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Exhaust gas temperature & pressure


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Before I start, yes I know I'm a total geek for doing this, but bear with me..... :eusa_whistle:

I was reading something a while back over on Apriliaforum (here here and here).

Basically some Aprilia Mille/Tuono owners have modified their exhaust collectors by welding on a couple of little link pipes between the front and rear cylinder downpipes:



The idea is it's meant to improve mid-range in a similar way to switching from single to dual exhausts.

Anyway, I got a bit bored at work so I created a CAD model of the exhaust and started running some simulations to see if I could see any difference.

To begin with I've taken a guess at things like the flow rate of the exhaust gas (which I reckon is the important bit - you can work it out based on engine size and rpm) and temperature (I reckon this will be less important, but has to be limited at least by the melting point of steel or titanium).

Does anyone know any ballpark figures for this kind of stuff? E.g. temperature and pressure of the exhaust gas at the headers (i.e. where they're bolted to the engine). I'm still having a think about pressure, but temperature has to be "a few hundred" degrees. What's reasonable - 250'C? 500'C? 750'C? How hot does the metal get? I've seen videos on YouTube of titanium exhausts glowing red hot so maybe that's a clue.

Also does anyone have any information on four stroke engine exhaust gas properties (i.e. thermodynamic properties)? At the moment I'm just using air which I reckon is OK as a first stab, but I realise the composition of exhaust gases is clearly not air! So far my web searches have drawn a blank.

So there you go - I'll post some colourful pictures if I find anything interesting :rock:


Matt :rock:

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Not sure iof this helps or confuses, but it is along the right lines, even though not bike specific:


Gas pressure is not an easy one, but very roughly:

P(kPa) = (L*S*Q^2*3.6E6)/D^5 + P.s + P.b


P (in. H2O) = (L*S*Q^2)/(187*D^5) + P.S + P.b


P = Back pressure (kPa), (in. H2O)

psi = 0.0361 x in. water column

kPa = 0.00981 x mm water column

L = Total Equivalent Length of pipe (m) (ft)

Q = Exhaust gas flow (m3/min),(cfm)

D = Inside diameter of pipe (mm),(in.)

S = Density of gas (kg/m3), (lb/ft3)

P.s = Pressure drop of silencer/

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The back-pressure will all depend on the design of the exhaust. i could not tell you what it is the the aprillia

The temperature is usually limited by the ability of the exhaust valve. This is typically no more then 800 deg C as above this you have to start using exotic materials to prevent the valve from mushrooming.

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