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| yet another cam question (custom oregon cam idea) https://slantsix.org/forum/viewtopic.php?t=46435 |
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| Author: | 65CrewCabPW [ Sat Sep 24, 2011 9:07 pm ] |
| Post subject: | yet another cam question (custom oregon cam idea) |
While looking at the list of grinds from Oregon Cam, I realized I didn't realy like any of them, as 244 degree duration seems small. And, added to that, running a propane carb, I need more intake than average, since the propane carbs have an inherent resistance. I had talked to them some time back, several months ago, and the cam guy said to use 218 ( a 260 cam) for my truck, but as I looked at it I get the impression I'm going to regret that, as it has a little too much overlap and not enough bottom end grunt. I need a strong idle, and good "off idle" response for driving a truck around in the rough. So, what if I used the intake off of 218 and the exhaust from 819? Set to a 108 or 106 lobe centerline and advanced 4 degrees? It would be duration @ valve lash 260 I 248 Ex, .440 x .437 lift. @ .050 225 I, 218 E I know the narrow cl tends to have more overlap and is more peaky output, but it should be "all on" early enough, and I don't care about power above 4000, so advancing it should get me a cam real strong in the 1800 to 3400 range....??? |
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| Author: | Joshie225 [ Sat Sep 24, 2011 9:58 pm ] |
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I was thinking 819 intake lobe and 818 exhaust. I know it looks a bit small on the advertised duration, but the lobes have good acceleration so the duration at .050" is good. Decent lift for such a short duration too. Nice tight lash too. I'd use 108° lobe centerline advanced 4° myself. |
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| Author: | 65CrewCabPW [ Sat Sep 24, 2011 10:13 pm ] |
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Quote: I was thinking 819 intake lobe and 818 exhaust. I know it looks a bit small on the advertised duration, but the lobes have good acceleration so the duration at .050" is good. Decent lift for such a short duration too. Nice tight lash too. I'd use 108° lobe centerline advanced 4° myself.
People who have used the RV15 cam say it has good low end grunt, and this combination seems to be closer to that than the other combo. My take is that I need the extra intake for dealing with the propane carb. I am hoping someone has some meaningful input on this... Whatever cam is in the motor now, it seems to have some kind of strange overlap mismatch... or something. Loosening the valve clearances clear out to 22 and 25 (int and exh) will get a relatively smooth idle, but if I tighten it up to something that gets me quiet valve train, the engine will miss on 3, 5, and 6 at idle, in a strange, uneven 8 cycle mode. When I got the motor, the lash was clear out at .035 to .040, and the valves on those cylinders had very strange looking seats, even the intakes. And, it had the same stupid lope to the idle, which was exaggerated by small valve leaks. So, I want to change to a known value, and it since it costs the same no matter what grind, I thought i'd try to do the "best possible" and leave it at that. I have 10.5:1 measured compression ratio (not dynamic), and it's dedicated to propane. |
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| Author: | Joshie225 [ Sat Sep 24, 2011 10:38 pm ] |
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You want more intake duration on a slant because the exhaust port flows proportionally more than the intake. I really don't think with the carburetor you're using that you have serious intake restriction. |
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| Author: | 65CrewCabPW [ Sun Sep 25, 2011 8:26 am ] |
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Quote: You want more intake duration on a slant because the exhaust port flows proportionally more than the intake. I really don't think with the carburetor you're using that you have serious intake restriction.
Ok, I'm confused. ALL diaphragm type propane carbs have signficant restriction. Yes, I oversized a little, which helps, but just the method by which they work creates a signficant loss of performance. My head is not signficantly ported, though a bowl hog was used (and I blended in the seats and smoothed things up a lot) and some removal of the worst uglies in the intake ports was done. The engine feels choked down all the time, even if you're just punching it in neutral. Thus, my sort of obsession with attempting to improve breathing and air density in the engine. |
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