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.

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Joshua

I bet he has ! Lou will let us know shortly.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Yes, I would like to see the Dyno Sim results, that would save me the effort of putting it in myself,,,
As a guess with a range of four degrees among the cams for the intake closed event, I would expect the differences between the three to be very subtle. The cam that closes the intake valve the soonest (lowest IVC value ABDC ) will have the highest dynamic compression and should have the best torque at the lowest rpm. The cam with the latest intake valve closing ( highest IVC value ABDC) will have the lowest dynamic compression, but since the intake is open later should have better power at higher rpm.
I would go through this cam duration balance exercise as a last step in the cam selection process to fit a torque curve to a specific gear ratio / rpm target.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
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Takes awhile to figure out (I'm still working on figuring it out), but it's free (used to cost $)........

Scroll to the bottom
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You'll need this: (tutorials examples come with free download above)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

And probably even less noticable in a car. The final ET of the 3 in a drag race is all I worry about.
I think the "whole package" is really important. Engine/transmission/torque convertor/rear.

I would like to see someone do the testing for sure! Go for it Josh.

On my street engines I have run the same duration/lift on both lobes. On my race cars I have taken the advice of Lou, through Cameron Tilley and use about 8 degrees less exhaust duration.

FYI- I'm pretty sure the cam in Ryan's car is the same on both lobes. Gone 11.00 too.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

I tried several combinations of intake and exhaust duration on the blue Valiant. I can't say there was really a detectable difference. Rick is right that I settled on a equal duration cam that I believe is still in the car. I agree that the flow numbers indicate that a cam with more intake duration is indicated, I believe the Hyper Pak cam was 276 intake and 268 exhaust.

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If you didn't drive it there, it's not a street car.

This would be a cool study. I believe Tilley has tried 20-30 big cams (260 @ 0.050" and bigger) on his roadrace NA motors and found the ones with more int dur were better, but not by a lot. Probably 2-4% better. Hard to do a good back-to-back, but you can do it on an engine dyno in a day or two with 2-3 pairs of hands on the job. We did this in 2005 with I think 3 cams and a couple of heads. We did not play with cam timing on each cam, which would have been nice. If it were me, I might try the same int dur and just vary exh dur in 2-3 steps, maybe one "even", one 6 deg more on exh and one 8 deg less on exh. Will think more... This will be quite dependent on head, desired HP curve, and CR. Tilley had well-tuned 45 mm webers, which are probably the best you can do w/o IR EFI...

Lou

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Home of Slant6-powered fun machines since 1988

Over cam it, over converter it, over gear it, and tune with the shoe polish on the windshield.

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Official Cookie and Mater Tormentor.

Well spoken, from a true drag racer... Too much thinking can just stop you from getting out there!

Lou

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Home of Slant6-powered fun machines since 1988