howto figure out cam specs from heads/intake

[jetuomi]

Hey, I was wondering what the math is behind the calculation of cam specs, (for example) how does FTI take head / intake specs, and make a custom grind? what's the math?

so I have 180cc intake volume, 60 cc exhaust volume, 250/195 flow @ .550 edelbrock heads, rpm2 intake, and they came up with a cam of .5472/.5408 lift, 220/228* duration.. how do you take account for the larger intake volume than an AFR by adjusting duration, and how do you maintain intake charge velocity?? how do they tie that in with the intake runner length..? etc, etc...

thank you !! bring on the theory, I'm ready for it..

 

[331 cobra]

Cams try to optimize the ram effects present in the intake and exhaust of an engine.

Intake runner length, cross section, engine displacement and cam timing all effect the RPM where your ram effect will happen, and also how strong the effect will be.

Similar things happen in the exhaust controlled by the exhaust port, header primary size, primary length, cam timing, collector style, cats, mufflers, crossover pipe, etc.

Then there is overlap to consider. That opens a whole new can of worms.

Yes there is math, but I doubt that someone like Ed Curtis really uses it. I think cam grinders usually rely on trial and error and borrowing profiles and specs to come up with good combinations.

I recall a magazine article about the ram effect in hilborn style fuel injection. They could not make their results match the formulas. The trends were duplicated, but a fairly signifigant fudge factor had to be applied to make things line up.

On the other hand, there is engine simulation software that is very close to the real world. I'm sure they use all of the math, and all of the various fudge factors that are required to make it close to the real world. I would not be surprised at all if the cam grinders are using that.

I'm sorry that I didn't give you any equations, but it's so much math that I would not attempt it, and then you never know if your corrections are going to be correct. My own personal philosophy is let someone else do the engineering for you, so I would either buy a custom cam, copy some one's combination or call a company like comp cams that will recomend an appropriate cam for you, but grill them about their decisions and figure out why they chose a specific grind for you over their other products.

Or else you could buy the software, but I have heard that some of it is very expensive.

 

[Michael Yount]

Go to the link below if you want theory (and practice) and read every tab - especially "cam truth" and "confusion factor". The math behind it is much more complex than most understand. In fact, all the numbers normally thrown around to describe a cam (duration, lift, LSA, overlap, etc.) are simply the resulting calculations of knowing what the optimum valve timing is. The valve timing is the key - and the models/programs to get there are as much art/experience as they are math.

http://www.wighat.com/fcr3/

 

[9150ho]

jetuomi, whats the lsa on that cam?

 

[jetuomi]

jetuomi, whats the lsa on that cam?

112* LSA

 

[jetuomi]

Go to the link below if you want theory (and practice) and read every tab - especially "cam truth" and "confusion factor". The math behind it is much more complex than most understand. In fact, all the numbers normally thrown around to describe a cam (duration, lift, LSA, overlap, etc.) are simply the resulting calculations of knowing what the optimum valve timing is. The valve timing is the key - and the models/programs to get there are as much art/experience as they are math.

http://www.wighat.com/fcr3/

cool, thanks, there is a lot of reading !