[Steve C.]

I'm posting this info below from Harold for general interest only. Since this was posted many years ago we have seen a lot of discussion regarding the amount of additional exhaust duration, dual pattern verse single pattern, the design of the exhaust port, etc. And as always a very solid point about our typical modified exhaust systems on the car, certainly a factor in the situation.

Many years ago I asked Harold at Ultradyne when he was posting here on PY about this subject of a single pattern vs. a dual pattern. Here was his reply:

There are two different items at work here.
1.---On most cylinder heads, whenever I have around a 75% exhaust-to-intake ratio, I use an 8 degree bigger exhaust cam. If the ratio is under 70%, I use 12 degrees, and around 80%, only 4. Whenever the ratio gets around 85%, single pattern cams seem to work as good as anything. Rarely do reverse-pattern(intake bigger than exhaust) cams work.
The two times they do, blown alcohol and turbo-charged, are both for putting heat into the engine or the exhaust.
2.---At the same time, it depends upon what you want the engine to do. Single pattern cams have better bottom-end, dual pattern cams have better top-end. At low RPM, the longer power stroke of a single pattern cam puts more torque into the crank. At high RPM, the most important thing is getting exhaust gas out of the engine. You can't get more charge in if old exhaust gas is still in the chamber. This is why the torque curve makes a sudden down-turn at peak horsepower. The exhaust cam has suddenly become inefficient about getting the old exhaust out, and some gas is retained and trapped, and the intake cannot fill completely because of this extra exhaust gas hanging around.
So, for the best overall power curve, on the average you want a dual-pattern cam, and around 8 to 10 degrees more exhaust. However, if a lot of bottom-end is your goal, or you have heads with a high exhaust/intake ratio, a single-pattern cam will work better.
Was this confusing enough? This is based on 30 years(1972-2002) of cam design and application...
Let me know if you have further questions...
UDHarold

Also something to consider was Harold's comments at the time made with a factory iron head in mind or also including higher flowing aftermarket cylinder heads.

.