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Exhaust Pipes: Is Bigger Better?

Last updated 11/06/2016

Imagine you’re a design engineer for a vehicle manufacturer. You’ve been tasked with increasing the power output of an engine and squeeze out a little more fuel economy too. It’s the same engine as previous model, but for the new model which needs a bit more power and economy for marketing reasons. However this engine is already several models old. It’s reaching the end of its life and the upcoming model is the last model before the engine is retired and the all new engine supersedes it. You’ve already de-bottlenecked it several times to squeeze out more power. Have you run out of options? What about a bigger exhaust?

How much would a bigger exhaust cost relative to a completely redesigned engine? I’d suggest that once a bigger exhaust is integrated into a production line, the extra cost would be almost nothing. A bit of extra metal in the larger diameter pipe? Bugger all. A complete new redesign is a huge expense.

So it seems a bigger exhaust would be an attractive solution for manufacturers trying to squeeze extra power and economy out of engines. If you were the engineer would you exploit this solution? I know I would. Unless it didn’t work.

Even in the final run of a particular engine, manufacturers do not increase exhaust size. Why? Because bigger isn’t better! The exhaust diameter coming out of the factory is already optimised for that engine. It’s common sense. Why wouldn’t the manufacturer exploit such a cheap and easy optimisation opportunity?

Flow velocity is important in exhaust pipe design. Exhaust gases are emitted in pulses. One pulse every exhaust stroke. Small pipes yield a high flow velocity. High flow velocity increases inertia of the flowing gas and smooths out the stop / start nature of gas pushed out by the engine. As the gas continues to flow between exhaust strokes, it creates a low pressure at the exhaust manifold which helps to suck out exhaust gases on the next exhaust stroke. This reduces the energy required to clear the cylinder of exhaust and re-accelerate the gas residing in the exhaust pipe already, thus reducing the energy wasted pushing out exhaust and improving the efficiency of the engine. Further, smaller exhaust pipes means the mass of gas held within is less. This translates to less energy required accelerating the mass of gas in the exhaust pipe with each exhaust stroke. Again engine efficiency is improved.

Smaller exhaust pipes present a greater resistance to steady state flow. So smaller isn’t necessarily better either. It’s a compromise between resistance to flow, flow velocity and mass of gas residing in the pipe. There is a sweet spot. An optimal compromise. The manufacturer’s design is based on this optimum. A bigger exhaust pipe isn’t necessarily better. Even without any understanding of exhaust physics, the proof is that manufacturers do not exploit bigger exhausts to improve performance. Why else wouldn’t they? Imagine how much extra metal is involved in a slightly bigger pipe. Not much, and steel is cheap. Manufactures don’t increase exhaust size because it doesn’t universally improve performance or efficiency.

The optimal exhaust size is a function of exhaust volume. The greater the exhaust volume, the greater the optimal exhaust size. Exhaust volume increases with engine rpm and fuel delivery. So ideally you’d have a variable sized exhaust pipe that increases in size as rpm and fuel injection quantity increases. For a fixed exhaust pipe size then you need to pick a compromise. The manufacturer does this – they will pick an exhaust size that is somewhere in the middle, optimized for the rpm when the engine develops it’s maximum torque and optimised to deliver the greatest area under the power curve. This will provide the greatest possible torque whilst maximising average power over the entire power curve. Maximum torque and low rpm performance are favoured because this is what makes the biggest impact for normal day to day driving.

A larger exhaust changes the shape of the power curve. It makes it more peaky, but the area under the curve is reduced. Average power is less but peak power is more. This will present as an extra couple of kW of peak power on a dyno run as it provides lower resistance at peak exhaust volume. However, looking at the entire rev range, the larger exhaust has deviated from the optimal size and average power over the full rpm range will be reduced. Maximum torque may be less and low rpm will produce less power. Fuel efficiency will also be worse, since most of the time your engine operates in the bottom half of its rev range.

A common mistake in terminology that people make when describing the relationship between exhaust pipe diameter and performance is that the engine requires a certain “back pressure”. Actually back pressure is a bad thing. It should be minimised. Engine’s don’t require back pressure but installing an exhaust pipe that is too big does negatively impact engine performance as the paragraphs above explain. I guess the back pressure explanation is a simplified way of saying bigger isn’t necessarily better (which is true).

Often an argument for a larger exhaust improving fuel economy is that if it improves power by a few kW then you should be able to get the same power as with the factory exhaust by using a bit less fuel. This isn’t correct. The increase in power output is occurring at high rpm only. At low to medium rpm the engine is less efficient and will use more fuel. Do not get a larger exhaust if you want to save fuel!

What about vehicles with turbos? Some people believe vehicles with turbos always benefit from larger exhaust pipes. A turbo is not a magical device that perfectly and completely isolates the upstream pipe from the downstream. It’s just one pipe with an obstruction in it, no different to the affect of a catalytic converter or muffler. What comes into the turbo must come out of it. The fitting of a turbo does not circumvent the explanations above. A larger exhaust downstream of the turbo will still hold more mass of gas which will be harder to accelerate. It will still yield less exhaust velocity and therefore less ability to continue to suck out exhaust between exhaust strokes. There MUST still be compromises involved otherwise the larger pipe would be there already. Do you honestly believe that vehicle manufacturers, with their hundred million dollar development budgets, years of cumulative experience, access to the best people in automotive design and an extremely competitive and mature automotive industry, would produce turbo charged vehicles with exhaust pipes that are always too small? If yes I have some property to sell you in the desert of Western Australia, it has great development potential.

Optimising a turbocharged reciprocating engine is different to optimising the energy extracted from a turbine. They are two different design problems. In a turbocharged engine the turbo is a parasitic load that wastes engine power. That waste needs to be minimised so that overall engine efficiency is optimised. You want the turbo to consume the minimum possible energy required to give you the desired quantity of air whilst simultaneously designing the exhaust system to minimise the energy required to clear the exhaust gas. This maximises engine efficiency. In a turbine engine, like on aircraft, you want to maximise energy transfer to the turbine. This confuses some people into thinking a bigger exhaust pipe is better on turbocharged engines.

I’ve read about people with turbo charged vehicles who validate that low end performance and fuel economy suffer with a larger exhaust pipe. This is a more reliable finding than those that confirm a larger exhaust pipe is better in every possible way due to the effect of placebo and confirmation bias.

What if you have a diesel performance chip? Chips dump more fuel in the engine to get more power. This translates to an increase in exhaust volume across the entire rev range. The optimal exhaust size will be larger than the optimal size for less exhaust volume. So if you have a chip you may benefit slightly from a larger exhaust. Same applies to any other performance enhancement that dump more fuel and air into the engine.

Some power gains (of maybe a few kW) can be had across the entire rpm range through less restrictive catalytic converters and mufflers / silencers and less bends in the exhaust pipe. The power gains are due to simply putting less stuff in the way of the exhaust flow and are unrelated to pipe size. Increased emissions and more noise are negative consequences to these types of modifications.

Replacing the catalytic converters and mufflers with high performance versions yields misleading dyno results. Firstly, if a bigger pipe was also installed, the vendor will have you believe that some of the improvement in power was due to the bigger pipe, even though at low rpm the bigger pipe actually made it worse. Secondly, the vendor will attribute part of the increase in power to the catalytic converters and mufflers being better than the old. Actually it may be simply that the old parts were clogged with soot. Installing new standard parts may have yielded a similar improvement in power. To correctly understand the impact of the high performance parts you’d have to compare the performance to brand new stock parts.

If you are always running your engine at high rpm (for example if you are racing) then you may benefit from a larger exhaust. You will get poorer fuel economy, lose low end power and your maximum torque may be reduced but you’ll gain high end power. High rpm yields high exhaust volume which benefits from the reduced resistance to flow of a larger exhaust. So if you’re interested in slightly more high end power at the expense of low end power and economy then get a bigger exhaust. For me, I need a tool that takes me camping and fishing at minimal expense. This maximises my camping and fishing capability. So my exhaust will be staying stock forever.

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See also:

Design Compromise

Emission Systems – Worth Tinkering?

Diesel vs Petrol

Diff Locker vs Traction Control

Do You Need a Suspension Lift Kit?

How Often Should I Service My Car?

How to Improve Fuel Efficiency

Why No Diesel Performance Chip?

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56 thoughts on “Exhaust Pipes: Is Bigger Better?

  1. Would it not need more boost ? For your comment about chips increasing exhaust volume.

    • Yo Theodore not sure what you mean. Chips make more power, more power means burning more fuel and more exhaust gas volume. Some chips might crank up boost on a turbocharged engine, some engines might have some spare air already, you can dump more fuel in and sacrifice efficiency and still get a bit more power even if it’s overfuelling for the amount of air. Diesels usually always have plenty of spare air. Doesn’t matter though. Any way you do it makes more exhaust gas volume.

  2. Should read ‘End of its life’ not ‘End of it’s life’..

  3. Hi Joe, My thoughts on the turbo version in this article are: There is no low pressure effect when the engine has a turbo because the turbo itself acts as a restriction in the exhaust system (the exhaust gas backs up against the turbo wheel) and so that part of the equation no longer exists. As for having a large pipe after the turbo, you want to reduce the resistance to flow (as you mentioned) as to not impede the rotation of the turbo exhaust wheel. In this situation, big pipe is good, you dont have any enhaust manifold low pressure benefits so the best thing to do is reduce any restriction to the turbo spooling.

    Fellow WA bloke here!, and loving your website. I only just came across it today. Hopefully I will have my 40 series back on the road soon…

    • Hey Dave yes the turbo is a restriction but the exhaust flow will still exit the turbo in pulses and exhaust velocity still impacts performance. Turbos are a 90 year old technology. I doubt such a mature technology, combined with hundred million dollar development budgets, would yield consistently undersized exhaust pipes across every make and model of turbo charged vehicle. Impossible.

      40 series ay. Those old cruisers are awesome. I love strong, functional, simple stuff. Good cars they are. I could picture myself chugging around in one of those.

  4. the belief that “why wouldn’t they have done that already ” I think it’s quite misplaced. Manufacturers are trying to squeeze maximum revenue out of each vehicle, and I think you find if they can make the exhaust smaller, it will save many dollars overs thousands of vehicles.
    What about headers? They cost bugger all extra to manufacture than the standard exhaust manifold, and they really do make a difference! However, bigger all times thousands of vehicles equals more profit.
    I certainly don’t place my trust in companies to look out for my best driving interests.

    • Hi Perry are you disputing well published, universally accepted exhaust theory? You don’t think exhaust pipe size is a compromise? On what grounds? Can you explain why flow velocity doesn’t impact performance?

      Or if you accept that exhaust pipe size is a compromise then you accept that a larger exhaust makes low rpm less efficient. It can’t be circumvented. There’s plenty of info on the net to validate this, do some research.

      How expensive is a bit of steel pipe compared to a few hundred million developing a new engine? Or putting in an engine bigger than necessary to compensate for an exhaust too small, if you sum that up over thousands of vehicles? Manufactures are going to great lengths to get small benefits in efficiency and performance because they are the key marketable parameters for their vehicles. But they won’t exploit a slightly bigger steel pipe? The leaders of this highly competitive and mature industry are putting in engines too big and developing new engines and technologies unnecessarily and coming up with direct injection and variable valve timing and ridiculously high pressure fuel systems and hybrids and engine management systems sophisticated enough to run the space shuttle to squeeze out a bit of extra performance and efficiency when all they need to do is put in a bigger bit of steel pipe?

      You trust the after market vendors over the original manufacturer? You prefer ambiguous claims and lack of performance guarantees as your measure of trust? “Up to xxx improvement in fuel efficiency.” You won’t see “Up to” on an original manufacturers specification. They are held accountable for what they publish. Unlike after market vendors who claim anything to lure in the gullible consumer.

      • All of this is true but you’re not considering that the “compromise” you’re referring to has to factor in noise and pollution restrictions too. Which you obviously ignore when you’re tuning your vehicle for maximum performance alone.

        Regarding the headers, I think that Perry made the right example. Stock cast headers are very cheap to make whereas aftermarket stainless steel headers, with a single pipe for cylinder and carefully designed to have the same length for every cylinder are not. Of course if you buy a Ferrari you can bet that Ferrari engineers already squeezed out every bit of power out of the engine (still, taking into account noise and pollution, which is why they’re the same people that sell you performance upgrades “not for road use”) but if you have a 70hp commuter my bet is that they probably sacrified a couple of hp in the name of fuel economy or for a couple of bucks less in the price tag. Which is a big deal on cheap cars, as opposed to the Ferrari.

        So you’re right when you say that a larger exhaust pipe alone will probably do less worse than good, but you can’t be that peremptory either 🙂

  5. Hi joe ! Great articles.. Love your passion and the fact you are willing to share your knowledge. I JUST bought my 08 hilux sr5 turbo diesel and I’m wanting more power , but I’m now put off chips and modules . Do you have a suggestion ? Perhaps a high flow turbo or ?! Ahh I’m confused..

    • Hey Nicholas the only way to get more power is to dump in more fuel and air. There’s already a bit of extra air from the factory so a chip is a good option to dump in more fuel if you’re just after modest gains. Then if air becomes the bottleneck you get bigger turbo. Then the injectors become the bottleneck and you need bigger injectors.

      Why have you gone off chips?

      • Hi joe , ok .. I’ve gone off them because I had a theory they can’t be good for the motor, and you confirmed the theory.
        BUT I have found a second hand ‘ diesel smart’ brand module claiming ” Diesel smart tuning module that’s plugs into injectors changing duration not rail pressure !
        Been on my car for around 12 months
        Also has a separate interface to prevent over boost

        Manufacturers specs
        These are some of the key features of the Diesel Smart Module.

        Module is fully programmable.
        Holds 4 individual programs that the customer can change at will.
        Module fuel is adjustable upto 30% +/-
        Can be Optioned with
        Customer has the ability to adjust fuel at will.
        Module plugs directly into each injector.
        Module changes injection timing, not rail pressure (like a chip)
        Tuned for Australian Economy.
        Diesel Smart modules have total control over the entire injector including Injection Timing, Pulse, Width & Current.
        Up to and over 30% gain in Power and Torque.
        The very latest in advanced digital technology.
        Experience the smoothest power delivery & performance.
        Simple plug-in installation IN MINUTES!
        INSTANT results!
        High quality RoHS compliant modules are made in Germany.
        Gold plated module connector contacts.
        Australian owned company.

      • Doesn’t matter which way you do it, they all wear out your engine faster. You can’t get something for nothing. The more you increase the power, the more the wear and the higher the failure rate. The method doesn’t matter.

        For modest gains a chip that only increases pulse width is pretty good. Like the diesel smart, or steinbauer. But if you want more then you need to increase rail pressure too. And then crank up the air. And then bigger injectors. In terms of failure, it’s more related to how much you increase power by, rather than the method. To the engine it’s all the same – dumping in more air and fuel.

    • Not sure where you live but these guys in Perth are turning tubo diesels into tyre shredding beasts! http://gturbo.com.au/ I look at some of the dyno sheets on their Facebook page in awe…. unfortunately there is very little I can do with my old mechanical injection Toyota 3B wth aftermarket turbo.

  6. Ok … Well I may opt for the ‘ diesel smart ‘ .. If u say it’s the safest . Recommend a k & n air filter ? And money wise a 3 inch exhaust for 1100$ seems too expensive after reading your article, I may just go for the module:-/ OR spend the money on Xrox bull bar and air locker ( in future )

    • I have no direct experience with k & n filter. There is a good study published online that demonstrates the factory paper filters are the best in terms of filtering capability. And the power gains from k & n are slim to none. So in my opinion not worth it. Some people like them.

      To figure out what you should spend money on, first figure out what are you trying to achieve and what makes you happy. Do you want to explore the outback, go offroading, camping, etc? In this case performance does not help achieve your goals. You’d do no performance upgrades but you would get a diff locker and maybe bull bar. This optimises your finite resources to achieve your goals. Are you racing? Then go for more performance upgrades.

  7. Yeah I hear u joe . Thanks for the replies! I actually love the outback, and escaping away from civilization. and I’m leaning towards no performance upgrades, and taking into consideration a xrox bullbar and a rear ? Air diff locker ..
    Curious about your car as ours are only separated by a year and mine is automatic, . How many kms have you done? How long have you had it for? Will you upgrade?

    • I bought mine in 2011 with around 60k on the clock, now it’s done 130k. Current plan is to never upgrade, I’ll keep it until it dies. For two reasons. One – upgrade won’t help me achieve what I want to achieve. It will only serve to make me poorer and able to afford less experiences that I enjoy. Two – I really dislike modern cars. The whole philosophy behind their design is not right for me. To explain properly would be quite lengthy, I’ll be writing some articles about it in the future. I do touch on it a bit on the why hilux article. Also diff locker vs traction control alludes to my philosophy.

  8. Morning , yeah for sure ! I hear you ! I just bought mine with 68,000 on the clock ! I love it it is my pride and joy . But I’ve always loved hilux s .. I have no reason not to keep this for ten years , but I / we need to keep in check this insatiable need to consume , why buy another unless u need ? Appreciate the advice and talking sense to me practicality is the way now ! Getting a reverse camera stereo put in today or tomorrow ( I have a baby ) and I load tools in and out the back in tight car parks in the city . And I’ll enquire about a diff locker today ..

    • Yeah reverse camera is handy, I got one, came with my head unit. Hilux is long with a big bulging ass end.

      I plan to expand this site with many more articles around consuming and happiness and how they relate. Some of my articles already have that theme but I have heaps more to write. Will be a year or two until I get around to it, no time now.

  9. Many good points and great site, however I have to disagree in relation to turbo cars. Also, you seem to have completely ignored the fact that manufacturers have to comply with noise standards and many new car buyers don’t want a loud car. Fuel economy is also a big factor. Others who are after performance don’t mind the extra noise or fuel consumption.

    Manufacturers designs are definitely not optimum in all cases especially if performance is your first priority. Some cars are better than others from factory. For many buyers performance is not the biggest factor and this influences design.

    If a manufacturer can save 55c per car on a mass produced vehicle, and their research shows it won’t affect their sales targets or warranty repairs etc, you can bet your life they will do it every time. All they care about is the bottom line. There are thousands of examples of this in production cars. Just look at some of the things that go wrong with them out of warranty.

    Check out these dynos showing both torque and HP gains over the entire rev range by increasing pipe diameter from 2.5″ to 3″ on just the cat back. It’s a 1.6l turbo. No low or midrange losses. Bigger gains again when replacing the whole system, and the only reason they didn’t go bigger is excessive noise.

    http://blog.perrinperformance.com/3-exhaust-too-big-for-your-turbo-mini/

    Sure it’s a compromise and it’s not going to make your car quieter; however with respect, I feel that your comments especially in relation to turbos are misleading.

    • Hey Oscar thanks for your feedback. That link just looks like someone trying to sell 3 inch exhausts. Did they change the CATs? Did they change mufflers / silencers? If you take stuff out of the way of the exhaust flow then you improve performance across the board, but this is unrelated to exhaust size. How many runs did they do and how did they control variables related to ambient conditions and engine temperature? You can change nothing on a vehicle and get similar variability in dyno runs. I think there are too many variables to say the difference between the charts is solely due to bigger exhaust. With so many variables there’s no way I’m going to trust charts from someone trying to sell bigger exhausts.

      Yeah manufacturers will try to save 55 cents if they can. Hows 55 cents sound compared to a few hundred million dollars developing a new engine?

      Manufacturers will save 55 cents on a push button or something because it’s hard to identify that on a specification and it’s hard to market “we have the 55 cent more expensive push button”. But when it comes to power and fuel economy, the two most marketable and easily specified features of a vehicle, where it costs hundreds of millions of dollars to squeeze out better performance in an all new design, I think the manufacturer will spend the 55 cents.

      Noise I think is more governed by mufflers and silencers and not pipe diameter. In any case I don’t believe that, in turbo cars, bigger is universally better, regardless of what someone trying to sell bigger pipes tells me. The laws of physics don’t change when a turbo is introduced.

      What’s misleading is vendors exaggerating the benefits of their products, and people being convinced to buy something that, with the full picture, they probably wouldn’t buy.

      • outbackjoe.. can you instead show us all a dyno run of a turbo charged engine being diesel or petrol that lost power going into a oversized turbo back exhaust?

      • Hey Donald what do you mean instead? Instead of understanding the physics? Why would some arbitrary graphs of an isolated case be better? With so many variables and only a few percent change in output, you can produce anything you want with a dyno run. That is why such info is so misleading and causes people to waste their money. It’s better to understand the process. Not only is it more interesting and thought provoking than comparing numbers from a flawed experiment, it also provides the ability to understand different scenarios, understand the compromises in a design and appreciate the existing design. It means you can see past the marketing rubbish and only spend money where it suits what you’re trying to achieve. That’s the point of this article, to understand the general principles and concepts rather than compare an isolated case.

        Instead of asking for an isolated case what if you keep with the theme of the article and explain why exhaust diameter selection doesn’t involve trade offs with reference to the explanation in the article, if that’s what your point is.

      • Now I realise that legally it is not acceptable and I actually no longer have the vehicle but with a previous car I owned it gained 6kW measured at the rear wheels (dyno) by removing the catalytic convertor. This is nothing but a removal of an exhaust restriction which upped the power delivery to the wheels from 201kW to 207kW. Just my two cents.

      • Yeah getting stuff out of the way of the exhaust flow will improve performance slightly. It’s got nothing to do with exhaust pipe size though. It’s all covered in the article.

  10. Hi Joe, i enjoy your blog, great to see you putting your experiences and ideas out there. It’s good to see people having conversations on a blog where no one gets offended, just for once!

    I’ve got a physics and aeronautics background, and respectfully disagree with some of your points relating to combustion engine turbines and the effect of back pressure. I think some of these misunderstandings come from your freely interchanging of the terms flow, volume, air mass and velocity. I know you are not a numpty and know this already but a turbine or even a water wheel operates by the principle of pressure differential P1:P2 across a wheel allowing it to do work when harnessed (in this case creating boost, although it could be running a dynamo or sonething else). For a given static manifold pressure P1, the greatest work the turbine can possibly do will be at the lowest P2 on the outlet side. Flow, velocity, and air mass (in the exhaust system) after the turbine are all terms related to the creation of the final figure of P2. An exhaust can never ‘suck’ the turbine along no matter what the size or diameter. It is simply the restriction preventing the turbine from being more efficient and having yet more work available to be extracted from it. When the turbine is being used to make more power from the engine the rules do change. The exhaust cannot ‘scavenge’ the turbine, unlike in a non turbo combustion engine. It is always the restriction. The rules change.
    Now hang on i hear people say. I gotcha with this one. Why do fighter jet turbines have variable vane nozzles then? Why not just a huge dump outlet? Because in that (different) case you want the exhaust gas after the turbine to do further work, which is push against atmospheric air molecules. To do this you want maximum kinetic energy and so maximum velocity. A simliar case exists with design of water jets in firefighting equipment (which are driven by a water turbine) to throw maximum distance. You need a long tube for laminar flow. A third variable has been introduced.

    Similarly, in helicopter jet engines the exhaust gases are harnessed to a free spinning power turbine directly to the main rotor and now you have a P3 to contend with…a three stage turbine so to speak. It gets damn complicated then to keep all the wheels spining along happily! However, to keep the P2:P3 ration as high as possible the power turbine dump is as least restrictive as possible also to allow maximum diabatic expansion. Think sexy giant nickel chrome dump pipe ie AS350 or B214. Mmmmm…Bell 214….noice fully sick dumo pipe maaaaate!

    I am in complete agreeance that the diameter of these aftermarket exhausts is probably the least reason for any power increase observed -most likely the better flowing catalytic convertor or straight through exhaust box. And they are noisy. Hate em. Which probably is why manufacturers keep them smaller and quiet.

    This is a good summary, he writes very well.
    http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

    Anyway..a few more neurons saved from the scrapheap. Any brain exercise is worth it. I enjoy reading your articles. No offence taken if my reply is deleted. ..

    • Hi Craig cheers for your comment, interesting stuff. I don’t reckon anything in your post is in conflict with anything in the article. The point in the article is not related to optimising energy extraction in the turbo. The point in the article is that the exhaust flow comes into the turbo in the form of pulses, they exit the turbo in pulses, the exhaust diameter will affect those pulses and therefore affect how easy it is for the engine to get rid of exhaust gas. This is a separate issue to optimising the system for maximum energy transfer to the turbo. The design would take into account efficiency of getting rid of exhaust as well as energy transfer to turbo and a compromise is made. It’s a different design problem compared to purely wanting to extract energy from turbine. In a turbocharged engine the goal is not to maximise energy transfer to the turbo. Actually the turbo is a parasitic load that takes energy away from where you want it. You want the turbo to extract just enough energy to give the required boost. No more. Other than that there is still the goal of maximizing fuel efficiency and area under the power curve.

      The turbo can not perfectly isolate upstream and downstream. So I don’t agree that exhaust velocity downstream of the turbo has no impact on exhaust scavenging. Each downstream pulse helps each upstream pulse flow along, all the way back to the engine, turbo or no turbo.

      Actually pressure drop across the turbo would be quite complicated when the flow is in pulses, and a smaller exhaust downstream of the turbo could help increase the pressure drop across the turbo in the same way that it leaves a low pressure to more easily remove exhaust gas from the engine. If the higher exhaust velocity helps create a low pressure downstream of the turbo when the next pulse of high pressure arrives then you get a bigger pressure drop with the smaller exhaust. I’m just speculating, but my point is it’s more complicated than “bigger is better”.

      If a bigger exhaust facilitates more boost this does not equate to an improvement in efficiency. All it does is allow more air and fuel to be dumped into the engine. No different to any other way of dumping more air and fuel in there. It’s sort of like getting a smaller pulley for a supercharger. This increases the parasitic load on the engine and therefore overall efficiency is less. But it increases air and fuel dumping capability so overall power output is increased.

      Designing the intake and turbo for an engine would first involve figuring out how much air and fuel you want to put in there. Once that is determined you can proceed to design the turbo and exhaust to deliver exactly the required air. No more. It’s not about optimising the pressure drop across the turbo.

      If you want more power then bigger is better, both in terms of less resistance to high exhaust volume and probably for getting more turbo boost and therefore being able to dump even more air and fuel into the engine. However this is not universally better, since you may be more interested in low down torque, fuel economy, minimising capital cost and maximising engine longevity. That’s what I’m interested in, since it increases my fishing and camping capability.

      • Hi joe,
        I got motivated amd did a few bum dyno hill tests with same ambient temp, same load and as close to possible throttle openings. It was about a 35deg day, was a bit sweaty under the cruiser changing exhausts!
        I could detect no performance difference with the 3″ exhaust vs factory stock up my test hills either bum dyno or speedo reading. I don’t have an egt gauge.
        I’ve left the stock exhaust on now, and conclude that unless you are dumping more fuel in with a chip (which i won’t) the stock exhaust is fine. Mr Toyota knows his/her stuff. So there ya go!
        Keep up the interesting articles.

  11. Nice work dude. So the difference is less than you could feel or detect on the speedo. Yeah, Mr Toyota knows his stuff. Just to confirm, your cruiser is turbo diesel?

    • Sure is. 1hdfte 100 series. And leaving the engine stock as a rock. That’s how you get 500k out of them. You always want more power but seldom need more power I figure.

  12. Actually I have put a plug n go module on the accelerator. Much better to drive around town, the up to 0.8 sec of fly by wire throttle lag was never welcome, even less so towing a camper. Thoroughly recommend it on this particular vehicle. although I was pretty sceptical about their blurb.. On the off-road setting up billygoat bluff track recently, the less sensitivity setting was welcome when accidentally stabbing the throttle over bumps.

    • Last comment on this. Just had a chat with my dyno tuner about why my standard 1hdfte seemed to have less torque with the 3″ exhaust. Answer is that at lower rpm the reduced backpressure and better flow means the afr ratio is actually increased, as no more fuel is being added. So there you have it…putting a larger exhaust on a standard turbo diesel is probably a waste of money without a chip if you are chasing more power. The bum dyno can actually work sometimes!

  13. Hello, I’m a newbie towards diesels and I am getting an SUV as I simply can’t stand an MPV for the ever growing family. So, I’m about to own a KIA Sorento with a 2.2L turbo diesel engine that puts out 147kw/3800 rpm and 441.3nm from 1750/2750rpm.
    Test drove the car and its a no brainer for a 7 seater with lots of options thrown in. My only beef with the car is that it feels great off the line but I feel its lacking in the 80-100kmh whilst overtaking. With significantly more money, I would have gotten the Volvo XC90 which has 187kw/5500 rpm and 350nm from 1500-4800rpm on the tap from its 2L turbo petrol. It has more power, less torque but is significantly superior in that 80-100 range based on my test drive.

    Since you don’t recommend changing the size of the exhaust, would it be of value to install a mandrel bent system with a free flow muffler to help boost power towards higher rpms?

    Thanks for your patience

    • Hey Vincent I don’t not recommend changing the size of the exhaust. I explain that there are compromises. A bigger exhaust will give you a bit more top end power. If that’s what you want then it’s a good thing. Mandrel bent and free flow muffler will probably improve power a bit but I don’t think it will be noticeable.

      Your best option is to understand that there is no correlation between engine power and happiness, minimise expenditure on your vehicle, spend less time working and spend more time with family and enjoying hobbies. 147kW is a butt load of power. Too much power. Way more power than needed. Old diesels used to have 60kW and people were happy with that back then. The reason for the desire for more power is just down to marketing and consumer culture.

      If you really need more power then the only way to make a significant difference is to dump more fuel into the engine. Playing around with exhaust might give you a few percent increase in top end power, at the expense of reduced low end power and economy. The bang for your buck isn’t very good I don’t think, if it’s more power you want. First figure out what you are trying to achieve and then optimise your spending around that goal.

  14. Hi Joe, many thanks for your time and reply. I hear what you are saying with regards to engine power and happiness. However I do spend over an hour a day driving for work and ferrying the wife and kids. The aural effects of a low non droning rumble and power on tap when i need to overtake whilst I drive, would go quite far to minimise the mundane highway drive.

    I did think that 147kw and especially 441nm would more than suffice, but it was a tad disappointing during the test drive. Would I be right to say that perhaps peak torque occurred a little early at 1750rpm? And would having the peak torque kick in more at 3000rpm be the way to go? I was guessing that having a bigger pipe and less restrictive mandrel bent pipe would help. I’m not sure what the stock size would be, but would 2.5 inch sound sufficient? No chipping for me as it would void the warranty I suppose. Thanks again

    • Yeah a bigger pipe etc will help, but only by a few percent, and you’ll suffer poorer fuel economy and less low down torque. I dunno what size, the bigger the size, the poorer low down performance and fuel economy, and you get diminishing returns in terms of top end power. The sweet spot that maximises the area under the curve has already been chosen by the original manufacturer.

  15. Hello Joe,

    Was just wondering, i had my 2011 sr5 straight pipe exhaust done & noticed power loss at low-end rpm ranges.

    Would re-installing the cat & muffler regain the power loss?

    Thanks!

    • Hi Joel did you get a bigger pipe than standard?

      • Hi!

        Nope! I still have the standard sized pipes. Just did a de cat & muffler. Replaced them w pipes of the same diameter as the standard ones.

        Am thinking of reinstalling the stock mufflers/cats but would i get it back to normal state? Or do i have to do an ECU reset/remap.

        Thanks for your time & help!

      • Hmmm I dunno really. Taking out the cat and muffler shouldn’t do anything except make more pollution and make it louder. Not sure why you are suffering from loss of performance. I’d put the cat back in at least, so you don’t give cancer to the guy driving behind you.

  16. The torque lost is mainly from low to mid end torque range. But it seems to have better flow at higher rpms.

    What i would like to know is would i get my low end torque back after reinstalling the stock mufflers?

    • Yeah I don’t know. The cat and muffler shouldn’t noticeably impact torque. The symptoms you describe are consistent with increasing exhaust diameter and not consistent with removing cat and muffler. Maybe something else is happening. I don’t know what the impact of reinstalling would be, apart from less pollution and less noise.

      • Alright, thanks for the info!

      • no worries good luck hope u get ur torque back.

      • I just had my cat & muffler reinstalled. Feels like the low-end torque is back. Might be the scavenging effect! Never should have messed w em in the first place. Hopefully the air/fuel ratio is still in place.

        Thanks joe!

        Your website rocks!!

        Thanks joe!

      • Nice one. Good work for coming back with the result. I can’t explain it. Maybe the engineers have gotten so fancy that these boxes on the exhaust resonate with the exhaust pulses to reinforce them or something. I’m just making shit up. The point is that design compromise tells us that a change will have unintended consequences even if we don’t understand why.

        In design, when a restriction is artificially placed on something due to standards or whatever, eventually the engineers become so good at optimising and exploiting the restriction that it actually benefits the design. Or maybe not benefit the design, but the initial negative impact gets eroded away to almost nothing.

  17. Alot to take in & some over my head. I thought about getting a Turbo & my motor freshened up on a Petrol & still don’t know if the exhaust stays stock? Would it be a waste power wise to keep the same exhaust?Basically I wan’t a turbo but without any loud exhaust sound added, I like my petrol sound as it is nice & quiet.

  18. The turbo will significantly quieten the exhast, even once you increase the pipe size (as you should given that with a turbo, more air needs to flow). The turbo acts like a mufler in that the exhaust noise is essentially blocked by it (until the waste gate opens). Since putting the turbo on my landcruiser, I no longer run a mufler at all yet my exhaust is quieter than before I had the turbo, and I also upsized the pipework.

    • My impression was that factory cars are detuned for comfort, reliability and driveability.
      Diesels are a different beast to petrol. Whilst petrol turbos require more fuel are higher boost to reduce egt, diesel turbos require less fuel at higher boost to reduce egt. A bigger pipe by itself is not a great idea. A bigger pipe, with higher boost running properly tuned fuel and timing (not a generic powerchip) is going to be more powerful and economical.

      • Hey Blacky I agree about driveability since the factory config gives the greatest area under the power curve and gives plenty of low down torque. Can you explain how a bigger pipe with an engine tune would improve economy?

  19. It’s nice to learn that you should avoid getting a larger exhaust if you want to save fuel. That concept of bigger being better can be so misleading. Although, I’ll admit that for some things it’s right.

  20. So to simplify for a newbie reading this, For performance and speed go bigger exhaust and aftermarket accessories, For longevity, economy stick to standard running gear.

    • Hey Benjamin depends what you mean by aftermarket accessories (engine mods or bullbars or subwoofers etc), and depends what you mean by performance. Most aftermarket accessories make your car slower through increased weight and wind resistance. But modifications that make more power will of course make the car faster, that’s obvious.

      If by performance you mean ability of the vehicle of performing a job like take you to sweet camping spots then most aftermarket accessories make performance worse because they cost money to buy, make you use more fuel and make the engine not last as long. All of which reduces your camping ability.

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