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First off, let me try to explain how your intake manifold works. It's more complicated than you think! Surely by now you've read enough posts that say longer intake runners are good for low end torque while shorter runners are good for top end horsepower. I know that, now you know that, and Ford knows it too. That's why the 1999-2004 split port V6's have short and long runners. At approximately 3500 RPMs your PCM (computer) will open up the short runners via a butterfly valve called an Intake Manifold Runner Control (IMRC - 2001+). The engine uses the long runners at low RPM for more torque and the short runners at high RPM for more horsepower.

"Resonance" is defined as a vibration. It's the action of moving back and forth. Air in your intake manifold resonates or moves back and forth with the opening and closing of your intake valves. It does not move in a straight line from the air filter to the cylinder at all times as so many people believe. When the intake valve opens, air is pulled into the cylinder by the low pressure created from the downward movement of the piston. When the intake valve closes, the air in the intake is still moving forward towards the valve. There is nothing driving it but inertia at this point. It slams up against the closed intake valve and creates an area of high pressure. This high pressure is then reflected as a wave back up the runner away from the valve. When the pressure wave reaches the intake plenum (a common area shared by all the intake runners), it is then reflected back down the runner towards the valve again. If this pressure wave approaches the valve as it is open, then it will help flow more air into the cylinder. It will actually push more air into the cylinder than the cylinder could suck in by itself. The valve closes and the process repeats again. This whole process results in increased volumetric efficiency (more air entering the cylinders than normal). Since your engine is nothing but a large air pump, more air = more horsepower/torque. The intake runner must be properly sized (length and width) in order to enable the pressure wave to arrive back at the intake valve as it opens for the next cycle. The only drawback of this is that the "resonance tuning" is only effective in a precise RPM band.

An intake spacer will lengthen the runner which causes the air inside of it to have to travel a longer distance. The pressure waves also have a longer distance to travel. They travel at the speed of sound. This doesn't change! Since they have a longer way to travel, they arrive back at the intake valve later. At lower RPMs, obviously, the valves are opening and closing slower. The intake valve reopens later and this can be timed with the intake pulse (with the spacer) in order to create more torque at lower RPMs.

Also, the intake spacer is made out of a heat resistant material. It doesn't soak in engine heat as easily as the aluminum intake manifold will. Colder air = denser air. Denser air takes up less space which means you can fit more of it in the cylinder. Again, more air = more power!

A good intake spacer is made of a phenolic plastic material called G-10. There are other types but they are of a cheaper grade and won't last for long under your hood. The thermal conductivity of aluminum is 2.165 W/cm-K. G-10 is 0.003 W/cm-K. I won't bother explaining what this means in detail but the larger the number, the easier it is for the material to soak in heat. G-10 stays much cooler than your aluminum manifold. It resists heat over 700 times better than aluminum. Because the G-10 is in between the upper and lower manifolds, the upper won't conduct the heat of the lower as easily. Ever notice your car "feels" stronger in the morning when the engine is cold? That's because the intake is cool and hasn't soaked in the engine's heat yet.

Article written by 3.8mustang.com moderator KTBug

 

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