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305 Small Block Engine Build - The Other Small-Block

Richard Holdener Jul 1, 2010

Despite sharing the same designation as the ever-popular 5.0L from the Ford camp, the 305 Chevy is viewed as a second-class citizen in the Chevy hobby. This seems odd since it's a Chevy small-block under the skin, but in the performance world, just as in nature, bigger is almost always seen as better.


In reality, 305 Chevys (including our carbureted LG4 version) make great street engines and can be the basis for impressive performance machines. That is not to say that the 350 (by way of the increase in displacement) won't ultimately make more average and possibly even peak power (this depends on combination), but don't dismiss that 5.0L/ 305 you have sitting under the hood on the basis of a missing 45 ci.

Before getting to our test, we should take a look at a few of the common complaints about the 305. According to ramblings on the various forums, the first strike against the 305 is displacement. True enough, it is always easier to reach a given power goal with more cubes, but the major concern for any build up is usually dollars and not displacement. While heads, cam and an intake for the 305 will cost the same as a 350, the fact that you can retain your existing 305 short-block makes swapping over to the larger 350 much less cost-effective. If you are looking for an extra 100-150 hp, the stock 305 short-block (even if it is a high-mileage unit) will work perfectly with the planned upgrades.

If your 305 needs rebuilding or replacing, by all means grab a cheap 350 from a wrecking yard and start with that-or better yet step up to a larger 383 or even 406.

Problem number two with the 305 is the small bore size. The bore (3.736-inch versus 4.00 for the 350) can ultimately limit head flow (by necessitating smaller valves, which limits potential airflow), but not at the power levels for most street engines. Were we to build a 350hp small-block using either a 302 (4.0 x 3.0), a 305 (3.736x3.48) or even a 307 (3.875x3.25) bore and stroke combination, the difference in the power curves would be minimal. If we went looking for a high-winding combination capable of exceeding 500 hp (or more), then the larger bore offered by the 302 configuration would certainly come into play.

Unlike its brother the 307, Chevy actually offered performance versions of the 5.0L 305. The 230hp rating offered by the 1989 LB9 TPI motor pales in comparison to the current LS-based combinations, but back in the day, the TPI 305 with the five-speed tranny was a pretty hot set up and could rip of 14-second quarter-mile times (and near 150 mph top speeds) in stock trim. Introduced in 1985, the TPI motors garnered all the attention, leaving the carbureted LG4 and L03 (TBI) motors seemingly out in the cold. The TPI motors did offer some serious torque thanks to their extra-long intake runners, but power production quickly leveled off at the top of the rev range. Rated anywhere from a low of 145 hp to a high of 170 hp, the carbureted LG4 and TBI L03 305s were down by as much as 60 hp to their TPI counterparts.

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.

We liked the fact that in addition to the substantial flow increase, the Trick Flow Super 23 175 heads offered small chambers and valves along with aluminum construction to help eliminate the chance of detonation compared to their cast-iron counter parts. With no loss in compression ratio (something common on head upgrades with a 305), we expected big things from the Trick Flow Super 23 175 heads. We also liked the port volume of the Trick Flow heads, as the 175cc intake ports were not excessive compared to the production heads. It is important to point out that increased port volume does not always result in a loss in low-speed power or even port velocity, as velocity is a function of the flow rate versus the cross section. A larger port that flows a commensurate amount of air will actually produce greater port velocity than a smaller one with less flow (at least at wide open throttle). Larger port volumes can become lazy at part throttle since the flow rate is determined by the throttle opening and not the port itself.

Before upgrading the LG4, we ran it in stock trim to establish a baseline. Our tester consisted of a late-model (meaning hydraulic roller cam) LG4 originally equipped with the factory intake and matching Q-jet 4-barrel carburetor. Since it was missing the factory induction system, we simply installed a suitable cast-iron Q-jet intake and carburetor (from the Carb Shop) along with a set of 13/4-inch long-tube headers. The headers were chosen primarily for dyno fitment, but a set of 15/8-inch headers would work every bit as well and possibly offer slightly more torque. Run with an electric water pump (no accessories), long-tube headers and 37 degrees of total timing, the LG4 produced 224 hp and 308 lb-ft of torque. The mild cam, restrictive heads and factory induction system were equally to blame for the lackluster power numbers, but the 305 still managed to exceed 300 lb-ft of torque from 2,500 rpm (or lower) to 3,600 rpm. Obviously the factory induction system was designed with low-speed grunt in mind.

Knowing that the stock 305 heads and wimpy L03 cam were now holding us back, we pulled both at once and installed the Comp XM270HR cam and Trick Flow Super 23 175 heads. It is possible to upgrade the cam or heads separately, but available dyno time meant we had to be quick. If you have to pick one, we'd opt for the cam first, as the TFS heads might not offer a ton of extra power when saddled with the stock cam (less than 0.400 lift). The cam upgrade will require updating the valve spring package on the stock heads, and if you have to remove the heads for the valve spring upgrade, it is a good time to swap the heads as well.

The Trick Flow Super 23 heads were installed using a set of Fel Pro head gaskets (designed for the small-bore 305) and ARP head bolts. Installation of the heads required no change in pushrod length. We also took the liberty of installing a set of inexpensive 1.5 ratio roller rockers from ProComp in place of the stock stamped steel units. No self-respecting small block should be saddled with stamped steel rockers. The upside of the hydraulic roller cam motor is reduced friction and improved ramp rate to increase power for any given lift and duration combination, but the downside is extra expense (roller cams cost more than their flat-tappet counterparts). To keep costs down, we reused the factory hydraulic roller lifters and timing chain rather than replace them during the cam swap.

There are obviously ways to reduce the cost of upgrading a 305, by either porting the stock 305 heads or substituting a set of Vortec iron heads for the slightly more expensive Trick Flow aluminum heads. The downside of porting the stock heads is that they will never equal the flow of the Trick Flow heads, nor will the porting eliminate the potential for cracking (305 heads are thin, weak and prone to cracking, especially if you overheat engine). The Vortec heads are stronger than the 305s, but have their downside as well. The problem with the Vortec head swap is that extra work is required to allow for reasonable cam timing (above 0.450 lift). The extra work and expense required to accept any reasonable performance cam needs to be taken into consideration when considering the Vortec heads, as does the fact that (like the stock 305s), they are heavy lumps of cast iron that will be more prone to detonation than modern aluminum heads. This is a case of you get what you pay for, and the dedicated 305 heads from Trick Flow Specialties are a worthwhile investment.

The last obstacle in our 305 upgrade was the induction system. It was certainly possible to top the new Trick Flow heads and Comp cam with the stock Q-jet intake and carb, but we chose to replace them with a dual-plane, aluminum intake from ProComp along with our favorite dyno carb, the Holley 750 HP series. A low-cost alternative to the 750 HP will provide equal performance at this power level, but the 750 was on hand at Westech and was easy to tune thanks to an external Percy's Adjust-a-Jet system. Make sure to measure the available hood clearance when selecting a high-rise intake like the air-gap-style from ProComp.

With everything bolted in place, we ran the 305 in anger and were rewarded with peak numbers of 362 hp at 5,800 rpm and 353 lb-ft of torque at 4,500 rpm. Torque production with the upgrades exceeded 340 lb-ft from 3,500 rpm to 5,500 rpm, making for not only a broad torque curve but a 305 that no longer fell on its face after 5,000 rpm.

Measured peak to peak, we improved the power output by 138 hp and by over 170 hp out at 6,000 rpm. If 360 hp isn't enough, we certainly could coax 400 hp out of this combination by upping the compression (milling the heads) and/or stepping up in cam timing, but you start to trade-off things like idle quality, drivability and low-speed response for the stronger mid-range and top-end charge.

RPM - HP TQ (Stock LG4) - HP TQ (Modified LG4)

2,500 - 149 313 - NA NA
2,700 - 157 306 - NA NA
2,900 - 169 306 - 181 327
3,100 - 182 308 - 194 329
3,300 - 199 308 - 209 333
3,500 - 203 304 - 226 340
3,700 - 210 299 - 244 346
3,900 - 217 292 - 260 350
4,100 - 221 283 - 273 349
4,300 - 223 272 - 286 350
4,500 - 224 261 - 300 353
4,700 - 224 250 - 314 351
4,900 - 219 234 - 328 351
5,100 - 214 220 - 339 349
5,300 - 205 203 - 348 345
5,500 - 195 186 - 355 339
5,700 - 186 171 - 357 329
5,900 - NA NA - 359 319
6,100 - NA NA - 358 308
6,300 - NA NA - 349 291


Trick Flow Specialties
Tallmadge, OH 44278
Comp Cams
Memphis, TN 38118
The Carb Shop
Ontario, CA
Pro Comp Electronics
Ontario, CA 91761



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