Intake & exhaust runners

[greymouser7]

is there a thumb rule or formula that dictates a good size runner or scaled guideline regarding engine displacement size, RPM, and efficiency?

I don't think I would be able to use this in the near future but am very curious.

I am sure if you are building an SBC, BBC, SBF, BBF, or maybe even BBM that there are lots of options to choose from to get ball park close (in choosing runner size {selecting heads}) before porting.

Since I have time between deployments, I was wondering about the old FABO SBM debate of 302's on a very mild 360-367" build. They are tempting because the heads I cleaned up need a valve job.

 

[JBurch]

Foggy memory says that back in the mid to late 80's "Car Craft" published several articles on just those very subjects, printed graphs that crossed air flow, displacement, RPM, velocity, cross sectional area. The graphs while very general in nature did get across the affects of one variable to another. In the overall scheme of things, unless you are gonna cast your own heads, just go with what you have and don't spend alot of time thinking about it. It's way too easy to get wrapped up chasing "ideals", cars are assembled compromises.

If possible while on deployment, take an online course in fluid dynamics, it will help.

 

[Wyrmrider]

Depends on where you want your torque curve
stock works for stock rpm range
in general a better valve job and steeper bottom cut and blending the bowls helps flow but does not increase the range much as full porting does
Where is the narrow spot on those heads?
No use making the intake face bigger if already bigger than the pushrod pinch
raising the roof much without raising the floor does not help much unless you can relieve a mismatch with the intake
That head is different and there are threads on it-- google around and find some and report back
I wonder if the usual things like flat face valves and 30 degree seats on half inch lift builds apply

 

[greymouser7]

Depends on where you want your torque curve
stock works for stock rpm range
in general a better valve job and steeper bottom cut and blending the bowls helps flow but does not increase the range much as full porting does
Where is the narrow spot on those heads?
No use making the intake face bigger if already bigger than the pushrod pinch
raising the roof much without raising the floor does not help much unless you can relieve a mismatch with the intake
That head is different and there are threads on it-- google around and find some and report back
I wonder if the usual things like flat face valves and 30 degree seats on half inch lift builds apply

From what I read, 30 degree initial valve seat is for dump-truck low rpm performance -which is the direction of this particular build. 45 degree initial angle on a valve job is for the high bank, long track, high rpm race builds if I remember the literature.

I might stick with these heads but if I am going to pay for a valve job, I could probably buy 302 (318 closed chamber) heads for about the same money.
This particular build doesn't need operability past 4000rpm. Just plan on cruising with 2.76 gears

 

[T56MaxTorq]

This calculator, and their entire calculator page, will get you started.

Wallace Racing HP Calculator For Choke Point of Cross Sectional Area of Ports

 

[Wyrmrider]

30 degree seats are for half inch lift builds
Most Olds and Pontiacs had 30 degree seats including the HP ones
figure the "curtain" area
many are using greater than 45 on the race builds for several reasons
not only for flow but to hinder flow at low lift with long cams
for your rpm range 302 make sense