Why does adding manifold length does something different than adding more trumpet ?


The sensible choice for more Trumpet

In the  big yellow book David Vizard remarks that adding more bell to the induction system of a mini was no substitute for manifold length, but it did not go into much detail as to why. I always found that a bit odd and thought it had to do something with the difference in cross sectional area of the venturi in the carburettor causing a reflection . Then recently Dr Vannik commented on a thread somewhere on Speedtalk which explained it ( further)  . While the constriction of the venturi might still have some effect for how it reflects the finite pressure waves, the other reason is actually quite simple.

The propagation speed of waves in a gas (or a solid for that matter)  are dependent on the density. That is why when you stir a fizzy drink with a spoon it changes tone: the amount of co2 will go down and the density will go up hence the speed will increase and the tone will go up.

The density of a gas is dependent on temperature, that is why a hot air balloon does what it does. Hot air has a lower density than colder air and hence it will want to float upward. Maybe more correctly the denser air has to go down due to gravity. Anyway, we like cold air as it is more dense and pack more oxygen met volume unit, thus you mix it with more fuel and set it alight, releasing more energy.

If you wash your hands with alcohol ( or any volatile liquid, but they tend to be really nasty)  it will feel very cold as it extracts energy in the form of heat during the evaporation process. That is why foggers on an inter-cooler work or why a fire extinguisher can cool your beer quite quickly.`

Now put this all in one bowl and shake a bit and there is the answer:


The point where the fuel is added is the starting point of the evaporative cooling process..

The cold pipe will tune to a lower frequency for the same length.It is not a huge difference but a few hundred rpm’s is entirely possible

Adding more bell length will add more length to the intake as well. But if you have a limited amount of room and you need the maximum effective tuned length, you would be better off using a very short bell and all manifold.

Of course too cold is not good as well as you will get frozen carbs. I’m not sure if it is a myth but it seems that the idle gear was needed as they turned the engine around to have the carburettor at the back as they had problems with icing. Then again how about the 7 and 8 port engines ?

For a port injected car it does not matter much as the injectors should not cause much of a CSA change and the position of the fuel insertion will be the independent of the intake length

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