[72LuxuryLeMansLa.]

Originally Posted by Steve C.

UltraDyne has been brought up here. And Skip Fix once made this comment before...

"Harold was also a big proponent of matching the intake/exhaust duration split to the head flow"

Harold Brookshire was UltraDyne. And to further Skip's comment of matching the intake/exhaust duration split to the head flow here was a comment I made that is related:

Years ago I asked Harold at Ultradyne when he was posting here on PY about the subject of a single pattern vs. a dual pattern cam. 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.
UDHarold

( side note: There have been interesting conversations, and opinions, here with skip and others regarding reverse pattern cams. Remember the above was Harold's opinion )

Also related. If you call Bullet Racing Cams about a custom cam with UltraDyne lobes or their own lobes do not be surprised if they bring up the topic of a narrow lobe separation. As often Harold Brookshire did at UlltraDyne.




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Steve,

I am FAR from an expert! My question is this. Shouldn't the exhaust system come in to play here? Say you have an 80% exhaust to intake ratio on the head flow but instead of headers you run ram air exhaust manifolds..... Does this negate some of the exhaust to intake ratio of the heads?