Development of the Special 300 HP configuation was chronicled in a 2010 Corvette Restorer article that is posted on the Corvette Forum as a pdf attached to the following thread:
https://www.corvetteforum.com/forums...camshafts.html
I recommend you read it to understand the full story of the objectives and development of this configuration including other dyno tests. A primary objective was to increase top end power and revs of the 327/300 HP engine without affecting the strong low end torque and butter smooth 450-500 RPM idle in Neutral with a manual transmission and Drive with an automatic and without affecting visual appearance. The engine must also pass a NCRS Performance Verification test, which one did. The primary modifications are traditional "head massaging" to improve port flow, different valve timing to optimize performance with the new port flow characteristics, and optimizing the spark advance map for maximum detonation-free torque bandwidth.
This partcular L-75 is from a '64 Convertible that has been in the owner's family since 1984. The block is bored .030" and fitted with Federal Mogul L2166NF-30 forged pistons (SHP/FI type, 5.3 cc dome), and the compression ratio was limited to 10.2:1 with .041" thick composition head gaskets. The weak, first design 327 connecting rods were replaced with Eagle SIR5700 pressed pin rods that yield a reliable 7000 rev bottom end, and the original 3721SB AFB was rebuilt without modification to the OE rods and jets.
Typical Flint-machined 327 heads flow about 200/135 CFM at 28" H2O depression, 0.5" valve lift, yielding an E/I flow ratio of about 0.68. Typical "head massaging" techniques increase flow to the 220s/170s CFM range. These heads comfortablely met objectives with the following test results, CFM at 28" H2O depression.
Lift......... I flow........E flow.... E/I flow ratio
0.3"............185...........147.............0.7 9
0.4..............215...........173.............0.8 0
0.45............225...........179.............0.80
Note that with the actual true maximum rocker ratio of 1.44:1, maximum valve lift of OE cams is in the range of less than 0.400" to slighty less than 0.450".
Head massaging increases exhaust flow a greater percentage than inlet flow yielding a higher E/I flow ratio, and this value is important in establishing optimum valve timing. Typical OE and aftermarket camshafts have equal or greater exhaust duration due to the restictive exhaust port. Equal duration is best if the E/I ratio is near 0.75 and greater exhaust duraton than inlet if less than 0.75. E/I ratios greater than 0.75 will respond well to less exhaust than inlet duration with a late opening exhaust valve that lets cylinder pressure work on the piston longer. Like modern small block engines, the modest duration inlet event is retarded relative to traditional designs. In fact, other than less lift due to the flat tappets the McCagh Special camshaft inlet event has nearly the identical .050" lifter rise timing points as the LS3 camshaft!
The final WOT spark advance curve is as follows using the black springs from the Mr. Gasket 928G spring set, and a 15" B22 VAC is intalled in the original single point distributor along with a 28-32 oz. point set.
Revs....Total WOT advance
500............12
1500..........28
2500..........35
3000..........36 (maximum)
Attempts to increase total WOT advance further into the optimum range of 36-40 resulted in some low speed roughness. This is because the '64 distributor only offers 24 degrees centrifugal advance. For '66-67 total centrifugal advance was increased to 30 which allows less initial and low speed advance, and the '67 engine tested in the "Tale of Two Camshafts" article showed one percent better power across the range at 39 versus 36 degrees total WOT advance. The owner has 93 PON fuel commonly available, and the engine never detonated either in road testing or on the dyno.
Another significant difference between the early C2 327/300s ('63-'65) versus late ('66-'67) is the air cleaner. The later vesions' open element 14 x 3" filter offers virtually zero restriction. In fact the '67 tested actually showed a one percent decrease in power across the rev range when the air cleaner was completely removed. Those two silenceing snorkels on the early air cleaner sure look restrictive, and the dyno would quanify it.
The test plan called for two pulls with the air cleaner installed and if within about one percent established a baseline. Then at least one pull with the air cleaner lid removed. All pulls were planned to be 1000 to 6000 RPM (and we almost made it). Though the installed 215/70R-15 97W (168 MPH) Pirelli P6000s are about ten years old, they have spent virtually their entire life inside, look near new, and I was confident they would easily tolerate the near 140 MPH speed at 6000 with the 3.36:1 axle.
The Dynojet model 224C chassis dyno is a new model that replaces the 248C. The shop proprieter said the roller is smaller than the 248C, but the results are very comparable. The shop had a good external cooling fan setup, which gave confidence that the fan clutch would not tighten, which can cost considerable torque and power.
For reference, a typical Flint-built late 327/300 will make low to mid 190s SAE corrected RWHP at about 4500 and is all done at 5000. SHP/FI engines are in the range of 220-240 at higher revs. The test showed that the early air cleaner reduced peak torque by 3.2 percent and peak power by 6.1 percent compared to the run with the lid removed. The higher the air flow (RPM) the greater the loss. The owner is working on a simple and inexpensive solution to capture this lost power, and both the problem and solution are discussed in the following threads:
https://www.corvetteforum.com/forums...00-hp-327s.htm
https://www.corvetteforum.com/forums...p-engines.html
Perhaps more impressive than the L-79-like top end power with a smooth (500@ 18") idle is the prodigious low end torque, maybe even more than Flint-built. The objective was 90 percent of peak at 2000, and the test shows no later than 1200 and maybe even as low as 1000. Next time the pulls need to start at 700. The owner says this low end torque/power is even more useful and impressive than the additional top end power relative to his recollection of the original engine's characteristics.
A graph of the SAE corrected RWHP and torque (with and without the air cleaner lid installed) is attached, and a summarly follows. Without the air cleaner lid the pull is virtually identical to the '67 engine in the referenced article, at least within the narrow 3000-5000 RPM range that the '67 engine was tested. Getting off idle to redline pulls is tough for no logical reason.
90 percent torque bandwidth: <1200 to 4400, peak torque 281 lb-ft @ 3250
80 percent torque bandwidth <1000 to 5100
90 percent power bandwidth 3750 to 5800 (est.), peak power 217 HP @ 5100
Given the generally accepted driveline/tire efficiency of 0.85 for rear drive, manual transmissions with a direct drive gear, SAE net at the flywheel is 331 lb-ft and 255 HP. From my library of small block lab and chassis dyno tests my empirically derived net/gross factor is 0.89, which yields SAE gross estimates of 371 lb-ft and 287 HP. The advertised SAE gross ratings are 360 lb-ft @ 3200 and 300 HP @ 5000.
The best feature of this configuartion is the very broad torque and power bandwidths. It doesn't need a close ratio transmission. Wider ratios with a top gear that yields about 1500 at 60 would work just fine.
The glitch in the no air cleaner lid run is likely the secondary air valve opening of the AFB. The dyno system picks it up because it samples data at a very high frequency, but it happens too fast for the driver to feel. I've seen this with many other carbs. I don't know why the second baseline run does not show the same anomaly.
As a final note the old AFB showed a quite consistent WOT A/F ratio in the proper range - a bit rich down low, which is typical, finishing sufficiently rich to be below my 13.5:1 maximum requirement at peak revs. Note that removing the air filter lid leaned out the mixture about the same percentage as the increase in power due to the similar percentage increase in inlet air density, and that matches what physics tells us. Carburetors meter fuel based on air volume flow, not mass flow. That's why they run rich at high altitude if set up for low altitude.
Duke
https://www.corvetteforum.com/forums...camshafts.html
I recommend you read it to understand the full story of the objectives and development of this configuration including other dyno tests. A primary objective was to increase top end power and revs of the 327/300 HP engine without affecting the strong low end torque and butter smooth 450-500 RPM idle in Neutral with a manual transmission and Drive with an automatic and without affecting visual appearance. The engine must also pass a NCRS Performance Verification test, which one did. The primary modifications are traditional "head massaging" to improve port flow, different valve timing to optimize performance with the new port flow characteristics, and optimizing the spark advance map for maximum detonation-free torque bandwidth.
This partcular L-75 is from a '64 Convertible that has been in the owner's family since 1984. The block is bored .030" and fitted with Federal Mogul L2166NF-30 forged pistons (SHP/FI type, 5.3 cc dome), and the compression ratio was limited to 10.2:1 with .041" thick composition head gaskets. The weak, first design 327 connecting rods were replaced with Eagle SIR5700 pressed pin rods that yield a reliable 7000 rev bottom end, and the original 3721SB AFB was rebuilt without modification to the OE rods and jets.
Typical Flint-machined 327 heads flow about 200/135 CFM at 28" H2O depression, 0.5" valve lift, yielding an E/I flow ratio of about 0.68. Typical "head massaging" techniques increase flow to the 220s/170s CFM range. These heads comfortablely met objectives with the following test results, CFM at 28" H2O depression.
Lift......... I flow........E flow.... E/I flow ratio
0.3"............185...........147.............0.7 9
0.4..............215...........173.............0.8 0
0.45............225...........179.............0.80
Note that with the actual true maximum rocker ratio of 1.44:1, maximum valve lift of OE cams is in the range of less than 0.400" to slighty less than 0.450".
Head massaging increases exhaust flow a greater percentage than inlet flow yielding a higher E/I flow ratio, and this value is important in establishing optimum valve timing. Typical OE and aftermarket camshafts have equal or greater exhaust duration due to the restictive exhaust port. Equal duration is best if the E/I ratio is near 0.75 and greater exhaust duraton than inlet if less than 0.75. E/I ratios greater than 0.75 will respond well to less exhaust than inlet duration with a late opening exhaust valve that lets cylinder pressure work on the piston longer. Like modern small block engines, the modest duration inlet event is retarded relative to traditional designs. In fact, other than less lift due to the flat tappets the McCagh Special camshaft inlet event has nearly the identical .050" lifter rise timing points as the LS3 camshaft!
The final WOT spark advance curve is as follows using the black springs from the Mr. Gasket 928G spring set, and a 15" B22 VAC is intalled in the original single point distributor along with a 28-32 oz. point set.
Revs....Total WOT advance
500............12
1500..........28
2500..........35
3000..........36 (maximum)
Attempts to increase total WOT advance further into the optimum range of 36-40 resulted in some low speed roughness. This is because the '64 distributor only offers 24 degrees centrifugal advance. For '66-67 total centrifugal advance was increased to 30 which allows less initial and low speed advance, and the '67 engine tested in the "Tale of Two Camshafts" article showed one percent better power across the range at 39 versus 36 degrees total WOT advance. The owner has 93 PON fuel commonly available, and the engine never detonated either in road testing or on the dyno.
Another significant difference between the early C2 327/300s ('63-'65) versus late ('66-'67) is the air cleaner. The later vesions' open element 14 x 3" filter offers virtually zero restriction. In fact the '67 tested actually showed a one percent decrease in power across the rev range when the air cleaner was completely removed. Those two silenceing snorkels on the early air cleaner sure look restrictive, and the dyno would quanify it.
The test plan called for two pulls with the air cleaner installed and if within about one percent established a baseline. Then at least one pull with the air cleaner lid removed. All pulls were planned to be 1000 to 6000 RPM (and we almost made it). Though the installed 215/70R-15 97W (168 MPH) Pirelli P6000s are about ten years old, they have spent virtually their entire life inside, look near new, and I was confident they would easily tolerate the near 140 MPH speed at 6000 with the 3.36:1 axle.
The Dynojet model 224C chassis dyno is a new model that replaces the 248C. The shop proprieter said the roller is smaller than the 248C, but the results are very comparable. The shop had a good external cooling fan setup, which gave confidence that the fan clutch would not tighten, which can cost considerable torque and power.
For reference, a typical Flint-built late 327/300 will make low to mid 190s SAE corrected RWHP at about 4500 and is all done at 5000. SHP/FI engines are in the range of 220-240 at higher revs. The test showed that the early air cleaner reduced peak torque by 3.2 percent and peak power by 6.1 percent compared to the run with the lid removed. The higher the air flow (RPM) the greater the loss. The owner is working on a simple and inexpensive solution to capture this lost power, and both the problem and solution are discussed in the following threads:
https://www.corvetteforum.com/forums...00-hp-327s.htm
https://www.corvetteforum.com/forums...p-engines.html
Perhaps more impressive than the L-79-like top end power with a smooth (500@ 18") idle is the prodigious low end torque, maybe even more than Flint-built. The objective was 90 percent of peak at 2000, and the test shows no later than 1200 and maybe even as low as 1000. Next time the pulls need to start at 700. The owner says this low end torque/power is even more useful and impressive than the additional top end power relative to his recollection of the original engine's characteristics.
A graph of the SAE corrected RWHP and torque (with and without the air cleaner lid installed) is attached, and a summarly follows. Without the air cleaner lid the pull is virtually identical to the '67 engine in the referenced article, at least within the narrow 3000-5000 RPM range that the '67 engine was tested. Getting off idle to redline pulls is tough for no logical reason.
90 percent torque bandwidth: <1200 to 4400, peak torque 281 lb-ft @ 3250
80 percent torque bandwidth <1000 to 5100
90 percent power bandwidth 3750 to 5800 (est.), peak power 217 HP @ 5100
Given the generally accepted driveline/tire efficiency of 0.85 for rear drive, manual transmissions with a direct drive gear, SAE net at the flywheel is 331 lb-ft and 255 HP. From my library of small block lab and chassis dyno tests my empirically derived net/gross factor is 0.89, which yields SAE gross estimates of 371 lb-ft and 287 HP. The advertised SAE gross ratings are 360 lb-ft @ 3200 and 300 HP @ 5000.
The best feature of this configuartion is the very broad torque and power bandwidths. It doesn't need a close ratio transmission. Wider ratios with a top gear that yields about 1500 at 60 would work just fine.
The glitch in the no air cleaner lid run is likely the secondary air valve opening of the AFB. The dyno system picks it up because it samples data at a very high frequency, but it happens too fast for the driver to feel. I've seen this with many other carbs. I don't know why the second baseline run does not show the same anomaly.
As a final note the old AFB showed a quite consistent WOT A/F ratio in the proper range - a bit rich down low, which is typical, finishing sufficiently rich to be below my 13.5:1 maximum requirement at peak revs. Note that removing the air filter lid leaned out the mixture about the same percentage as the increase in power due to the similar percentage increase in inlet air density, and that matches what physics tells us. Carburetors meter fuel based on air volume flow, not mass flow. That's why they run rich at high altitude if set up for low altitude.
Duke
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