Subject engine began life as a 283/250 FI in a chassis with a Powerglide transmission and 3.70 axle. All major engine components - block, heads, FI system are original to the car. Considering the PG transmission, the overall objective was to create a "torque monster" with top end power a secondary consideration.
The block was bored to .060" over and a 3.48" stroke 350 crankshaft was installed for a displacement of 339 CID. Custom pistons were made for the 6-inch rods and the final CR is approximately 10:1.
The OE heads were massaged including installation of 1.84" valves on the inlet side. Flow testing revealed 173/135 (I/E ) CFM at 0.5" valve lift, 28" H20 test depression for computed isentropic flow efficiencies of (I/E) 46.6/47.3 percent
I felt both sides had potential for more flow with additional work, but the owner did not want to risk these rare OE heads with any more material removal.
The remaining issue was to develop valve timing that yields the flattest possible torque curve from the 1800 PG stall speed to the established design speed of 5500 RPM. As is typical with massaged heads the E/I flow ratio increased over OE indicating a need for greater inlet than exhaust valve duration. Consideration was given to using the Special 300 HP camshaft, but given the Powerglide, shorter duration was deemed necessary to maximize torque at converter stall speed.
Looking at avaliable OE lobes, the 3896929 "300 HP cam" showed promise by merely swapping the lobes to yield (I/E) 202/194 degrees duration at .050" lifter rise compared to 194/202 for the 929.
Initial lobe indexing established the EVC and IVO points the same as the 929 cam, which maintains the same 112 degree LSA and effective overlap to achieve OE idle quality/characteristics. The inlet POML was then varied to yield the flattest torque curve possible as predicted by Engine Analyzer 3.0.
The final POML indexing was established at 116 deg. ATDC on the inlet side and 108 deg. BTDC on the exhaust side compared to 108/116 for the 929. In hot rod lingo this cam would be considered "4 degrees retarded" and the 929 "4 degrees advanced".
The configuration was simulated at both SAE net and SAE gross conditions. Peak SAE gross torque was predicted at about 370 lb-ft near 3500 with a very flat curve and about 320 gross HP at 5000. The OE 929 cam showed nearly identical results up to about 4500 where it flattened out. Above 4500 the special camshaft pulled away - power continued to increase up to 5000 and was virtually flat to 5500.
An actual lab dyno test to SAE gross conditions yielded 364 lb-ft at 3500 and 316 HP at 5000, which is a mean piston speed of 2900 fpm - equivalent to 5800 revs on a 283 and 5350 on a 327. Unfortunately no pulls were made from 1500 (I again remind all to start pulls at 1500 on either lab or chassis dyno tests, and you may have to cajole the operator to do this as it is outside their typical "race engine" thinking.), but given the shape of the torque curve in the 3100-5000 test range, it likely makes close to 90 percent peak torque at converter stall speed. Torque at 5000 was an impressive 91 percent of peak. The curve is very flat and usable power should extend to 5500+.
The test was made with headers, but given the low valve overlap, they likely had little effect. In fact, with a low overlap cam, manifolds might actually make more top end power as the additional friction of routing each cylinder's exhaust gas through the long and small primary pipe creates more friction than can be offset with the modest wave dynamics harnessing that is achievable at high revs with a low overlap cam.
SAE net output as installed in the car with the OE exhaust, fixed steel fan, and SAE net air density correction should be near 340 lb-ft/275 HP with the same fat 90 percent torque bandwidth. As is typical of the era, the SAE gross ratings are likely overstated, and I estimate the actual SAE net output of the OE 283/250 FI engine at about 270 lb-ft and 200 HP, so this configuration should provide very spirited performance while maintaining OE appearance and idle characteristics.
This project is another data point proving that excellent performance can be achieved with vintage engines using vintage components to maintain vintage appearance and easily recognizable operating characterisitics such as idle speed and quality - a real "sleeper".
It's all in the details!
Duke
The block was bored to .060" over and a 3.48" stroke 350 crankshaft was installed for a displacement of 339 CID. Custom pistons were made for the 6-inch rods and the final CR is approximately 10:1.
The OE heads were massaged including installation of 1.84" valves on the inlet side. Flow testing revealed 173/135 (I/E ) CFM at 0.5" valve lift, 28" H20 test depression for computed isentropic flow efficiencies of (I/E) 46.6/47.3 percent
I felt both sides had potential for more flow with additional work, but the owner did not want to risk these rare OE heads with any more material removal.
The remaining issue was to develop valve timing that yields the flattest possible torque curve from the 1800 PG stall speed to the established design speed of 5500 RPM. As is typical with massaged heads the E/I flow ratio increased over OE indicating a need for greater inlet than exhaust valve duration. Consideration was given to using the Special 300 HP camshaft, but given the Powerglide, shorter duration was deemed necessary to maximize torque at converter stall speed.
Looking at avaliable OE lobes, the 3896929 "300 HP cam" showed promise by merely swapping the lobes to yield (I/E) 202/194 degrees duration at .050" lifter rise compared to 194/202 for the 929.
Initial lobe indexing established the EVC and IVO points the same as the 929 cam, which maintains the same 112 degree LSA and effective overlap to achieve OE idle quality/characteristics. The inlet POML was then varied to yield the flattest torque curve possible as predicted by Engine Analyzer 3.0.
The final POML indexing was established at 116 deg. ATDC on the inlet side and 108 deg. BTDC on the exhaust side compared to 108/116 for the 929. In hot rod lingo this cam would be considered "4 degrees retarded" and the 929 "4 degrees advanced".
The configuration was simulated at both SAE net and SAE gross conditions. Peak SAE gross torque was predicted at about 370 lb-ft near 3500 with a very flat curve and about 320 gross HP at 5000. The OE 929 cam showed nearly identical results up to about 4500 where it flattened out. Above 4500 the special camshaft pulled away - power continued to increase up to 5000 and was virtually flat to 5500.
An actual lab dyno test to SAE gross conditions yielded 364 lb-ft at 3500 and 316 HP at 5000, which is a mean piston speed of 2900 fpm - equivalent to 5800 revs on a 283 and 5350 on a 327. Unfortunately no pulls were made from 1500 (I again remind all to start pulls at 1500 on either lab or chassis dyno tests, and you may have to cajole the operator to do this as it is outside their typical "race engine" thinking.), but given the shape of the torque curve in the 3100-5000 test range, it likely makes close to 90 percent peak torque at converter stall speed. Torque at 5000 was an impressive 91 percent of peak. The curve is very flat and usable power should extend to 5500+.
The test was made with headers, but given the low valve overlap, they likely had little effect. In fact, with a low overlap cam, manifolds might actually make more top end power as the additional friction of routing each cylinder's exhaust gas through the long and small primary pipe creates more friction than can be offset with the modest wave dynamics harnessing that is achievable at high revs with a low overlap cam.
SAE net output as installed in the car with the OE exhaust, fixed steel fan, and SAE net air density correction should be near 340 lb-ft/275 HP with the same fat 90 percent torque bandwidth. As is typical of the era, the SAE gross ratings are likely overstated, and I estimate the actual SAE net output of the OE 283/250 FI engine at about 270 lb-ft and 200 HP, so this configuration should provide very spirited performance while maintaining OE appearance and idle characteristics.
This project is another data point proving that excellent performance can be achieved with vintage engines using vintage components to maintain vintage appearance and easily recognizable operating characterisitics such as idle speed and quality - a real "sleeper".
It's all in the details!
Duke
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