150 HP for a 305 Chevy Engine - Hate Me
Add 150 HP to the Chevy No One Likes
From the January, 2007 issue of Popular Hot Rodding
By Steve Dulcich
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.
When most guys see these 5.0...
When most guys see these 5.0 casting numbers on the back of the block, they figure the engine belongs on the scrap heap. With the small 3.746-inch bore, it is assumed these engines can't make power.
Although it's tarted-up with...
Although it's tarted-up with a two-piece timing cover (for a quicker cam change on the dyno), an aftermarket damper, and the dyno's electric water pump, this is nothing but a standard carbureted 305-cube stocker. In baseline form, the 305 was no powerhouse, making 221 hp at 4,800 rpm.
Up top, this heavy lump of...
Up top, this heavy lump of an iron manifold was a pretty common sight on 305 and 350 engines of the '70s and '80s, as was the familiar Q-Jet carb. While there is plenty of room for improvement in a hot street engine, it is a decent setup on a stock 305 with those awful heads.
While the COMP three-piece...
While the COMP three-piece timing cover doesn't add any horsepower, it allows the cam to be pulled without removing the oil pan, which is a giant time waster in a dyno test session.
Obviously, cam and valvetrain would be part of the strategy. With the engine's small-displacement and undoubtedly compromised compression ratio, too much cam would be counterproductive. Since this is a flat-tappet engine, we decided to stay with that type of camshaft, as opposed to a retro-fit hydraulic roller, which would have been considerably more costly. Had this been a higher compression roller block, we definitely would have gone with a more aggressive hydraulic roller camshaft. While erring too big would ruin the build, erring too small is also a mistake-one that would curb peak power. After much consideration, a COMP XE268H flat-tappet hydraulic was chosen. This cam allows a smaller engine to make power up into the 6,000-plus rpm range, yet its relatively short duration, at 224/230-degrees, preserves cylinder pressure and maintains acceptable street idle quality. For a general purpose real street cam, this stick is hard to beat. The final components of our 305 power package related to the induction system. This engine was a carbureted unit, and we intended to keep it that way. Later 305s had some form of fuel injection, but for all-out power and a broad rpm range, a properly chosen carb/manifold will out-gun the factory EFI stuff, and is far more cost effective than a trick aftermarket injection setup. The manifold decision was easy, since the Edelbrock Performer RPM AirGap consistently shows the capability to make good peak power, while having much better torque than a single plane. With the 180cc EQ heads, the AirGap would be a perfect match when considering the port sizes. To top the manifold, we decided upon a 650-cfm Speed Demon carb, a good general high-performance carburetor, in a flow capacity that seemed about right for an engine of this displacement.
We had essentially mapped out our entire modification plan before hitting the dyno, a basic heads/cam/induction package that we figured would breathe life into one of these underachieving mills. For a baseline combination, the engine was all stock, including the short-block, heads, cam, a cast iron GM two-plane intake manifold, and a Quadra-Jet carb. As the engine was docked to Westech's SuperFlow 901 dyno in this form, the engine's virtue was disparaged as is usual for 305s. The scuttlebutt indicated the engine would be well short of the 200 hp mark, except for one optimistic passerby who put the number at 275. Well, the engine was being tested bare of all of its factory smog equipment, through excellent long-tube dyno headers, and with an electric water pump in place of the factory belt driven accessories. My 225 hp guess proved to be pretty close, with the 305 delivering 221 hp at 4,800 rpm, and 291 lb-ft of torque at 3,200 rpm. Normally, we like to make changes one step at a time, but in this case, we planned to make the changes all at once. There seemed to be no use in trying to run the good heads with the tiny stock cam, and the big cam with the junk heads also seemed like a dog of a combination. Finally, we weren't going to add a good set of heads and a hot street cam, and then cap it with a stock iron intake. This program was more of a full power package, so with the baseline numbers in, we stripped the 305 to the bare short-block and got to work. The heads were bolted on using Mr. Gasket's thin 0.018-inch head gaskets, actually the same set that had been under the stock 305 heads. The COMP cam got a new set of COMP's standard hydraulic lifters, along with COMP's pushrods and their Pro Magnum steel rockers. Shaver's supplied the rocker studs and guide plates that work with these heads, and before long we were bolting on the Edelbrock Vortec AirGap intake and hooking the linkage to the 650 Speed Demon carb.
What would the 305 make? We had plenty of time for conjecture while the engine was being run for the cam break-in cycle. Even though the stock 305 had show better output than most of the detractors had anticipated, there was little confidence in the 305's potential. Jawboning in the cell predicted numbers in the low 300hp range. There were a few surprised expressions when the 305 cranked 367 hp at 6,000 rpm on the first pull. With some fine-tuning of the air/fuel ratio via a jet change to the Speed Demon carb, power inched up to 372 hp at 6,100 rpm. That's credible power for any street small-block; enough to make a Third Gen Camaro really scoot. Had we started with a higher compression hydraulic roller 305, as is typical of the later F-bodies, there likely would have been even a little more. The lesson here is that any engine can be made to perform with the right combination of parts, even those that the vast majority of people are reluctant to love.
With the baseline testing...
With the baseline testing completed, the engine crew at Westech began tearing into the 305, to make way for our power combination of cam, heads, and induction.
Beneath the heads, the 305...
Beneath the heads, the 305 had these production cast dished pistons. These pistons are pretty lame, costing compression ratio compared to some of the better high-performance 305s. In fact, our 305 is just a regular old style, flat-tappet, low-compression, carbureted unit.
Step one was to remove the...
Step one was to remove the stock factory cam, and put in its place a more appropriate COMP Cams stick. We used the XE268H, a hydraulic flat tappet with 224/230-degrees duration @ 0.050-inch, and 0.477/0.480-inch lift, on a 110 degree lobe separation. Idle quality was good, with 13 in-hg of manifold vacuum at 850 rpm.
Studs and guideplates were...
Studs and guideplates were supplied by Shaver Specialty Service. The guideplates have elongated holes, so that the position can be adjusted side to side for perfect alignment before they are torqued down.
COMP's Pro Magnum steel roller...
COMP's Pro Magnum steel roller rockers were used, and we tried both a 1.5:1 and 1.6:1 ratio. The 1.6:1 ratio seemed like it might just start coming on at the very top of our rpm range, but the valvetrain seemed to lose stability at just the point where the advantage would have become apparent. Another case where we wished we'd stepped up to Shaver's optional COMP beehive spring upgrade, which offers more spring load.
Since these heads are Vortec...
Since these heads are Vortec style, we gained the improved sealing gaskets up top, but needed to be sure to order a Vortec-style Edelbrock AirGap intake, and also needed center-bolt style valve covers.
A 650-cfm Speed Demon carb...
A 650-cfm Speed Demon carb seemed like plenty for a 305, promising a good balance of low-end performance and top-end power. The full-throttle manifold vacuum peaked at under 1 in-hg. at the top of the dyno pulls, showing that it had plenty of flow capacity on this engine.
What was our "Power Pack"...
What was our "Power Pack" worth on the 305? How about 372 hp at 6,100 rpm? However you feel about 305s, you've gotta love that.
Engine Master Dyno Test
305 CID Chevrolet
901 Engine Dyno
STP Correction Factor
Tested at Westech
| RPM || Stock TQ || Mod TQ || Stock HP || Mod HP |
| 2,600 || 285 || 290 || 141 || 144 |
| 2,800 || 283 || 297 || 151 || 158 |
| 3,000 || 287 || 320 || 164 || 182 |
| 3,200 || 291 || 328 || 177 || 200 |
| 3,400 || 290 || 334 || 188 || 216 |
| 3,600 || 284 || 337 || 195 || 231 |
| 3,800 || 279 || 340 || 202 || 246 |
| 4,000 || 271 || 346 || 206 || 264 |
| 4,200 || 264 || 349 || 211 || 279 |
| 4,400 || 256 || 349 || 214 || 292 |
| 4,500 || 252 || 350 || 216 || 300 |
| 4,600 || 250 || 348 || 219 || 305 |
| 4,800 || 241 || 344 || 220 || 314 |
| 5,000 || 233 || 342 || 221 || 326 |
| 5,200 || 221 || 343 || 219 || 339 |
| 5,400 || - || 337 || - || 347 |
| 5,600 || - || 330 || - || 352 |
| 5,800 || - || 324 || - || 358 |
| 6,000 || - || 322 || - || 368 |
| 6,100 || - || 320 || - || 372 |
| 6,200 || - || 314 || - || 370 |
Lightning Cylinder Heads
The EQ Lightning cylinder heads were a key component in making our engine modification package a success. Without a doubt, one of the worst features of the production 305 is the factory "smog" cylinder heads, which are choking even at a measly 305 ci of displacement. With decent heads, a small engine can breathe, and that allows it to take advantage of its rpm potential to make power. Without good heads, it isn't going to happen. These Vortec-style castings are just the ticket, with the right size valves, at 1.94/1.50, to fit and function in the smallish bores. Besides good flow, price is a key consideration when dealing with a 305. Enthusiasts do not typically regard these as their "ultimate" engine, so very few would spring for pricey race parts. However, if a substantial improvement can be had at a modest cost, why not go for it?
Shaver Specialty Service, a legendary name in the sprint-car engine world, is our local dealer for these EQ heads. Shaver receives the heads as bare machined castings, and prepped our set in record time to make this dyno test a reality. The heads were milled 0.030-inch for a little more compression with the small, dished-piston test engine. Shaver's completed the assembly by fitting their assembly package, including the valves, springs, retainers, seals and locks. The heads were also flow tested, showing very respectable numbers, particularly considering the valve sizes.
| Lift || Intake || Exhaust |
| 0.1 || 60 || 54 |
| 0.2 || 117 || 102 |
| 0.3 || 173 || 134 |
| 0.4 || 216 || 152 |
| 0.5 || 225 || 162 |
| 0.6 || 232 || 169 |
| 0.7 || 237 || 172 |