Let's talk about valve timing events and camshafts!
There are a few things we know about the slant six and can agree upon, but there is a conclusion drawn from one of these facts that I want to test.
Fact: The slant six head has a relatively better exhaust port than intake port as a percentage of flow.
Conclusion: Less exhaust-valve-open duration is needed than intake duration for peak power and efficiency.
This conclusion, and the fact that Chrysler used some cams of longer intake duration, has lead many of us to use cams with a longer intake than exhaust duration. Is this really for the best? Maybe a single-pattern cam works best? What about the traditional split duration cams with more exhaust duration? I don't know of anyone having tested this. Does anyone else? Maybe Cameron Tilley knows?
To get to the bottom of this I propose this test: Run 3 cams with essentially the same overlap, but with different duration splits to see what makes more power. Cam 1 would be a single pattern grind. Cam 2 would have 4° more intake duration and 4° less exhaust duration to give an 8° duration split. Cam 3 would maintain the same duration split, but with more exhaust duration. The idea here is to hold the overlap, and as many other factors as we can, constant to best isolate the effects of duration. All cams would be ground with a 106° LSA and run with a 102° installed centerline. The test engine would be a +.045" bored 225 (230 ci) with 9:1 compression sporting a ported head with oversize valves, 4 bbl intake and long tube headers.
I found a few cam lobes in the Oregon Cam Grinding catalog to come up with the following...
Cam 1: 240° duration @ .050" lifter rise, 280° advertised duration, .506" lift, 28° overlap
.050" lift valve events: IVO:18 IVC:42 EVO:50 EVC:10
Cam 2: 244°/236° @ .050", 282/282° adv., .516/.495" lift, 28° overlap
.050" lift valve events: IVO:20 IVC:44 EVO:48 EVC:8
Cam 3: 236/244° @ .050", 282/282° adv., .495/.516" lift, 28° overlap
.050" lift valve events: IVO:16 IVC:40 EVO:52 EVC:12
Yes, the compression ratio is a touch lower than ideal. The UEM Dynamic Compression Calculator gives 7.58:1 on cam 1, 7.48 on cam 2 and 7.675 on cam 3.
That's a lot to think about. And expensive to test. 3x the cams and lifters, 3x the dyno time and a bunch of labor. Anyone care what Dyno 2000 says the differences are in torque and horsepower? It's so little different that I don't see doing this in the real world.
Lou will let us know shortly.
