head flow vs cam choice

[Bronze66]

This question pertains to mild to stout street motors of 400-500hp. Is there a way to calculate what type of cam to use for X amount of head flow? For example if you have a set of heads that flow 260 cfm @28". How would one spec a cam to get the most efficient/max flow from the heads. I know the rpm range plays a part in this so let make a max of 6000rpms for this example. I realize that there are many parts that effect this. So all correct parts being equal to get the flow from the heads. Are there any calculators or rule of thumb to kind of get you close to what cam is needed or is it just experience that plays a bigger part in selection? The cam could be hydraulic or solid. Just trying to learn a little more.

[Ron E]

Assuming you're working with a port that will flow a peak of 260 CFM @ .600". If no additional porting is planned, and you want to get the most from the heads there are two methods that come to mind. One is to get a .600" net lift camshaft, and there are several cam gurus here who could set you up. Second, if .600"net is a potential longevity/durability problem, a competent head-porter can work your valve seats, and valve shapes to get more of the available flow at a acceptable valve-lift. Hope that helps.

[jmarkaudio]

The problem is that the same heads require different cams depending on the rest of the combination. Engine displacement, vehicle weight, gear ratio, intake, carb, header size...... you get the picture? This is why you call three cam companies and get three different answers. In most cases it still is based on an "educated guess" from past experience. For some of the pro's here, lots of dyno time.

[bigjoe1]

contrary to popular belief, the poorer the had flows, the MORE cam it will need to made the same HP, Example, wiith a real good head, 260 works very well, but with a week head, it might take 275 or 280 to get the same top end HP.

JOE SHERMAN RACING ENGINES

[Greg D]

Would it help to crutch an insufficient flowing head with a cam that has tighter LCA's?

[CamKing]

Would it help to crutch an insufficient flowing head with a cam that has tighter LCA's?

No.

You tighten up the lobe centers when you have too big of a port, and widen them when the port is too small.

[Bronze66]

Camking could you expand on that a little? Is that because the DCR will be higher? That's the type of rule of thumb I'm asking about. I realize you won't find the "perfect" cam but should be able to get real close on parameters of lca, lsa, duration,lift and lobe shape. The latter may not be realistic.

bigjoe1 are you talking about lower velocity of the port making it weak?

I'm not building anything at this time. It was just something I was thinking about and decided to ask the people that might know. I tend to like a tighter lca's for the street with a safe cr. Except when a cr of 10;1 or more is planned. I was thinking that you build a motor to make X hp. you then need X amount of flow to get there. That flow is ,partially handled, by the cam and the valve opening and closing events. As long as all other parts needed to achieve the flow wanted are used, there should be away to calculate a close ballpark cam to use to get it. This would be for basically off the shelf cams that the companies make. I'm assuming that a custom cam could change everything as well as rocker arm ratios. I quess the question could also be "what relationship to head flow does the cam have in common?"

[Greg D]

Would it help to crutch an insufficient flowing head with a cam that has tighter LCA's?

No.

You tighten up the lobe centers when you have too big of a port, and widen them when the port is too small.

I guess I am a bit confused, In a David Vizard article, he states The bottom line is that LCA's are related to how big a cylinder the intake valve has to feed. The bigger the cylinder in relation to the valve, the tighter the LCA needs to be.
Can you expand?

[bigjoe1]

I go along with the Vizard idea.On very high compression engines (13,14 or 15 to one) I spread the centers. On lower compression engines ( 10 or 11 to one) I tighten them up I do NOT agree with camking at all. I do NOT try to explain all this stuff, but I know what works. It sounds like everyone is trying to make more out of this than they should.
JOE SHERMAN RACING ENGINES

[OldSStroker]

Bronze,

X amount of hp requires a certain mass of air (and the associated fuel) to be processed each minute (or second or hour). A head (or port) that flows well requires less time (duration) each cycle to move a given amount (mass) of air thru it than a port that flows less well, just as Joe mentioned.

It always surprises me that many folks don't see this intuitively.

[CamKing]

Would it help to crutch an insufficient flowing head with a cam that has tighter LCA's?

No.

You tighten up the lobe centers when you have too big of a port, and widen them when the port is too small.

I guess I am a bit confused, In a David Vizard article, he states The bottom line is that LCA's are related to how big a cylinder the intake valve has to feed. The bigger the cylinder in relation to the valve, the tighter the LCA needs to be.
Can you expand?

Two things to remember about David, He get's paid by the word, and he doesn't build real race engines.

When your port doesn't flow enough air, where do you see the biggest loss in power?
What normally happens to the power curve when you spread out the Lobe centers?

When your port is too big, where do you see it hurt the power?
What normally happens to the power curve when you tighten the Lobe centers?

We change the lobe centers to band-aid the port's problem.

[Greg D]

No.

You tighten up the lobe centers when you have too big of a port, and widen them when the port is too small.

I guess I am a bit confused, In a David Vizard article, he states The bottom line is that LCA's are related to how big a cylinder the intake valve has to feed. The bigger the cylinder in relation to the valve, the tighter the LCA needs to be.
Can you expand?

Two things to remember about David, He get's paid by the word, and he doesn't build real race engines.

When your port doesn't flow enough air, where do you see the biggest loss in power?
What normally happens to the power curve when you spread out the Lobe centers?

When your port is too big, where do you see it hurt the power?
What normally happens to the power curve when you tighten the Lobe centers?

We change the lobe centers to band-aid the port's problem.

Vizard may get paid for his articles, but I just can't believe he would get the facts backward. If he did, he wouldn't be selling many articles/books.

Would the heads of a NHRA SS car using stock valves 283/305/327/350 = 1.720/1.500 , 1.840/1.500, 1.940/1.500 be considered restricting? I understand they use 100/102/104 LCA's.

Doesn't a ProStock car with monster valves and cavernous ports use 115-120 LCA's?

[Ed-vancedEngines]

I may get hated upon for what I am about to say but here goes. David Vizard as famous as he has become and as many tech articles and books as he has written is or was basically a writer for a perofrmance magazine, just like Dave Morgan was/is. Both guys have contribued much into the performance realm with their writings and their seminars, but at times they can also be wrong, the same as we all can.

This statement to me absolutely makes no sense in the world of big engines.

In a David Vizard article, he states The bottom line is that LCA's are related to how big a cylinder the intake valve has to feed. The bigger the cylinder in relation to the valve, the tighter the LCA needs to be.

By that statement I am wondering what he would think of what we and others routinely do with big bore cylinders and with huge swept volume cylinders.

Normal aspirated LCA are frequently wide in 500 cu in engines. LCA are mostly very wide in Big Bore Big Stroke engines, from 115 to as much as 120. I know of quite a a few that were LCA of 120.

Ed

[Greg D]

If someone knows him or his email address, maybe we could invite him here to explain his point of view? (I wouldn't mind learning)

Ed - Maybe he didn't mean the size of the cylinder alone dictates the LCA, but the size of the cylinder in relationship to the valve.

Does my example above hold water? (I could be missnig the boat here) a SS car with restriced heads uses narrow LCA's but a Pro Stock engine with huge good flowing heads uses wide LCA's.

[Ron E]

It seems like this good port, tight centers, vs. bad port spread centers would have to involve more than just separation. A good port could use more lift, less duration to go with the tighter centers, right? And, bigger ports are a little soft on the bottom-end, so they might like the tighter centers and less duration w/o increasing overlap in the process. Where, a bad port needs more time (duration), but not necessarily more lift or overlap, so a spread center goes with the recipe, as a bad port can mean a smaller port, which would have more bottom-end , but would suffer up high. So, that seems like the wide centers may help balance the combination out a bit.

Then again, cams confuse the hell out of me. The only reliable cam knowledge is have are phone numbers to people who know cams.