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Old 09-21-2008, 09:12 AM
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What gives? Cam Comparison

I'm trying to figure out how a cam with only 3 degrees less .050" duration on intake and 3 degrees more duration on exhaust can have such a lower and higher rpm limit(wider range) than another one.

______________________adv. dur_ .050"dur___in/ex lift_____range
Magnum________270HR__270/270__215/215__.500"/.500"__1800/5000

Xtreme Energy XR264HR__264/270__212/218__.487"/.495"__1200/5200

I would rather have a split dur. cam rather than dual pattern because I have hogged out the exhaust ports, and have a decent exhaust system, so I don't believe I need the added ex. dur., and I've read the extra ex.dur. costs low rpm torque. I've also read the Extreme Energy type are hard on valve trains and make noise, but I like the range(especially since I want to use my stock convertor).
How does the Extreme get 600 more usable rpm on the low end, and rev higher on the top end??? Your opinions please!

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Last edited by ssmonty; 09-21-2008 at 09:19 AM.
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Old 09-21-2008, 09:50 AM
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It would take someone with more camshaft knowledge than me to explain it.

Basically...the extreme cam is newer technology.
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Old 09-21-2008, 11:11 AM
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There are so many more factors than what's on the cam card. Ramp speed is a very important key in broadening the powerband. It provides more area under the lift curve and therefore more area under the torque curve.

The other thing you don't list is LSA. A broader LSA tends to make less peak TQ and HP, but it makes wider curves. In a light car, a tight LSA is more OK because you can access the peak numbers easily. In a tow truck, you need as much average torque across the range. The Magnum cams tend to have tighter LSAs. They're kinda like improved reproduction muscle car cams. Nothing special. The XE cams tend to have wider LSAs.

Another factor is perception. A tech at Comp Cams looks at a dyno chart from both cams and sees similar results, but the faster ramps and wider LSA of the XE cam (plus the fact that its the cam they're pushing these days) and they advertise its powerband a little broader.

I also wanted to discuss your split cam and your exhaust ports. Typically you want a split pattern cam to help a crippled exhaust. Factory exhaust ports don't flow much, so you leave them open longer. Once you port the exhaust side, less duration is needed. The best way to tell how much you need is with flow numbers. Someone here will remember the I/E flow ratios at which split cam patterns are a benefit. But you're right... there are benefits to getting the flow you need with less duration. That is more of a factor on the intake lobe, but the exhaust duration will afffect reversion and scavenging; two very important events that happen during the overlap.

I don't know your application, but the most advanced lobe designs over at Comp are the XFI series. Some of them have a very wide LSA to make EFI computers happy, but if you can get one custom ground on a narrower LSA, you'll be happy with the power. After that, the XE line has very nice lobes that work well.
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Old 09-21-2008, 11:29 AM
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What he said ^^^

Quote:
I would rather have a split dur. cam rather than dual pattern because I have hogged out the exhaust ports, and have a decent exhaust system, so I don't believe I need the added ex. dur., and I've read the extra ex.dur. costs low rpm torque. I've also read the Extreme Energy type are hard on valve trains and make noise
This does not make for sound judgement whewn it comes to camshaft choice. Just because you 'hogged out' the exhaust ports does not mean they will do anything better, or worse, than from where they were. Combustion end gas flow is not dependent on hogged out ports. Depending on what youj did to your heads, you may have hurt them as opposed to help. Maybe you take and have them flowed for a better idea of what the I/E ratio is. Then, you could make your choice with something other than what you believe. Not trying to be a harda**, JMO.
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Old 09-21-2008, 11:43 AM
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I'll give it a shot and try to keep it simple.

First you need to look at the online cam card (specs) and see the whole story.
Both cams have the same 110* LSA lobe separation angle and are recommended to time the same at 106* ILC intake lobe centerline.

Let's use the 270HR as the primary standard:
50* of overlap with a description of noticeable lope at idle, which generally speaking enhances the scavenging effect.
The XR264HR has 47* of overlap with a milder idle.

The 264 intake valve has 6* less duration and opens 3* later and closes 3* sooner. At .050 lift the intake valve has 3* less duration because it is a shorter duration lobe than the 270, The earlier closing traps more cylinder pressure for a slightly higher cranking compression, which favors the low end.

and the exhaust has 3* more duration at .050 because it is a faster opening/steeper ramped lobe than the 270.

The combination of these factors makes the cam favor the low end due to the increased cranking compression and lean towards higher rpm increases because the exhaust valve opens sooner and stays open longer, which favors higher rpm exhaust flow.
Over all the 264 will have a flatter and broader torque curve (wider rpm range) even though it might give up a few pounds of peak midrange torque in exchange for a gains at either end.

Lift is similar and varies due to the steepness of the lobe ramp.
Note that exhaust valves are not as sensitive to lift due to the hot expanding gases trying to get out.
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