402 Big Block Rebuild

[Adam Laney]

Me and my father are going to rebuild a 1972 402 Big Block, Numbers are 3999290 (for the block) and 399241 (for the heads). I was wondering how far this engine could be bored out and what kind of intake manifold and carb we should get for it? I have not been able to find very much information on the web about what can be done with this engine.... If anyone here can halp me, I'd be very grateful

Cheers, Adam

[Mark #28455]

Re: 402 Big Block Rebuild

Big block 402's have been bored to 4.25" if there is no core shift, but it is safer to stick with .060 over. If you really want to go bigger, you can get a late model 454 block for about $250.

Your intake selection depends upon the car you're putting it into. For a stock hood, 1968 and later Corvettes need a low-rise intake so you'll likely have to choose a factory intake - cast iron is a lot cheaper and flow is essentially the same. As far as I know, they are all spread bore design.1967 and earlier can use a high rise intakes, available in both spread and square bore.

Don't go too big on the carb, about 750CFM is all you need and vacuum secondaries work best for street driving if properly tuned.

What car is it going into?
Mark

[Adam Laney]

Re: 402 Big Block Rebuild

76 Corvette, stock hood.

[Mark #28455]

Re: 402 Big Block Rebuild

You'll likely need the low rise intake that came stock with the motor. You can still get decent power for street, especially with a set of 1 3/4" headers like the Hedman ones, using a stock type dual exhaust like the 1974 and staying with a mild cam similar to the Comp Cams 268 hydraulic. If you make these changes, your car will not be emissions legal, but will be a lot faster than the original 350. You may end up spending a lot on brackets and pulleys (use the stock 1974 454 stuff) I would price it out first, you may just decide to try the 400 small block instead (with vortec heads if you really want a budget screamer).

On the other hand, I stopped running small blocks in 1988 and am still having a blast.

Good luck,
Mark

[Joe L.]

Re: 402 Big Block Rebuild

Adam-----

The only 4 barrel intake manifolds that will be compatible with your 399241 heads and will fit under a 1976 Corvette hood are the following:

GM #3919849 (aluminum-spread bore-dual plane)

GM #3937793 (aluminum-spread bore-dual plane)

GM #3947801 (aluminum-spread bore-dial plane)

GM #3955287 (cast iron-spread bore-dual plane)

GM #6263753 (cast iron-spread bore-dual plane)

GM #336789 (cast iron-spread bore-dual plane)

GM #346242 (cast iron-spread bore-dual plane)

Edelbrock Torker II #5061 (aluminum-square flange-single plane)

All of the above-referenced GM manifolds are discontinued. The aluminum versions were Corvette-only, are difficult to find used, and are expensive when you do. The GM cast iron manifolds were used for both Corvette and other Chevrolet applications. They're much easier to find and much cheaper.

The Edelbrock manifold is still available from Edelbrock. It's a square flange manifold that requires a Holley or Carter carburetor. Plus, it has no provisions for a choke, so an electric chole carb must be used with it. As described, this is a single plane manifold. I don't recommend single plane manifolds for the street since too much low end torque is lost using them.

[Adam Laney]

Re: 402 Big Block Rebuild

Are there any other heads available that will allow me to have a different intake manifold? We're rebuilding this engine from the ground up, new pistons, cam etc etc, so new heads are no problem if there are better ones around Thanks for the response too

Adam

[Joe L.]

Re: 402 Big Block Rebuild

Adam-----

If you were to switch to rectangular port heads, then the only 4 barrel manifold that would fit would be the 1970-71 LS-6, GM #3963569. This manifold is diffiuclt to find and expensive, too. It's the ONLY square flange, dual plane manifold that will fit under a C3 hood. So, if you were to change heads to the rectangular style, then you manifold options become even more limited.

[Mark #28455]

Re: 402 Big Block Rebuild

The better stock head castings for a big block are the 049 or 781 sastings from the mid 1970's. They have an open chamber combined with a full sized oval intake port (not like the later "peanut ports"). If you are willing to work a little to get head gaskets, the late model vortec heads are excellent. They are easily identified by their full size oval intake ports, but NO heat crossover. They also have a heart shape combustion chamber and a huge swirl ramp in the intake port bowl.
If you choose to use them, DO NOT remove the swirl ramp, use stock size valves and only pocket port and gasket match them.I have had these flow bench tested against a fully ported set of 049 castings with the larger 2.19 and 1.88 valves and they flowed just as well to .500 lift and were only a little off at .550 lift - perfect for a street cam. Due to the 100cc chamber, you would need flat top pistons.
Do NOT use excessive ignition advance with the vortec heads, they only want a max of 28 to 30 degrees (with the vacuum advance blocked) due to the tremendous mixture turbulence.

Good luck,
Mark