Large split duration cams
 

I've been chatting with several vendors and have gotten a cam recommendation from them. Most if not all of the cams had a relatively large split (6-8*) for a good flowing CNC'd cathedral port head.

Back in the day I remember single pattern cams were the shit (224R, 228R, TR224 etc.) After that the craze went on to run small split cams (224/228, 228/232, 226/230 etc.) Lately tho I've seen splits get bigger and bigger, like running a 224/230 or a 226/234 or a 230/236.

I understand that rectangular port heads like LS3s or LS7s need more exhaust duration to make up for the lack of exhaust flow compared to intake flow, but why is that ideology starting to creep into cathedral style heads?

For example, taking a 226/230 cam that many run, what would be the difference in power and delivery if you extend that exhaust duration another 4* and run a 226/234 keeping everything else similar? Will it make more power up top, will it carry the power further rather than falling off quickly, will it create more low end power?

The larger exhaust lobe has always been around. G5X3 was one of the best performing cams... 234/242...

It carries power past peak better.

I don't think 6 to 8* split is large at all. Overlap is good when it comes to CNC's cathedral port heads.

I never said it was big, just wondering how these would perform vs. a cam with small split. Overlap you can get by advancing the cam too or getting it ground with tighter LSA. Is extending the duration of the exhaust timing better than lowering the LSA to achieve overlap?

Ok cool that's sorta the answer I was looking for. So it only really affects power after peak? Does it do anything for under the curve or peak?

I'd say both. My cam is on a 110 LSA and has 8* split. Overlap equals 11 degrees and it hauls the mail with my AI heads.

Rise of the Phoenix, when are you going to get that thing dyno'ed man?!

Dyno day that I was going to attend on the 12th got canceled due to his dyno being broke. It's a Mustang dyno, but it's local to me, which is a plus. He's going to let me come up and dyno it once he's got it fixed.

When Geoff at EPS came up with mine, he said on the phone that his experience playing with them was positive likely due to an effect were escaping exhaust gasses were helping pull in the intake charge.

He mentioned it would have a reasonable peak HP RPM but would carry well way past peak.

I think the wide splits are also popular with nitrous.

Thunderstruck hit the nail on the head..

The later EVO (exhaust valve opening) creates an optimal overlap cycle where the fresh intake charge is pulled through from the later evacuation of the exhaust. That helps create more power.

The peak is unaffected. Intake duration, ICL (intake centerline), and manifold runner design/head CSA (cross-sectional area) determine peak. The later EVO just allows it to hang on past peak.

Power under the curve is a result of actually increasing compression or holding the intake and exhaust valves closed longer to create more torque. Reverse split cams do this and make excellent power under the curve but generally fall off quickly. Same for smaller duration, tight LSA cams.

My cam is an EPS 234/242 111+3. It should peak around 6300 and hold til 7k. Maybe more since I'm running short travel lifters, which usually provide another 400-500RPM. So I plan to spin this to 7400.

There are no absolutes when it comes to duration "splits" or overlap.

The minute you think that, you'll have to find a cure for tunnel vision. :)

Mine peaks around 6200 but has went faster every time I raised the shift points. I stopped going higher at a 6700rpm shift (shift occurrence, not commanded) for fear of my 98 stock bottom end/rod bolts.

Car with this cam and some mildly touched up 799s (intakes were hand ported, exhausts were only smoothed) showed a considerable improvement over cam only 230/224 reverse split. So far it has gained bests of 6mph and .57 in ET. I think it will go faster still with a perfect pass in good air.

As I said, you have to take into account compression, head CSA and port volume, manifold runner design, etc. That contributes greatly to how a cam behaves and performs. We see that all the time with LS3 heads, especially on smaller bores. They are sensitive to reversion.

But there are generally accepted ideas on cam design.

If you're geared right, holding power past peak generally makes a car faster.