Mr. Duck,
Apologies for all the questions, I do enjoy the math behind camshafts and would like to learn more from someone experienced such as yourself:
I had already ordered a custom cam from Matt and it is on the way to me, I was curious how you would adjust a cam for a larger cubic inch, without upgrading heads? For example, the "She Gone" cam posted above, what would you change for a bigger cubic inch?
I've also always heard that generally, PDS cams should not have a lot of overlap, but recently I was told differently and that overlap is fine and can actually make more power if timed correctly, would you mind educating me on this?
Also, how does one calculate required lift for a cam? For example, if you were to spec out a cam for the build above assuming the 4" stroke and 4.030 bore, how would you figure out what the exact lift required is? I understand it has to do with the flow numbers and VE of the motor, but I can't find much information online regarding this.
Thanks.
Drew
First, welcome to the forum.
Matt makes some nice cams, so your most likely going to be happy. . .
Matt should answer some these questions also, even if he does not agree with me.
Maybe some real life experiences from his dyno, versus track performance. . . .
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Regarding all of your questions. . .
I don't think we will get this covered in one post..lol
First, you must determine how much HP you want.
That then tells you how much air / cfm the engine will require.
Generally speaking, with an efficient engine it takes about 1.5 cfm to produce 1.0 fwHP.
Again, generally speaking, once you understand how much air the engine will require, you have at least 9-variables your going to be dealing with. 1) Piston speed. 2) Cylinder Head Flow. 3) Cam Lift. 4) Cam Duration. 5) Cylinder Head Runner Volume. 6) Overlap Requirements. 7) ICL. 8) LSA 9) Compression Ratio.
1) Piston speed is linked to your engine rpm and stroke.
2) Valve lift is linked to cylinder head flow and volume of cylinder.
3) Lobe duration required is linked to piston speed.
4) Cylinder head runner volume is linked to overall air cfm / flow requirements.
5) Overlap requirements for an NA Engine depend on several different variables, such as cylinder head flow versus engine rpm, versus actual lobe area required. Compression is very important here.
6) Typically speaking with a Roots Blower, we use a larger cylinder head having more runner volume, as well as a larger Minimal Circular Square Area (MCSA).
7) ICL is mostly dependent regarding HP Requirements, cylinder head flow & velocity profiles, runner volume. Or, how soon can we, or how soon we desire to fill the cylinder.
8) The LSA requirement of a race engine is tied to the actual lobe area required, versus the velocity of the cylinder heads.
9) Static Compression Ratio required is tied to LSA, overlap, engine rpm, as well as VE% and fuel.
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Okay, one of your questions today. . . .
"Also, how does one calculate required lift for a cam? For example, if you were to spec out a cam for the build above
assuming the 4" stroke and 4.030 bore, how would you figure out what the exact lift required is? I understand it has to do with the flow numbers and VE of the motor, but I can't find much information online regarding this."
-simply put-
After understanding how much HP you require and at what engine rpm your engine would be required to be spun to, we would flow your heads. We would then understand how much 'useful' lift could be put into the engine. If we fall short of max lift requirement, we first attempt to fill in the gap with duration.
If everything fits, then fine. . . .
If not, we would desire to increase the flow at those higher lifts by porting the head accordingly.
We desire for the valve lift, duration, runner volume and MCSA, CFM & Velocity profiles to all work together at peak engine rpm and resultant piston speed, generated over the first ~74° of piston movement (ATDC) on the intake cycle.
The faster the piston moves; the more depression will be generated across the intake valve. If the piston moves too fast the cylinder head will then move into velocity choke.
If the piston moves to slow, the cylinder head flow might move into velocity stall.
We desire to see a constant velocity of air and fuel across / through the curtain area.
Cheers
