305 Small Block Engine Build - The Other Small-Block

Richard Holdener Jul 1, 2010 0 Comment(s)

Despite that deficit, the LG4 and L03 305s respond very well to performance upgrades. In fact, the TPI induction system will soon become the power restriction, as a carbureted 305 will easily make more peak power (but less peak torque) than a typical TPI set up. The factory TBI induction system leaves a little to be desired, but replacing the TBI with a carburetor is a simple enough matter (though first check with state smog regulations). (For you TBI lovers out there, we do have some of that tech coming.)

To take full advantage of the power gains offered by the mods performed to our 305 test motor, a good set of long-tube headers and free flowing exhaust are a must, as is a true (and free-flowing) cold air induction. On the dyno, our carburetor received an unrestricted supply of cold air, but every effort should be made to provide a dedicated ambient air source in the car as well. We ran a set of mufflers on our long-tube headers to simulate a free-flowing street exhaust, but figure huge power losses with the factory exhaust system.

That brings us to the factory heads and cam. According to our information, factory cam timing for the various 305 combinations checked in as small as 0.350-inch lift and just 179 degrees of intake duration (measured at 0.050). That, my friends, is one wimpy small-block cam. Add to that the fact that many 305s came with a static compression ratio of just 8.6:1 (thanks to dished pistons) and a set of 305 heads equipped with 1.84/1.50 valve combination. The 305 heads (624 and 416 casting numbers are good examples) offer small 56-58cc combustion chambers to keep compression ratio up on the small(er) displacement 305. The valve and chamber sizing is important when choosing suitable replacement heads for the 305, ditto when choosing an acceptable street cam.

To illustrate just how effective the much maligned 305 is for performance usage, we decided to upgrade a stock LG4 using suitable, street-friendly modifications. In choosing our performance components, we selected them based not on some pie-in-the-sky peak power number, but with real driving in mind (though all testing was performed on the engine dyno). It is certainly possible to produce even more peak power with a wilder cam, ported heads and increased static compression (or even displacement for that matter), but our combination was chosen every bit for drivability as much as maximizing power production.

To that end, we elected to run a very streetable XM270HR (our test mill was a later hydraulic roller version) that offered a 0.495/0.502 lift split, a 218/224 duration split and a 112-degree lobe separation angle. An interesting alternative would be the 268XFI H13 with a 0.570/0.565 lift split, a 218/224 duration split and a 113-degree lobe separation angle. Our Trick Flow Specialties heads would certainly be able to take advantage of the higher lift offered by the XFI cam, but we stuck with the low-lift XM270HR grind.

Speaking of cylinder heads, the stock 305 iron heads obviously had to go, but choosing a suitable replacement was actually pretty easy. Looking for the ideal combination of flow, valve sizing (for the small bore) and combustion chamber size (to eliminate any loss in static compression), we went right to Trick Flow Specialties. Given its extensive listing of available SBC heads, we weren't surprised to find a set designed specifically for the 305. Its Super 23 175 heads featured all the variables to help make our 5.0 a success, including 56cc combustion chambers, a 1.94/1.50 valve package and 175cc Fast-as-Cast runners that offered an impressive 245 cfm of airflow from the intake and an equally impressive 192 cfm from the exhaust. That compares to less than 200 cfm for the typical 305 factory heads.




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