panhandle
Contributor
Posts: 226
Reg: 08-01-07
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05-02-12 02:59 PM - Post#2221638
Can someone explain to me how a small circle cam can maintain a certain valve lift if is smaller in diameter than a standard cam.
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 grumpyvette
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
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05-02-12 03:38 PM - Post#2221645
In response to panhandle
http://forum.grumpysperformance.com/viewtopic.php?...
read this link
if you can,t read the link let me know please
| " " IF YOU CAN'T SMOKE THE TIRES FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK !" |
Edited by grumpyvette on 05-03-12 07:19 AM. Reason for edit: No reason given.
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 MikeB
Senior Member
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Loc: Plano, TX
Reg: 08-28-03
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05-02-12 06:05 PM - Post#2221706
In response to panhandle
Sure, lift at lobe is the difference between base circle diameter and the measurement across the widest part of the lobe. For a cam with .300" lobe lift, those numbers might be:
base circle: 1.200"
widest part of the lobe: 1.500"
OR on a small base circle cam: 1.100" and 1.400"
The smaller base circle, in this example, will give you an additional .050" rod-to-cam lobe clearance in a stroker motor. However, I guess the base circle could be offset giving you .100" clearance. Anybody here know if that's done?
"Higher lift cams also have smaller base circle diameters than stock lift cams."
In fact, on a dual pattern cam with more lift on the exhaust lobe, that lobe will have a smaller base circle. In a small block, that means all push rods can be the same length.
I think there must be some standard lobe dimension at 50% lift, because both low-lift and high-lift cams (within reason) can use the same push rod length and maintain proper rocker arm geometry.
Real Hot Rods have a Clutch!
1955 210 2dr: 327, Brodix IK180 heads, Jones cam, Muncie M20, Wilwood front brakes
1969 C-10 pickup: 350, TH350
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Edited by MikeB on 05-02-12 06:22 PM. Reason for edit: No reason given.
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busterrm
Contributor
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05-02-12 06:44 PM - Post#2221729
In response to panhandle
Can someone explain to me how a small circle cam can maintain a certain valve lift if is smaller in diameter than a standard cam.
Panhandle, small base circle cams are used to help with stroker clearance issues as Mike said. If you use one, make sure its a billeted core, they are much stronger and are far more unlikely to fail. The base circle of the cam isn't a issue if you have the correct length pushrod, once the valve is brought to zero lash and preload is applied you will not know any difference. Small base circles are mostly used in roller cams.
==== BOB ====
If I can't smoke the tires I want more!
1976 Chevy Nova
2007 Chevy 1500 1/2 ton
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Edited by busterrm on 05-02-12 06:46 PM. Reason for edit: No reason given.
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CNC BLOCKS N/E
Senior Member
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05-03-12 04:22 AM - Post#2221850
In response to busterrm
If you are using a .300 lobe or a .420 lobe the lifter still rises to the same height of the lifter bore but the .420 lobe uses a smaller base cirle cam then the .300 lobe.
The .420 lobe is a small base cirle cam but does nothing for cam to rod clearance. Now you take the .420 grind it to a smaller base cirle to lower the nose of the cam to clear the rod.
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MikeB
Senior Member
Posts: 9388
Loc: Plano, TX
Reg: 08-28-03
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05-03-12 07:53 AM - Post#2221920
In response to CNC BLOCKS N/E
Exactly. Just because a .420 cam has a smaller base circle than a .300 cam, that doesn't mean the .420 cam is a "small base circle" cam.
Are you sure the max lifter rise is the same on standard base circle cams with various lifts? That means lifters would sit much lower when a higher lift cam is on the base circle. Wouldn't that require changing pushrod length to keep roller tip to valve stem contact pattern centered?
Real Hot Rods have a Clutch!
1955 210 2dr: 327, Brodix IK180 heads, Jones cam, Muncie M20, Wilwood front brakes
1969 C-10 pickup: 350, TH350
My car pictures
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CNC BLOCKS N/E
Senior Member
Posts: 662
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Reg: 12-12-03
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05-03-12 09:26 AM - Post#2221958
In response to MikeB
Exactly. Just because a .420 cam has a smaller base circle than a .300 cam, that doesn't mean the .420 cam is a "small base circle" cam.
Are you sure the max lifter rise is the same on standard base circle cams with various lifts? That means lifters would sit much lower when a higher lift cam is on the base circle. Wouldn't that require changing pushrod length to keep roller tip to valve stem contact pattern centered?
Take any stand base cirle cam and put a straight edge from journal to journal you will see the lobe is about .010 shorter then the jounal. You tell me how high the liftet will be in the lifter bore???
I would have to say its going to be the same height between a .300 or .450 lobe!!!!!
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