Lurked long enough

LILCBRA said:

A dual pattern cam will help maximize the exhaust side to more closely resemble the intake flow. Right or wrong, here's how I think of it: the intake valve opens a certain distance for a certain amount of time and allows X amount of air/fuel into the cylinder. Then, the compression and power cycle produce X amount of exhaust. Since nothing was added, you still have to expend the same volume of exhaust as you'd want to bring air/fuel into the cylinder for the next intake cycle, so the exhaust valve needs to be the same size and open the same distance and for the same length of time. Since it's almost physically impossible (there are exceptions, of course) to match the valves and ports size for size, the only way to help equalize them is to adjust the amount of lift and length of time that they're open. So a dual pattern cam generally has longer duration and more lift on the exhaust side to help equalize flow.

I've shared this video here before, as I know a couple others have, I think it's a good watch to help get a better handle on cam selection.


He gets into single/dual pattern cams at around the 35 minute mark.

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FastDriver said:

I've seen a rule of thumb that when you divide the exhaust flow by the intake flow, & then every percent less than 75% wants 1.5 degrees of exhaust duration @ .050".

Maybe this gives a decent idea of why split patterns work so well on GT40 style heads. Let's take the GT40X heads, for example. Skip to the second list, at the bottom, on this:



So, at .600, it'd be around 69.8%, which would need about 7.7 degrees. The XE274HR, a pretty good cam that would have manners similar to a B-cam is 224/232 @ .050". 8 degrees!



So voila! And on top of the right split, it has 1 degree more LSA, which I'll bet makes it even more street friendly than the B-cam. The extra lift should help a bit, too.

Now, most that are familiar with nitrous cams know that they generally have even more duration on the exhaust side, and that jibes with the idea that injecting nitrous is equivalent to having an even bigger intake CFM flow.

Just some food for thought for ya.

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General karthief said:

Well, you'll likely find if it's a stock head/cam and gearing it will be a dog (with a broke leg) around town, moves the RPM range up and you'll likely lose torque down low, needs different heads and cam (and prolly a gear change) to be useful.
JMO

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FastDriver said:

You're on the right path with your thinking in #1. Adding duration @ .050 and gives more time to both clear the spent gasses, & adds overlap to scavenge to get the intake charge started. Don't overlook the added lift, which means it's going to flow more during that time, too. Yet, the same adv exh duration means that the comp gets there due to more aggressive lobes. The 274HR is def going to make better power on a GT40 head. All of that on the same LSA.

A little offbase in #2. It's not going to bring in "almost as much," it will definitely bring in more. It will expel more, and it will more efficiently make use of the exhaust charge to initiate the intake charge.

For driveability, look at overlap. B cam has 0 at .050" but 60* at .006. 274HR has 4* @ .050" but only 54* at .006, again due to the faster ramp rates. A bit of a conflicting signal. My guess is about the same driveability, or maybe a slight edge to the 274.

B-cam advantages? Softer ramp rates means easier on the valvetrain and more compliant of weaker springs. Cheaper. Can say you have the almighty B! *Might* squeeze out or equal the 274 on the low end.

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FastDriver said:

B-cam memes are just for fun. Split pattern is definitely better in this application. Custom from Ed would be better. Xe 274HR is not a bad selection, otherwise, IMO.

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