Re: 65 Fuelie - 11:1 compression - Octane Booster?
Using the 0.050" valve events to calculate DCR is meaningless because the piston is still generating significant pressure at that point in the stroke. It is common practice to use SAE J604D (AKA: "advertised" or "seat-to-seat" ) duration because pressure build effectively ends at this valve lift value, although the rate of valve closure will cause some variation in results as well. Durations are never measured at a tappet lift point; they are always measured at a valve lift point and SAE J604D is 0.006" for a mechanical tappet and 0.004" for a hydraulic tappet. To further complicate matters, different manufacturers use different timing points in which to define their cams, and one very popular one is 0.015" valve lift.
That chart is a guide and is useful in noting the relative intake valve closing points of the more popular vintage SBC cams, especially considering the fact that the rate of closure impacts the rate of pressure build and causes a certain amount of leeway around using SAE J604D as the calculation point. The absolute points must be considered in light of what I mentioned above, especially valve actuation rates at low lift. I built mine with an honest 11.3:1 SCR and have no issues. The engine develops between 230 and 240 psig during a "warm, dry" cranking pressure test. The engine in its present form uses a tight lash/fast ramp solid roller cam of 286/292 (@ 0.015") 248/254 (@ 0.050") and 0.604"/0.608" lift with 1.6:1 full roller trunnion rocker arms.
In your case, if your block has not been decked then you have a whopping 0.025" +/- 0.010" (nominal deck) + 0.038" gasket = 0.063" +/- 0.010" quench, which will result in much earlier detonation (since detonation begins in the quench zone) than an identical engine built with a more acceptable quench of 0.040. SBC were originally designed with 0.025" (nominal) + .021" = .046" quench. Mine is built with - 0.004" + 0.038" = 0.034" quench. Retarding the cam will NOT affect your engine's idle , at all, since idle vacuum is affected by valve overlap, valve lift and valve actuation speed (solid tappet lash affects both duration and valve lift, as does "zero lashing" your hydraulic cam). I don't recommend retarding your cam unless you compensate by adding more static compression. Retarding the cam by 4 degrees will result in the intake valve closing 4 degrees later, which will decrease your engine's dynamic stroke and thus its DCR. I recommend retarding the cam only in instances where the subject engine suffers from detonation, and decreasing SCR is not an option.
Using the 0.050" valve events to calculate DCR is meaningless because the piston is still generating significant pressure at that point in the stroke. It is common practice to use SAE J604D (AKA: "advertised" or "seat-to-seat" ) duration because pressure build effectively ends at this valve lift value, although the rate of valve closure will cause some variation in results as well. Durations are never measured at a tappet lift point; they are always measured at a valve lift point and SAE J604D is 0.006" for a mechanical tappet and 0.004" for a hydraulic tappet. To further complicate matters, different manufacturers use different timing points in which to define their cams, and one very popular one is 0.015" valve lift.
That chart is a guide and is useful in noting the relative intake valve closing points of the more popular vintage SBC cams, especially considering the fact that the rate of closure impacts the rate of pressure build and causes a certain amount of leeway around using SAE J604D as the calculation point. The absolute points must be considered in light of what I mentioned above, especially valve actuation rates at low lift. I built mine with an honest 11.3:1 SCR and have no issues. The engine develops between 230 and 240 psig during a "warm, dry" cranking pressure test. The engine in its present form uses a tight lash/fast ramp solid roller cam of 286/292 (@ 0.015") 248/254 (@ 0.050") and 0.604"/0.608" lift with 1.6:1 full roller trunnion rocker arms.
In your case, if your block has not been decked then you have a whopping 0.025" +/- 0.010" (nominal deck) + 0.038" gasket = 0.063" +/- 0.010" quench, which will result in much earlier detonation (since detonation begins in the quench zone) than an identical engine built with a more acceptable quench of 0.040. SBC were originally designed with 0.025" (nominal) + .021" = .046" quench. Mine is built with - 0.004" + 0.038" = 0.034" quench. Retarding the cam will NOT affect your engine's idle , at all, since idle vacuum is affected by valve overlap, valve lift and valve actuation speed (solid tappet lash affects both duration and valve lift, as does "zero lashing" your hydraulic cam). I don't recommend retarding your cam unless you compensate by adding more static compression. Retarding the cam by 4 degrees will result in the intake valve closing 4 degrees later, which will decrease your engine's dynamic stroke and thus its DCR. I recommend retarding the cam only in instances where the subject engine suffers from detonation, and decreasing SCR is not an option.
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