More exhaust?
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More exhaust?
Suppose, for a minute, that you're working on a Vortec 350. You have the heads off, you have the cam out. Should you invest in having the heads machined for 1.60" exhaust valves, or just spec a cam with more exhaust duration and/or lift? Why?
Yes, choose the bigger valves. Not only will it improve the short turn radius after blending the bowls, it will also allow either more low end torque with the same top end power, or more top end power with the same low end torque.
Adding duration to the exhaust to crutch poor flow either increases overlap, which hurts off-idle no matter what, and only helps midrange and top end if you fit long tubes, or it requires opening the exhaust valves sooner, which makes the exhaust louder, it hurts off-idle, and runs into problems with the valve train if running massive backpressure because of turbos.
The bigger valves will get you closer to the limits of what these cheap heads can give while giving better low end and more average power than more exhaust lobe with small exhaust valves.
Also, more-agressive lobes can cause valve float at modest RPM even with beehives and titanium retainers. Those inverted-radius lobes are not for 5500 RPM no matter how much duration they have. Not in SBCs, no matter which heads.
Yes, choose the bigger valves. Not only will it improve the short turn radius after blending the bowls, it will also allow either more low end torque with the same top end power, or more top end power with the same low end torque.
Adding duration to the exhaust to crutch poor flow either increases overlap, which hurts off-idle no matter what, and only helps midrange and top end if you fit long tubes, or it requires opening the exhaust valves sooner, which makes the exhaust louder, it hurts off-idle, and runs into problems with the valve train if running massive backpressure because of turbos.
The bigger valves will get you closer to the limits of what these cheap heads can give while giving better low end and more average power than more exhaust lobe with small exhaust valves.
Also, more-agressive lobes can cause valve float at modest RPM even with beehives and titanium retainers. Those inverted-radius lobes are not for 5500 RPM no matter how much duration they have. Not in SBCs, no matter which heads.
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Re: More exhaust?
Suppose, for a minute, that you're working on a Vortec 350. You have the heads off, you have the cam out. Should you invest in having the heads machined for 1.60" exhaust valves, or just spec a cam with more exhaust duration and/or lift? Why?
Yes, choose the bigger valves. Not only will it improve the short turn radius after blending the bowls, it will also allow either more low end torque with the same top end power, or more top end power with the same low end torque.
Adding duration to the exhaust to crutch poor flow either increases overlap, which hurts off-idle no matter what, and only helps midrange and top end if you fit long tubes, or it requires opening the exhaust valves sooner, which makes the exhaust louder, it hurts off-idle, and runs into problems with the valve train if running massive backpressure because of turbos.
The bigger valves will get you closer to the limits of what these cheap heads can give while giving better low end and more average power than more exhaust lobe with small exhaust valves.
Also, more-agressive lobes can cause valve float at modest RPM even with beehives and titanium retainers. Those inverted-radius lobes are not for 5500 RPM no matter how much duration they have. Not in SBCs, no matter which heads.
Yes, choose the bigger valves. Not only will it improve the short turn radius after blending the bowls, it will also allow either more low end torque with the same top end power, or more top end power with the same low end torque.
Adding duration to the exhaust to crutch poor flow either increases overlap, which hurts off-idle no matter what, and only helps midrange and top end if you fit long tubes, or it requires opening the exhaust valves sooner, which makes the exhaust louder, it hurts off-idle, and runs into problems with the valve train if running massive backpressure because of turbos.
The bigger valves will get you closer to the limits of what these cheap heads can give while giving better low end and more average power than more exhaust lobe with small exhaust valves.
Also, more-agressive lobes can cause valve float at modest RPM even with beehives and titanium retainers. Those inverted-radius lobes are not for 5500 RPM no matter how much duration they have. Not in SBCs, no matter which heads.
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Re: More exhaust?
Perform the math, considering the circumferential area of a valve at a given lift to a larger circumferential area at a lesser lift. Then consider the bowl area after running a fly-cutter in there to increase the diameter to match the larger valve. Then consider the valve mass and peak velocity at a given RPM under a given valve lift. The results should make the choice easier, if not more expensive.
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