Guys, I need a little tutoring to answer why companies design manifolds in different ways.

I want to know how the following factors effect the behavior of the manifold:

• Runner Length
  Runner Inner Diameter/shape
  Carb mounting plate design (open vs split)
  Other important factors

For example, why does the offy 4bbl manifold behave differently than the clifford?

How is the offy 2x1 different than one of these 4bbls in terms of design. I was thinking about the CFM of 2x1bbls vs. a single 390, and how the intake design differed.

I was reading Frank Raso's website about the pressure differentials and how lower atmospheric pressure allows more air in, even when carbs are flowing the same amount of CFM. Unfortunately, the page he had about the CFM for each model of carb has a broken link, so I'm not sure how to compare a 1bbl, 2bbl and 4bbls carbs.

Thanks!

Just pondering away

MJ

http://victorylibrary.com/mopar/intake-tech.htm

That page has some information.

There is also a couple of 'mainstream' popular books in modifying intake manifolds and have a nice explanation section.

That would get you started, you may have to look around a bit to find the 'best answer'.

-D.Idiot

The offy performs differently because it lets you use the stock exhaust manifold carb heat provision.

I also believe the size of the intake runner diameter is smaller on the Offy giving better throttle response at lower RPMs (I could be completely talking out of my butt on this assertion though). Clifford intake runners have a biggetr diameter which increases overall flow, good for higher RPM racing applications, but not so good for regukar street driving.

Thanks DI, reading through the mysteries on that page. I appreciate it.

Reed, thanks for the little difference. I knew the characteristics were different, and if runner length wasn't the difference, then diameter seems to make sense. So at WOT and high rpms, the offy is restricted by its smaller diameter runners, but runs better when you don't need as much air at lower rpms.

Which is going to lead me to read and find out why torque is better with lower cfm, etc HEHE

Runner length affects the behaviour of the column of gas moving through the manifold. A particular length pipe resonates at a particular frequency (which is what makes pipe organs, trombones and flutes work). Same effect works to provide a "sonic ram tuning" effect at particular (calculable) engine RPMs. Hence the long length of the Hyper Pak intake runners. Runner cross-sectional area affects gas velocity: Any given volume of gas will pass through a smaller tube faster than a slower tube. This is the Venturi effect. It's why your carburetor works, and it's why the wind is so much fiercer when you walk between two buildings than when you walk out in the open. Of course, there is a limit to this effect: Beyond a certain volume point, the smaller tube will restrict the flow. Runner shape, both along the runner path and in cross-section, affects flow as well. Some shapes produce turbulence while others facilitate smooth flow.

Unless you have a port fuel injected engine, you have a "wet" manifold: The gas flowing through the intake is a fuel/air mix. Runner length, size and shape affect the degree to which fuel drops out of the mixture on its way from the carb to the intake, which in turn affects not only cold engine driveability but also cylinder-to-cylinder evenness of mixture distribution. Sharp bends and long runners aggravate the dropout effect. A smaller cross-sectional area to the runner = higher velocity = less dropout and more even mixture distribution = better low RPM torque.

That's it in a very small nutshell; there's obviously more.

_________________
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Dan you da man! Thanks!

So the smaller cross section allows for more velocity, which lowers fuel drop out, and thus better torque at low end. vice versa for long runners, but when the carb is at WOT and high rpms, there is sufficient velocity to flow more fuel and air to get more power. I think I've got a rudimentary understanding here!

BTW the building/wind tunnel example=very nice

MJ