Just joined? Please introduce yourself.
Page 2 of 2 FirstFirst 12
Results 11 to 20 of 20

Thread: BBC build roller cam?

  1. #11
    Registered Member
    Join Date
    May 2012

    Member #:625
    Posts
    3,409
    Quote Originally Posted by 55 Tony View Post
    So I had the gear terminology backwards.

    With a high stall converter, will it have to rev to near the stall speed to take off easy or moderately from a stop? Or will it just rev higher when I get on it hard? That is a major deciding factor in cam selection. I don't care at all if it revs when I
    Old cam: http://www.compcams.com/Company/CC/c...?csid=388&sb=0
    New cam: http://www.compcams.com/Company/CC/c...?csid=444&sb=2
    New one has numerically lower lift, but with it being roller just how much of a step up is it? Or isn't it?
    I'm sure you'd rather see this one, but I really hate the high stall.
    http://www.compcams.com/Company/CC/c...?csid=445&sb=0
    Having once ridden in a truck with a high stall converter (don't know how high it was?) I hated that it had to rev to move out of it's own way. I've never driven anything with a high stall converter and keep asking the question:
    nail it, I just don't want it to have to rev taking off conservatively. Or am I looking for something they don't make?
    As the owner of 3 convertors of the 4 to 5 thousand stall range, it sounds like you are describing a slipping trans more than stall speed. They will move vehicle long before rated stall speed.

    Rickl is 100% correct about compression ratio, see my first reply. you need pistons more than a roller cam or aluminum heads.

  2. #12
    Registered Member
    Join Date
    Apr 2012

    Member #:571
    Posts
    4,671
    With a high stall converter, will it have to rev to near the stall speed to take off easy or moderately from a stop?
    No, as markm said. But it will slip more at cruising speed.

    Or will it just rev higher when I get on it hard?
    Yes.

    The difference between the first two cams is splitting hairs, there is simply not much difference.

    The 3rd cam is more aggressive.

    Seems to me that based on your expectations something smaller than the first two cams makes sense, along with a 2000 rpm stall speed converter.

  3. #13
    Registered Member
    Join Date
    Nov 2015

    Member #:2764
    Posts
    649
    Quote Originally Posted by Rick_L View Post
    No, as markm said. But it will slip more at cruising speed.



    Yes.

    The difference between the first two cams is splitting hairs, there is simply not much difference.

    The 3rd cam is more aggressive.

    Seems to me that based on your expectations something smaller than the first two cams makes sense, along with a 2000 rpm stall speed converter.
    I'm trying not to sound like an idiot but I'm afraid I'm failing badly.
    Maybe I described the high stall converter wrong, it was long long ago, like over 30 years ago. I guess what I meant is that there was a lot of slipping at lower rpms. To me it sounded like the motor was working and we were hardly moving uphill and it was at low speed. (it was on back roads in the mountains)
    I don't know how to take the last comment? Suggesting something smaller than I already have? No, I want more, my current cam (the first one) is in great condition, I want to change it to get more hp/torque. My current converter is supposed to be 2300 to 2500 stall speed, if it's actually as advertised, would it on fast idle at 2K going into reverse jerk and chirp the tires on a smooth garage floor? Sorry for my dumb sounding questions, I'm not a motor head and am trying to learn. If my current converter is acting as advertised, then I see no problem going a notch higher. Would going to 9.5:1 and that last cam be a worthwhile step up, maybe not for you, but for a rookie like me?

  4. #14
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    10,835
    I always thought that static compression ratio was independent of head size or cam specs. How does one affect the other? Both the heads and cam affect how much air you can get into the cylinder, but compression ratio only affects how much it's squeezed. Why does a particular head or cam require a different compression ratio?
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  5. #15
    Registered Member
    Join Date
    Nov 2015

    Member #:2764
    Posts
    649
    Quote Originally Posted by chevynut View Post
    I always thought that static compression ratio was independent of head size or cam specs. How does one affect the other? Both the heads and cam affect how much air you can get into the cylinder, but compression ratio only affects how much it's squeezed. Why does a particular head or cam require a different compression ratio?
    I believe static c/r is independent of the cam, and rod length. Static compression has to figure the total area of the combustion chamber, so it's highly dependent on the head profile and how much open area there is. More area, open heads = less compression.

  6. #16
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    10,835
    What I meant by "head size" was the size of the ports and valves, not the combustion chamber volume.

    I also don't understand why a particular cam has to have a particular stall speed converter. If it won't idle under 1000 RPM or has no torque down low, I get it...but I wouldn't get that much of a cam.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  7. #17
    Registered Member
    Join Date
    Apr 2012

    Member #:571
    Posts
    4,671
    Chevynut, the comments about compression ratio in so far in this thread are what you want, not what you have.

    Compression ratio is simply the ratio of the maximum volume and the minimum volume of the cylinder. The minimum volume is the volume with the piston at top dead center, and maximum volume is the volume at bottom dead center. The minimum volume is influenced most heavily by the combustion chamber volume, but it's also affected by the piston (popup or dish and valve clearance reliefs), the deck height, the head gasket thickness, and the volume formed by the piston and cylinder down to the top ring.

    The stall speeds are not what's required, but what's recommended and needed for performance. If you have a big cam that kills low end, you need a high stall converter to let you get into the range where you do make torque and power. It's not that it won't run without it, just that you won't get the full potential performance. Just like big cams kill street torque, high stall speeds slip a lot at street cruise conditions. You can go too far with both.

  8. #18
    Registered Member
    Join Date
    May 2012

    Member #:625
    Posts
    3,409
    matching stall speed to cam is more important than rear gear ratio.

  9. #19
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    10,835
    Quote Originally Posted by Rick_L View Post
    the comments about compression ratio in so far in this thread are what you want, not what you have.
    I was referring to COMP cam's recommended use for the came. Why do they recommend a particular compression ratio with a particular cam? That's what I didn't understand. COMP says:

    "Hydraulic-Great for Street Machines. Use headers and 9:1 compression. In 396-402 use 2500 stall, lower gears. Rough idle.:"

    Why does 9:1 work any better than 10:1? They seem to be tying the cam to a certain compression ratio.

    Compression ratio is simply the ratio of the maximum volume and the minimum volume of the cylinder.
    I know what compression ratio is.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  10. #20
    Registered Member
    Join Date
    Apr 2012

    Member #:571
    Posts
    4,671
    More compression ratio is always better as long as the fuel used will support it. Increased compression ratio increases performance under any operating condition. After around 13 or 14:1 the gains become close to asymptotic but they are still there.

    One of the consequences of a big cam is that it lowers cylinder pressure at low rpm because the valves are open so long. So when a cam maker lists a cam as being good for 9:1 compression, they are saying that it doesn't bleed down cylinder pressure at low speeds as severely as other choices.

    Dynamic compression ratio is another way of stating all this. It is an arbitrary measure of compression ratio and cam timing - an attempt to show both compression ratio need and how big the cam is. It's generally accepted that you don't want to drop much below around 8:1 dynamic compression ratio, and that around 8.5:1 is optimum for street performance. Smaller cams and higher static compression ratios are how you increase dynamic compression ratio.

Page 2 of 2 FirstFirst 12

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •