selecting intake ports

[grumpyvette]

I frequently get questions about realistically selecting cylinder head port size.

Ok lets look at a few factors, first, the port can flow only up to the limits of the restrictions imposed on the flow, if the cam timing, duration & lift limit the valves max flow to levels below what the port can at least in theory flow, that’s your limit, if the exhaust savaging is not matched that will also limit your max flow, and while the port size/volume in CCs gives you a rough way to gauge its not an exact relationship to flow potential, the MINIMAL CROSS SECTIONAL AREA will also have a big effect on the flow potential, as will the angle of the port in relation to the valves ,the valve dia. and the combustion chamber shape and shrouding of the valve curtain area.

OK lets assume your selecting heads for a 383 SBC, in most cases you’ll find the max effective rpm with that stroke will be between about 6200rpm-7000rpm(depending on the application and matched components due to limits on piston speed & stress.

lets say 6500 rpm, at 6500rpm a single cylinder will in theory need 90 cfm in air flow if the valve was fully open 100% of the time , but the truth is that out of the 720 degrees in the cycle you’ll be very hard pressed to have 250 degrees of meaningful flow into the cylinder, that jumps the minimum flow required to about 259cfm at 100% efficiency, and that assumes the valve instantly opens and closes, and can flow that as a minimum, but the valves have both mechanical limits to lift and acceleration rates, so while the valve and port might flow enough at peak lift its unlikely at lower lifts.

keep in mind that flow below about .050 lift is minimal and your valves only at peak lift for a very short time, at 6500 rpm, the valve goes from fully closed to fully open to fully closed in under 1/60th of a second at 54 times a second.

lets look at curtain area, if you have 2.02" intake valves that reach .600 lift that in theory is 3.8sq inches of flow curtain, at max lift but your AVERAGE flow curtain is more like 2.3 sq inches (look at the info and calculators below)

 

 

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

 

 

http://www.rickwrench.com/bunchofcalculators.html

 

http://www.bgsoflex.com/intakeln.html

 

http://www.rbracing-rsr.com/runnertorquecalc.html

 

here’s a chart FROM THE BOOK,HOW TO BUILD BIG-INCH CHEVY SMALL BLOCKS with some common cross sectional port sizes

(measured at the smallest part of the ports)

...........................sq inches........port cc

edelbrock performer rpm ....1.43.............170

vortec......................1.66.............170

tfs195......................1.93.............195

afr 180.....................1.93.............180

afr 195.....................1.98.............195

afr 210.....................2.05.............210

dart pro 200................2.06.............200

dart pro 215................2.14.............215

brodix track 1 .............2.30.............221

dart pro 1 230..............2.40.............230

edelbrock 23 high port .....2.53.............238

edelbrock 18 deg............2.71.............266

tfs 18 deg..................2.80.............250

 

Potential HP based on Airflow (Hot Rod, Jun '99, p74):

Airflow at 28" of water x 0.257 x number of cylinders = potential HP

or required airflow based on HP:

HP / 0.257 / cylinders = required airflow

 

http://www.rbracing-rsr.com/runnertorquecalc.html