Big Block cam concept

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 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.

Two flaws I found w/ the ol' Desktop Dyno/2000 sw packages, comparing the programs' resuls w/ actual dyno tests of different cams:
1. DD always "made" broader power with wider LSAs
2. DD always "made" more HP with extra exhaust duration

"Real world" results simply didn't align w/ the programs' results.

What gear / weight / converter / etc.? I had good results w/ an Isky 260-something advertised single-pattern on 108 LSA in a mild 383 many years ago.

It was all done by 5200, but pulled the car around nicely for a 383 4-speed B-body. Even ran mid-14s at almost 100 MPH on crappy street tires.

Single pattern cams work just fine in Mopar engines. Maybe a dual pattern cam is better, but maybe it isn't.

I've had great results with the old MP .528 in several motors over the years. I also run a single pattern roller in my 514 dyno motor which makes more than 900 hp. So mild to wild, single pattern cams can work just fine.

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.

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

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

The computer programs are generally correct but far from dead on. They are IMO best as a learning aid &/or a more then likely probable outcome from changing a cam from one to another.

Take it with a grain of salt and understand that when comparing cams, what the graph reads is what will likely happen between the cams. The actual power output numbers should generally be ignored. But not shunned as hocus pokus. The program is good for what it is. An inexpensive look into probable outcomes.

Compare your cams with as much information as you can put in.
Note the torque and HP line differences. This is how you can help yourself make a better choice.

Split duration cams aid the engine in poorer flowing heads & restrictive exhaust. It helps clear out the cylinder more of the spent fuel. This aids in power production. So making use of one in a stock or highly modified ride can very well be helpful. But not mandatory for all engines.

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.

The exhaust system matters also. With a stock system/manifolds extra exhaust duration may help it breath. The same engine with headers and good pipes might do better with the single pattern.

When I called Engle and had them grind a cam for me a few years ago they seemed to think that Mopars didn't need a dual pattern cam.

No pro builder here but every motor I`ve ever built has had Isky split duration cams with great results so far.