The common wall side is the side of the intake port that has the head bolt running thru the two intake ports that are right beside each other...side by each...... You need to raise the whole roof of the intake port so its taller.
right from the guide boss/bowl all the way to the intake flange.
Use the 1205 gasket as a guide. Common wall port wall porting width limit is that head bolt hole between the two intake ports. It would be nice to not bust thru into that cylinder head bolt hole.
this and the thinning shaping-streamlining of the intake valve guide boss (one side of it anyway) is primarily done with a long shaft cutter working from the intake flange side.
The other side of the valve guide boss in the bowl is done from the bowl side, for the most part.
Leave the 1st inch of the intake port, for last. Most people get carried away there cause its easy to see and reach.
You need to carve two strong flow paths around the valve guide boss, in the bowl. Just like in the pic.
and generally make the roof of the intake port taller.
There is very little flow at the floor of the intake port where those little casting bumps are.
This area only needs cleaning up. Do not lower the floor of the intake port. The critical part of the floor of the intake port is the short side radius shape and width as it aproaches the valve bowl.
The shape the "short side" decides the flow balance between a low lift friendly port job or a port job that is high valve lift friendly. The short side radius shape causes the air to hug it as it turns.
If the air does not hug the shape, it separates and creates turbulence in the port.
On a stock GM SBC you only ahve what you have to work with to reshape it. It is not optimum.
and won;t be when you are done, without epoxy or port welding.. You have to work with whats there.
All 8 intake ports don't flow the same stock as cast and they won't all flow exactly the same when you are done. And if you attemp to chase that and get them all exactly the same, you will probabily F*&^% it up.
+/- 2-3-4 cfm port to port variance don;t mean squat. its the shape and overall ports performance that counts.
Just look at a big block chevy head. They do not all flow the same at all. Runs just fine with 4 long ports and 4 short port that enter the chamber differently.
On all SBC intake manifolds the intake manifold runners DO NOT FLOW THE SAME.
Do not chase port to port flow little differences. The point is to make a better Stock head, not a prostock CNC head. ALL the ports will flow a ton more than stock. Thats the point.
Do not keep running back and forth to the flow bench. Port the whole port to the desired finished shape.
1000's of stock SBC heads have been ported before. You are not going to invent anything.
once you are all done porting the ports then you can check the ports flow on a flow bench. for braggin rights.
On a street motor witha stock SBC head and a stret cam the important zone and a good benchmark is the flow @ .400" lift.
If it flows good at .400" it will make good power. The absolut peak hgih lift port flow is not that critical and won't tell you the engines real power by the number alone. Its all about flow balance. Low lift flow is important too. Especially with smaller street cams where the valve never gets that far away from the seat (relatively). If you get the intake port up to 230-240cfm peak you did a great job and it will make very good power in a street motor. It will make well over 400hp on a decent 350.
somewhere around 217 to 224@.400" indicates a good small block chev street port on this type of OEM SBC head ,too.
If you think you are the next Joe Mondello, your not and you won;t reinvent the SBC racing head.
you will get to a point where you just trade off low lift flow for high lift flow by trying to be a hero.
The funnel venturii shape just under the valve job in the bowl is critical to low lift flow. remember air wants to follow a curved surface. that curved surface venturii shape under the valve job helps the air come around the intake valve into the port at lower lifts. and sets the port curtain area velocity profile as the valve moves away from the seat.
This venturii shape just under the valve job should be approx 88% of the diameter of the valve.
The head in the pic is a finished 083 head with a 2.08" valve. It flows well over 260cfm at high lift.
And they don't all flow the same.
Takes a .600"roller cam to use the high lift flow.
Its how much horsepower and torque the engine actually makes and how well that engine accelerates up thru the power curve that counts. Not the flow bench numbers.
Last edited by F-BIRD'88; 01-26-2013 at 02:47 PM.