Camshaft- single/double pattern

[cazsper]

It seems quite a few here have upgraded their engines in the past, not to mention the few that are currently planning upgrades in the future. When it comes to picking a camshaft, there are dual and single pattern cams. When would one use a single pattern cam over a dual pattern cam? Can there be an equivalant single pattern to a dual pattern? (For example, 232/240 @ .050 vs 236/236 @ .050)

 

[TommyK]

The theory is that the greater the disparity in intake to exhaust flow a particular cylinder head displays, the greater the benefit of a split duration (and sometimes split lift) cam design. An iron 4V head has a great intake port and an exhaust port that needs help. That is why many people recommend a split duration cam with a bias toward the exhaust side for those heads. Some times as much as 16 degrees. A 2V head has more evenly matched intake and exhaust flows which benefits less from a split duration cam. That is why many people use single pattern cams with 2V heads.

This goes both ways. A head with a poor intake and a great exhaust port might benefit from a split duration cam with the bias toward the intake side.

This is why custom cam grinders are interested in your head's intake and exhaust port flow (among many other things) when specifying a cam. It is also why even with a mild build it may be worth talking to a custom cam grinder when selecting a cam. This is especially true for Fords because most of the off the shelf grinds were developed for Chevy engines.

 

[cazsper]

Interesting.. Thanks.

 

[c9zx]

As is usual, TommyK's post gives you the straight story on the subject. For SCR of about 9.5:1 to about 13:1 the exhaust should flow about 75% of what the intake flows. As SCR increases the % decreases gradually. By 16:1 the percentage drops to about 65%. The change is due to the additional pressure in the cylinder when the exhaust valve opens. This is ONLY a rule of thumb and not precise by any means but, it is a good place to start. As an example my heads flows 305 CFM on the intake and 198 CFM on the exhaust at .600 lift, that is 65%. I added duration and lift to the exhaust lobe. If I were using manifolds and mufflers instead of headers and mufflers I would have added more to the exhaust. Chuck