Port flow and cross section area

Walline · Post by **Walline** » Mon May 03, 2004 4:43 pm

The more research I do, the more I believe, flow numbers do not mean much.( I am also very interested in reverse flow at low numbers) If two heads flow the same numbers but have different cross section area, does the smaller head always make more power, or do you just have to test? So for a new porter like myself, how do I know what size to keep a head, I would imagine cubic inch and RPM, compression all play a factor. Is it just trial and error? I am still unsure how to figure cross section at various parts of the runner. Also where can I buy the rubber stuff to make molds, of runners? Any info would be great! Thanks Everyone!

Post by **maxracesoftware** » Wed May 05, 2004 3:24 pm

If two heads flow the same numbers but have different cross section area, does the smaller head always make more power, or do you just have to test?
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Air_Velocity_fps = ( Flow_CFM * 2.4 ) / Cross-sectional_Area_SqInches

the "smaller cross-sectional area" intake port doesn't always make more HP or even more peak Torque !!! so be carefull not to make port too small or too large

the cross-sectional area is "tied" to engine CID and desired RPM range

the correct cross-sectional area is also determined by sonic choke conditions

EngineTech1 · Post by **EngineTech1** » Thu Jun 17, 2004 11:29 pm

You are headed in the right direction with your interests in head porting and reverse flow. As far as flow numbers not meaning much you're headed in the wrong direction there. It is popular among people who can't get heads to flow much to say it doesn't matter much anyway but no good head porter would actually tell you that flow isn't important, it is just not the only, or main concern when evaluating a port design. I've had conversations of this nature many times and I've not seen any evidence to the contrary yet. In general if you were to take a head that dyno'd a certain amount of power on the dyno and you added material to that port to reduce the cross section in areas that didn't hurt flow then you will almost always pick up power, especially if you can shrink up the port and increase flow without changing valve size or causing the port to go turbulent or stall on top. There are other things to consider when designing a port, alot of the tuning of the port can be used to effect the powerband of the engine depending on what you need it to do and what kind of car or style of racing you will be doing.

speedtalk · Post by **speedtalk** » Fri Jun 18, 2004 3:09 pm

Hi dano73327, thanks for joining the forum. Looks like you'll be a real asset.

EngineTech1 · Post by **EngineTech1** » Sat Jun 19, 2004 1:04 am

Thanks Don, I just found this site and I am much more impressed with it than some of the others.

Guest · Post by **Guest** » Sat Jun 19, 2004 9:50 pm

right on danno, but do you think that port that you just shrunk up without losing flow might have problems pulling the same rpm even if the flow bench showed no other evidence to the contrary. and how much does port "shape" (and how the port feeds the cylinder) matter over raw flow numbers.

EngineTech1 · Post by **EngineTech1** » Sun Jun 20, 2004 12:10 am

Shrinking up the port without a loss in flow should not negatively affect the piston speed that the head will support on that size bore unless you cause some kind of turbulence or separation under the same test conditions. Shrinking up the port without a loss in flow will effectively be increasing the efficiency of the port and increasing airspeed and inertia, as long as it doesn't also cause overspeed and separation over the short turn it should increase ve throughout the range and make more power. It should even allow you to shorten up cam timing slightly if drivability is a concern and you are happy with your power output.

Like I said before raw flow numbers aren't the only factor for evaluating a port design. Port "Shape" is a result of getting the correct cross section and radii from removing material in areas that increase airflow. I'm not sure what you mean by "how the port feeds the cylinder" but if you increase port efficiency if should "feed the cylinder" better by increasing VE.