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AeroCoupe · Monday at 11:04 AM

Pretty vague on the intake. Ask for a flow sheet on it at 350 cfm doesn't mean anything without knowing the inches of water column. I am also not a huge fan of just flow rate, what was the port velocity? Slow velocity means lazy down low and high velocity means less lazy. You would need some AFE 205's or 225' or the like to keep up with the intake and at that point you better have some cubic inches and an rpm range from 3500 to 8000 to make it work optimally. Stick with an Explorer upper and lower and if you want to jazz things up send them to TMoss to port the lower and clean up the upper.

VERGAZ1 · Tuesday at 3:18 AM

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.

View: https://youtu.be/JPAeepqrY-0?si=CS1R2U2ARzOtryvE
He gets into single/dual pattern cams at around the 35 minute mark.

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.

That dude's video is very informative as were both of your posts. This is the first time I've ever had to consider any of these numbers and calculations so I have just a couple dummy questions.

1. when you don't have an extended exhaust valve opening duration, does the unexpelled exhaust stay in the combustion chamber and block fresh air and fuel from coming in? is that why you want to get the extra time for that valve open so it all can get out before the cylinder is reloaded with explosive stuff? why don't they do that from the get-go? (genuine question not sarcasm)

2. that second cam from comp cams the 224/232 means the exhaust valve puts out even more than what's taken in so there's no lingering fumes for the next intake cycle? so it's bringing in almost as much volume on intake and spitting out a lot more to run smoother but also deliver similar power output at full throttle and at the same time better parking lot manners? I like a lopey cam but I don't think a small block Ford requires it ive just always heard about the "B" cam and it being a Day 2 type of upgrade

thanks to all of you for your answers

VERGAZ1 · Tuesday at 3:30 AM

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

ok I'll keep on the explorer intake/heads hunt then thank you. it looked cool is the only reason I asked

FastDriver · Tuesday at 5:30 AM

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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