This serves to illustrate for comparison only, some relative differences between 4 of the more popular SBC camshafts:
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Re: Cam Quick Sheet
Joe, looking at your chart, the Duntov (110 lobe spacing) and the L79 (114 lobe spacing) have the least overlap and yield more compression (cylinder pressure) and should have the most bottom end. True?
The other two must be longer duration overall, yes?
Does closing the lobe centers spacing raise cylinder pressure on an after market profile (more area under the curve at .050" lift) that has less overall duration with faster rise/close rates such as Comp 270H grind compared to L79?
The Comp 270 (110 center spacing) has more at .050" but less overall duration vs the L79
Most charts list L79 cam intake and exhaust open and closing timing at full duration (.006") not at .050" which has been the industry standard. Do you have L79 I&E open/closing timing at .050" ? -
Re: Cam Quick Sheet
Gene,
At low RPM, valve overlap has no effect on cylinder pressure rise. At low RPM, pressure rise is strictly a function of when the inlet valve is considered effectively closed, which is generally @ J604D. At high RPM both intake and exhaust impulse tuning take over, so more valve overlap causes more charge to be rammed in and more exhaust to be sucked out by virtue of negative pressure waves traveling within the inlet and exhaust tracts. If high overlap cams are used with inefficient induction/exhaust systems, then all that overlap amounts to nothing more than charge dilution, with almost no pressure wave tuning effect. So, yes, the 097 and 151 will have more bottom end and less enhancement at top end.
Closing lobe centers, all else being equal creates more valve overlap, with the effects explained above. Valve overlap also causes the "lumpy" idle because of charge reversion at low RPM. It also causes an engine to develop less idle vacuum, for the same reason. All else being equal, a larger displacement engine will have more tolerance for narrower LSA's than smaller displacement engines.
Here's duration specs on those cams:
097 cam.............J604D: (270/270)................0.050" valve lift: (228/230)
151 (hyd)...........J604D: (290/290)................0.050" valve lift: (222/222)
178 cam.............J604D: (295/310)................0.050" valve lift: (242/254)
346 cam.............J604D: (310/310)................0.050" valve lift: (254/254)
For more about durations see:
post #18Last edited by Joe C.; February 6, 2011, 07:06 AM.Comment
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Re: Cam Quick Sheet
as i have posted before the older GM solid lifter high performance cams were designed for open exhaust because back then the racers had to use the "stock" camshaft but they could use open exhaust and GM wanted to make sure the racers had a advantage.Comment
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Re: Cam Quick Sheet
Joe,
Thank you for posting these comparisons.. Very helpful to see it all in one spot.
tcComment
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Re: Cam Quick Sheet
anyone ever see or use one of these ???Comment
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Re: Cam Quick Sheet
How many times have I explained this? You CAN'T directly compare .050" lifter rise timing/duration numbers for hydraulic lifter and mechanical lifter cams.
Theoretically all of the first .050" lifter rise for a hydraulic cam is converted into valve lift. On mechanical lifter cams the first .008 to .020" lifter rise is clearance ramp so the valve doesn't even move.
I've previously stated more than once what the lifter rise timing/duration, net of the clearance ramps, is for all the OE mechanical lifter camshafts, and these data are what can be used to compare to any hydraulic lifter cams, including OE.
Garbage in, garbage out. This "comparison" is not only worthless, it's totally misleading.
DukeLast edited by Duke W.; February 6, 2011, 03:39 PM.Comment
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Re: Cam Quick Sheet
See! There you go again making a blanket statement believing that no one will point out your error. Would that ".008" - .020"" statement apply to mechanical lifter cams whose clearance ramps are smaller/larger than .008/.020? Both lobes of the so called "30-30" cam perhaps? The exhaust lobe of your beloved LT1 cam?
Did you even bother to reference the linked post, because it is there that I explain the difference in "seat" timing between hydraulic and mechanical tappet cams? Your obsessive-compulsion to be obeyed unconditionally makes you leap before you look!
Your biggest problem is that you are poised to criticize others, blindly, for the sole purpose of trying to make yourself look like the only person on God's green earth who knows anything about anything.......in this case, camshaft theory.
I enjoy talking to you because you are one of the few people that I can have an intelligent and stimulating conversation with, when it comes to engine design. When I eventually make the mistake of believing that you have finally become an adult and realized that everything is not "all about you and you alone", then you disappoint me and make the predictably inane remarks again.
You're uncomfortable with the thought that there's somebody here who can check you when you get too far out of line. You prefer it when you have the podium all to yourself, like a disturbed high school teacher leading a group of impressionable and eager teenage girls who need a father image.Last edited by Joe C.; February 6, 2011, 09:36 PM. Reason: correction: change larger/smaller to smaller/largerComment
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Re: Cam Quick Sheet
Boy do you come on like a barking dog. Joe has shared information he has on cam specifications. Nobody is doing any comparison of one to the other (solid vs hydraulic).
Information is shared of the specifications not readily available on GM cam profiles as the GM profiles are not completely defined in standards used today.
Thank you very much and in the future please relax a bit before posting.Comment
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Re: Cam Quick Sheet
The .050" lifter rise specifications for all the OE GM mechanical lifter cams are readily available at the NAPA Web site for the Sealed Power/Speed Pro exact replacements.
Any combined listing of these specs - whether mechanical lifter cams only or mechanical and hydraulic cams invites comparisons, but such comparisons are apples and oranges, which can lead to erroneous conclusions.
Mechanical lifter durations - net of the clearance ramps - that can be used to make direct comparisons between the mechanical lifter cams or mechanical lifter cams and any hydaulic lifter cam are as follows.
In. dur./Ex.dur./In. POML/LSA
Duntov: 220/220/108.5/110.5
L-79: 222/222/110/114
30-30: 239/239/110/114
L-46: 224/224/114/114
LT-1: 231/239/110/116
L-72: 231/231/108/114
'67-up base engine: 194/202/108/116
McCagh Special: 202/194/116/116
LSA can only be used to compare overlap for cams of equal duration. The base engine and McCagh Special cams have a LSA of 116 deg. - same as the LT-1 cam, but the former two only have 0.9 sq-in-deg effective overlap and the LT-1 is 8.5 because of its much longer duration, and the difference is reflected in the idle behavior. The 30-30 cam has 11.5 sq-in-deg, which puts it into the "racing cam" ballpark, and is one reason why the 30-30 is so torque-shy below 3500-4000.
The LT-1 inlet lobe is the L-72 lobe on a slightly smaller base circle. The LT-1 exhaust lobe is the 30-30 lobe indexed four degrees earlier.
For those who love to hate the LT-1 cam - it doesn't leave much left to love.
DukeLast edited by Duke W.; February 7, 2011, 02:36 PM.Comment
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