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BIG BLOCK CHEVROLET - TECH HIGHLIGHTS

BIG BLOCK CHEVROLET - TECH HIGHLIGHTS
 

The Big Block Chevrolet has become the standard for the serious street machine, heavy duty towing, and for many classes of racing competition. It is used in any application that demands brute horsepower The large displacement of these engines allows them to be quite forgiving, and even a fairly wild package will deliver surprisingly good driveability if carefully tuned. If the intended use is for a towing application, this engine can be optimized to produce tremendous low end torque. At the races you will often see the race car and the tow vehicle using this same engine design. Despite the big block's popularity, there remain a few areas that require careful attention to ensure success.

 

One of the strengths of the Chevy big block is also a source of much confusion. The variety of cylinder heads and pistons available allows the selection of parts which may not be compatible. While we can apply some general rules, the ONLY way to be sure that your chosen combination will work is to check piston to head clearance during the engine's trial assembly, and then to modify the piston domes if needed. The general break point is in the use of "open" verses "closed” chamber type cylinder heads. If you are not sure which you have, check with an experienced machinist

Flat top pistons are usually safe with any cylinder head. Some domed pistons, such as the L2240NF for 396's, the L2383F for 402's, or the L2300F for 427's, can have their domes machined off, which lowers the compression ratio and allows the use of most available cylinder heads. Pistons designed for closed chamber use, such as the L2328F for 396/4O2 engines, the L2349F for 454's, or the L2239NF and L2268F for the 427, may also work with open chamber heads, but you MUST check piston to head clearance due to cylinder head variations. Pistons intended for open chamber heads usually will not work with closed chamber heads unless they are modified significantly. Examples include the L2465F and the L2399F, both intended for use in 454 engines having open chamber type heads. Using the pistons designed for your heads gives better results than trying make others work. Modification of pistons will make them ineligible for return, so be sure of your work. There are definite limits to the amount of material you can remove without compromising the strength and durability of the piston. Do not assume that all pistons can be modified. Many are too thin to allow major dome alterations. The Federal-Mogul Tech Line can answer questions regarding the amount of material that can be safely removed. Carefully read the Piston Tech Section before making any modifications.

Compression ratios listed in this catalog are now calculated figures, not the factory "advertised" numbers. Thus the information shown for various O.E. combinations may differ from data published in the past. We have not changed the pistons, but we now provide compression ratio information "as installed". All ratios are calculated with a standard block and a gasket thickness of .0385". One other piston related area of concern is the possibility of interference between the piston skirt and the crankshaft counterweights, which is a common problem in some 454 engines. The solution involves machining down the outside diameter of the counterweights and balancing the assembly.

 

When installing a performance camshaft, the primary concerns are the clearances between pistons and valves, and the prevention of valve spring coil bind. Piston to valve clearance must be at least .100", and must be checked when making a change to the cam, cam timing, pistons, heads, or rockers. Valve springs must have an additional .060" of available travel at maximum valve lift, or they will bind at operating speeds. VSS-7504R is a special spacer to replace valve rotators, which are not recommended for high performance use. Long slot or roller rockers are also required with some cams - check for clearance.

The big block Chevy uses different length pushrods for the intake and the exhaust. Guide plates are required, and must match the diameter of the pushrods that are being used. When using billet roller cams it is necessary to use a thrust button (PIN CSB3) and a bronze distributor gear. Finally, it is important to note that 1965 and 1966 engines require that a 3/16" groove be machined into the center of the rear cam journal for proper lubrication. Absence of this groove will result in engine failure. Use the 1404M cam bearing set if using a grooved journal cam in a later model block.



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