small base circle cams

Discussion in 'Cams, Heads and Valve Trains' started by grumpyvette, Nov 2, 2010.

  1. grumpyvette

    grumpyvette Administrator Staff Member

    hey grumpyvette ,
    you still around? i would like to ask you about a small base circle cam.I just bought one and measured the lobes on it compared to my other cam which is a edelbrock HR 234/238 duration @ 50
    and lift 539/548 ,the new cam is a extreme HR 242/240 duration @ 50,and the lift is 540/562.And the lobes seem to be a little smaller then the edelbrock cam,which is suppose to be smaller in duration. and lift.I don,t understand this unless it has something to do with small base circle cam.Its for stroker engines which you probably know already.I thought just the base circle of the cam would be smaller.Can you explain this to me? No body on CF could really answer it for me.





    lift on the valve is controlled by the distance the lifter travels between the cams base circle to the peak lift on the cam lobe, small base cams lift the lifter the same distance as the standard cams, but the smaller base circle allows the cam lobes distance from the cams center-line to be less to provide more cam lobe to connecting rod clearance.
    a small base cam may have a .900 inch base circle the standard cams will be closer to a 1.060 but several manufacturers use a 1.125" to 1.150" base circle. keep in mind theres several different cam core materials and heat treatments used by various manufacturers
    [​IMG]
    [​IMG]
    your cams lift, is the result of the lifter movement, or distance it travels from the cams base circle, where the valves seated, to the point in the cams rotation where the lifters moved along the ramp surface fully up on the nose of the cam lobe where the valves at full lift.
    example
    lets say in this case we compare two imaginary cams
    a standard cams base circle is 1.125" and
    your cams running on a .900 base circle
    both cams have a .560 valve lift and run with 1.5:1 rockers
    so both cams will need to move the lifter .374"
    that means the standard cam lobe will be 1.125"+.374" or 1.499" from the cams base to the cam lobe nose
    that means the small base cam lobe will be .900"+.374" or 1.274" from the cams base to the cam lobe nose
    which is significantly smaller,
    small base circle cams are generally only used when connecting rod clearance necessitates there use

    [​IMG]
    [​IMG]

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    your correct, its the circle the cam lifter seats on when the lifters NOT on the lobe and the valves seated, but keep in mind the lobe lift will remain the same for a given valve lift, so reducing the distance from the rotational center-line of the cam to the base circle also reduces the diameter of the arc the cam lobe makes as it rotates, so reducing the base circle effectively reduces the diameter of the rotating cam, ,lobes nose, giving greater clearance to the rods and crank as they rotate, past each other.
    when you go to a small base cam a billet core is almost mandatory
    [​IMG]

    Base Circle, or the heel, is the round portion of the cam lobe. This is where the lifter rides while the valve is closed. A high spot in this area is called Base Circle Runout. If the runout is more than .001 on hydraulic lifter cams the valve will be off of its seat while the lifter is on the runout area. Poor performance and burnt valves will result from this. Small Base Circle Cams have the lobes ground down to the core diameter to give extra clearance for connecting rods used on stroker cranks. Higher lift cams also have smaller base circle diameters than stock lift cams.

    Billets and Cores are the blank shafts that the camshafts are made from. Cast Cores and Proferal Iron Billets are used for most flat tappet camshafts. Steel Billets are used for roller tappet camshafts.




    Ok i understand that ,so i only thing to make up the difference would be longer pushrod,right? I was thinking the lobe part of the cam that i want to run would be bigger then the old cam,and that that base circle was just the part of the cam that was smaller.Its the whole cam lobe thats smaller and you make up the difference with longer push rod.Is that right or am i still off?




    yes its common for a longer push rod to be required but thats NOT always required, because in a few cases the rocker geometry allows you to just take up the clearance with the rocker adjustment on the rocker stud threads, but youll ALWAYS need to verify the geometry



    greg_moreira posted this info
    verify during pre-assembly that you lifters match the lifter bore and oil passage geometry,
    the main issue, although even with a .900 base...most lifters are fine. The problem is if the oil "band" drops low enough in the lifter bore that it actually comes out the bottom and just allows oil to leak out into free space(not inside the lifter bore). That creates a pretty large internal leak/oil pressure drop.

    Ive also heard reported that in some cases the band is located low enough that it doesnt line up well with the internal oil passages...thereby creating a restriction for other lifters in sequence.

    Either way...its not something you wanna deal with(finding out what happens if the lifter is too short), but usually not a problem with a .900. I cant speak for all lifters out there...but the typical crower or isky lifters work well on a .900 without issue. There may be other brands that dont have as much luck just depending on where the band is. Easy to check though. Just put the lifter on the lobe at the base circle and see where it ends up!

    comp sells a lifter that is stock height...but the band is moved to compensate for the smaller base circle. Not much of a fan of most of their stuff though. I like the pressurized oiling to the needle bearings as opposed to most of theirs which only directs oil to the sides of hte lifter wheel.

    I've heard their highest end lifters now have oiling directly to the axle/needle bearings, but cant say for sure on that
    ."
    [​IMG]


    [​IMG]
    rods that use bolts with nuts like pictured below will be weakened if excessively clearance ground
    [​IMG]
    stroker profile rods offer more clearance to cam lobes, and yes the stroker clearanced profile rods are available in both (h) and (I ) beam designs
    [​IMG]
    [​IMG]

    generally its a minor easily done clearance job
    [​IMG]

    if you wonder why I suggest using SCAT (H) beam style cap screw connecting rods vs stock or most (I) beam designs this picture should show the increased cam to connecting rod clearance

    bits of related info you'll need

    viewtopic.php?f=50&t=1249

    viewtopic.php?f=52&t=181

    viewtopic.php?f=50&t=428

    viewtopic.php?f=44&t=38

    viewtopic.php?f=52&t=2746

    viewtopic.php?f=52&t=1376

    viewtopic.php?f=52&t=126

    viewtopic.php?f=52&t=697

    viewtopic.php?f=55&t=4590

    viewtopic.php?f=44&t=700

    viewtopic.php?f=69&t=519
     
    Last edited by a moderator: Nov 29, 2017
  2. grumpyvette

    grumpyvette Administrator Staff Member

  3. kelley555

    kelley555 Member

    Grumpy,great explanation of the small base circle cams.The best I heard.I have exprience with the Edelbrock hyd. roller cam #2201 or 2204, I'll share it . I used this cam to check rod to cam interference in my 400ci (3.75" stroke).Edelbrock advertises 283-400,so their thinking either Gm zz383 rods(stroker clearanced) or a smaller rod for the 3.75 strokes.The base circle on the lobes are 1."115 & 1."125,smaller,but not as small as the 1.050 or the .900 base circle cams.I've got it logged in my records which lobe is which.Even using the lighter I-beam type 4340 rod provided it provides the same width at the interference point as the Eagle 4340 type h-rod.The rods, have to be barely clipped for interference.That is what a stroker clearanced rods is.The rods have to be clipped in the clearancing stage to prevent the balance of the motor messed up. I checked the width & radius of a set of Callies Compstar rods;SGI /Eagle type 4340 h-rod & the SGI 4340 I-beam(used for lighter targeted bob wts. for balancing ),all are about the same.My options were:#1 if I had not previously balanced my assemble & spiral locked the rods to the pistons,I could have clipped the rods before I balanced the assemble & not used such the small .900 small base circle cam that effects the valve train geometry plus strength of the cam.The new heads of today,one better check how the rockers are running.Small base cams,decking the block & head ht. affect the pushrods length.The Edelbrock cam had the same specs,maybe for a later model cam.It made plenty,514hp in a 10:1 383ci 1x 4 Afr headed street engine.Check Gilbert Chevy crate motor specs.It's not a billet core,but a lot of companies use the "austemper" core on their small hyd.roller cams for the street.You have to keep a little closer eye on this cam,but it's affordable,watch the spring psi on these cams,you can regulate it & get slightly higher psi you need to make the hydraulic roller cams work. #2 use a .900 base circle cam,the down side the cam HAS TO BE A BILLET core cause(expensive) or it will flex, & it'll be weaker.This gives you adequate rod to cam clearance,even when if a rod bearing spins & eats the bearing.A bigger base circle cam can allow the cam to hit the rodsif the clearances are tight.Can you say !Grenade!Later Jr
     
  4. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member


    lets look at this
    you may not like the answers, but I doubt you can dispute the logic,
    the answer simply comes down to making an informed choice as too, what system of maintaining your valve train stability is most dependable and durable

    lifter's, in the block lifter bores , ride on the cam lobes and the dog bone style retainers and those lifter while less expensive, would not be my personal choice.
    lift on the valve is controlled by the distance the lifter travels between the cams base circle to the peak lift on the cam lobe, small base cams lift the lifter the same distance as the standard cams, but the smaller base circle allows the cam lobes distance from the cams center-line to be less to provide more cam lobe to connecting rod clearance.
    a small base cam may have a .900 inch base circle the standard cams will be closer to a 1.060 but several manufacturers use a 1.125" to 1.150" base circle. keep in mind theres several different cam core materials and heat treatments used by various manufacturers
    your cams lift, is the result of the lifter movement, or distance it travels from the cams base circle, where the valves seated, to the point in the cams rotation where the lifters moved along the ramp surface fully up on the nose of the cam lobe where the valves at full lift.
    example

    lets say in this case we compare two imaginary cams
    a standard cams base circle is 1.125" and
    your cams running on a .900 base circle
    both cams have a .560 valve lift and run with 1.5:1 rockers
    so both cams will need to move the lifter .374"
    that means the standard cam lobe will be 1.125"+.374" or 1.499" from the cams base to the cam lobe nose
    that means the small base cam lobe will be .900"+.374" or 1.274" from the cams base to the cam lobe nose
    which is significantly smaller,
    small base circle cams are generally only used when connecting rod clearance necessitates there use

    [​IMG]
    [​IMG]

    will they work? yes they will, but the stock lifter design and spring spider retainer was not designed for lifts over about .530 nor rpms over about 5500 rpm, as always a bunch of research helps long term durability, Ive seen more than once engine have issues using them,and the link bar lifer design is the preferred route.

    LESS THAN IDEAL
    [​IMG]
    [​IMG]


    YES
    [​IMG]

    use of a small base circle cam allows more connecting rod to cam lobe clearance but it also makes the lifters sit a bit lower in the block lifter bores thus the upper edge of the lifter has a bit more chance of lifting the lifter retainer or dog bone keeping the lifters roller centered on the cam lobes.
    look at the pictures in these threads I linked , and sub-linked too.



    just read the linked info, look at the pictures and ask the cam company you,ll buy the cam from their advise,
    on what lifters , and valve springs,
    they would suggest be matched too the cam,
    you've selected and installed,
    that will provide the best long term durability,
    and valve train stability and make a logical choice,

    but keep in mind that it gets very expensive very rapidly if your lifter on that cam lobe does not stay in its intended alignment on the cam lobe or the valve spring allows that lifter to loft or float and loose contact with the cam lobe surface at higher rpms,
    your very likely to find that if you are building a SBC engine with a 3.75" or greater stroke crank, and if you ask any cam manufacturer for a cam suggestion, they will most likely suggest a small base circle cam design,
    Id suggest this is almost universally done as they are simply not aware of your particular engines current internal engine rotating assembly too cam clearances between the cam lobes and the connecting rods,but are very familiar with the fact that the vast majority of people who order cams either are totally un-aware of how to accurately check internal rotating assembly clearances,or they correctly assume many people will simply rotate the engine and if they don,t feel anything bind, will assume there.s no clearance issues.
    and are dealing from long experience, gained from dealing with less than knowledgeable customers, thus taking the theoretical route that provides the lowest chance for you to run into clearance issues by suggesting use of the smaller base circle cam design.



    yes theres a few links and sub links...
    DON,T IGNORE THEM!
    reading them could save you hundreds of dollars,
    and weeks of wasted effort


    http://garage.grumpysperformance.co...er-lifter-install-direction.11398/#post-52208

    http://garage.grumpysperformance.co...ulic-roller-lifter-selection.5522/#post-16620

    http://garage.grumpysperformance.co...rect-valve-spring-load-rates.4680/#post-12650

    http://garage.grumpysperformance.com/index.php?threads/small-base-circle-cams.3810/

    http://garage.grumpysperformance.co...rect-custom-length-pushrods.14241/#post-72353

    http://garage.grumpysperformance.co...et-lifters-and-cam-core-specs.2166/#post-5840

    http://garage.grumpysperformance.co...ng-pressures-don-t-work-well.1489/#post-16234

    http://garage.grumpysperformance.co...train-clearances-and-problems.528/#post-57678

    http://garage.grumpysperformance.co...cost-vs-a-flat-tappet-design.3802/#post-54090

    http://garage.grumpysperformance.co...t-for-specific-applications.10162/#post-40008

    http://garage.grumpysperformance.com/index.php?threads/cam-wear-articles-you-need-to-read.282/

    http://garage.grumpysperformance.com/index.php?threads/valve-train-clearances-and-problems.528/

    http://garage.grumpysperformance.co...ring-installation-questions.12833/#post-66460

    http://garage.grumpysperformance.co...e-springs-and-setting-up-the-valve-train.181/

    http://garage.grumpysperformance.com/index.php?threads/checking-piston-to-valve-clearances.399/

    http://garage.grumpysperformance.co...ng-pressures-don-t-work-well.1489/#post-36984
     
    Last edited: Nov 28, 2017
  5. Indycars

    Indycars Administrator Staff Member

    I'm not sure which Scat rods you are using, but I have their standard weight forged rotating assembly
    and the same Crower 00471 cam and their lifters in a Dart SHP block. Now this is about 4 years old
    now, so I can't be sure they have not changed something. You can see from the photos that I had plenty
    of clearance.

    Found my purchase documentation. I ordered the following ....

    Scat Forged Crankshaft CH383RM16, 4-350-3750-6000 1,419.80 4340 Forged, Std Weight
    Forged Connecting Rod 2-ICR6000-7/16, Bushed, ARP 8740 7/16” x 1.400 cap screws for stroker clearance

    CamToRodClearance1649.jpg

    CamToRodClearance1650.JPG

    FinishedRods_00342.jpg

    CamInBlock01_2370.jpg
     
    Last edited: Nov 28, 2017
  6. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member

    thanks for re-posting the great related pictures
     
    Shane oakhill likes this.
  7. Shane oakhill

    Shane oakhill Phat 49

    Thanks Indycars, I was slowly going through your build to verify my thoughts. I have the scat Stroker clearances 5.7 rods and the current lunati standard cam works fine. I am going to use the crower tie bar lifters which although a bit more expensive seem to be high quality which is fine with me, ...do it right, do it once , pay once!! I am interested to know what thoughts are about this cam, my build is very similar to yours. My truck is 3500 lbs with me in it but it’s not a daily driver. I know grumpy doesn’t like it under a certain rpm , do you have a clip of your t bucket idling or driving? Thanks in advance
     
    Last edited: Nov 30, 2017
  8. Indycars

    Indycars Administrator Staff Member

    I'm a lot lighter than you at about 1800 lbs. I can give up some low end, to gain some at the
    higher rpms since the tires are spinning most of 1st gear. Next time I might move up to a
    little hotter cam. If I did it would require a higher stall speed converter than the 2800 rpm
    I have now. Which would also require a better trans cooler ...... it's the domino effect.

    From idle at 800-850 rpm, it's instantly more than the MT tires can handle, but at 3200 rpm
    it's starting the sweet spot and up to 6200 rpm.

    I"ve got about 30 YouTube videos. Here are a three for your enjoyment.





     
    Shane oakhill likes this.

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