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Thread: Selecting the camshaft. . .

  1. #1
    Senior Member
    Join Date
    Mar 2010
    Posts
    883
    Being a good ole southern boy I will keep this simple. Feel free to add ask questions at anytime. I will have to do this as time permits, but for some us, author included, I'm not boating so I have time. This is not the "ONLY WAY" or the RIGHT WAY. I just want to give you some basics on what to look at and how to go about making an educated guess on what stick needs to go in your engine
    Here goes.
    Most of us start out with basic combo in mind. For sake of this thread and an application we are all familar with we are going to use a 502 CID engine as our "Engine to Cam".
    We need first to select a Max RPM that way we can get our first basic calculation down. REQUIRED CFM FOR ENGINE. For most part we run about 5500 max so we will use it. The formula as follows:
    (CID/2) * 6500 rpm/1728 per cubic foot = need air flow
    (502/2)*5500/1728= 798.90
    Now we break that down to individual cylinders
    Total CFM/ No. of Cyl= CFM Requirement per cylinder.
    798.9/8cyl= 99.86 CFM
    Now we know how much CFM we need to feed our 502 at our max rpm. Next we will calculate the needed cylinder head flow and cam duration to achieve this.
    Any questions?

  2. #2
    Blown 472
    You mean you dont call for the latest trick out, trick o tha week wiz bang cam? now I am going to have to send mine back.

  3. #3
    Blown 472
    Being a good ole southern boy I will keep this simple. Feel free to add ask questions at anytime. I will have to do this as time permits, but for some us, author included, I'm not boating so I have time. This is not the "ONLY WAY" or the RIGHT WAY. I just want to give you some basics on what to look at and how to go about making an educated guess on what stick needs to go in your engine
    Here goes.
    Most of us start out with basic combo in mind. For sake of this thread and an application we are all familar with we are going to use a 502 CID engine as our "Engine to Cam".
    We need first to select a Max RPM that way we can get our first basic calculation down. REQUIRED CFM FOR ENGINE. For most part we run about 5500 max so we will use it. The formula as follows:
    (CID/2) * 6500 rpm/1728 per cubic foot = need air flow
    (502/2)*5500/1728= 798.90
    Now we break that down to individual cylinders
    Total CFM/ No. of Cyl= CFM Requirement per cylinder.
    798.9/8cyl= 99.86 CFM
    Now we know how much CFM we need to feed our 502 at our max rpm. Next we will calculate the needed cylinder head flow and cam duration to achieve this.
    Any questions?
    What about barometric pressure?

  4. #4
    Senior Member
    Join Date
    Mar 2010
    Posts
    883
    Blown, if you behave and are real good I may take boost and covert it for you to barometric pressure.
    Chris

  5. #5
    schiada96
    Chris
    I'm running a engle tc7 in my boat now I was running a crane 134241
    twin turbo draw thru. After changing cams the boat got lazy. Can you recomend a cam. 4.5 bore 4.0 stroke bbc 188 heads.21 ft schiada rc 45 gears in the v drive 11.375 x 16 p 3 blade menkins. The boat runs 93-95
    now at 5300 rpm
    Thanks
    Jeff
    Oh yeah it's making a little over 1bar of boost

  6. #6
    76BARRON
    just get a 3/4 race cam an FO- GETTA-BOU-DIT!

  7. #7
    Senior Member
    Join Date
    Mar 2010
    Posts
    883
    Well after that 2nd race looks like we will be busy at the shop. . .back to our "Selection".
    We are dealing with a 4 stroke engine so we all agree we have 720 degrees of rotation to complete all 4 cycles. The intake valve for the most part opens a little before TDC while the exhaust valve is closing. The intake continues to open during the intake stroke and after BDC for a smidgin. Now as the intake valve closes there is something that occurs called the RAM effect but we will discuss that later. . .someone just remind me.
    Okay so the time the valve is open is what we refer to as intake duration. Now we can input selected camshaft durations to figure CFM we need to feed our 502 at max rpm:
    cam duration / 720 degree = percent of time to get the air per revolution
    Any questions?

  8. #8
    Taylorman
    Great thread. Next lesson please.

  9. #9
    Taylorman
    (CID/2) * 6500 rpm/1728 per cubic foot = need air flow
    (502/2)*5500/1728= 798.90
    What do you mean by per cubic foot?

  10. #10
    Senior Member
    Join Date
    Mar 2010
    Posts
    883
    Okay, I do need to back up a little and give some credit. This "camshaft" math is what we named Ghetto Math coined by a fellow by the name of Erik K. Sharp guy and it comes from a thread that I was apart of on another board.
    Alright. . .now we can proceed. You now need 2 things. A camshaft book for reference and flow numbers off your heads. . . perferablly numbers from your heads but "printed" numbers will work.
    We will use a stand lobe that is 230 @.050"
    We will use the Canfield 310 head with the following flow numbers:
    Lift Int Exh
    .300 204 176
    .400 266 205
    .500 318 237
    .600 347 260
    Okay we have the following:
    (502/2)*5500/1728 = 798.88
    798.88/8 cylinders = 99.86 cfm per cylinder
    Now lets pick an intake lobe to try. Use the duration @ .50" I will use a cam lobe with 230 @ .050. By dividing the cam duration by the the total degrees of crank rotation of the 4 cycles we get the % of time we have to pack the cylinder for 100% VE
    230/720 = .31 (= percent of time to get the air per revolution)
    We now use the % of time we have calculated and divide the cylinder CFM requirement by it to come up with the intake port CFM flow that is required to fill our 502 at Max RPM
    99.86/.31 = 322 cfm (needed air flow divided by percent of time to get the air in per rev is cfm needed)
    So we need 322 cfm and the head at .500" cam lift is 318 CFM. Hey we are right there, but the question is do we run the boat at 5500 rpm all the time?
    Okay, I'll stop here for today. Any questions? Please post. Is this helpful? Let me know. No question is to stupid and I will answer to the best of my ability. Blown since you have been good I will give you the equation to convert boost to Barometric pressure: (PSI x 2.035789617) + 29.92
    Chris

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