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Ken Gratton18 Jul 2013
NEWS

The manifold virtues of Mazda3

SKYACTIV exhaust – "race header tied in a knot"
One of the overlooked technical features that sets apart the new Mazda3 from the current car's SP20 variant is the exhaust manifold. 
Along with weight reduction and a lower drag coefficient, it's one of the principal reasons the new car's combined-cycle fuel consumption is now below the 6.1L/100km figure set by the current SP20. 
Exhaust manifolds are not glamorous – and the new SKYACTIV 4-2-1 unit takes ugly to a whole new level. It's so ugly that Mazda doesn't even publish press pics of it. 
But we did get the chance to see one during a technical presentation for the new car in the US last week. Mazda's self-styled 'resident nerd', David Coleman, presented the manifold during his presentation concerning the new model's technical specification. Coleman, whose actual job title is apparently 'Manager, Vehicle Evaluation and Technobabble', began with a chart showing the comparative torque curves of the new 2.0-litre engine and the SP20's. 
"On paper the 2.0-litre SKYACTIV-G engine looks like it has about the same output as it did in the previous car... [but] lower down in the rev range you have a much bigger jump in torque with this version of the engine... And the reason for that is it's exactly the same engine, but we've changed the exhaust manifold to the manifold that the engine was designed for in the first place."
Showing the SP20 manifold first, Coleman explained that it was a conventional 4-to-1 unit that placed the catalytic converter closer to the engine's exhaust ports for the sake of faster warm-up and cleaner emissions. 
"But the downside to this," he continued, "is that we end up with a lot of cross-talk and interference between the cylinders."
The firing order of a four-cylinder engine, he explained, is 1-3-4-2, which leads to exhaust valves open simultaneously in the case of the number one and number three cylinders – number one closing just as number three is opening. 
"The way exhaust flow comes out of the cylinder, when you first open the valve there's this huge pulse of pressure and energy from the combustion going on in there. Once that pulse comes out... the rest is sort of wafting the exhaust gas out. So most of the energy is at the beginning of the exhaust stroke."
That initial pulse exits at the speed of sound and bounces around in the conventional exhaust manifold, like a bullet ricocheting, finding its way up the runners back to the other exhaust ports before it makes its way out of the manifold. It's travelling so fast that during that bounce it can push back the 'wafting' exhaust from the number one cylinder and enter that cylinder – occupying space within that cylinder prior to the next intake stroke. 
According to Coleman, that's where uncontrolled detonation ('knock') can occur. And for that reason, car companies reduce the compression ratio of their engines for the sake of engine longevity. The exhaust fumes bouncing back into the (number one) cylinder can make the environment within the cylinder hotter when the new charge enters. 
The new manifold, which Coleman describes as "a traditional race header, tied up in a knot", has allowed Mazda to raise the compression ratio for the SKYACTIV engine from 12.0:1 for the SP20 to 13.0:1 for the new engine. 
And like everything about SKYACTIV, the new manifold is simple in the way it works. The longer runners and a series of valves and collectors slow the bounce to the point where even with the engine running down to 2000rpm – and the pulse still travelling at the speed of sound – the pulse will not reach the number one cylinder's exhaust port before that cylinder's exhaust valve closes. 
What, you were expecting maybe something more exotic?
Picture courtesy ecomodder.com

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Written byKen Gratton
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