This serves to illustrate for comparison only, some relative differences between 4 of the more popular SBC camshafts:
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:
https://www.forums.ncrs.org/showthrea...84229&uid=5383
post #18
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:
https://www.forums.ncrs.org/showthrea...84229&uid=5383
post #18
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