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Automotive Automotive intake question

  1. Feb 2, 2004 #1
    If I can somehow increase and decrease the size of my intake tube with respect to engine's rpm (high rpm= bigger intake, low rpm=small intake to increase air velocity, about how much hp would I gain. Better question is would I gain any at all[?]
  2. jcsd
  3. Feb 2, 2004 #2
    Why would decreasing your intake size at lower RPM increase your power output?

    Air intake comes from the vacuum action of the piston acting as a plunger.

    If your intake is too small, you choke your mixture and your power output will be adversely affected.

    If you increase your intake size it allows more air into the cylinder.
    More air in = more air to compress = greater expansion upon ignition = more power.

    Increasing it more than necessary makes no difference at all since it only takes in as much as the plunger action sucks in.
    Although intake size is sometimes limited for fuel consumption. If your fuel pump will only put out enough fuel to efficiently ignite in X amount of air volume, then increasing the size of your intake without increasing your fuel pump capacity could have a negative effect.

    Reducing it at lower RPM would, at best, make no difference at all and at worst, choke your supply and make the engine work harder on the intake stroke causing you to lose power in both ways.

    To increase power you have to increase how hard the piston gets pushed back after ignition.
    If you use a supercharger (or turbo) the air pressure in the cylinder is increased (as well as changing the amount of fuel per stroke. generally) therefore when the mixture is ignited, the piston is pushed back with a greater force.
    Another way to increase power is to cool the air coming in. The cooler the air is, the more dense it is, so when heated upon ignition it expands more and does so more rapidly.
    The third standard way of increasing the power is increasing the size of the cylinder itself (through boring the engine or adjusting the stroke with different cams and such).
    Then you have increasing the efficiency of the explosion ignition itself (through better plugs, wires, ignition coils, custom timing etc).
    And finally, reducing back pressure from the exhaust system (though if you reduce it too much you run the risk of hammering your valves).

    Of course there are endless exotic techniques and tricks (such as forcing the engine to run hotter or cooler than standard, power chips, different manifold materials... the list goes on and on) but those five general approaches are your best bets to focus on.
  4. Feb 2, 2004 #3
    Although forced air intake systems (such as superchargers, turbochargers and ramjets) do increase air velocity the increase in power does not come directly from the increased air velocity, but the increased air flow.

    Think of a garden hose.
    If you put your thumb over the end the water may come out faster, but less water comes out of the smaller opening.

    You want more air to be taken in, not less air taken in faster.
  5. Feb 2, 2004 #4


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    There is actually a nice resonance effect that you can take advantage of if you're clever enough by changing the lengths of the intake runners. BMW is I believe the only company which has a fully variable intake manifold. Look it up for more info.

    Answering how much you'd gain from this is an impossible question. It depends so much on the size of the engine and how it is tuned. This technology is really more for broadening the torque curve rather than improving peak numbers very much though.
  6. Feb 2, 2004 #5
    Not quite true. If your intake is too large, this slows down the gas velocity and reduces the cylinder efficiency (I think due to the lack of swirl on a slow moving mass of gas).

    I race motorcycles, and engines that are 'over-carbed' are often less rideable than ones with smaller intakes.

    As a general rule (if their is such a thing with tuning two-strokes...) smaller intakes increase the engines pick-up and acceleration, and bigger carbs increase top speed... but at the expense of narrowing your power band. The big carbs work better at peak power and revs, as by then, the intake velocity is sufficient to get the cylinders working properly.

    As for exhaust length and intake tract length, these can make a massive difference. Changing your exhaust pipe length by only a few mm will noticeably change the engine characteristics. Also, with two-strokes, you want your inlet tract to be as short as possible.
  7. Feb 2, 2004 #6

    So yes it will make more torque and hp if I can find a why to varies the size of my intake with respect to engine rpm?
  8. Feb 2, 2004 #7
    Possibly.... but all my experience is with very peaky, high revving two-strokes. Your best bet is to make up various size manifolds and book some Dyno time at a local tuning shop......
  9. Feb 2, 2004 #8


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    I'm not an automotive engineer, or a fluid dyamics specialist, but resonant gas behavior can probably explain a lot of the issues. (This means that what I say might be completely incorrect.)

    Although it's convenient to think of the the air-flow in an engine as continous, it's actually pulsed. Since air is compressible, it also acts as a spring, and it has momentum.

    Effectively, an exhaust pipe/manifold acts like a pulse jet engine. When the cylinder is vented, there is a high-pressure shockwave that travels along the exhaust, and pushes enough air out that the interior of the exhaust pipe is at less than atmospheric pressure. As a result there is a second front of air that comes back into the exhaust pipe and can slam into a cylinder that is trying to vent. Since the cylinder must now work harder to vent there is a decrease in efficiency.

    Choosing an appropriate exhaust length can actually create a situation where the reverberation from the previous venting provides extra suction which can improve both horsepower and torque - although I have no clue to what degree.

    This effect is probably most pronounced in situations where there is a straight pipe running from the engine out, and where the equipment is usually in a narrow RPM band. A well tuned exhaust should also make less noise. I expect that the characteristic bop-bop-bop that cars make when they are at low RPMs is the result of cylinders firing at frequencies too low for the exhaust system to be efficient.

    There is potential for a similar situation at the intake manifold -- whenever air is taken in a shockwave travels through the intake manifold. If the timing is such that the pressure peak hits when the next cylinder is taking in air there will probably be an increase in compression which also leads to more power and more torque.

    Assuming that the shock waves travel at approximately the speed of sound (which we all know varies with temperature and pressure), leads t'o the conlusion that an exhaust pipe that has length equal to the time between firings multiplied by the speed of sound or longer. That way the when the second cylinder vents there should be maximal underpressure from the previous cylinder's shockwave. It's also possible to use other odd fractions of the that length, but it's much more sensitive to length changes since even fractions of this length are especially bad.

    If we assume that the speed of sound is 280 m/s the calculation comes out to:
    [tex]\frac{Strokes*16800}{RPM * Cylinders} meters.[/tex] (or approximately three times that many feet)

    You should probably err on the side of length and use your use your lowest operating RPM in the formula. Also remember that this formula is for the number of cylinders per manifold.

    I recommend that you experiment, and ask someone who has more experience than me, but I would guess that the optimal length for the exhaust pipe on a 2 stroke 2 cylinder engine at 8000 RPM would be somewhere around 3 feet. At lower RPM the pipes would probably want to be longer. By comparison, a 12 Cylinder 4 stroke engine at 8000 RPM wants a minimum length of only 1/2 foot or so.

    If you do experiment, I would be most interested to hear your results.
  10. Feb 4, 2004 #9
    An excellent reply Nate TG. It certainly is resonant gas behaviour.

    My racing bike has a power band from only 8,000 to 10,300 rpm. At anything below 7500rpm it barely runs. Opening up the throttle at lower revs just ends up spraying jets of fuel out of the carbs. The whole back of the bike is a real mess after a race. However, as it approaches 8,000rpm, the exhaust note changes, the induction note changes and it suddenly flies forwards like a rocket. In the power band, the pulses of air in the carbs rams the fuel air mix in to the cylinder (sort of...) and the return wave from the exhaust in the expansion pipe pushes unburnt mix back up the exhaust pipe into the cylinder. At any revs above or below the power band, everything is out of sync.

    Here is a rather pathetic video of the carbs when the engine is being revved in the pits...

    Revving Suzuki

    As you can see, masses of fuel mix are thrown OUT of the carbs towards the rear of the bike. When the revs are correct and the gas pulses in sync, then this is rammed into the cylinders instead!

    My bike is a Classic Racer (1967) and so is not allowed modern developments that increase the power band width. Modern two strokes have variable length exhaust pipes that change as the revs rise (they use a valve that opens or shuts a chamber in the exhaust port)

    Also reed valves in the inlet tract reduce blow back and helps the power band spread a little more.

    Incidentally, my 250cc bike averages about 15 miles per English gallon of petrol. (4.54 litres)
  11. Feb 7, 2004 #10
    Dynamically tuned exhausts and saxophones

    So, in essense, what they are is woodwind instruments.

    Last edited by a moderator: Apr 20, 2017
  12. Feb 8, 2004 #11
    Buy an Induction Kit and an Exhaust kit and then have them fitted by someone who knows what they are doing, this will increase the performance of your car. But you have to get both, either one on its own will have a negative effect.
  13. Feb 11, 2004 #12
    The following page shows a nice animation of how a tuned exhaust two-stroke works (2nd pic from top). This doesn't apply directly to four-strokes.

    http://www.coolskunk.com/two_stroke_engine.htm [Broken]

    It takes a minute or two for the animation to load.
    Last edited by a moderator: May 1, 2017
  14. Feb 12, 2004 #13
    Nice site - the animation does take ages to load but it is an excellent illustration of the two-stroke principle.

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