If those are production 5.7" Chevy rods, then the builder must have used a cam with reduced diameter base circle. Stock rods in a 383 will hit some of the cam lobes, so you can either replace the rods with units that will clear (such as the Scat #25700P) and use full-size cam lobe circle or use a reduced lobe circle cam if you are going to use 5.7" production rods.
I'm going to give you some advice.....
1. Do not buy ANY parts of ANY kind until you have a complete engine build plan written down on paper that everyone on this board has read and approved.
I learned a long time ago not to buy a pig in a poke and hopefully you have just learned that lesson also. Never buy a complete motor that you have not seen inside of. Much cheaper and smarter to buy individual parts and pieces at current market value and engineer the build yourself with help from fellows like the ones on this forum.
If you're going to use aluminum heads, they will allow a max static compression ratio of about 10.5:1 on pump gas. More SCR than that could get the motor into detonation, and you've already seen the results of that.
You don't have to build the motor at 10.5:1 though, you can build it at mostly anywhere between 9.0:1 and 10.5:1, depending on the camshaft characteristics you are looking for. For instance, a 9.0:1 to 9.5:1 build would allow the use of a mild camshaft, resulting in good street performance with good fuel mileage and good manifold vacuum for decent fuel mileage and for operating power brakes and other vacuum-operated accessories. The wilder you go with the cam timing, the more concessions you will have to make and the steeper gears you will need in the differential, because the cam will be operating in a higher range of rpms and so will require stiffer gears.
When planning a build, begin with your stack of parts. Add the piston compression height, the rod center to center length and the radius of the stroke together and find the STACK.
For instance, you might use a 1.425" piston compression height, 5.7" rod length and 1.875" crank radius to arrive at a stack of 9.000". Or you might prefer a longer rod, using a 1.125" piston compression height, 6.0" rod length and 1.875" crank radius to arrive at a stack of 9.000". Here's the rod I would use if I were going to build a long-rod motor, it is clearanced to miss the cam lobes of a standard base circle camshaft....
Oftentimes, beginning with a new set of quality rods that are ready to use will be cheaper than re-conditioning production Chevy rods, particularly if you add magnaflux to find cracks and quality aftermarket rod bolts and nuts. Then you still have to address the clearance problems at the cam.
Now, you have a stack of 9.000" that you need to fit into the block, with the proper piston deck height to allow the use of a composition gasket and still engineer the squish to 0.035" to 0.045".
Airflow Research recommends a Fel-Pro 1003 composition head gasket with their aluminum heads, so that is what I would go with, no matter whose aluminum heads I used. The bore is 4.166" and it compresses to 0.041". If you use steel shim head gaskets with aluminum heads, the differential in movement between the iron block and the aluminum heads will cause the heads to move around. If you use a steel shim head gasket, there will be insufficient "cushion" to absorb the movement and the heads will suffer fretting
, wearing away of the head surface because of the relative softness of the aluminum against a relatively different-moving steel gasket.
I like to recommend a very tight 0.035" squish on a SBC, so in this case, with a 9.000" stack, I would cut the block decks to a new block deck height of 8.994" after having the machine shop verify that the main saddle bores are all round and parallel with each other. Get the mains dialed in first, that's the foundation of the motor, then cut the decks. The stack of 9.000" and the block deck height of 8.994" will allow the piston crowns to "pop-up" out of the bore by 0.006", allowing a squish of 0.035" with the 0.041" thickness of the composition head gasket.
Such a motor, with 64cc heads and 12cc pistons, would exhibit a 10.34:1 static compression ratio
. With the aluminum heads and using a 0.035" squish as outlined above, this motor should run very well without detonation on pump gas.
A cam with 0.050" duration of 227 to 230 on the intake side would be a good choice.
Moving to an 18cc piston would allow a 9.72:1 static compression ratio
, allowing you to use a milder cam for a more street-friendly operation. A cam with a 0.050" duration of 214-216 on the intake side would be a good choice.
Using an 18cc piston with 70cc chambers would make 9.17:1 static compression ratio
, which would like a cam with a 0.050" duration of 204-206 on the intake side.
Here's where I'm getting the cam ranges from. I put this tutorial together after thoroughly investigating the recommendations of Crane Cams engineers.....
A dual-plane, high-rise intake manifold such as the Edelbrock Performer RPM (not the Air Gap model) will make more hp and torque (power under curve) in a street motor than any other manifold you could bolt onto the motor. I don't like the Air Gap model, even though it will make a few more hp than the standard RPM. The open gap on it will not allow heating of the air/fuel charge in cool or cold weather and can contribute to driveability problems. If you're going drag racing and want an extra couple of hp, use the Air Gap. If you're building a street motor where driveability needs to be addressed, use the standard RPM.
The RPM, together with the Weiand Stealth 8016 and the Holley Street Dominator 300-36, were copied from a design that Chevrolet produced for the 1968 Camaro 302 Z-28 motor and farmed out to Winters Foundry to be cast in aluminum. The 8016 and 300-36 have been discontinued from current production, but can be found used on craigslist, racing junk and local swap meets. Another current production manifold that will work well is the Weiand 8150.
An 850 carb would make the most power on a 383, but may be overkill on a street motor. A 750 with vacuum secondaries might be my choice for such a motor, along with a good set of 1 3/4" long-tube headers. Choose only headers with a flange thickness of 3/8" to help prevent the flange from curling up like a potato chip from the heat of operation and spitting out the exhaust gaskets. Install an H or X pipe immediately after the collectors to equalize flow from each side of the motor and sweeten the exhaust note. Actually, a Rochester Quadrajet would be my carburetor of choice on any street motor. It would require a spread-bore intake manifold such as the Edelbrock RPM #7104. If using a Quadrajet interests you, get your hands on Cliff Ruggles book and learn how to rebuild and tune them.....
Cliffs High Performance Quadrajets :: Qjet Carburetor Rebuild Kits, Parts, Quadrajet Rebuilding, Quadrajet Parts, Bushing Kits, Carb Tuning
There now, your turn....