The end result is that you need to use the cam profile or rocker ratios to compensate for head flow. If your heads flow less than 75% on the exhaust versus the intake then you need to compensate - i.e. Indy EZ1's from all the info I can find only flow 60% on the exhaust. A split pattern cam or a higher ratio exhaust rocker would be in order.

As for the "factory" analogy, that's just BS! The cars came off the line with enough power to get you in trouble and the engineers knew damn well that anyone wanting to improve things would have headers and a cam at the top of the list.

The pro stockers would love to get more ex flow but since power is primarliy limited by the ability to get air and fuel into the engine they make a very big intake valve, this means you can only run a small ex valve and that means you have to run more ex duration.

As usual: everyone speaks, no one listens.

just an uneducated response with my
Flow sheets show different flow percentages at every level of flow, not to mention that taking the highest lift flow numbers isn't a good bench mark as most motors don't have enough lift to get there any way.
Efficiency of the motor must effect the percentage ratio needed? A highly efficient intake system with lots of inertia must have different requirements than a basic bracket motor, or worse yet a stock type motor.
Efficiency of the header design must play a big part.
In the end, every combo will be unique to itself, having its own sweet spot.
look at the number of cams that are tested on high end engine builds like Prostock or other professional or comp category's and that will give you a clue as to how well a rule like that works. Any one want to buy a used roller cam for a big block? I have three for sale,,,,,,,,,,,,

I am all ears....

sorry but im ears
i m very dumb on alot of issues
somethings i know alot
my fear is sometimes i get confused between the two

It would be nice to approach this question with an open mind. I know this is Moparts and the general feel of the site has evolved in a different direction but where better to try to revive the old atmosphere...lol

Here's my opinon...I think as with most of engine theory, one has to look at how far one is willing to go. If the one perfect lobe relationship is indeed individualistic to every engine and use, then we could use a pyramid analogy with the perfect relationship and max power is the top or point, poor relationship and lost power is the base, and we're looking to find the relationship that best fits our engine, that's affordable, and that's as close to the point as possible. In terms of techonolgy, a builder can use the port, the valve job, the manifolding, and the cam to optimize the airflow into and out of any engine. There is no perfect relationship unless you are talking identical builds, otherwise the variables add up and "your results may vary". FAST guys have to overcome a bunch of things and do some strange things to run well. I wouldnt call them, or the perfectly scienced out race engine good examples of what to do. Andy's the closest because he's testing his stuff, in a running environment, one part, one change at a time. Like Jim says, modern cam lobe designs are even making what we use to compare lobes confusing with assymetrical lobes and duration measurements that don't specify if there's more on the opening ramp or closing ramp...lol. Over all, I think in terms of exh flow out, you dont need as much lift or duration if the support parts all work well for your intended rpm range. The further you get from that point, the more generalities are assumed and generally, most engines of any sort benefit from more exh duration and lift because the rest of the parts create issues impeding exh efficiency and flow.

My last cam was a straight pattern! why? cause thats what was recomended...

My new cam is a split pattern! why? cause thats what was recomended ...

My Way too complicated and 'application specific' to discuss beyond generalities.

As the motor gets larger relative to the original port cross section (think Stroker) it's much more of a challenge to get enough air IN to the motor and if you can't get enough in there's less proportionally you have to worry about getting OUT. If VE is falling by default so are the pumping forces. The tuning objective for just about any motor is get to optimum VE as early as practical for the application/load/gearing and keep it there (as close to peak) for as long as possible/practical.

The intake closing event effects overall torque and power curves more than any other single event....for a given application you more or less select the cam for the type of engine tuning (powerband) you're looking for, the exhaust timing and lobe spreads are selected from there.

Also remeber it's harder to fill the cylinder than to empty it...you have to induce the air/fuel into the hole past the valve typically with little or nothing more than Atmospheric pressure....but once it's detonated those expanding gases are LOOKING for somewhere to go....where would they go if your exhaust valve didn't open? They'ed go out somewhere, woudn't they???? The exhaust pattern/duration is a tuning means aimed at tuning the pressure (as much as possible) and to ensure adequate evacuation without upsetting the next intake charge. Backpressure through the system (as the high pressure gases return back to atmosphereic) acts like a resistive force that somewhat slows the rate of acceleration of the crank, which in turn generates the draw on the intake charge....but that resistence is also summed along with all the other resistive forces acting on the crank....namely the recipricating/rotating masses as well as the drag from the rings, bearing and valvetrain.

Also as Jim correctly illustrates the @.050 duration only tells part of the story, many cam grinders use a "lazier" exhaust pattern in an effort to trap cylinder pressure a bit longer where as with the intake you want to snap it open to get to full lift sooner and then snap it back shut before the piston sweeping back up past BDC bleeds combustion pressure back into the intake tract.

I,ve always loved Polysheric's posts but I wish he'd get a little thicker skinned; it's a forum afterall and if I ran off every time somebody disagreed with me (even if they were wrong ), I'd have been gone a long long time ago. Or maybe in myu case I'm just too hard-headed to take a hint that nobody wants me around!!

There was an old vaudeville joke about what facts are worth vs opinions.
2 guys are hiring a new secretary, but they want to be sure she can do the job, so they devise a simple test.
Q. "how much is 4 and 4?"
Applicant #1: "8"
Applicant #2: "44"
Applicant #3: "it depends on how tall he is"

They discuss it:
"Well, only one got the right answer, but the others seemed pretty sure. What should we do?"
"Hire the blond with the big t*ts".

The answer was already given, and yet the dialogue continues...

Poly,

Well as is often the case....you delete your posts before I ever see the thread

Maybe you can repost the 'right answer' or PM it to me.

In the mean time maybe the well endowed blonde I hired as my secretary can keep me busy while I'm waiting

I do know that many generally accepted laws of physics and thermodynamics don't always (but mostly they do...and a LOT more than in other 'bowtie' car forums I've seen) seem to apply here, those professors and professional engineers I work with everyday must be idiots!!

My mistake, the shelf MM 305 is a dual pattern. The MM 293 I have in the motor at the moment is a single pattern cam.

I will say that while Jim has had excellent results with that MM 305, I'd be willing to bet some bucks that a sharp cam guy could grind up a single pattern cam for Jim's motor that would make his car faster. I would also assume that another equally sharp cam guy could then figure out a dual pattern cam that would once again improve the performance.

On a related front, I've been doing some dyno testing on my 514 motor with split pattern cams and split rocker arm ratios. So the cam has different intake and exhaust lobes, and the rocker arms are 1.85 on the intake and 1.70 on the exhaust. I might drop the exhaust all the way down to 1.55 next time I run some tests. So there are a couple of more variable to be tossed into the punch bowl.

I'll go back to what I said earlier in this thread: I believe the subject is too complicated for anyone to fully understand. You can make plenty of power with either type of cam. If you have the money and time for testing then you can start to optimize around your specific combination. You can continue to optimize until you run out of money.

The question I always ask people when they say they're exhaust needs to be bigger....I say realtive to what?....maybe the intake is too small to begin with!!

As this post gets kicked around, useful info pops up, Sure every statement does not apply for all & everything but one can pick thru it and see what fits, It is to bad that jeff pulled his reply, I didnt see it also

It is interesting that the big power-adders (Supercharging, Turbocharging, and NOS) all work on the principle of packing more oxygen into the combustion chamber. Turbo even does it at the detriment of exhaust flow, yet a big percentage of performance cams favor the exhaust side...