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Want to set up a turbocharger for KTM 390 engine

  1. Jun 19, 2015 #1
    Hey friends,

    My college team takes part in college level racing events. They currently use a KTM 390 engine(race limit is 600). WIth the 20mm restrictor rule we can generate around 30-33hp after some tuning, after the restriction of course. Now we want more power. We need it. So we have 2 options- turbo or super. Super reduces the already short life span of the engine, and adds weight that cant be recovered by the power it generates. So turbo looks like the only option. Now the problem is that the rules say that we must build the turbo ourselves, from scrap. Turbo parts allowed. So i am thinking of building one, but i wanted to know whether it will work or not? And what turbo is suggested, and what could be the possible increase in power? Please give supporting links, that also show how to make it and tune it. Also, if you feel super is possible, then please suggest with supporting info and links.
    P.S. Please DO NOT suggest getting a bigger engine, as that option is ruled out. We need to build a turbo or super.

    thanks in advance.
  2. jcsd
  3. Jun 19, 2015 #2


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    Welcome to the PF.

    What reading have you been doing so far about turbochargers? Can you post links to that reading, and ask any specific questions you have about that reading?

    Also, have you considered using a 3-D printer to help you make some of the parts for the turbocharger? What temperatures do the parts typically have to endure?
  4. Jun 19, 2015 #3

    jack action

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    I suggest you used this calculator to estimate the potential of your engine (read the theory to understand how it works).

    The restrictor will impose a limit on the airflow rate based on this equation:


    Divide by ρ0 to get the volumetric flow rate. Assuming a discharge coefficient Cd of 1, SAE J1349 air conditions (99 kPa & 25°C), the maximum flow rate is 226.5 m³/h for a 20 mm restrictor.

    If you play around with the calculator, assuming a BMEPd of 12 bar, you'll notice that you cannot get more than 100 hp out of your engine, no matter how you modify your engine.

    You have a 1-cyl engine, hence you're doing your intake process during ¼ of your cycle (With a 4-cyl or more, the engine is doing it throughout the cycle, as every cylinder are not doing the process simultaneously). So, in theory, your engine pulls its air during only ½ revolution instead of 2 revolutions, which means the flow rate could be 4 times as high as the one from a 4-cyl+ of the same displacement and rpm. Your limit would be 56.6 m³/h instead of 226.5 m³/h (In fact, it will be a little higher as the vacuum created inside the manifold will still pulls some air even after the intake valve is closed). That means that your 100 hp limit just dropped to 25 hp! (I told you a little more and you got 30-33 hp). That is, if you pull the air with the piston itself.

    What you can do, is put a box at the inlet of the intake manifold and feed that box with an air pump. That box will act as a sub-atmosphere to feed the engine and the air pump will fill the box continuously instead of just during ½ revolution. This way, you can reach again your 226.5 m³/h limit imposed by the restrictor.

    Although you can now feed 226.5 m³/h, your engine has a speed limit and the inlet pressure (boost pressure) might be a limiting factor too. So if the engine is too small, it might be impossible to accept that much air. If you play around with the calculator, you'll find out that this is your case. With extreme modifications (I mean extreme), one might bring your engine to 65-70 hp.

    Turbocharged or supercharged?

    Personally, I think I would prefer a supercharger to a turbocharger for the air pump. A positive displacement compressor rather than a dynamic compressor.

    The supercharger is a relatively easy install compared to a turbocharger. The supercharger will not affect the air & exhaust flow of the engine. So you can tune the engine manifolds independently from the air pump. With a turbocharger, the flow affects the turbocharger and the turbocharger affect the flow. If you mismatch the parts, you can end up with something worse than what you started with. Since you have no reference for your engine turbocharged (especially with a restrictor), a lot of costly (time & money) testing will probably be involved. Worst, if you never done it before. With a supercharger, it is merely a question of selecting the right pulley ratio and it can be easily estimated with calculations.

    In any case, an intercooler will be helpful.

    A turbocharger is a dynamic compressor (DC) and, as I already mentioned, a positive displacement compressor (PDC) might be better. The PDC will raise your power curve through the whole rpm range, which I think you will need for your competition. With a DC - even as a supercharger - it will perform best at a certain rpm only. This bring us again to costly testing.

    I assumed you had this engine. By reverse engineering, I found out it had BMEPd of 12 bar and a volumetric efficiency of 92%. By putting a supercharger, you can keep the 92% VEun-boost, drop the BMEPd to 9.6 bar (for power consumption, maybe better) and raise the pressure ratio to 1.8: You get 50 hp with 138 m³/h of air flow. Look at that, the Ogura TX02 (they have a US office as well, so a possible sponsor) can produce 130 m³/h at a pressure ratio of 1.8!

    For a turbocharged version, your VEun-boost will probably drop to 80% (due to the un-tuned exhaust length), the BMEPd to 10.5 bar (maybe better) and, with a pressure ratio of 2, you will get the same power output as the supercharger. Though, you might be able to reach a 2.8 pressure ratio for an extreme case, giving you 60 hp. But remember, that is peak power. Your average power within your rpm range might not be as good as with the supercharger. Sorry - unlike a supercharger - I can't easily give you an available compressor/turbine combination that would fit those scenarios. You would need to match compressor & turbine maps with the engine estimated airflow and pressure (told you it was complicated).

    On the plus side for the turbocharger, note that its higher BMEPd means that it will have a better fuel economy compared to the supercharger.
  5. Jun 19, 2015 #4


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    It's not clear what you mean here. Are you trying to build a turbocharger which is weightless?

    It's a pretty poor supercharger that can't generate enough extra power to carry its weight around.

    If the supercharger reduces engine life, what makes you think a turbo won't do so as well?
  6. Jun 21, 2015 #5
    Thanks for the info @jack action . I will use it.
    I will just make my question clear. We need to 'build a turbo or super', but you can use second hand turbos and supers, unused ones from cars. 3d printing is not possible, plus our budget is not high. I was suggesting using the compressor part of a turbo(some unused turbo from a car, small turbo) as a belt driven blower-supercharger, but they disagreed. But with your article, i might just be able to convince them. But do you my idea will work?
  7. Jun 21, 2015 #6
    And yes, supercharged or turbo charged air to the engine must be cooled with ambient air only- thats what rules state.
    EDIT:- @jack action I stay in India. Indian college, so that sponsor is not possible. plus we can build the super from scrap supers and turbos. Possibly taking the compressor part of a turbo and using that, belt/chain driven. Will that work?
  8. Jun 21, 2015 #7

    jack action

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    It does work and it is used on aftermarket vehicles. Below you can see an image of a centrifugal compressor built to be belt-driven. Note that there is an integrated gearbox to attain the proper compressor rpm:

    http://www.vortechsuperchargers.com/images/superchargers.jpg [Broken] ​

    The reason you don't see it on OEM vehicles is because it gives a very peaky engine, i.e. it performs best only at high rpms. Below, you see how the boost is built-up throughout the rpm range.

    http://www.timskelton.com/lightning/race_prep/powertrain/images/eaton_v_kb_v_turbo_v_cent.jpg ​
    Eaton and KB are positive displacement compressors, Vortech is a centrifugal compressor (supercharger) and there is a turbo for comparison. The only drawback of the PDC is its low adiabatic efficiency which translate into higher power consumption and higher heat addition to the air flow. An intercooler would help removing the unwanted heat from the air flow.

    You might be interested in this website about supercharging small engines. They even show how to use a smog pump as a supercharger. Because in the end, an air pump is an air pump; The only difference being that some do the job more efficiently than others (You want a pump that is designed to build up pressure rather than one that only displaces air).

    And as I'm writing this down, I'm thinking you might not be too penalized with a pump that is not good at building up pressure as you will basically fill a void (when the engine isn't in the intake process) rather than constantly pushing more air than what the engine naturally pulls in. So your intake pressure might not be as high as one could imagine with typical uses (i.e. without a restrictor).
    Last edited by a moderator: May 7, 2017
  9. Jun 24, 2015 #8
    @jack action Actually, using turbo compressor as super is the only option for us. You think it will work? Also, assume we get 55 hp, how long do you think the engine will last? Because we use 1 engine for 2 years.
  10. Jun 24, 2015 #9

    jack action

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    It can work but it is not the easiest way, especially when you have little means. Careful selection of the compressor, removal of the turbine, designing and building proper gear reduction are your principal challenges. If you aim at higher-than-stock air flow, then there's the tuning of the fuel and ignition systems that may need to change.

    Assuming your stock engine makes 44 hp, the modifications will not have any impact on the engine if you have a similar output. Usually, stock engines can take about a 25% increase with no major modifications, so 55 hp doesn't look worrisome. Then again, I don't have experience with that particular engine. I would worry a lot more about the failure of the turbocharger transformed into a supercharger.
  11. Sep 20, 2016 #10
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