Exhaust Port on Heads
10-17-2005, 04:19 PM
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Exhaust Port on Heads Exhaust ports on LS1 heads are anywhere from 1.57" to 1.60", my question is, would it be beneficial to a H/C engine to port match the exhaust ports to the size of long tube headers, such as 1.75" or a 1.875".
Since the intake ports flow usually over 300cfm on after market heads and then the exhaust ports only flow roughly 230-240 cfm on these engines, the restrictive stock intake manifold would let the exhaust ports flow perfectly, but if a ported FAST 90/90 was used and the intake manifold now flows near 280-290 cfm, wouldnt the 1.57" and 1.60" exhaust ports be restrictive on a H/C engine?
Since the intake ports flow usually over 300cfm on after market heads and then the exhaust ports only flow roughly 230-240 cfm on these engines, the restrictive stock intake manifold would let the exhaust ports flow perfectly, but if a ported FAST 90/90 was used and the intake manifold now flows near 280-290 cfm, wouldnt the 1.57" and 1.60" exhaust ports be restrictive on a H/C engine?
10-19-2005, 09:02 AM
#4
Good luck fitting a valve that big in there. Simply increasing diameter does not necessarily mean youll increase flow. And even if you do increase flow, you stand a good chance of lowering velocity. Just like on the intake side velocity helps you out bolstering your low end torque through proper scavenging much the same way headers work in general. Matching the port exit to the header is a good idea, but opening up the valve throat, even if it were possible, would likely hurt performance.
10-19-2005, 12:57 PM
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Originally Posted by GuitsBoy
Good luck fitting a valve that big in there. Simply increasing diameter does not necessarily mean youll increase flow. And even if you do increase flow, you stand a good chance of lowering velocity. Just like on the intake side velocity helps you out bolstering your low end torque through proper scavenging much the same way headers work in general. Matching the port exit to the header is a good idea, but opening up the valve throat, even if it were possible, would likely hurt performance.
So wouldnt a larger lift/duration camshaft on the exhaust perform as if the exhaust ports were larger?
10-19-2005, 01:05 PM
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yes to your Q on lrgr lift/duration to an extent, to much can hurt flow, however match porting will smooth exhaust flow which will improve engine performance.
fuerzaws6
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10-19-2005, 02:12 PM
#7
Yes, keeping the exhaust port open longer allows more exhaust gasses to exit, but conversly, that means there either has to be less time with both valves closed after the power stroke, less time for the intake valve to fill the cylinder, or simply more overlap which raises the powerband and affects idle quality. The name of the game is finding a good compromise. There's a reason why these components were designed the way they were - Im sure they did extensive testing and found a good compromise of valve sizes, port flow and cam profile.
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10-19-2005, 04:10 PM
#8
Originally Posted by Brodiemans
Exhaust ports on LS1 heads are anywhere from 1.57" to 1.60", my question is, would it be beneficial to a H/C engine to port match the exhaust ports to the size of long tube headers, such as 1.75" or a 1.875".
Since the intake ports flow usually over 300cfm on after market heads and then the exhaust ports only flow roughly 230-240 cfm on these engines, the restrictive stock intake manifold would let the exhaust ports flow perfectly, but if a ported FAST 90/90 was used and the intake manifold now flows near 280-290 cfm, wouldnt the 1.57" and 1.60" exhaust ports be restrictive on a H/C engine?
Since the intake ports flow usually over 300cfm on after market heads and then the exhaust ports only flow roughly 230-240 cfm on these engines, the restrictive stock intake manifold would let the exhaust ports flow perfectly, but if a ported FAST 90/90 was used and the intake manifold now flows near 280-290 cfm, wouldnt the 1.57" and 1.60" exhaust ports be restrictive on a H/C engine?
Also, Do NOT open an exhaust port opening to the size of the header....BIG mistake. You will kill the airflow of the port by creating a port that flows less (due to destroying the short side radius), and obliterate the velocity by having a much larger "hole" that flows even less air. Also, the engine is more likely to experience increased reversion due to the perfect match of the exhaust port and header flange. The typical small mis-match is beneficial at helping to reduce the reversion common with aftermarket performance cams and the overlap they create.
It's easy to turn an aluminum race head (or intake manifold) into a pretty paperweight with the wrong approach to modifying it...
Tony M.
Last edited by Tony Mamo @ AFR; 10-19-2005 at 04:36 PM.
10-23-2005, 09:34 PM
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What Tony said, but some builders match/align the header tubes to the tops of the ports, but always leave the bottom of the port untouched, to form an anti-reversion step-up to the pipe diameter. (Not sure what to suggest if the tube is smaller than the port!)
10-23-2005, 09:36 PM
#10
Originally Posted by MadBill
What Tony said, but some builders match/align the header tubes to the tops of the ports, but always leave the bottom of the port untouched, to form an anti-reversion step-up to the pipe diameter. (Not sure what to suggest if the tube is smaller than the port!)
10-24-2005, 08:49 AM
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Originally Posted by Brodiemans
Exhaust ports on LS1 heads are anywhere from 1.57" to 1.60", my question is, would it be beneficial to a H/C engine to port match the exhaust ports to the size of long tube headers, such as 1.75" or a 1.875".
Since the intake ports flow usually over 300cfm on after market heads and then the exhaust ports only flow roughly 230-240 cfm on these engines, the restrictive stock intake manifold would let the exhaust ports flow perfectly, but if a ported FAST 90/90 was used and the intake manifold now flows near 280-290 cfm, wouldnt the 1.57" and 1.60" exhaust ports be restrictive on a H/C engine?
Since the intake ports flow usually over 300cfm on after market heads and then the exhaust ports only flow roughly 230-240 cfm on these engines, the restrictive stock intake manifold would let the exhaust ports flow perfectly, but if a ported FAST 90/90 was used and the intake manifold now flows near 280-290 cfm, wouldnt the 1.57" and 1.60" exhaust ports be restrictive on a H/C engine?
Also, dont think that it is common for LS1 heads to flow 230+ on the exhaust side, unless they have the pipe extension. An ls6 head usually tops out in that range, but you are doing well to hit 220 on an LS1 exhaust port.
Brandon
Brandon
