I am making an intake manifold for my 12a and I am debating on combining the primary port runners into one large 2" runner instead of 2 smaller runners. The main reason is, it is always such a pain in the *** to crush and weld the runners together for the primary ports. It rarely comes out nice looking and you always get a harsh angle as it enters the motor.

Are there any draw backs to combining these two port runners into one runner?

This intake manifold will have a plenum with a single throttle body so it seems that it would not matter since they would already be combined (kind of) in the plenem area anyways.


But maybe there was testing done that I havnt seen?


ps. Would it make a difference wether it was N/A or Turbo?

 

All the intake manifolds i have seen have devided runners, so that leads me to think that the pulses from the two ports mixing in the intake runner isn't a good thing.

From what little i know, when deciding on a runner design you want the correct size to get as much air velocity as you can for your desired power band, and the length is to get the timing right so the pulses from the closed ports help push air into the open ports.

If the primary ports share the same runner i think you would loose power.
Atleast on NA application, when you have a turbo, boost make up for bad intake design so it's not that important unless you are going all out.

 

i've never seen an intake with 1 runner like that, i don't think its a good idea either, although i'm not sure why.

Mazda has a graph of plenum volume vs VE, and smaller is better.... the S2000 has a small plenum too

 

I have seen a few like this. But all were on turbo cars. So plenum/runner design isnt so critical. These cras were making big power too.

I would like to know why it is a bad idea or will not work as well as split runners... I can't really think of or find anything that proves or disproves it. I can guess and rationalize why it will or will not work, and just do it anyway. I'm not trying to break any records with this particular car.

 

Hell, the engines don't seem to like the primaries sharing a plenum on the OEM manifolds where they join 10-15 inches upstream of the engine.

 

Kahren made and intake with the primary runners combined....it made good power as well

https://www.rx7club.com/2nd-generation-rx-7-1986-1992-parts-194/fs-custom-intake-manifold-s4-s5-na-834592/

Nevermind it is still divided. His method looks a lot nicer than two crushed tubes though. Food for thought

 

the link did not work?

 

How about putting a divider into the tube? Simlar to the renesis center iron exhaust.

I could have this divider in the center of the tube keeping the runners seperate until it reaches the plenum area. This would give each primary runner a mirrored D shape runner.

 

Seems this would only work for high powered applications were drivability isn't important. Depending on how the rest of the intake is setup, you would loose a ton of velocity through the combined primary ports in the low rpms.

 

https://www.rx7club.com/2nd-generati...-s5-na-834592/

It looks like he cute a tube in half, added a divider and welded everything back together.