Try this experiment the next time you are sitting around the garage or online, talking about small block Chevy performance. When everyone is talking about DZ302s, 365 hp 327s, and any number of 350 Chevys, casually mention the 305!
Chances are great that you will experience one of two things, plenty of laughter or outright ridicule. Few (if any) enthusiasts will step in to defend the much-maligned 305 small block!
In truth, the small block Chevy has been offered in a variety of different displacements, ranging from a diminutive 262 cubic inches (3.671 x 3.10 in. bore x stroke) to a full 400 inches (4.125 x 3.75 in.). Many confuse the limited production 262 small block with the more popular 265 displacement (3.75 x 3.0 in.), but the little 262 was rated at just 110 horsepower—kind of pathetic for any self-respecting V8.
Mixed in with all the various other displacements (267, 283, 302, 307, 327, and 350) was the 305, often referred to in metric circles as the Chevy 5.0L.
Despite its dismissal from enthusiasts, the 305, unlike many of the other small blocks listed here, was actually offered in performance applications.
In fact, the 305 all but became the go-to V8 for GM applications in the 1980s and 1990s. Not as legendary as the DZ302s or LT-1s of yesteryear, yet it was the L69 and LB9 305s that took the battle to the 5.0L Mustangs of the era.
Exploring the Chevy 305’s Potential
Given the extensive usage and even performance heritage of the 305, why does it have this less-than-stellar reputation among enthusiasts? The 305 is a capable performer, but suffers because of both its dimensions and aftermarket support.
What do we mean by this statement? Well, dimensionally speaking, the 305 displacement came from a combination of a 3.736 inch bore and the 3.48 inch stoke (stroke length shared with the larger 350). Though things like cams, rockers, and induction systems that fit all of the other small blocks work equally well on the 305, cylinder head selections to fit the small-bore size were limited.
Most aftermarket head manufacturers were quick to offer performance cylinder heads for the 4.0 inch bore 302, 327, and 350 motors, yet few stepped up and made heads specifically sized to work with the smaller 3.736 bore on the 305. Notching the top of the bore to fit some of the larger heads on the 305 was a possibility, but the bore sizing also had the potential to reduce head flow and hinder performance. Stock, or even ported stock, heads had no trouble supporting your typical (low-rpm) TPI 5.0L, the small-bore combo needed real head flow to make power!
The other consideration for enthusiasts was that, given the availability of motors in the wrecking yard, it was very easy (and cheap) to simply step up to a larger (big bore) 350.
Where does that leave the many 5.0L 305 owners looking for more power?
Some Good Chevy 305 Aftermarket Performance Upgrades
Well, the big-bore guys might have more offerings, but that doesn’t mean 305 owners can’t have some fun too.
To demonstrate what is possible with a 305 small block, we upgraded a factory 5.0L Tuned Port motor. Our upgrades were applied to an original (and zero mile!) LB9, but replacing the stock heads, cam, and intake can be performed with equal (possibly better) results on any of the many carbureted 305s. Note the later Vortec 305s (LO3) came with Vortec specific heads and attending intake bolt pattern.
To illustrate the power potential of a top-end (HCI) upgrade, we enlisted the help of the gang at Comp Cams, Trick Flow Specialties, and Holley.
Though head choices were limited for the small-bore 305, Trick Flow Specialties did offer a set of 175cc Super 23 degree aluminum heads designed for the 5.0L. The heads featured a 1.94/1.50 valve package, 56cc combustion chambers and a valve package designed for our hydraulic roller spring package. Rather than go whole hog, we decided to make the combo a decent driver and chose our cam accordingly.
The powerful, but drivable, Comp XFI268 cam offered a .570/.565 lift split, a 218/224 degree duration split and 113 degree LSA.
This dynamic duo was combined with a short(er)-runner, Holley (Weiand) Stealth Ram induction system to replace the factory TPI system. The combination was designed to enhance both flow and rpm potential of the 5.0L small block.
Modified Chevy 305 Dyno Testing
To test the potential of the proposed combination, we first ran our stock 305 LB9 TPI motor on the engine dyno with long-tube headers, no accessories and a Holley HP management system.
The lack of accessories and induction, combined with an open exhaust and optimized tune, is why this motor made more power than the rated 215 to 220 hp (in stock form) for the LB9. Run in this manner, the TPI LB9 305 produced peak numbers of 267 hp at 4,700 rpm and 333 lb.-ft. of torque at 3,700 rpm. It should be obvious from these numbers (especially the peak engine speeds), that GM designed the long-runner, tune-port motors for low-speed torque.
After backup runs repeated the numbers, we tore the LB9 apart to make way for the Comp cam, TFS heads and Holley Stealth ram induction. The Stealth ram was equipped with 80 pound Accel injectors, since we would later be running this motor under boost (in the form of a TorqStorm supercharger).
After installation of the new components, the peak power numbers jumped to 367 hp at 6,000 rpm and 349 lb.-ft. of torque at 4,800 rpm. The new combo was making peak torque at a higher engine speed than the stock motor made peak hp. Despite the mild cam timing, the HCI upgrade improved the power by a solid 100 hp, though there was a loss in torque way down low (it is tough to beat the TPI off-idle).
Best of all, with plenty of head flow left (enough to support near 500 hp), this 305 just needs more cam (and maybe intake) to chalk up even more power.