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The magic number, here, is 75%. You want your exhaust port to flow @ 75% of your intake port. Most stock heads (especially BB mopars) don't do this, so cam manufacturers figured they can compensate for the heads shortcomings by extending the duration on the exhaust side.

What has happened now, is that split pattern cams have just become the norm, and the general public percieves they are getting a better engineered product with varying #'s on the card (not really knowing why)

Most good aftermarket heads do support the 75% number, however.

The key when choosing a cam is knowing what your heads will flow, and complimenting that curve with the cam. With good heads that flow properly, there is a strong case to be made that you'll maximize the power available from a given duration and lift spec. with a square pattern cam. Your intake, carburetor, and header selection also plays a role here.

I conferred with Member, Jeff Patterson, when choosing my cam, and we both agreed that my combo seemed to call for a square pattern cam, so that's what we dialed up from Engle.

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WHAT HE SAID.

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Testing has show for a given intake profile they will give a little more power with a longer exhaust duration.

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The magic number, here, is 75%.

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Opinions are just that...

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I won't argue that point.

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Hey Budget , what does 'do not feed the troll' mean?

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It means I'm in kind of a crappy mood and this is just the sort of thread that could cause me to post of bunch of smart@ss comments in my replies.

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I spent a bunch of money dyno testing camshafts on my 470 low deck motor. It had exhaust manifolds and lots of people told me that I would need extra duration on the exhaust side to compensate for the manifolds. Turns out that the very best camshaft for that engine was the old school Mopar .528 solid flat tappet.

I suppose if I was given unlimted funds I could find a camshaft that was even better than the .528. And perhaps, after hunreds of dyno pulls the very best cam might well be a split pattern cam. But I called it quits after spending a couple of grand on cams and dyno time.

As far as I know, nobody knows the answer to this question. It is fairly easy to get a 90% correct answer to the question, but I don't think anyone has ever really figured it all out. If they had, then there wouldn't be any need for all of the dyno testing that goes on every day at all of the engine shops around the world.

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Right then , take it the jurys out for single pattern grinds.

Used 14 degree split when on the bottle , but that's a different story.

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Hey Budget , what does 'do not feed the troll' mean?

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It means I'm in kind of a crappy mood and this is just the sort of thread that could cause me to post of bunch of [Email]smart@ss[/Email] comments in my replies.

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I wanna know who's the TROLL.

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I wanna know who's the TROLL.

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That refers to ME. Although I tell my kids my parenting role model is Shrek and he's an ogre, not a troll.

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Testing has show for a given intake profile they will give a little more power with a longer exhaust duration.

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Whose testing? I've seen some that show that the proper single-pattern cam will provide more power through the entire range. So, who do you believe?

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The magic number, here, is 75%.

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Sorry, there is NO MAGIC NUMBER. That "75%" rule is outdated.

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Opinions are just that...

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I won't argue that point.

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Well, if opinions are "just that", then I'll consider yours to be the low side of the analogy. You've certainly provided plenty of facts to support your rant, there..... (oh, maybe not) Until you can show me actual numbers that say the 75% rule is untrue, I'm going to consider your opinion completely arbitrary. Perhaps I should have called it a "rule of thumb", not a "magic number", this I'll concede. But the facts remain. This number is based on the physics of volumetric transfer. I don't think these mathematical equations "expire" with time. Why does just about every internal combustion head ever manufactured feature an exhaust valve approx. 75% of the intake? You may really mean to point out that this rule of thumb isn't foolproof every time??. To which I'd still ask you to come up with a fact based equation that outweighs it. Bottom line, heated exhaust gas needs approx. 75% of the volumetric space to travel at the same velocity as the cooler, wetted intake charge. If a heads flow capability doesn't support that mathematical certainty, then adjusting the exhaust valve duration to compensate has become the accepted path that cam manufacturers have followed.

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According to the writeup on Joel Nystrom's 67 GTX in MuscleCar Review last month, He reportedly runs heads that have been custom ported to flow 420 on the intake, and 315 on the exhaust.... (at this point I would ask you to do the math on that relationship).

The same writeup notes that he employs a camshaft that features 4% more duration on the exhaust side. This makes sense, since his hemi runs 1250 cfm worth of dual AFB on the intake side, but stock exhaust manifolds on the exhaust side.

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420/315 what does a head done for super stock flow?