When it's all said and done, be prepared to invest a considerable amount of time and money into a 301 that might not run much better than a comparably-equipped 400. After considering all that's required, it's easy to see why so many hobbyists ultimately opt for a larger displacement engine as the foundation of their performance build. You've stated that you chose to begin with a 301 for personal reasons, and while it may be a somewhat challenging task to complete, it still sounds like you have a fun project ahead!
Powering Up The 403
I love your magazine and featured vehicles! I'm new to the Trans Am modification segment and have a few questions. I own a '77 K-code (California emissions) Trans Am with 89,000 original miles. The car runs excellent and the Turbo 350 transmission was recently rebuilt, but the motor is untouched. Unfortunately, as we all know, the 403 wasn't regarded as a powerhouse and it seems that it's not a popular engine for adding power. Is it worth the effort and funds to modify this motor?
If so, what modifications do you recommend if I wanted to see an additional 80-plus hp and/or torque, or should I just drive it stock until the motor gets tired and do a swap? Will the modifications make for an unreliable setup? I'm not looking to be the quarter-mile champ but would love to end up with a great sounding/running T/A that isn't afraid every time a late-model Honda Civic pulls up next to it at a stoplight. Thanks.John VaccaroScottsdale, AZ
Rocky Rotella responds:
The 185hp 403 Olds was the only powerplant available in Trans Ams bound for California or high-altitude areas in 1977. As you've found, its performance in stock form was fairly anemic and leaves much to be desired. That doesn't suggest it wasn't reliable or its performance can't be improved upon, however.
Common modifications include opening the Shaker, richening the carburetor, and recalibrating the distributor. While these certainly increase performance, they're likely only good for 20-25 hp. Knowing you desire much more, I posed your question to Terry Fritsch of FCR Performance in Omaha, Nebraska. He has successfully raced Olds-powered vehicles for years, and is very familiar with extracting additional output from the 403.
Fritsch says your T/A's original 403 is an excellent basis with which to achieve your performance goals. With nearly 90,000 miles, however, it likely needs a complete rebuild. Fritsch suggests increasing its bore by 0.024 inch to a total of 4.375, which boosts total displacement to 407 ci. The stock connecting rods and crankshaft should be completely adequate at your power level, and a main girdle isn't necessary. A replacement Melling oil-pump is sufficient, but Fritsch prefers to add a 3/16-inch shim under the regulator spring to increase pressure.
Your T/A's original cast-aluminum pistons contain a 20cc dish, which yields a compression ratio of 8:1 when combined with the stock No. 4A heads' 83cc combustion chambers. Fritsch suggests a set of Keith Black hypereutectic pistons (No. 405)-which he helped design-that contain a dish of just 5 cc. This would boost compression toward 9.5:1 when using a 0.040-inch head gasket, and the engine will remain pump-gas friendly.
Your No. 4A cylinder heads are completely adequate, but Fritsch says to have them rebuilt using top-quality components. He also suggests increasing intake and exhaust valve diameter from 2.00/1.50-inches (stock) to 2.07/1.62. This requires that the combustion chambers be relieved to unshroud the larger valves and maximize airflow. The castings should then be milled only enough to bring them back to 83 cc. No additional gasket matching or port work is required, but smoothing any sharp corners in the ports left behind after the valve cutter is also suggested.