I know that with a fuel/air plenum you want the divider walls to be rounded with a nice radius. But with a fuel injection system where the divider walls will only see air is a knife edge wall better for flow?

With my intake system I can do it either way. It will come out of the machine shop with a knife edge. For the curious this is on my modified TPI intake system where the two runners are fully siamesed into an oval shape. At the point where the siamesed runners meet the intake manifold I have to split the runners up hence the question.

 

After some further internet research with no direct answers I have decided to do the radius to the wall dividers. I know this will work based on the carburetor plenums.

 

its complicated !

Sharp Divider or Entry = makes the Total Induction System Length seem longer

Large Radius Divider or Entry Area = makes the Total Length seem shorter.


if you Total Induction System Length was close to being perfect
for your operating RPM Range,...and then you either made the Entry
or Port pair Divider thinner/sharper ...or thicker/radiused...
you will significantly "alter" the Total Tuned Induction Length


so depending if a Length is slightly too short,
it might like a thinner/shaper Divider or Entry Area with not much radius
and Vice/Versa

another aspect=>
a sharper/thinner entry or divider will usually pickup bottom-end Torque
and may hurt hi-rpm HP

a radiused/thicker entry or divider will usually pickup hi-rpm HP
and hurt very low end Torque numbers

in the NHRA SS Class with Chrysler small block 318/340/360's
the old Offenhauser Intake with very sharp dividers was faster down the
Dragstrip and often made more Torque + HP...than the much better
flowing newer Edelbrock Victor Intake that had generous radiused thicker
dividers.

it wasn't until the Heads got better and RPM range went higher
that we finally went faster with Edelbrock Intake

a few more things=>
you can grind one side of the divider to a sharp edge
biasing it to one port...and the rest of that same divider be a little more
radiused to the other side favoring the other port if need be to even out
Fuel Distribution + Flow

or you can grind a sharp edge right down the middle of the Divider
sort of like forming a leading edge triangle .

i know what you are working on is a little different, but
i just posted a few things you can think about .

Another thing to look at is the siamezed very sharp dividers
some 4-valve heads have near start of Bowls
it just depends on what the manufacturer is trying to accomplish ?

 

Larry this is excellant, excellant information! Thank You for an early Christmas present. I have searched and searched the internet with no clear answers.

In my situation with only turning to maybe 6300rpm depending on the dyno results and according to the Wallace Racing formulas my runners are a couple of inches short. So I will go with the sharp divider wall to "gain some length".

One last question if you have the time or anyone else. I have Siamesed two runners together for one big oval comming out of the plenum and stopping just short of the intake manifold and the soon to be sharp divider wall.

Will the oval runners act as an extension of the plenum or will they act as one big runner with the CSA being over twice the size of a single runner? Just wondering how it would effect the "port tuning"? Not sure how to spell Heimholtz.

PS: I would think this information should help the LT1 manifold guys looking for longer runners and more low end torque.

 

Remember....a sharper/thinner Divider or Entry area will most times
"hurt" CFM AirFlow Numbers...so you might gain very bottom-end Torque
and loose top end HP because total induction system now Flows less
at higher RPMs ...its complicated.

you need a FlowBench , snap gauges, maybe a few port molds,
Pitot Probe, Runner Length computer calculations, etc.,
then real world Dyno + Track tests to sort it all out !