Re: Big Block cam concept
[Re: Alchemi]
#1815320
04/30/15 12:22 PM
04/30/15 12:22 PM
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Joined: Jan 2003
Posts: 10,021 Frostbitefalls MN (Rocky&Bullw...
gregsdart
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 10,021
Frostbitefalls MN (Rocky&Bullw...
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I wouldn't vary too much from what you see recommended in cam catalogs, save for maybe a narrower LSA than most cam companies grind them on. If they show 268 duration for xxxx rpm range, in a 383 that is probably about what you need. Bigger motor, more duration. Keep this in mind, that the bigger the motor, the narrower the LSA needs to be when using the same valve sizes. So a 383 motor with 2.14 intake valves might work great at say 110, but a 500 inch stroker might be much happier with a 106 or even less! The narrower LSA provides the overlap to feed all those extra cubes through the same valve overlap/valve size setup. In a lot of cases, cam companies grind cams with too much LSA, to cover up the problem of guys picking too much cam to start with. But the cost is a motor that performs, OK, but not to its' peak potential for what it is.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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Re: Big Block cam concept
[Re: Alchemi]
#1815829
04/30/15 09:43 PM
04/30/15 09:43 PM
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Joined: Nov 2006
Posts: 1,460 oklahoma
forphorty
pro stock
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pro stock
Joined: Nov 2006
Posts: 1,460
oklahoma
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Been looking at low end sticks for my 68 383 powered Fury in the stock 256 up to 284 durations and have had a play with the dyno 2003 software (similar to camquest)
One of my discoveries is that mopar heads exhaust to intake ratio's suck, they are nowhere near the "magic" 80% figure, some of the newer ones come close (the trickflows are almost spot on) but the flow figures on stock and ported stock heads are usually 70-75%.
As I am stuck with my 906's with cleaned up ports for the foreseeable future, a dual pattern camshaft makes sense to "fix" the flow differences.
Currently the 268/280 or 255/275 grinds look not too bad for my purposes but of course as I have the dyno software I have been playing and have come up with this: Duration 252/284 lift.462/.484 ICA 109.8 LCA 111.9 ECA 114 Overlap 44.3 It swaps a few HP from 5k to 2k vs the 268 cam, has the torque figures of the 255 cam and has better VE figures than either, all this is computer output though.
So is this a viable camshaft with 32 degrees of exhaust duration more than the intake? If not, Why? I thought the old ballpark ratio to shoot for was around 70%? Have you ever seen the intake/exhaust flow ratios on some of the old Ford engines? Awful. Some were below 60% I believe.
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Re: Big Block cam concept
[Re: Alchemi]
#1815860
04/30/15 10:23 PM
04/30/15 10:23 PM
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Joined: Jan 2003
Posts: 10,021 Frostbitefalls MN (Rocky&Bullw...
gregsdart
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 10,021
Frostbitefalls MN (Rocky&Bullw...
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The percent intake to exhaust ratio depends on the build. A low compression engine needs a lot more flow percentage than a high compression engine. At 8/1 the ratio might be best at 80 percent, at 16/1 it might only need 60 percent
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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Re: Big Block cam concept
[Re: Alchemi]
#1816093
05/01/15 04:30 AM
05/01/15 04:30 AM
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Joined: Feb 2014
Posts: 234 Brisvegas, Australia
Alchemi
OP
enthusiast
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OP
enthusiast
Joined: Feb 2014
Posts: 234
Brisvegas, Australia
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Thanks lots guys
The fury should come in at around 3950 lbs with me in it give or take - im no featherweight, stock converter (i think - it was replaced/rebuilt in '81 so a little unknown till i get the motor back together), 2.76 diff - have a 3.23 pumpkin sitting here but a bit torn as to put it in or not, I live in a rural area and will be mainly doing highway miles.
The motor has the stock low comp pistons with the 9cc dish, but its a 68, so the pistons are .003 above deck (on average - my decks are .005 & .006 out of square in opposite directions, in the process of equalizing the heights on the 906's chambers and cutting roughly 50 thou to get the comp up to 9.6:1 (8.6 as it sits now), have a performer intake to go on and have to source a small thermoquad or maybe a 625 street demon.
I thought the software was a bit out with the extra duration - i havent seen any cam like that advertised anywhere, but then theres lots of ppl out there with "secret" cams, so i though i would ask
I will have a bit more of a look and pick one with a 108-110 lsa
Thanks again for the thoughts guys
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Re: Big Block cam concept
[Re: Rob C]
#1816310
05/01/15 01:36 PM
05/01/15 01:36 PM
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Joined: Jan 2003
Posts: 16,376
dogdays
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 16,376
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I believe that all Mopar stock cams were dual pattern. Except for a late 360 roller cam, every one had more exhaust duration. The street cam lines were the same. The drag race cams were all single pattern. I believe that they figured racers would use headers.
The Isky website used to have an interesting piece on single vs dual pattern cams.
When circle track racers started using the 308 and Mag heads, they found an advantage using a single pattern cam because the exhaust ports were so good.
Several years ago Street Rodder magazine built a buick nailhead as a project engine. One of the things I remember about that build was the fact that the nailhead exhaust ports were very poor and couldn't really be made better. Stock buicks ran exhaust durations around 238 degrees at 50 lift in order to crutch the exhaust port.
R.
MY APOLOGIES to anyone who tried to slog through the Isky website because of what I said. I just checked them out and Wow! what a lousy website. Looks like it's been updated recently, too. Oh, well.....But I did find both B and LA cast iron rocker arm sets for $450.00.
Last edited by dogdays; 05/01/15 05:51 PM.
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Re: Big Block cam concept
[Re: Alchemi]
#1816518
05/01/15 05:27 PM
05/01/15 05:27 PM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
I Live Here
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I Live Here
Joined: Jul 2004
Posts: 19,318
State of confusion
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No pro builder here but every motor I`ve ever built has had Isky split duration cams with great results so far.
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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