I scanned this from the January 2011 Car Craft magazine and thought it might help with this often asked question. Seems many of the younger guys are willing to attempt a build-up on their own but are stuck with the lowly 305. Here's the mag's technical editor Jeff Smith's response to this question from a reader in the "What's Your Problem" section......
THE 305 CONTROVERSY, REVISITED—AGAIN
Ed Shagman; Corona, CA: This appears true of your magazine as well as your competition: A reader asks about building his 305 small-block, and the answer is to dump the 305 and start with a 350. This is an ongoing path you all seem to take. Why not change course and build a stout 305 new or a junkyard dog to prove it's possible? Also, why is it that Ford fans routinely build 281-inch two-valve engines with tiny 3.5inch bores and get 400 hp or more, and the same magazines joke about the Chevy's 3.75-inch bore? i think a 305 can be built to 450 hp and more, so maybe you should come up with an engine builder's competition. It would be *itchin'!
Jeff Smith: It seems we have to deal with this question about once a year. Some people just want to build a 305—and that's fine. But let's go over why that's not the best idea if you're building an engine to make good power. You brought up bore size, Ed, so let's start there. The reason the Ford guys build the 281-inch mod motors with their tiny 3.5-inch bores is because that's all they have to work with within the engines available for the late-model Mustangs. Even the 5.4L truck engine only has a 3.5-inch bore, and the new 5.0L is only slightly larger at 3.029 inches. The mod engine was designed with a 3.93-inch bore spacing, so a 4.00-inch bore is physically impossible. But if the bore spacing were wider, you can bet your last lug nut that any engine builder would be using a 4.00-inch bore. When you're forced to work with an engine with a tiny bore, you make the best of what you have.
One reason tiny engines like the Ford mod make good power (besides three and four valves per cylinder) is because the builders are willing to spin those overhead cam engines to the moon. The classic equation you should have tattooed on your forearm is Horsepower =TorquexRPM/5,252. Using this equation, if you make 30O Ib-ft at 4.000 rpm, the engine only makes 228 hp. Take that same 300 Ib-ft and apply it at 7,500 rpm and the number jumps to 428 hp. That's great if you like to spin the engine that high. This also means you should shift at closer to 8,000 rpm to maximize acceleration. All that rpm is abusive on parts— much more so than shifting at 6.000 rpm. But even beyond that, a tiny, 281ci, normally aspirated engine that makes 420 hp at 7,500 rpm would fae a miserable street engine because it would deliver minimal torque below 5,000 rpm. Then you have to put a deep gear in the car to make it accelerate.
Let's stick with this displacement idea for a moment longer. Let's use a typical 350ci small-block that makes 1.1 Ib-ft per cubic inch. Multiply 350ci x 1.1 = 385 Ib-ft. which is fairly typical of any mild small-block. GU s 353 H.O. crate engine is rated at 380 Ib-ft at 3,800 rpm and 330 hp at 5,000 rpm. While enthusiasts are drawn to the horsepower numbers. I prefer to look at torque and horsepower because torque is what accelerates the car. A larger-displacement engine will always make more torque as a result of its size. That's how engine designers made power back in the '20s—with giant engines that ran slowly. They made monster torque—just no horsepower. Now let's take that 305ci engine and multiply 305 x 1.1 = 335.5 Ib-ft of max torque. We're down roughly 50 Ib-ft.and that's just at torque peak. So a 305 up against a 350 (even at the same horsepower) will always lose to the larger engine because the 350 will make more torque.
I completely agree that a 305 can be built to make 450 hp. That's not too difficult. The engine is limited to smaller heads with 1.94-inch valves because larger 2.02-inch valves won't clear the cylinder wall. You're also going to give up roughly 10 cfm or more of flow because the small bore shrouds the valves and reduces flow. You can make 450 hp, but this will require a much-longer-duration camshaft to make torque at higher engine speeds—around 6,500 rpm. A longer-duration camshaft merely moves the torque peak higher in the rpm curve. To make 450 hp at 6,500, this engine has to make 364 Ib-ft at that engine speed. Most street engines create a span of 1,500 rpm between peak torque and peak horsepower, which puts peak torque at 5,000 rpm. That means that at 4,000 rpm, the engine is barely making 320 Ib-ft. Compare that with the mild 350 H.O. engine that makes 380 Ib-ft at 3,750 rpm—or roughly 60 Ib-ft more torque at this lower engine speed. That additional torque is there every time you touch the throttle, making the larger engine much more fun to drive. Europeans have long suffered with tiny little engines to get decent mileage because gas prices in Europe are so high. But in the good old USA, we've always driven big cars with big engines that make tons of torque. That's why there are still people who think the 5OOci Cadillac engine is worth massaging. Those engines make stupid torque.
We could also talk about how the 350ci engine parts are substantially cheaper because of the insane volume those engines enjoy. Compare replacement piston prices between a 350 and 305. The 305 piston is smaller, so it requires less aluminum, yet the 350 pistons are less expensive. That's because the 350 pistons are sold at ridiculously high volumes, which means the price comes down.
So what have we learned? Can you build a 305 to make 450 street horsepower? The answer is yes. Will it run as quickly in the quarter-mile in the same car as a 450hp, 350ci engine? No, because the 350 will make more torque that will help acceleration. So the bottom line is the 305 will cost more to build and won't make as much torque as a 350. Now I'll agree that if you live on a desert island and all you have is a 305 to build, go for it. But given a choice, I'll always go for more displacement to make more overall
power. I understand that engine-building decisions are often based on what you have lying around. All I'm saying is that you can find Chevy small-block engines on almost every street corner in America, so unless you enjoy spending more more to make less power, the answer seems obvious. OK, the emails can start flooding in!
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