150 HP for a 305 Chevy Engine - Hate Me

I'm not sure what's tougher, being a 305 Chevy, or being a 305 owner. It's all abuse and insults, and never any respect. Talk about building or hopping up a 305 and guys will generally shun you, or even boldly accuse you of being a fool. You'll hear, "Why mess with that junk motor? You're wasting your time." Wouldn't it be nice to lay waste on their stuff instead, with a few discrete bolt-ons? How about a bolt-on package that will add 150 crank hp, and still pull a good 13 in-hg of manifold vacuum at 850 rpm? And what if all the stuff comes in cheaper than what it would take to build the average high performance 350 short-block, and all the parts will transfer over to a larger engine and work great if you ever decide go bigger? We know there were tons of 305s built in the 1970s right through to the mid '90s, many of them in cars with definite performance undertones, most notably GM F- and G-bodies. We'll go out on a limb and presume that there are quite a few of these guys on a working man's budget who'd love to spring for a fresh up-sized engine, but have to make do with the 305 that's in it now.

No engine modification effort will be successful without giving it plenty of thought and planning. First, we really had to think about why the 305 has such a reputation as a pooch. We deduced right away that all of these engines were built during an era when horsepower took a back seat to other considerations, and were generally choked with all sorts of add-on emissions do-dads. Factory 305s came with anywhere from a low of about 130 hp, to a high water mark of 230 hp for the hot Camaro LB9 injected engines in 1990. Most 305s made right around 140-150 hp as the factory net rating, and that isn't the kind of output legends are built on. We also identified that the engine is small, and has a particularly small bore at 3.736-inches, while the stroke is the same as a 350 at 3.48-inches. The small bore is an immediate red flag to most guys; but is it really when things are looked at in proportion to the engine's size? In fact, the factory 305 bore/stoke ratio is actually slightly better than a stock 454 Chevy, and way better than any of the 4.030x4.000-inch strokers. Conjuring the Power Pack Actually, with the right heads, there is nothing inherent in the bore to stroke ratio of an engine of this displacement that will cripple cylinder filling, especially in the street rpm range of under 6,500 rpm. We had just the heads in mind for this project, the 180cc intake runner EngineQuest (EQ) replacement Vortec castings. These heads are machined for 1.94/1.50-inch valves, which are proportionally large for a 305-cube engine, and their low cost makes them a natural for a budget conscious 305 build. The flow of these castings is a major step up from restrictive stock 305 heads, and we have seen firsthand that they can support excellent output, even on a larger 350-cube engine. On a 305 engine, their 233-cfm of intake flow, if taken in proportion, would be the equivalent of running a 290-plus-cfm head on a 383 small-block combination. Our 305 should be more than happy with the flow these heads provide.

Shaver's Specialty Service supplied the EngineQuest cylinder heads, which retail for about $225 each as bare castings. We had Shaver's mill the heads a modest 0.030-inch, to reduced the chamber volume to 60cc, so that we wouldn't lose compression ratio compared to the typical production small-chamber heads used on these engines. Shaver's assembled the heads with a budget 1.94/1.50-inch valve combination, along with convention single performance springs with dampers. Later in testing, we wished we'd stepped-up to Shaver's extra-cost COMP No. 26918 beehive spring package, which would have allowed even more rpm, and possibly allowed the engine to make more peak power.

The cylinder heads from Shaver's were the cornerstone of our 305 modification plan. Besides the heads, we only intended a few other complimentary mods, intending to use a factory 305 short-block for the testing. Our 305 test engine had been stashed unused and unloved on a pallet rack at Westech Performance Group for years. It wasn't until the casting numbers at the back of the block were verified that they were even certain of what the engine was. It took considerable persuasion to build enough enthusiasm in the Westech crew for them to even consider forklifting it down. It turned out to be a low-end 305 with dished pistons and a regular flat-tappet cam. Some of the high-end 305s came with flat-top pistons for more compression ratio, as well as factory hydraulic roller cam setups. Our engine isn't ideal to show off the 305's potential, but if nothing else it does represent the typical 305 that people love to hate.