I bored the holes in my housing today on my housings. And before I make my sleeves, I am trying to decide on the shape I should blend the port to. Or just leave it alone. I have seen MANY shapes but would be nice to have a breakdown of different style shapes and what benefits they might have as well as there draw backs.


D shape, Hour glass (RB & MFR style), square, regular circle, ect...?

 

well mazda had round ports in 77, and went to a D shaped port in 79. i've heard that round has a little more midrange than a D shape.

if you like the round port will open and close a little slower, so it actually should be a little more friendly. mine are big and rectangular, and the engine is pretty tame though, so i dunno, and i've never run anything else, although i'd like to!

 

i'd prefer the D shape ports with the less radical squared off opening edge. squaring off the opening edge doesn't necessarily make anymore power, just creates more turbulence in the intake.

the D shape gets you a little more charge density over the O shape ports. O shape ports are obviously the easiest to create.

 

OK, lots of questions here, since this is my first PP set up... bare with me.


After lots of thinking and taking into account both posts. I think I will be going with a D shape port. But one thing I cant seem to figure. It probly dosnt matter much, but I like to get over involved and make everything count, even when the advantages are minimal... Does it make a difference if the "D" has a radius or close to an actual D at the corners? Basically, Should I file the D bottom edge to be as much of a real corner as possible, Or leave it rounded from the carbide burr?

And should the opening edge of the D be radiused or be as close to a 45* angle as possible. The idead of letting the air gradually come in through the port with the use of a radius sounds good. But a burst of air from opening to the full port all at once sounds just as interesting...


Also, is there any advantage to machining a taper into the sleeve instead of using just a regular strait tube?I have seen the Racing Beat and Speedsource PP housings. Both have a tapered sleeve. My plan was to have it 1.750" at the port face (Before I port it to shape Whichwill open it up to around 2.000" when adding in the radius edges and blending.) Tapered out to 1.875". I'm going to use 2.000" OD pipe with 1/16 inch wall making them 1.875" ID, give or take a little. The sleeve will be around 3-3.5" long at its longest part. Just enought to leave about 1.000" petruding from the housing to attatch couplers for the intake runners. To me it just makes sense to have a good taper right before the port face to increase velocity.

Thanks

 

for the port shape i would do a radius on the bottom. the port opens fast enough (you'll see) that slowing it down a little is probably good. actually depending on the port timing the rotor might be right there too, the rotor really is the thing controlling port flow at the opening.

i don't know about the taper, although your ID of 1.875 should be just right.

 

the taper would also act similar to the radiused port, multiplying the effect of smoother airflow into the engine.

i have no experience with squared off ports but from all evidence i have seen from them in these engines the benefits aren't great or even really all that noticable. mainly it just creates a rougher idle and less economy, so if you want a beefier sounding engine then by all means, if you want an engine that works well all around i would radius the leading edges and taper the intake port.

squaring off the trailing edge is probably going to also be almost unnoticable, the stock ports clap shut with rather a rather square trailing edge. the more surface area you remove the more air you can cram in, so filing the trailing edges square probably would be beneficial for slightly improved top end performance. but we're talking less than 1% over just beveling them with a burr.

 

Thanks for the tips. I should be done with my sleeves by this weekend. And start in on the port shaping.

Now one other thing that i have never been 100% sure about it is where to measuring intake length from. When figuring the length, do i measure from the throttle plates or veloicity stack end? This of course is in reference to itb and weber set ups.

 

most commonly it would be block to throttle body flange, the butterflies are a negligible distance from the flange on most carbs. everyone is going to use slightly different length measuring techniques.

 

Most commonly yes, but I doubt that's the correct way. Im guessing it woukd be best to start at the port face and end at the throttle plates for the most accurate math.

But what confuses me is when the throttle is wide open, wouldn't it be best to calculate the pulse from the tip of the velocity stack?

 

if you're doing total volume calculations yes, rotor housing inner edge to butterflies, if you're making adjustments to a current manifold, easier to compare just the intake manifolds.

the velocity stack length will also have an effect on the power but a little more difficult to get real measurements from since most stacks are flared and the intake velocity isn't accurate at the opening.

some setups move the throttle body, some move the velocity stacks, both methods work but have slightly different results at mid and low ranges.

 

Mazda goes from the tip of the velocity stack to the trochoid surface. although it doesn't matter too much as long as you realize that we could measure from different places!

Mazda started @400mm went to 370mm, and then the variable length setups. shorter will peak later, and if you make yours adjustable, you could really make yourself happy.