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Ask Away! with Jeff Smith: A Brief History of the Airflow-Restricting Chevy 305

(Image/Paul Ward – Pexels)

How did Chevrolet build the 305 cubic inch small-block? Is it a mash-up of 283 and 350 parts? Or is it just slightly larger than a 302 Chevy engine? — L.M.

Jeff Smith: “Mash-up” is a good term to use with any discussion of the 305 or 307 engines.

Actually, the 305 is a small-bore, long stroke engine created when General Motors officials decided back in the ‘80s to phase out the V8 in favor of the V6.

Then, the plan was for the 305 to be the biggest V8 you could purchase.

The problem was that the only V6 making decent power was the turbocharged Buick V6. The other engines were more than a disappointment when it came to making power — they were just too small.

The 305 wasn’t much better. Ultimately, the 350 c.i.d. V8 got a reprieve and now the horsepower wars have us at more than 700 horsepower in production engines.

From 1956 to 1966, the 283 was Chevy’s workhorse V8 powering millions of cars and trucks. It was just a 3.875-inch bore version of the 265 and was the hot V8 until 1962 when the 4.00-inch bore 327 arrived.

By the ‘70s, the 307 replaced the 283 as the base V8 but it didn’t do well because of its 283-size bore and long stroke.

Chevy should have learned from this engine but they didn’t, choosing instead to go a cheaper route by eventually going to the 305.

Displacement for Small Block Chevy Production Engines


As you can see from our displacement chart, the 305 uses the same stroke crank as the 350 even though the cranks are not interchangeable because the counterweights are heavier on a 350 crankshaft due to larger, heavier pistons.

The long stroke helped this small-displacement engine make torque at low speeds, but it was hampered by its tiny 3.736-inch bore.

The hot-dog 305 engine in the mid 1980s was the L69 carbureted engine used in early third-gen Camaros. While considered a hotter engine than its LG4 counterpart, it was only rated at 190 hp. To put this in perspective, a 2-bbl 283 back in 1966 was also rated at 190 hp.

While the rating systems changed to net power, if you were to put a 4-bbl on a 283 with matching compression to the L69 of 9.5:1, the 283 would win the power contest every time.

The difference is the 283 sports a slightly larger bore, but a shorter stroke. That combination works much better for overall power. The larger bore improves breathing while the shorter stroke reduces piston friction.

Unfortunately, the number of 283 small-blocks out there are dwindling fast. Nobody wanted the 283 because the 327 was both larger and better.

We’ve addressed the limitations of the 305 in several previous questions, so we won’t go too deeply here.

You can build a 305 to work on the street but as we’ve mentioned in the past, these small-bore engines restrict inlet air so badly that you can’t run a 2.02-inch intake valve head because the valve will literally crash into the bore. That’s not a good place to start when building a performance engine.

Conversely, if you just need a mild small-block to putt around town and chase parts, then the 305 will do the job.

There are always a few enthusiasts who feel the need to “defend” this engine. If you think the 305 deserves attention, then please feel free to build one.

But from a simple airflow standpoint, a 302 small-block with a 4.00-inch bore and a short 3.00-inch stroke is slightly smaller in displacement but has a 0.264-inch larger diameter bore! That allows large valve heads and enough flow to make decent power.

In the mid to late ‘60s, thousands of 283 engines were treated to a 0.125-inch bore increase from 3.875 to 4.00 inches which were termed 301’s since the bore size penciled out to 301.59 cubic inches. Chevy later combined the 327’s 4.00-inch bore with the 283’s steel 3.00-inch stroke crank and termed it a 302.

And the rest is history.

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  1. Have you heard about the Aussie GM 308 V8? 4 inch bore, short stroke and produced more power than the 327?…….

    • Ooh Steve, you don’t wanna go there, I spent much of my early days removing these piles of junk and replacing with small blocks. The aussies did know what they were doing with them, but the effort wasn’t worth it, believe me

  2. Travis Jones says:

    Not to argue on behalf of actually building a 305, but comparing the as cast 327/302 “fuelie” 461 heads to the Vortec 5.0l L30 heads, the L30 heads flow much better particularly in the mid lift of the camshaft. Conversely the small bore 4.8l or 5.3l Generation III/IV small blocks wipe the floor with comparable displacement factory headed SBC’s, the primary reason is the heads.

    The 305 was bad because of the heads that were bolted to it, not because of the bore/stroke combination. Just like the L31 Vortec heads are the best factory heads for the large bore motors, the L30 heads are the best for a small bore engine. It’s amazing what 40 years of engineering research can do for a motor.

    I would argue that an L30 headed 305 would handily beat a 302 with fuelie heads if a hydraulic cam is used and we were to use engine masters style scoring. Sure you could put a solid lifter cam in the 302 and rev it to the moon like an old trans am racer, but I don’t think it would be enough to beat a vortec 305 with a healthy cam.

    • Daniel Right says:

      302 had 11 to 1 compression too,the 305 suffered because it came out with emission controls, low compression and old school heads initially.

  3. stan macklin says:

    I love seeing what people can do with low displacement.

  4. Jeff the muffler guy told me that there was gas coming through my exhaust pipe and I immediately thought the EGR valve would that be a great start

  5. The 305 is an engine designed in 1974 and first used in 1976. It wasn’t designed in the 80s. It was an econosmog engine. Also, the 283 ran from 1957-1967 and the 307 replaced it for 1968, running through the 1972 model year.

    Getting back to the 305, it was overlooked by almost all enthusiasts because the 350 was also plentiful and cost the same to build, while yielding better results.

    Today, 305s are a dime a dozen, and with modern technology cylinder heads can be respectable performers in their own right. They can be built to produce horsepower north of 300, with torque also well north of 300 meaning it can be more powerful than the 350s of the day. (same goes for the 350 today – technology can be good!). 2.02 valves won’t fit in the 3.736″ bore of the 305, true … but you can flow more with today’s heads on 1.94 valves than you could with any factory 2.02 heads produced.

    Using technology from the time the engine was designed, yes it’s a bit of an airflow disaster. Using today’s tech, not so much.

  6. You also can’t compare horsepower measurements from the 1960s to the measurements used since 1972. The 190hp 283 was measured at the crankshaft, with no accessories. Just pure brake horsepower. The 1972 up measurement is NET horsepower, which is at the flexplate with all accessories included. Therefore a 190hp 305 is more powerful than the 190hp 283, because parasitic losses from the alternator, smog pump, power steering pump etc are accounted for in the net horsepower rating. Not to mention the 305 will out-torque the 283. No replacement for displacement.

    Horesepower is definitely useful but it doesn’t tell the true story of the engine’s performance. Torque is what breaks momentum and gives inertia to the vehicle, not horsepower.

  7. I have to take exception with the remark about the 307. It’s “reputation” of being no good/worthless/a dog etc probably comes from the fact that it was only available in 2 bbl/single exhaust form.

    I had experience with two of them back in the 70’s. The first one was equipped with the early fuelie heads (1.94″ intakes and a point in compression bump), a TRW TP153 hydraulic cam 232 degrees on the intake at .050″ lifter rise, a 1962 Corvette Rochester fuel injection system and dual exhausts (through factory exhaust manifolds) in my ’72 Nova. It would eat stock big block (and some modified) muscle cars for lunch. It would happily rev to 6700 and wouldn’t start to pull until 3600 or so. Many a big block owner couldn’t believe a 307 could run that good. I wonder how much of an improvement headers would have made? Nevermade a quater mile pass with it but those who rode in it said it felt like high 13’s. All that with a 3.07 axle and three speed on the tree.

    The second was in a ’69 Camaro convertible. With only a Rochester Q-Jet, headers and dual exhausts, that Camaro would run mid to high 15’s like clockwork. It still was the stock cam and low compression engine underneath.

    The 307 responded to mods like any other Chevy small block of the time. I still have one squirreled away for some future project.

  8. Ralph A Corbin says:

    Hello n I just brought a 91,92 305 but here’s the thing it’s a
    Jasper with rollers cam i think n it’s Clean inside Now
    I have a 91 RS has been sitting for 10yrs just a little smoke when starting valve seals Now I would like to put that Jasper into
    Should I just go with new Cam Kits timing chain, lifters, put don’t
    OR should put head gaskets to hmmmm Hello n Help

  9. While Chevy,specifically, did not have a V-6 305, you left out that GMC did. From 1960-74, GMC had various versions of the V-6 305, and even the 351, 401, and 478.

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