In 1955, Chevrolet's storied engineers put their heads together knowing their recent sensation, the 265 cubic-inch V8 engine could not adequately power future, heavier trucks and cars. Due strictly to its cubic-inch displacement, it simply did not produce enough torque. It was also reported that its overall day-in and day-out reliability in present trucks at continuous high rpm, first-gear use had become a critical issue. Try as it may, the 265 could only do so-much. A larger powerplant was urgently needed. T honchos gave the engineers the green flag and in a blink, a new larger V8 engine "evolved". As designed, it would be enlargeable to increased cubic-inch displacement as well as higher compression ratios. Its exterior size/dimensions would allow it to fit within the inner fenders of passenger cars. It would also be able to operate various bolt-on power accessories with little performance decrease. Wait a second, why didn't GM just use the larger engines available at Pontiac, Oldsmobile and Cadillac? There were lots of reasons. Availability, block strength, possible detonation under severe load conditions are but a few. By mid-1957, Chevrolet Engineering liked its coded "W-Motor" engine. It was designed from a "clean sheet of paper" like the 1955 265 small-block was and it too had few, if any, weaknesses.
1958 3x2 348 with A/C.
The need for different compression ratios weighed heavily on the engineer's minds. Word was if the engine was to have "in-head" combustion chambers (like the 283) it would have meant much costly chamber retooling for different compression ratios. As a direct result, approval was given to have the 348's combustion chambers in the upper "wedge" portion of the cylinders. By comparison, the 283's block decks were at 90 degrees to the crankshaft centerline. The 348's were at 74 degrees - thus making a wedge-shaped, 16-degree combustion chamber between the top of the piston and the slightly recessed flat head face.
Even though many different carb numbers were used, the 1958-61 tri-power 348 engines look
Further, a relief would be cut into the top each cylinder wall to lower compression - for truck usage only. For higher compression in passenger cars, the cylinder walls would be left alone and various height piston dome configurations would be created. As things happen, the 348's cylinders ended up being just about impervious to pre-ignition / detonation due to having no casting irregularities or hot spots. Yes, the block was very well engineered for its intended use - hard core, low vehicle speed / high engine rpm grunt work. With 1.94-inch and 1.65-inch diameter intake and exhaust valves, its breathing capability was labeled "excellent". Yes, the 348 was originally conceived for Chevy trucks. That was good! It was tuff-built. Being a "truck" engine has always been a great thing.
As introduced in 1958, the new "Turbo Thrust" 348 big-block was 1.7 inches longer and three inches wider than the 265 / 283. It was also said to be 0.80-inch lower due to a flat intake manifold. Its cylinder bore size was 1/4-inch larger (4-1/8 versus 3-7/8) than a 283 and its stroke was 1/4-inch longer (3-1/4 versus 3.00). The 348 crankshaft was also 12 pounds heavier. In all, a 283 engine weighed in at about 500 pounds while the 348 was 625. Much of the 348s extra weight was in the block design - put there specifically for strength. The rest were in the heavy-duty heads.
|HP||CAM||Comp. Ratio||INDUCTION||CAMSHAFT||V. LIFT (I/E)|
|250||Hyd||9.50||Carter WCFB 4bbl||3744901||.400/.412|
|265||Mech||9.50||Carter WCFB 4bbl||3755946||.406/.412|
|280||Hyd||9.50||3 Roch 2 bbl||3744901||.400/.412|
|300||Mech||11.00||Carter WCFB 4bbl||3755946||.406/.412|
|305||Mech||11.00||Carter WCFB 4bbl||3764664||.407/.414|
|305||Hyd||11.00||Carter AFB* 4bbl||3744901||.400/.412|
|315||Mech||11.00||3 Roch 2 bbl||3755946||.406/.412|
|320||Mech||11.25||Carter AFB* 4bbl||3755946||.406/.412|
|335||Mech||11.25||3 Roch 2 bbl||3755946||.406/.412|
|340||Mech||11.25||Carter AFB* 4bbl||3755946||.406/.412|
|350||Mech||11.25||3 Roch 2 bbl||3755946||.406/.412|
|* High rise aluminum intake manifold|
Here is the cylinder wall of a 348 truck block. All were notched for a lower compression r
Two new 348 engines were initially offered. The 250 horsepower "Turbo Thrust" version had a Rochester 4GC four barrel carburetor and produced a maximum of 355 foot pounds of torque at 2,800 rpm. The other was a "Super Turbo Thrust" and had three two-barrel carburetors. It produced 280 horsepower at 4,800 rpm and 355 foot pounds of torque at 3,200 rpm. Under normal driving, only the center carburetor metered fuel and air to the cylinders. When the throttle (also known as "foot feed" in 1958) was opened 60-degrees, a vacuum switch opened the front and rear carburetors. Each two barrel carburetor flowed around 225 cfm while the Rochester 4GC four barrel carburetor flowed about 450 cfm. The extra 225 cfm "tri-power" air flow equaled 30 horsepower. Both engines featured the same heads and hydraulic lifter camshaft. Chevrolet publicly limited engine rpm to a respectable 5,400. Most of the Brand-X engines generally peaked out at 5,000 rpm.
Here's terry Prince putting a hole shot on the factory '62 405hp 406 Ford Galaxie 4-door f
Originally designed as a high torque powerplant for monster trucks, its rod/stroke ratio and valve / port sizes made it very responsive to performance modifications even though its piston and wrist pin assembly weight totaled about 1-1/2 pounds. What the 1958 348 engine offered was a broad torque band that made manual transmission cars accelerate very well from 2,000 to 5,000 rpm.
Note: many felt the 348 was slow when bolted to a 1.90:1 first-gear ratio Powerglide transmission. This ratio, combined with a low, 1,400 rpm torque converter stall-speed gave the heavier '58 little off-idle "punch". With a three or four-speed manual transmission, it was a whole different story.
Positraction became available in 1958. Most of the other brands did not have it for a few more years. Believe me when I tell you that a stick-shift 348 with a Positraction rear end made it almost always a winner on street and drag strip. Anyone without a Positraction was at a distinct disadvantage. Most everything that was brand new back then had never even been heard about before. The only way to beat a Positraction car was to pre-load the chassis. But only the top drag racing minds of the day knew that.