About a week ago I mentioned my review of original GM drawings revealed that the LT-1 inlet lobe eccentricity is exactly identical to the big block SHP mechanical lifter lobe eccentricity, which are the same for both inlet and exhaust. Other than having different base circles they are the same lobe! Also, the LT-1 cam uses the 30-30 lobe (again, they are the same for both inlet and exhaust) phased four degrees earlier.
In a somewhat related thought, I wondered if the LT-1 cam could be improved on an engine with massaged heads by using the inlet lobe on the exhaust side and further refining the phasing.
In OE form the "big port" (461. 462, 186, 492 et al.) SB heads have relatively restrictive exhaust ports with an exhaust to inlet flow ratio of about 65 percent. The "ideal" ratio is about 75 percent, and lobes of equal duration on both sides are generally best if the flow ratio in in this range. If less, a longer duration exhaust event is usually best via early opening, and if the ratio is higher than 75 percent, a shorter exhaust duration via later opening is usually best.
Flow tests of SHP heads indicate that the improvement on the exhaust side is relatively more than on the inlet side, so a good pocket port job with three angle seats and a top cut on the inlet valve will usually place exhaust flow in the range of 80-90 percent of inlet flow.
The use of the 30-30 exhaust lobe on the LT-1 cam, phased four degrees earlier, was dictacted by the relatively restrictive production exhaust port, but when the heads are reworked, the early opening exhaust valve is not necessary, and closing it later can improve low end torque with little or no sacrifice in top end power. You want cylinder pressure to work on the piston as long as possible, which dictates delaying opening, but at some point this will increase high rev exhaust pumping loss and reduce top end power. Somewhere there is an optimum point for every engine configuration and the better the exhaust port flow, the longer you can delay exhaust valve opening.
Since the various OE lobe profiles are known by the cam manufacturing industry, this opens the possibility for a "custom cam" using the proven reliable OE lobes that are easy on the valvetrain by mixing and matching them along with adjusting their phasing to detemine valve timing that maximizes peak power and torque bandwidth on a production SHP engine with well massaged heads, but NO OTHER CHANGES in order to maintain original external appearance. My investigation included the LT-1 lobe, 30-30 lobe, and Duntov exhaust lobe.
All the simulations (using the Engine Analyser program) were run modeling the OE 2.5" manifolds and pipes with low restriction mufflers that generate no more than about 3.0 psi exhaust backpressure at peak revs. The exhaust system configuration is a BIG factor in selecting valve timing, so these data and conclusions ONLY apply to the OE exhaust system. Headers with muffler and especially with open exhaust dictate different timing, particularly the inlet opening and exhaust closing that determine overlap, but since my philosophy is to leave the visual appearance unchanged, I didn't investigate the headers/mufflers/open exhaust case farther than necessary to determine that optimum overlap is usually different.
One of the general conclusions I have reached is that when using the OE manifolds and exhaust system, effective overlap must be limited to about 3.5 sq-in-deg or average output across the range is reduced.
The inlet valve closing point has the biggest impact on peak power. Closing it later shifts the torque curve up the rev scale, but this, of course, reduces torque at low revs. My basic criteria is to not have less than 80 percent of peak torque arrive by about 2000 (correcting for the 20 percent low value predicted by EA). The basic methodology is that once the inlet valve opening and exhaust valve closing (which determine overlap) are established, you delay the inlet opening until low end torque is at the lowest acceptable level, then experiment with the exhaust opening point to gain the last bit of torque bandwidth.
The Engine Analyser program tends to yield fairly accurate top end power, but is about 10 percent high on peak torque and 20 percent low on bottom end torque for SHP class engines, so I take this into account when computing torque bandwidth. I like EA because it has a number of good output diagnostics like inlet Mach index, effective valve overlap, exhaust/inlet flow ratio (which takes into account both flow and valve timing), and dynamic compression ratio. It also allows the user to specify rocker ratio, and I use my measured value of 1.44:1 (at peak lift). Actual rocker ratio varies throughout the lift cycle, but EA only allows a constant ratio.
So after experimenting around with lobes and phasing my conclusion is that the best combination is the LT-1 inlet lobe, retarded three degrees, combined with the Duntov exhaust lobe at its OE indexing. This provided both the best low end torque and the best top end power, but the differences between the various lobe combinations and phasing were so small relative to the LT-1 cam - averaging one percent and no more than two percent total range variation - that I don't think it would be worth pursuing, however, if anyone wants to proceed to building a custom SHP cam for an engine with pocket ported heads and production inlet and exhaust systems, the above description is probably the best starting point.
The above combination yields a fairly wide lobe center - much wider than than afermarket cams with similar duration, and it's always been my observation that aftermarket cams have too narrow lobe centers i.e. too much overlap. Perhaps they were developed only with headers and open exhaust, which is typical of lab dyno configurations - not the OE manifolds and street legal exhaust systems. It's also noteworthy that modern LS engines have cams with very little overlap, and less duration (but greater peak lift) than vintage SHP cams. They make their excellent torque bandwith and power via very high flowing heads and very efficient inlet and exhaust systems. The cams feature low overlap and modest durations with very aggressive flank acceleration profiles that are allowable with roller lifters and stiffer valvetrain components without having to use excessive spring force.
The basic approximate specs for the above cam are as follows:
Duration at .050" lifter rise (include ramps): 242/235
Duration at .050" lifter rise above the top of the clearance ramps: 229/222
(Note: When comparing a mechanical lifter cam duration at .050" to a hydraulic cam, use .050" above the top of the clearance ramp! The aftermarket DOES NOT do this, so mechanical lifter cams cannot be compared to hydraulic lifter cams. The mechanical lifter cams will always appear "bigger" than they really are depending on clearance ramp design. Hydraulic cams have very short/low clearance ramps, so their .050" lifter rise will usually convert to actual valve rise. Not so on mechanical lifter cams because a good deal of the .050" lifter rise above the base circle is clearance ramp.)
Inlet/exhaust "centerlines", LCA: 116 ATC/112.5 BTC, 114.25
(Note: The actual point of maximum lift for the inlet lobe is 113 degrees ATC since the LT-1 lobe has six degrees asymmetry. The above specified inlet phasing is three degrees retarded from the OE LT-1 inlet lobe. The Duntov lobe is symmetric and the point of maximum lift and centerline are coincident. Phasing is the same as the OE Duntov cam.)
Gross lobe lift: 0.3057"/0.2665"
I'm having some difficulty reconciling the various OE valve clearance specs with the actual geometry of the clearance ramps and may revise my recommended lash settings for both the 30-30 and LT-1 cams.
Duke
In a somewhat related thought, I wondered if the LT-1 cam could be improved on an engine with massaged heads by using the inlet lobe on the exhaust side and further refining the phasing.
In OE form the "big port" (461. 462, 186, 492 et al.) SB heads have relatively restrictive exhaust ports with an exhaust to inlet flow ratio of about 65 percent. The "ideal" ratio is about 75 percent, and lobes of equal duration on both sides are generally best if the flow ratio in in this range. If less, a longer duration exhaust event is usually best via early opening, and if the ratio is higher than 75 percent, a shorter exhaust duration via later opening is usually best.
Flow tests of SHP heads indicate that the improvement on the exhaust side is relatively more than on the inlet side, so a good pocket port job with three angle seats and a top cut on the inlet valve will usually place exhaust flow in the range of 80-90 percent of inlet flow.
The use of the 30-30 exhaust lobe on the LT-1 cam, phased four degrees earlier, was dictacted by the relatively restrictive production exhaust port, but when the heads are reworked, the early opening exhaust valve is not necessary, and closing it later can improve low end torque with little or no sacrifice in top end power. You want cylinder pressure to work on the piston as long as possible, which dictates delaying opening, but at some point this will increase high rev exhaust pumping loss and reduce top end power. Somewhere there is an optimum point for every engine configuration and the better the exhaust port flow, the longer you can delay exhaust valve opening.
Since the various OE lobe profiles are known by the cam manufacturing industry, this opens the possibility for a "custom cam" using the proven reliable OE lobes that are easy on the valvetrain by mixing and matching them along with adjusting their phasing to detemine valve timing that maximizes peak power and torque bandwidth on a production SHP engine with well massaged heads, but NO OTHER CHANGES in order to maintain original external appearance. My investigation included the LT-1 lobe, 30-30 lobe, and Duntov exhaust lobe.
All the simulations (using the Engine Analyser program) were run modeling the OE 2.5" manifolds and pipes with low restriction mufflers that generate no more than about 3.0 psi exhaust backpressure at peak revs. The exhaust system configuration is a BIG factor in selecting valve timing, so these data and conclusions ONLY apply to the OE exhaust system. Headers with muffler and especially with open exhaust dictate different timing, particularly the inlet opening and exhaust closing that determine overlap, but since my philosophy is to leave the visual appearance unchanged, I didn't investigate the headers/mufflers/open exhaust case farther than necessary to determine that optimum overlap is usually different.
One of the general conclusions I have reached is that when using the OE manifolds and exhaust system, effective overlap must be limited to about 3.5 sq-in-deg or average output across the range is reduced.
The inlet valve closing point has the biggest impact on peak power. Closing it later shifts the torque curve up the rev scale, but this, of course, reduces torque at low revs. My basic criteria is to not have less than 80 percent of peak torque arrive by about 2000 (correcting for the 20 percent low value predicted by EA). The basic methodology is that once the inlet valve opening and exhaust valve closing (which determine overlap) are established, you delay the inlet opening until low end torque is at the lowest acceptable level, then experiment with the exhaust opening point to gain the last bit of torque bandwidth.
The Engine Analyser program tends to yield fairly accurate top end power, but is about 10 percent high on peak torque and 20 percent low on bottom end torque for SHP class engines, so I take this into account when computing torque bandwidth. I like EA because it has a number of good output diagnostics like inlet Mach index, effective valve overlap, exhaust/inlet flow ratio (which takes into account both flow and valve timing), and dynamic compression ratio. It also allows the user to specify rocker ratio, and I use my measured value of 1.44:1 (at peak lift). Actual rocker ratio varies throughout the lift cycle, but EA only allows a constant ratio.
So after experimenting around with lobes and phasing my conclusion is that the best combination is the LT-1 inlet lobe, retarded three degrees, combined with the Duntov exhaust lobe at its OE indexing. This provided both the best low end torque and the best top end power, but the differences between the various lobe combinations and phasing were so small relative to the LT-1 cam - averaging one percent and no more than two percent total range variation - that I don't think it would be worth pursuing, however, if anyone wants to proceed to building a custom SHP cam for an engine with pocket ported heads and production inlet and exhaust systems, the above description is probably the best starting point.
The above combination yields a fairly wide lobe center - much wider than than afermarket cams with similar duration, and it's always been my observation that aftermarket cams have too narrow lobe centers i.e. too much overlap. Perhaps they were developed only with headers and open exhaust, which is typical of lab dyno configurations - not the OE manifolds and street legal exhaust systems. It's also noteworthy that modern LS engines have cams with very little overlap, and less duration (but greater peak lift) than vintage SHP cams. They make their excellent torque bandwith and power via very high flowing heads and very efficient inlet and exhaust systems. The cams feature low overlap and modest durations with very aggressive flank acceleration profiles that are allowable with roller lifters and stiffer valvetrain components without having to use excessive spring force.
The basic approximate specs for the above cam are as follows:
Duration at .050" lifter rise (include ramps): 242/235
Duration at .050" lifter rise above the top of the clearance ramps: 229/222
(Note: When comparing a mechanical lifter cam duration at .050" to a hydraulic cam, use .050" above the top of the clearance ramp! The aftermarket DOES NOT do this, so mechanical lifter cams cannot be compared to hydraulic lifter cams. The mechanical lifter cams will always appear "bigger" than they really are depending on clearance ramp design. Hydraulic cams have very short/low clearance ramps, so their .050" lifter rise will usually convert to actual valve rise. Not so on mechanical lifter cams because a good deal of the .050" lifter rise above the base circle is clearance ramp.)
Inlet/exhaust "centerlines", LCA: 116 ATC/112.5 BTC, 114.25
(Note: The actual point of maximum lift for the inlet lobe is 113 degrees ATC since the LT-1 lobe has six degrees asymmetry. The above specified inlet phasing is three degrees retarded from the OE LT-1 inlet lobe. The Duntov lobe is symmetric and the point of maximum lift and centerline are coincident. Phasing is the same as the OE Duntov cam.)
Gross lobe lift: 0.3057"/0.2665"
I'm having some difficulty reconciling the various OE valve clearance specs with the actual geometry of the clearance ramps and may revise my recommended lash settings for both the 30-30 and LT-1 cams.
Duke
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