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Header Crossover Tubes


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Im not sure if this article has been shared on here before. But incase you were wondering why the fj-09 header has crossover tubes give this a read.
If I were to install an aftermarket system I would want to make sure the header includes this feature.
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You can get similar results sometimes from a larger ID pipe.
The stock exhaust ports are of a certain size. (I don't know it, but I bet someone here could find out.)
The stock pipes are of a certain ID.
The stock pipes are tuned to have crossover pipes at a certain distance from the valves to avoid the bad pressure wave, and increase torque.
I found on many turbo applications, with "log" and even header style exhausts, that you could improve low end power/torque by NOT porting the exhaust out as much as possible to fit flush with the header.
Leaving a 2mm or even larger "lip" nice and sharp to catch any reverse pulse flow trying to go back into the head improved the low end considerably. Common lore is to match port the intake and exhaust, so the fluid dynamics can get maximum flow, with minimum induced turbulence that can hamper flow, and thus power.
I ended up porting the "log" style cast manifolds, opening them up 2 or 3 mm past the exhaust gasket. And then port the exhaust port 1 or 2 mm smaller than the gasket. This resulted in two steps, the port, the gasket and the bell opening into the manifold to stop reversing flow pulses.
Top end power never suffered. On a turbo you just are force feeding the engine at that point, and even a crappy head/port design will flow plenty. It's off power, off boost, at low RPM when this really would make the engine run smoother and with more low end power. (Less diluting the intake charge as pointed out in the article.) Better throttle response was a side benefit.
I think on here, the top performing aftermarket exhaust has larger tubes, and no crossovers. It has plenty of power, and I did not remember seeing a lack of Torque on the dyno sheet.
I would guess that the larger tube is creating that step around the exhaust port, and that "limits" the pulse reversion similar to how the stock crossover tubes work. The best system might be to have both. Larger tubes, and crossover pipes. But you would have to measure the volume of the tubes, v/s stock, and then locate the crossover pipes at the right distance from the valves, or you would get poor results.
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