Ive had many guys ask me about which cylinder heads to select and they seem genuinely concerned about making the wrong choice, between two very similar aftermarket head designs.
theres lots of calculators on this site you can use to limit the choices to the correct range and as long as you realize the cam, and displacement will effect the port flow and that the port can only flow up to the rate the valves opening allow it too, you should get close to a decent selection.
now I'm not about to say the results won,t change with the choices you make, but with some research you'll find that as long as your dealing with a similar cylinder head design the results won,t be totally different if you select a slightly larger port than ideal or a cam that's a bit to small,
lets compare two similar brodix heads and a cam that has a max lift just under 570 lift etc.
obviously as the displacement,of the engine you select increases and the average rpm band of the engine increases the port speeds increase and the cross sectional area of the ports needs to keep pace, or it can be a restriction
http://www.brodix.com/heads/ik.html
lift.....................brodix 180cc...............brodix 200cc
0.200................138..............................123
0.300................188...............................181
0.400................234..............................230
0.500...............243................................253
0.600...............245................................261
total average flow reached at .570 lift should be close too
185cfm for the 180cc and and 184cfm for the larger 200cc ports
when you add each flow number below peak twice and the peak flow once and average the results
(remember the valve reaches the peak lift once but opens past the lower lift levels twice)
the difference in cross sectional areas will tend to favor the smaller port at lower rpms and the larger port at the higher rpms so the gearing and cam timing and compression along with the exhaust scavenging and intake port flow rates will more than cancel out any slight advantage either port size might have had, the difference in port cross section might be as much as 4%-6%, but that also won,t mean a great deal other than a slightly lower restriction for the larger port, and a change of about 150rpm-200rpm in the engines SWEET SPOT IF ONLY THE PORT SIZE MATTERED, but the other factors, like compression, and combustion chamber valve shrouding also come into play and the real results will be in the 2%-3% range in most cases, intake runner design, compression ratio, cam timing, exhaust scavenging all effect results slightly
the short answer is that youll get a slightly more responsive combo with smaller ports but risk restricting flow and hurting the peak and near peak hp levels if you cant maintain the volumetric efficiency with the slightly smaller port cross section, while the slightly larger port gives up a bit in responsiveness it allows the engine to pull higher in the rpm range before the ports restrict flow, so the cars drive train and gearing will effect results because they do in large part determine where in the rpm range the engine spends most of its time, your gearing and cam selection will potentially have a greater effect than the heads port size, and the intake manifold and headers selected will have a major effect on where the port flow rates peak or function the most efficiently
JUST one more point, if your looking at the flow numbers of the two heads it should be obvious that if the cam you've selected has under .500 lift the smaller port heads have a slight but noticeable flow advantage, and if the cam had over about a .650 lift the larger ports should walk away from the smaller ports, especially in a larger displacement, with a fairly long duration cam, but depending on the combo,the larger ports could easily be the best possible choice, far lower in the rpm band than you might suspect their advantages to come into play.
generally a dual plane intake with its limited plenum volume will compensate for slightly larger than ideal port size to a large degree and a single plane intake with its larger plenum volume tends to compensate slightly for under size ports.
it makes little sense to stick a long duration ,high lift cam in a small restrictive intake port combo where the port becomes a restriction just as the cam timing starts to in theory optimize the engines volumetric efficiency, or to stick a low lift short duration cam, matched to a significantly larger port head where the cam timing won,t allow the port to breath at the rpm range its designed to flow,but both combos can be helped significantly thru matching the components used and minor differences in what you select can be compensated for to a large degree, (within limits) if you understand the concepts
http://www.wallaceracing.com/runnertorquecalc.php
http://www.wallaceracing.com/header_length.php
http://www.wallaceracing.com/chokepoint.php
http://www.wallaceracing.com/machcalc.php
http://www.wallaceracing.com/flow-rpm.php
http://www.gofastnews.com/board/technic ... uding.html
http://www.gofastnews.com/board/technic ... lumes.html
well worth reading thru the links and sub links
viewtopic.php?f=52&t=322
viewtopic.php?f=52&t=796
viewtopic.php?f=52&t=1977
viewtopic.php?f=52&t=333&p=407#p407
worrying about the difference between a 190cc and a 200cc sbc , or 315cc and 320cc bbc port size if they flow similar numbers is basically wasted effort because of several factors the port size alone won,t really effect, if given a choice between , BRODIX,TRICKFLOW,AFR, DART, EDELBROCK,ETC, theres several factors like the quality of the castings, the quality of the valve train, the cnc port work, and other machine work , that Id suggest looking at more than the port size alone
bbc oval ports making 697hp
http://www.popularhotrodding.com/tech/0 ... /dyno.html
theres a good deal of info on assembling a 383 -406 sbc in this thread you might need
viewtopic.php?f=69&t=261&st=0&sk=t&sd=a&start=10
http://www.gofastnews.com/board/technic ... lumes.html
http://www.gofastnews.com/board/technic ... areas.html
up to the rpm range where the port cross sectional area becomes a restriction to flow a smaller port is usually superior
for torque produced, larger ports tend to be less responsive and produce slightly lower torque so you need to match the flow to the displacement and rpm range
theres lots of calculators on this site you can use to limit the choices to the correct range and as long as you realize the cam, and displacement will effect the port flow and that the port can only flow up to the rate the valves opening allow it too, you should get close to a decent selection.
now I'm not about to say the results won,t change with the choices you make, but with some research you'll find that as long as your dealing with a similar cylinder head design the results won,t be totally different if you select a slightly larger port than ideal or a cam that's a bit to small,
lets compare two similar brodix heads and a cam that has a max lift just under 570 lift etc.
obviously as the displacement,of the engine you select increases and the average rpm band of the engine increases the port speeds increase and the cross sectional area of the ports needs to keep pace, or it can be a restriction
http://www.brodix.com/heads/ik.html
lift.....................brodix 180cc...............brodix 200cc
0.200................138..............................123
0.300................188...............................181
0.400................234..............................230
0.500...............243................................253
0.600...............245................................261
total average flow reached at .570 lift should be close too
185cfm for the 180cc and and 184cfm for the larger 200cc ports
when you add each flow number below peak twice and the peak flow once and average the results
(remember the valve reaches the peak lift once but opens past the lower lift levels twice)
the difference in cross sectional areas will tend to favor the smaller port at lower rpms and the larger port at the higher rpms so the gearing and cam timing and compression along with the exhaust scavenging and intake port flow rates will more than cancel out any slight advantage either port size might have had, the difference in port cross section might be as much as 4%-6%, but that also won,t mean a great deal other than a slightly lower restriction for the larger port, and a change of about 150rpm-200rpm in the engines SWEET SPOT IF ONLY THE PORT SIZE MATTERED, but the other factors, like compression, and combustion chamber valve shrouding also come into play and the real results will be in the 2%-3% range in most cases, intake runner design, compression ratio, cam timing, exhaust scavenging all effect results slightly
the short answer is that youll get a slightly more responsive combo with smaller ports but risk restricting flow and hurting the peak and near peak hp levels if you cant maintain the volumetric efficiency with the slightly smaller port cross section, while the slightly larger port gives up a bit in responsiveness it allows the engine to pull higher in the rpm range before the ports restrict flow, so the cars drive train and gearing will effect results because they do in large part determine where in the rpm range the engine spends most of its time, your gearing and cam selection will potentially have a greater effect than the heads port size, and the intake manifold and headers selected will have a major effect on where the port flow rates peak or function the most efficiently
JUST one more point, if your looking at the flow numbers of the two heads it should be obvious that if the cam you've selected has under .500 lift the smaller port heads have a slight but noticeable flow advantage, and if the cam had over about a .650 lift the larger ports should walk away from the smaller ports, especially in a larger displacement, with a fairly long duration cam, but depending on the combo,the larger ports could easily be the best possible choice, far lower in the rpm band than you might suspect their advantages to come into play.
generally a dual plane intake with its limited plenum volume will compensate for slightly larger than ideal port size to a large degree and a single plane intake with its larger plenum volume tends to compensate slightly for under size ports.
it makes little sense to stick a long duration ,high lift cam in a small restrictive intake port combo where the port becomes a restriction just as the cam timing starts to in theory optimize the engines volumetric efficiency, or to stick a low lift short duration cam, matched to a significantly larger port head where the cam timing won,t allow the port to breath at the rpm range its designed to flow,but both combos can be helped significantly thru matching the components used and minor differences in what you select can be compensated for to a large degree, (within limits) if you understand the concepts
http://www.wallaceracing.com/runnertorquecalc.php
http://www.wallaceracing.com/header_length.php
http://www.wallaceracing.com/chokepoint.php
http://www.wallaceracing.com/machcalc.php
http://www.wallaceracing.com/flow-rpm.php
http://www.gofastnews.com/board/technic ... uding.html
http://www.gofastnews.com/board/technic ... lumes.html
well worth reading thru the links and sub links
viewtopic.php?f=52&t=322
viewtopic.php?f=52&t=796
viewtopic.php?f=52&t=1977
viewtopic.php?f=52&t=333&p=407#p407
worrying about the difference between a 190cc and a 200cc sbc , or 315cc and 320cc bbc port size if they flow similar numbers is basically wasted effort because of several factors the port size alone won,t really effect, if given a choice between , BRODIX,TRICKFLOW,AFR, DART, EDELBROCK,ETC, theres several factors like the quality of the castings, the quality of the valve train, the cnc port work, and other machine work , that Id suggest looking at more than the port size alone
bbc oval ports making 697hp
http://www.popularhotrodding.com/tech/0 ... /dyno.html
theres a good deal of info on assembling a 383 -406 sbc in this thread you might need
viewtopic.php?f=69&t=261&st=0&sk=t&sd=a&start=10
http://www.gofastnews.com/board/technic ... lumes.html
http://www.gofastnews.com/board/technic ... areas.html
up to the rpm range where the port cross sectional area becomes a restriction to flow a smaller port is usually superior
for torque produced, larger ports tend to be less responsive and produce slightly lower torque so you need to match the flow to the displacement and rpm range