Originally Posted by 454C10
I have read the following "general guide lines" about what more exhaust duration is good for:
1) weak flowing exhaust port (less than 70% of intake port)
2) stock exhaust manifolds and/or restrictive exhaust system
3) heavy car with a small engine, like 4500 lb truck with a 350.
4) cars with automatic transmissions
5) N20 or supercharger equipped engines
A cam with more exhaust duration with have a rougher idle than a cam with less exhaust duration as long as the cams are ground with the same lobe separation. The longer duration exhaust cam will have more overlap, and the amount of overlap effects the idle more than anything.
454C10 is spot on. Exact. There may be other factors involved, but in general a dual pattern cam is used where the exhaust/intake flow ratio is in need of help.
All things being equal, its better (in terms of torque curve production) to port for better E/I ratio and run a single pattern cam. For instance, if you have a poor E/I ratio and run a 214/224 cam, then you port the exhaust for an optimum E/I ratio and switch to a 214/214 cam, the torque curve will be generally flatter, peak hp will be higher, and the RPM at which the torque peaks will be a tiny bit lower... but in this example, only by about 50 rpms.
To determine whether or not you need or should use a dual pattern or single pattern cam for your setup, find the flow rates for your heads and use them to determine if you need a dual pattern for your application. All engines are different... BBCs optimum E/I ratio is different from a Ford Windsor.