Won’t asymmetrical manifolds result in uneven running?

Won’t asymmetrical manifolds result in uneven running?

Non symmetrical intakes area really not an issue, especially on a forced induction setup. Essentially you are pushing the charge in one end, it WILL come out the other.

CB Turbo systems all use asymmetrical intakes. 99% of all OEM production cars have asymmetrical intakes, this includes ALL of your carbureted V8’s and most out-of-the-box performance cars. Granted on an N/A setup it will make a small difference, but even then, it truly is negligible.

The incorrect assumption generally made is that on each intake stroke the cylinder has to pull the fuel all the way from the carburettor and so the asymmetrical setup will result in a lean condition in some cylinders as the fuel has to travel further. At face value this seems like a reasonable assumption, but in reality what is overlooked is that the intake is already completely full of charge and so the charge is already present on the upstream side of the intake valves. The fuel is also present in metered quantities (read:same distribution) throughout the charge (remembering that it is all metered from a single point – the carburettor ) plus the piston will draw in the required volume of charge every time irrespective of how far away the carburettor is, as by its very nature the volume of the cylinder is fixed.

The other assumption is that one set of cylinders is nearer to the intake / carburettor and so will somehow draw the intake charge away from the other cylinders, or that somehow the charge will favour the nearer set of cylinders. Again, each piston will draw in the same volume of charge as the cylinder size and piston stroke is fixed, the cylinders are also not in competition with each other as with a 4 cylinder otto cycle only one cylinder is on the intake stroke at any one time.

The only real difference that the asymmetrical input will make to the charge is that there will be slightly lowered cylinder pressure at the end of the intake stroke due to the extra drag in the longer intake due to Bernoulli’s principle, however in reality this difference is negligible, especially when you consider that in this case the manifold far outflows the requirements of the intake as the tee junction is over 2 and half times the area of the stock 35mm intake valve. You would see more of a difference on a system where the intake is restrictive (system diameter less than intake valve size), but that is not the case here. This also only applies to a normally aspirated engine where the piston draws the charge into the cylinder,  rather than a forced induction system where the charge is pushed through the system into the cylinder and all cylinder pressures are normalised at the boost level.

In addition to this the charge is then compressed at an order of magnitudes higher than the static cylinder pressure by the compression stroke. The net difference is so negligible that you could not detect it on a compression test. The larger difference is the extra work that the engine has to do due to the increased drag, which simply results in less power, but again, the difference really is negligible.

It is also worth noting that the characteristic of an unbalanced engine is increased vibrations, uneven idling and uneven power delivery. An extreme case is when it’s only running on three cylinders due to a plug or plug lead issue, which most people have experienced at some point. So by this very definition if uneven intake runner lengths was such as issue, it would be easily detectable by a simple before / after comparison and duly noted in those with direct experience. The reality is with our systems that the opposite is experienced. The engine is much smoother due to BETTER fuel atomisation and delivery.

Perceived issues due to the asymmetrical intake length on forced induction engines is basically just an internet myth perpetuated by those who do not understand the principles of operation. Which is essentially is a good summation of about 80-90% of all things on the internet 😀

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