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intake manifolds

672 Views 13 Replies 9 Participants Last post by  55_327
I am getting ready to build a 360 sbc out of a 880 4 bolt block,its been machined lined ,decked bored,and I recently bought some aluminum heads for it ,these heads will fit either vortec style intake manifold or conventional ,my block does have third bypass hole for the water pump and mechanical fuel pump hole.To me it seemed simple to just set it up like old school 350 build but Im wondering if I might be missing something.or over thinking it,and another thing is Ive got my mind set on a air gap manifold,do I have to have it? motors going in a 4x4 jeep.there's other cheaper intake manifolds that do the same. other than the cooling part.I guess I should add that the runners are 200cc 68cc chambers,2.02 -1.64 valves,flat top forged pistons, .500/.510 cam, thank you
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Air gaps perform better where the engine revs are kept high and the weather is warm. The reason being that engines need finely misted fuel as liquid in globs doesn’t burn well making for a late burn and erratic running to lost power.

There are two solutions to getting around this one mentioned is keeping the revs up around 3000 or higher which creates enough violence in the manifold flow to prevent fuel pooling thus evening out the cylinder to cylinder mixture and presenting a finely divided fuel mist in the mixture flow that is easy to ignite and burns sufficiently fast in the chamber. The other short of fuel injection is to heat the incoming air or the intake manifold itself the latter being the OEM’s favorite way. This in their hands is sufficient to actually vaporize the fuel. While this burns nice and makes for a stable running engine between start up cold and running hot it does cost power in that vaporized fuel expands a lot (you’re essentially making it into the equivalent of hydrocarbon steam) so this expansion takes space which reduces the air volume in the flow to the cylinder thus power is lost. The companion problem and the one the hot rod community seems transfixed on is the loss of air density itself but while a valid concern this is not the sole problem.

So the decision on an air gap needs to have some rest upon how the engine gets used and the typical weather you operate it in. The combination of cold and especially if combined with high humidity can be problematic especially at revs under the torque peak. A partial answer here is simply an unheated intake but one that closes the valley with the runners such that the manifold at least sees engine operating temperature. I run this combination with aluminum heads on my daily driver to where much of the year is cool to cold and wet, it is something of a PIA in city traffic till the engine is good and warmed up.

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