Why do many DOHC Ford engines have problems with #5 piston and/or conrod once an aftermarket plenum is fitted?
This is because of the plenum design and the placement of the throttle body to the front. Flow bench testing has revealed there can be a flow difference of 10+cfm per runner, which equates to a leaner mixture in the cylinders with the higher flow. These are the cylinders toward the rear of the engine and vary depending on the manufacturer of the plenum. We find the factory plenum is very even in flow (all runners within 5%) due to the central intake location. However the runner length and plenum volume is too small to support power greater than 550kW.
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