Quick question about the ports on our intake manifolds. I noticed that the ports on the manifold themselves are smaller than the actual intake ports on the irons. I take it Mazda did this for reason. Is thats the case, is port matching the manifold to the irons a no no?


Thanks, Ed

 

I did this on my 12a. Just be careful not to go into the cooland ports.

 

hog your manifold out, there is power hiding in there

 

the step edge there is to stop port flow reversion. without that step there port flow back into the manifold would dramaticly increase. I opened mine up a little still leaving the step there.

 

I ran no coolant in my intakes. With this 4port I used 12a plates with 85 SE rotors and housings. I believe the intermediate plate is from an early 12a's because the port is long and narrow like the 79's. Even if I wanted to, my atkins manifold ported to the max would only match up about 3/4's of the way.


Thanks, Ed

 

The step is there to fight reversion, yes.

But there is more.

If you open the port exit to the size of the runner in the engine, you greatly increase the difference in length between the short (inside/"top") and the long (outside/"bottom") walls of the runner. This introduces in not so technical terms a metric shitload of turbulence and vortices (reverse flow, like a whirlpool in the port!) and you kill flow everywhere.

Also, use some common sense. The port ain't that big under the carb, why does it need to be huge at the gasket surface. Taken further, the carb venturis and throttles are tiny, so you don't *really* need to make the top area larger, either. Just remove the big ***** and smooth out the sharp edges into nice radii and call it a day. Nothing hurts flow more than idle hands and a die grinder.

 

Thanks for the info guys. Also I should have stated that I'm using a RB dellorto 48. I have 2 intakes to play with. The first is an RB 2 piece wrap around and second is an Atkins straight shot. To me the Atkins manifold makes more sense to match port but there is not enough meat. I just dont want to choke the motor of the air it should be getting.


Thanks, Ed

 

The Lake Cities (Atkins is making them now?) is your best bet. This manifold *is* safe to open up to the size of the runners in the center housing (*) but if there's not enough metal then you need to add some. Nothing new in the world of hotrodding. Make sure the divide where the single port from the carb splits into two has a nice fat radius on it, no sharp edges.

(*) - Not all the way. You *always* want a minimum of .040-.060" step out in the direction of flow.

 

Thanks!



Ed