If you are confident that the air/fuel ratio is on the safe, rich side, you need to start with some ignition timing curves. As a baseline, set your total spark advance at 3,500 rpm at 32 degrees. With this total timing spec, you may need to limit the spark advance in the mechanical advance. To get good slow-speed engine performance, the initial spark timing at idle should be 16-18 degrees. You'll need to limit your mechanical advance in the distributor to approximately 7 degrees. You can either do this with bushings supplied by MSD for its distributors or you'll need to weld up the advance mechanism in an HEI style to limit its travel. All this work can easily be done by Performance Distributors, or you can use your engine and a timing light as a distributor machine. We don't recommend the latter unless you're very experienced in all facets of distributor modification. We do it all the time on the engine dyno.
The spark plugs and spark advance specs we've given you will get you in the ballpark. If, after these changes, you are still experiencing timing knock issues, mix a batch of super unleaded with 108-octane leaded racing gas. Mix your gas at a 50/50 mix and this will give you approximately 100-octane gas. This, I'm sure, will take care of your spark knock challenge. You should be able to find a combination of spark timing, air/fuel mixture, and spark plug heat range to keep your engine happy. Also, keep that thing out of knock for the first couple of thousand miles. When your engine goes into knock, it is building tremendous combustion heat and pressure. This is wreaking havoc on your poor engine's attempts to break in properly. Good luck!
Seems Like a Lifetime
Q: I am on the 11th year of a frame-off restoration of a '63 Impala SS Convertible. Five years ago the engine was completed as follows: a 400 GM small-block making 377 cid with a 350 crank, 6-inch rods, 10.2:1 hypereutectic flat-top pistons, a Comp Cams XE268 hydraulic cam, zero-decked and balanced. The upper end uses AFR 180 CNC aluminum heads with 1.5 stainless steel roller rockers, Edelbrock EPS intake and 1401 600-cfm carb, and a HEI distributor. The exhaust is 15/8-inch full-length headers, 21/2-inch exhaust through Flowmaster 50 series mufflers. Transferring the power is a close-ratio T10 four-speed, 3.70:1 rear gears with a post. Tires are 26-inch-tall P235/55R16 Goodyear Eagle 1 NASCAR street tires. A second rear upper control arm was added for traction. My guess is the car with me in it will weigh right around 4,000 pounds.
Over the course of the build, I don't know how many people have told me what they think I should have done differently powerwise-350, 383, 406, different cams, carbs, heads... I have tried to build for a larger car that will make me proud of its performance on the street. Since completion may be several years away, can you tell me what kind of power numbers I might expect, or how to improve on what I have? Since I have no intention of adding a rollcage to my convertible, a time at a dragstrip will be hard to come by. What could I expect at the track, and at what rpm should my heavy car be shifted for the best quarter-mile performance?
A: It seems we have something in common. Between 1985 and 1996 my '57 Chevy race car was parked, since everything seemed to get in the way of its completion. I was running the car in 1985 when the engine expired. That gave me a chance to update it with several things I'd been wanting to do to it. I basically had to threaten myself that if it sat for 10 years I would sell it! (If you did the math, it was parked for 11.) All I know is good things take time to build. Your Imp will be done when you're happy with it.
I really like the engine combination for your car. I think it will be perfect for a heavy four-speed car like your 'vert. The camshaft is mild enough to build great torque and will carry the power where it's fun to row the gears. Everyone has a perfect build in mind, but it stays there until we put the parts together and run it. You should see an easy 420 hp by 420 lb-ft of torque. The horsepower peak should fall right around 6,000 rpm with the Comp XE268, and we'd shift it right past 6,000. The AFR heads are going to want to breathe and will carry the power upstairs with your relatively mild camshaft.
After you get the car running you may want to install a little larger carburetor. However, for a street cruiser, the 600-cfm carb will give you great street manners and decent fuel economy.
As for a quarter-mile e.t., that is a little tougher. Four-speed cars can be rough to launch without a ton of wheelspin or bogging the engine. It's very safe to say your Imp should run in the 13s at 100-plus mph. Weighing 4,000 pounds is nothing; your engine will pull this around with no problem.
Get your Impala done and do some top-down motoring for all of us!
Technical questions for Kevin McClelland can be sent to him at firstname.lastname@example.org.