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Exhaust manifold 4-2-1 versus 4-1 article in Mazda Skyactiv

ben yip

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Most of you probably know about the difference between 421 and 41 exhaust manifold differences, but do you know this? This article is quite useful if you are considering changing header configuration

The Mazda Skyactiv engine produces the highest compression ratio of 14:1, one of the design is using 4-2-1 exhaust mainfold instead of 4-1, the explanation is quite interesting


Surprisingly, Mazda uses a very conventional approach to do that: a long, 4-to-2-to-1 exhaust manifold. On a typical inline-4 cylinder engine with short, 4-to-1 exhaust manifolds (the first picture), once the exhaust valve of Cylinder 3 opens, its exhaust pressure waves (grey area) flows through the short manifolds to the exhaust valve of Cylinder 1, which is at the end of its exhaust phase. This pumps some exhaust gas back into Cylinder 1 and becomes residual exhaust gas. When the engine is running at low speed (2000 rpm in the first picture), the exhaust pressure wave arrives Cylinder 1 early enough to cause high percentage of residual exhaust gas. As engine rev rises, the opening and closing of valves speeds up as well, thus the exhaust pressure waves of Cylinder 3 reaches Cylinder 1 at later stage, causing lower percentage of residual exhaust gas. In short, from low to mid-range engine speed the level of residual exhaust gas is pretty high for this engine configuration.

In the case of Skyactiv-G's 4-2-1 exhaust manifolds (the second picture above), exhaust pressure waves from Cylinder 3 has to travel a long way to reach Cylinder 1, by the time Cylinder 1 has already, or nearly completed its exhaust phase. Therefore the level of residual exhaust gas is much lower than the previous case, especially for low to mid-range rpm. As a result, the Skyactiv-G engine attains lower temperature in its combustion chambers and allows a higher compression ratio to be used.

Well, if the principle is so simple, why not others discovered already? It's not that simple, of course. One critical drawback of the long 4-2-1 exhaust manifold is that it takes relatively long time to heat up the NOx catalyst during cold start. In fact, this is exactly the reason why most modern production engines have abandoned this exhaust configuration - with the exception of high-performance engines which may use thin-wall fabricated stainless steel exhaust manifolds to compensate for its extra length. On cost-conscious mass production engines, cheap cast-iron exhaust manifolds are still the norm. Its extra mass and surface area absorb a great deal of heat and delay the proper functioning of catalyst. This causes difficulty to comply with emission regulations.

Article in


That also means that my car might not be able to pass emission test!!

and I understand why my header is designed like this, the distance travelling in cyclinder 3 to cyclinder 1 is much more longer, producing less residual exhaust gas


Edited by ben yip
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