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First off, I am not looking to try this nor am I suggesting that anyone else try it.

Why do the upper intake manifolds have 6 runners (just referring to the 6 cylinders right now) rather than having it be one big runner that extends all the way across? The same thing for the lower manifold? What if it did not have separate runners and just had one opening that extended across 3 runners (both sides)?

My assumption is that it would create uneven air flow to each of the pistons...if not, would this even be feasable or would it require fuel injectors the size of beer cans to maintan the A/F ratio?

Hmmmmm....
 

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Its probably uneven flow. Some engines do use one manifold without runners but thats when the intake goes right between all of the cylinders. If we had an intake manifold that had the opening at the top-side (look at the engine)
 

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Vacuum is not the issue - that is a side effect of what the OP is describing.. without runners, you effectively have created a Tunnel Ram - these device work on higher end engines, that do not have to be streetable - Usually pro stock engines or race track engines are designed around little to no runners.. they normally make a lot more power, at the expense on all low end torque and HP...

Runners create port velocity, without it, there is no intake pulse, the runners are the staging area for the next flow of air in to the cylinder - and believe it or not, there is a lot of science that goes in to how an intake runner effect performance of an engine. ( look up Helmholtz Theory) The 2.7 with no runners, would not run very well, and would be a daily nightmare...

The stock 2.7 upper intake is designed to maximize torque and HP in the normal driving ranges - 2500-4000 rpm... removing runner length, moves that up in to the unusable ranges ( even though some with aftermarket intakes will argue about torque and HP being made at redline, the normal person doesn't drive around bouncing the rev limiter all day)
 

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^ I think he's right. :)
 

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Vacuum is not the issue - that is a side effect of what the OP is describing.. without runners, you effectively have created a Tunnel Ram - these device work on higher end engines, that do not have to be streetable - Usually pro stock engines or race track engines are designed around little to no runners.. they normally make a lot more power, at the expense on all low end torque and HP...

Runners create port velocity, without it, there is no intake pulse, the runners are the staging area for the next flow of air in to the cylinder - and believe it or not, there is a lot of science that goes in to how an intake runner effect performance of an engine. ( look up Helmholtz Theory) The 2.7 with no runners, would not run very well, and would be a daily nightmare...

The stock 2.7 upper intake is designed to maximize torque and HP in the normal driving ranges - 2500-4000 rpm... removing runner length, moves that up in to the unusable ranges ( even though some with aftermarket intakes will argue about torque and HP being made at redline, the normal person doesn't drive around bouncing the rev limiter all day)
+1 and a cupcake for you! (metaphorically speaking)
 

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UnHoyTib is right....there is a lot of science that goes into not only runner, but also the whole intake. Everything from the throttle body, the plenum, and especially the runners. I could talk all day about this subject as I have designed intakes for Formula SAE cars the past four years. Essentially, as UnHoly hinted at, the runners are tuned for certain acoustical effects (you can think of them as pressure waves bounce off the valves). Like tuning forks, different runner sizes will be "excited" at different frequencies. These frequencies can be correlated to an RPM value where max torque is achieved by the peak of the pressure wave striking the valve when it is open. There are many theories, Helmholtz being the most famous and basic, but also the least accurate for engines that rev higher than 4000 RPM. I think the method propsed by Engleman would be most appropriate for our cars in which out intakes are shown to resonate at 4800 to 5000 RPM (with a secondary peak around 3500). One large runner would be off the map!
 
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