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I Air flow velocity - big to small and vice versa

  1. Dec 27, 2017 #1
    Hi there,

    prob a simple question for you physics gurus, but this application is for a race car, and question relates to air flow velocity through a pipe, in particular exhaust and intake.

    Now, my thinking for the exhaust (push) is that by going from a small diameter to a large diameter will create a vacuum/vortex, i.e. going from a high pressure to a low pressure, therefore sucking the exhaust gases out. was thinking of having the mid section of the exhaust 2.5", and the tail section 3".

    Same principle with the intake, but in reverse, as I need air velocity to increase, hence going from large diameter to small diameter, remembering though this is a suck.

    Its the exhaust scavenging affect.

    I did some quick calcs

    Engine capacity - 5948cc @ 7,000rpm = 116.7 rps
    Exhaust is every 2nd revolution therefore = 58.35 rps
    5948 x 58.35/sec = 347L/sec of gas vol/velocity

    the 2.5" pipe has 12672cc (12.7L) of volume per 1 m
    the 3" pipe has 18249cc (18.2L) of volume per 1m

    I guesstimated length of pipes to be 3.5m x 2 (dual pipes).

    The variables are the header design, mufflers and catalytic converters.

    This may or may not help with my question about the theory of going from small pipe to big pipe (vice versa) to increase gas scavenging

    Any theories?

    Thanks, Mark
    Last edited by a moderator: Dec 29, 2017
  2. jcsd
  3. Dec 28, 2017 #2
    I also calculated flow rate using on-line calculator

    2.5" pipe = 109.5 m/sec. Theoretically would take a gas particle 0.032 secs to exit the 3.5m pipe
    3" pipe = 76.09 m/sec. Theoretically would take a gas particle 0.046 secs to exit the 3.5m pipe
  4. Dec 29, 2017 #3


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    Counterintuitively, if you add an exhaust diffuser (going from a smaller diameter to a larger diameter pipe), the pressure will actually increase as the flow slows down. However, far and away, I'd expect the largest impact the exhaust would have would be based on flow losses in the pipe, and in this aspect, larger will always be better. I suspect running a smaller pipe would rob you of power, no matter what you do with it downstream. Also, you'd obviously want the least restrictive cat and muffler possible.

    Another thing to note is that you're dramatically underestimating your exhaust velocities. Your calculation would be right for intake velocity, but the exhaust has a couple of complicating factors. First of all, the number of moles of gas that come out is not the same as what went in, thanks to the chemical reaction of the burning fuel. Secondly, the temperature of the exhaust will be very high, significantly increasing your gas volume relative to what was initially put into the cylinder. As a quick back-of-the-envelope calculation, assuming complete combustion and pure octane as your fuel, you're looking at about 16 mol CO2 and 18 mol H2O per 25 mol O2 entering the combustion chamber. Since oxygen is about 21% of the atmosphere, you have about 128 mol gas exiting your engine for every 119 that went in, so about a 7.5% increase. In addition, assuming an exhaust temperature of 800C, you'll get about a 3.6x increase in volume compared to the intake, for an overall increase of about 3.85x. So, rather than 347L/s of exhaust, you have more like 1300.

    Finally, your diameters sound way too small (which you can probably guess already, based on the fact that 1300L/s would be more than 400m/s through your proposed 2.5" pipe). A 6L race engine at 7000RPM should easily be making 550-600hp, if not more. For that kind of power, you'll want a minimum of a 4" diameter single pipe, or 3" duals.
  5. Dec 29, 2017 #4
    Thanks cjl,
    Good point about the EGT and increase in pressure. I didn't think about that.

    Another stuff up in my calcs is didn't factor in dual pipe set-up, my calcs are for a single pipe, so the flow rate could be halved. Of course due to V8 design the 2 banks work independently of each other (well wrt exhaust), so may make calcs easier to theorise. Also forgot to mention I have an x-pipe that joins the 2 banks. I do currently have high flow cats and muffs. I currently have dual 2.5" mandrel bend that is on a 347ci (5.6L) race motor, and feel this is fine for this application.

    I was thinking of running a dual 3", and have asked on racing forums for their opinion. Its a mixed response, most going bigger is better, some make a good point that the dual high flowing 2.5" is fine. Another consideration is extra weight and clearance the 3" will bring.

    Anyway, I posted this on the physics forums to get a more scientific way of looking at it, rather than a petrol head, and remembering main question is about pipe flow dynamics - going from small to big will act as a scavenging effect, much like how headers/extractors work, which incidentally each pipe is a tuned length design, that exit into a larger pipe (collector), out through into the main exhaust. They work by scavenging (sucking out) the exhaust gases. It has been tested that running no pipes loses power. Even those drag cars with pipes just exited the motor are tune length and there is a science behind the length, and race teams hold this testing secretively. Another example of scavenging effect is the expansion pipe on a 2-stroke. As we know 2 stroke are inefficient in removing burnt gases (why atrocious at emissions, that plus burning oil), so an expansion pipe (often seen on those bloody mopeds), helps in sucking the exhaust gases out.

    Anyway, my first coffee for the morning has just kicked in.
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