to match your fluid flow.
  8. 5gmirage97

    5gmirage97 New Member

    i assure you back pressure has nothing to do with performance... i have multiple (different levels of modding, with different exhaust sizes) dyno sheets that clearly show that whether it be 2", 2.25", or 2.5" piping, all with very good headers keep in mind to optimize the scavenging (RPW Steet 4-1's, and MP SFT 4-1's) and with every diameter, engine produced more power with my post dp exhaust dump/cutout open vs it being closed.
  9. Liom the Chef

    Liom the Chef Member

    Are we talking peak kw output or the entire curve?
  10. Trondabron

    Trondabron Administrator Forum Administrator

    I think you will find that back pressure does effect performance. Swap one of your 2.5inch gaskets for a piece of sheet metal with a 1inch hole. Your performance will drop.

    I think you will find that at power levels that our cars typically experience, (relatively low, being low capacity engines) that the difference in pipe size flow will have less of an impact than say the exhaust components that go along with them, mufflers, and hotdogs and cats etc.
  11. 5gmirage97

    5gmirage97 New Member

    entire curve/range.. no question.. happened every time ive been to dyno.. my last dyno was 181 with cutout open, 174 with it closed.

    and yes obviously a 1" pipe is going to hender performance.. but i assumed we were all in the same thinking of only upgrading exhausts, not downgrading them.
  12. donki

    donki Administrator Forum Administrator

    We all agree getting more air+fuel in, and out efficiently will give more power, true?

    1st Case in point: motorbikes.

    Every think or wonder why Harley and other big cruiser bikes with straight pipes burble and pop and backfire all the time...? Not very efficient as the poping/back firing is fuel being wasted and not burning. Its a bit different with bikes due to most being carby's, and smaller engines requiring a little more fine tuning compared to cars, but its engine basics...

    2nd case in point: Formula 1 cars.

    Scavenging plays a massive role in the manifolds, just like in cars, and it would be rightly so: a 2.4L V8 revving up to 18,000rpm? Almost incomprehensible how fast the gases would be bucketing out of the pipes.
  13. aviomen

    aviomen Member

    ENOUGH Said pplz, all i am interested to know is what piping size is best for my CE :)
  14. donki

    donki Administrator Forum Administrator

    2.5" for turbo applications.
    2" or up to 2.25" for everyday use.

    Get a semi-decent sports muffler with either setup, and youre good to go
  15. peregrine

    peregrine Active Member Lifetime Premium Member

    As each cylinder fires there is a gap before the next one and so on, so an extractor manifold is attached to the out flowing gasses to even up the flow of this pulsing and in fact endeavor to use the vacuums formed by pushing the next positive burst into the vacuum formed by a previous stroke to form an extraction effect.

    Then pipe diameter is the next issue larger is generally better but only to a point dependent on engine size and or practicality.

    Next a big hurdle, causing heaps of back pressure but essential is the catalytic converter.

    Last but certainly not least the Muffler with its inevitable increase in back pressure, major point of attack and the easiest and cheapest way to alter back pressure and exhaust tone. The muffler in order to reduce noise to acceptable (legal!!!) limits has either baffles or packing materials and whatever else including its size. All alterable.

    Exhaust systems don't cause back pressure? On the race track everything possible is done (no mufflers etc) to keep the air flowing through as smoothly and efficiently as possible.
    That's my simplistic look at this situation.
    Last edited: Oct 31, 2015
  16. peregrine

    peregrine Active Member Lifetime Premium Member

    Anyone ever thought of placing a free flowing fan in the exhaust possibly try tip at rear easiest.
    Driven by exhaust pressure pulse, continue rotating on vacuum, extract, to even up and smooth flow?

    Edit rather than new post. Just requiring momentum of power pulse to smooth following vacuum. realize fan diameter would need to be larger than pipe to compensate for restriction it would cause. Maybe this would be better placed before the muffler.

    Sought of like the benefits naturally gained from a turbo in evening up exhausts flow but without the compressor for induction and benefits of it being where it goes and therefor able to be placed down the rear end.

    This would possibly be a worthwhile patentable item if it works!!!! Good luck.
    Last edited: Feb 20, 2016
  17. donki

    donki Administrator Forum Administrator

    Turbo's use that effect to increase operational efficiency.

    Essentially, the fan would spin nowhere near fast enough to assist, which is why when you start fiddling with turbos they start making all kinds of odd sounds.

    Grabbed from another website:
    My car has a 3.7 litre engine and the diameter of the single exhaust pipe at the back is 6 cm. The engine can rev to a maximum of 7,000 rpm.

    Here are my calculations:

    At 7,000 rpm my engine is spinning 116.7 times per second.

    As it is a 4-stroke engine, each cylinder is displacing the exhaust gases into the exhaust system once in every two revolutions. Therefore, the engine is putting out 3.7 litres of exhaust gases 58.3 times per second. That is a total volume of 215.8 litres per second.

    The cross-sectional area of the end of my exhaust pipe is pi*(3*3), which is 28.3 square centimetres. To contain 1 litre of air, a cylinder with this cross-sectional area would need to be 35.4 cm long.

    Therefore, if my engine were displacing 1 litre of gas per second the gas would have to flow through the end of the exhaust pipe at 35.4 cm per second.

    To displace 215.8 litres per second the gas would need to travel along the exhaust pipe at 76.32 metres per second. That is 162 miles per hour, or 260 kilometres per hour!
  18. frosty7

    frosty7 Active Member

    Merge collectors, headers that increase in diameter and tuned length runners who can tell me what I need???
  19. peregrine

    peregrine Active Member Lifetime Premium Member

    don't know how this got into this yellow part? If the air coming out of the exhaust is enough to spin a turbo fast enough,. why wouldn't the same amount of air be enough to turn a fan over fast enough here? Same volume just cooler and not driving a compressor
    Last edited by a moderator: Feb 20, 2016
  20. donki

    donki Administrator Forum Administrator

    Corrected the yellow part for you.

    The reason it wouldn't be a smart idea (it could work, dont get me wrong), is because if you power the fan externally it would need an outstanding supply of energy to spin fast enough to constantly extract the massive amounts of gases.

    If the fan is just stationary and siting within the exhaust to simply spin when the gas passes it, and then to suck air when the engine in in vacuum like you said, there next issues are the drag of the fan (how long will spin after gas stops passing through, and how much will it slow if theres a vacuum on the engine side and a positive amount of gas post-fan), as well as whats called the Venturi effect (or in other words, the amount of resistance the fan would introduce to the exhaust system VS a proper tuned length exhaust system where each pulse of gas sucks the next pulse out.

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