cam selection
- Thread starter mustang302
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Michael Yount
Mustang Master
What kind of head/valve size/chamber configuration? The answer to your question is different depending on which head is used. A 2.02 intake valve is gonna reduce clearance compared to a 1.90 valve. If the head has been milled - clearance gets reduced. The TFS head has the valves sitting in different locations relative to the piston than other heads do - that changes clearance. Different thickness head gaskets are available - that changes clearance. And peak lift has nothing to do with the clearance you've asked about because at peak lift the pistons are well down the cylinder. Lastly, all these dimensions are a bit different from engine to engine - even with the same components. The only way to know how much clearance you'll have is by doing some measuring with the short block, head and head gasket you're gonna use on yours.
If you look at flow numbers associated with different heads, some flow relatively better or worse on the intake or the exhaust side. If you have a head that is weak on the exhaust flow compared to the intake flow, one way to help compensate for that imbalance is with more exhaust duration. Similarly, if the exhaust manifolds or downstream piping are limited, sometimes that can be compensated for with more duration on the exhaust side of the cam. For engines that have boost (super/turbo charged) or nitrous, getting O2 into the cylinder is relatively easier than getting it out because you're forcing it in under pressure, or adding it via oxygen-rich nitrous. Many cams spec'd for boost or nitrous have more exhaust duration to aid the exhaust side of the equation. It's a bit of an oversimplification - but it gives you an idea of what some of the variables are that could cause one to select a cam with more duration on the exhaust side.
If you look at flow numbers associated with different heads, some flow relatively better or worse on the intake or the exhaust side. If you have a head that is weak on the exhaust flow compared to the intake flow, one way to help compensate for that imbalance is with more exhaust duration. Similarly, if the exhaust manifolds or downstream piping are limited, sometimes that can be compensated for with more duration on the exhaust side of the cam. For engines that have boost (super/turbo charged) or nitrous, getting O2 into the cylinder is relatively easier than getting it out because you're forcing it in under pressure, or adding it via oxygen-rich nitrous. Many cams spec'd for boost or nitrous have more exhaust duration to aid the exhaust side of the equation. It's a bit of an oversimplification - but it gives you an idea of what some of the variables are that could cause one to select a cam with more duration on the exhaust side.
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