Adv. Duration vs. .050" Duration - Chevelle Tech
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Old Dec 11th, 04, 11:19 PM
1967chevelless396 1967chevelless396 is offline
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OK cam guru's, which duration helps engine performance the most? When should I consider one over the other when selecting a cam?

Thanks, Charles
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Old Dec 11th, 04, 11:47 PM
pdq67 pdq67 is offline
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Location: Columbia, MO, USA
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You can't produce any power until the intake valve is closed!!

But the faster it rises which is indicated by big .050" numbers helps too...

I personally am a fan of the old school, low lift cams b/c of all the trouble that high valve spring pressures cause but I do know I am leaving power on the table too in a "balls-out" motor application...


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Old Dec 12th, 04, 11:18 AM
UDHarold UDHarold is offline
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The engine sees the total valve lift curve, not a number like .050" duration. In fact, the .050" duration that occurs before TDC is harmful to the engine, not beneficial--Reversion, you know...
I consider the intake valve opening point and the exhaust valve opening point to be of equal importance. The 3rd most important is where the intake valve closes, the least important is where the exhaust valve closes.
Other cam companies DISAGREE with me, not all, but many.
To answer your question, the engine sees seat durations, not .050 durations. And off-set, the difference between opening and closing sides, makes a huge difference also. This tricks the engine by reducing reversion, while maintaining a good high-lift area. This is the way I've designed cams since 1977.


Harold Brookshire
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Old Dec 12th, 04, 2:44 PM
greg_moreira greg_moreira is offline
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Here is the purpose for .050 numbers. Its basically a real good reference for comparing different camshafts. Lets say you are comparing some crane camshafts to some comp camshafts for example and they both have 230 degrees duration at .050. Its safe to say that your average crane cam with 230 degrees duration is gonna compare fairly close to your average comp cam with 230 degrees at .050. Comparing advertised numbers can be tricky, cause one company might measure from .002 and the other from .006, so even if the cams actually do have the same advertised duration in real life, the cam card will show different numbers cause of the different points at which the duration was measured. But .050 is always .050 so thats why its a good enough comparison for your average application.
Now, in your case where you say you want to really optimize the engines power, like Harold said, duration at .050 should not be the only number you should use for comparison. Thats why I said two 230 cams will act similar, not identical. To really know how one camshaft will act compared to another, you need to know everything Harold was talking about in the previous post, and even more than that. Look at the thread about the new voodoo cams. Harold explains why even with seemingly similar numbers, his Voodoo camshafts would be a better running cam in the right application, and its not all seen just in the numbers(at least duration numbers), you've really gotta disect everything to see where the difference is. Knowing how to use all this info will help you better optimize an engine combo, rather than just coming pretty close by only comparing things like peak valve lift or duration of any kind.
Lastly, just for good comparing witout going into too much depth, look at advertised duration numbers, and .050 numbers and peak valve lift, and compare those numbers to each other. Also, compare those numbers to the same numbers of another camshaft. For example, one cam has 274 advertised duration at .004, it has 232 at .050, and .501 lift. The other cam also has 232 duration at .050, but at .004 it has 288 advertised, and .480 valve lift. With these numbers you can see that although they have the same duration at .050, the first cam that has less advertised duration and more peak valve lift. This can indicate that the first cam would have a more aggressive lobe that accelerates the valve faster. That tells you that even with less advertised duration, and the same .050 numbers, this first cam can be capable of building better cylinder pressures, while moving more overall air through the intake valve due to the faster acting, more agressive lobe profile. It basically creates more area under the lift curve. In the end, I would still ask the advice of your cam maker to be sure, but id doesnt hurt to have a real good idea prior to getting on the phone with your cam grinder of choice.
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