The Forgotten Olds 403
Tim Campbell; Morristown, TN: Many people have the Olds 403 engine in their car and would like to upgrade. When you do the research on the Olds 403, you get a very broad range of opinions on the web ranging from "the blocks are no good" to "these engines can be made to produce some decent power." How about some help? Most of us would like to keep the Olds 403. With what you guys at Car Craft have done before with other oddball engines, we know you could build good horsepower and great torque. We are not talking about a race engine, but a true driver that is reliable-something you can beat on. We would love to hear that our engines have potential and that building them is worth it.
Jeff Smith: We called our Olds guy, Dick Miller at Dick Miller Racing (DMR), to get his input. He mentioned that even an average street Oldsmobile 403 would benefit from the typical improvements involving heads, intake, headers, and cam timing without having to go too deeply into the engine. We'll also assume that you would prefer to retain the stock torque converter and rear gear, which means being conservative with the cam timing. The stock Oldsmobile 403 enjoys the benefit of a giant 4.351-inch bore diameter with a relatively short 3.385-inch stroke. Miller says when Olds opened up the bore on these '70s-vintage engines, it intruded into the main webbing area requiring larger areas around the main webbing that weakens the crankcase. This isn't an issue with a mild street engine, but he recommends his five-cap halo-style main cap girdle for any Oldsmobile 403 motor that will see more than 6,000 rpm. The production Oldsmobile 403 also suffers from the typical late-'70s handicap of lame 8:1 compression by way of large, 83cc chambers.
One popular budget approach is to use 350 Olds heads to pump up the compression. Olds heads are identified by a large number cast into them. The 350 heads with a 5A casting offer a 68cc combustion chamber, the 6 head measures 70 cc, and the 7 head comes in at 69 cc. Miller says these 350 heads are the best, but there are still caveats. The universal problem with reviving old cast-iron heads is that by the time you've invested in new guides, a valve job, new stainless steel valves, filling the heat riser passage, and filling the depression between the center exhaust ports to prevent header leaks, you've got more money into a set of stock iron heads than the price of a pair of brand-new aluminum Edelbrock heads. DMR sells a set of Edelbrock aluminum Olds heads for $1,675, but keep in mind that these heads also require aftermarket aluminum roller rockers that will also pump up the price roughly another $250. The iron heads might be cheaper once they're rebuilt, but the flow probably won't be as good. Moreover, Edelbrock has revised the Olds head with improved flow, the combustion chamber is larger at 77 cc, but that will still gain some compression over the stock 83cc heads.
Assuming you will stick with a stock converter and gearing, Miller suggests a relatively mild hydraulic flat-tappet cam along the lines of a 219/233-degrees-at-0.050-tappet-lift cam with 0.476/0.508-inch lift and a lobe-separation angle of 110 degrees. This cam sells for $209 (PN 210-4) and will make some serious torque when combined with the Edelbrock Performer RPM intake you mentioned (PN 7111) and a set of 13/4-inch headers. All these parts taken as a whole represent a sizable investment, but you can do this a step at a time, spreading the cost out over a year or so, and still drive the car between mods.
Dick Miller Racing