Max Torque at Low RPM?
Scott Weaver; via CarCraft@carcraft.com: I love reading your Ask Anything column! I have always paid close attention to your cam recommendations and the relation of cam duration versus torque and at what rpm that torque occurs. Most stock small-block Chevy cams are around 190–200 degrees of duration with around a 5,000 rpm redline, while most street performance cams run around 230 degrees of duration. Is it possible to exploit a short-duration cam (like 170–180 degrees) for more torque at lower rpm, say, with a 4,000-rpm redline? I'm thinking of something like a 350ci small-block that would put out around 180–200 hp and 400-plus torque at around 3,000 rpm, something that would be right at home in a work truck. Or am I overthinking cam duration?
Jeff Smith: There's nothing wrong with your idea, Scott. In fact, that's why the OEMs use short-duration camshafts. But there are details and implications galore that make for a fun bench-racing session that you can use to amaze your friends with your newfound knowledge. Basically, torque is most closely tied to displacement. The easiest way to make a large amount of torque is with a large-displacement engine—it's really that simple. That's why those old Cadillacs from the '30s used huge engines. They couldn't spin them to make horsepower, but they made awesome torque due to their large displacement. But I think what you are asking is how much torque can a small-block Chevy make? A working estimate for torque and horsepower is to multiply the displacement times a number that ranges from 1.1 to 1.25. This is admittedly a rather large range; we'll get into that a bit later. For now, let's take a typical 355ci small-block and apply the entire range estimate for torque. On the low side, 355 times 1.1 equals 390 lb-ft of torque. The high-side estimate is 355 times 1.25 equals 444 lb-ft. That's a difference of 54 lb-ft of torque. The difference in power between these two estimates is based on how well the engine breathes. But first, let's show the effect when we try different displacements. A 302ci engine making 1.1 lb-ft/ci will make 332 lb-ft, while on the high side it can make 377. If we then take a 406ci small-block, it has a range of 447 to 507 lb-ft. Even at a mild 1.1, a 406ci small-block Chevy, Ford, Chrysler, or AMC has the potential to make great torque. That's the beauty of displacement and the truth behind the oft-heard line, "There's no replacement for displacement."
So if your goal is to make as much torque as possible with a short-duration camshaft, there are some trends worth inspecting closely. We mentioned that a good-breathing engine would make good power. This puts a premium on efficient airflow in and out of the engine. Another good idea is compression. If all you do is add compression to an engine, you will make it more efficient, and it will make both more torque and more horsepower. But here is where you have to be careful. There is an important relationship between cam timing and compression. What you don't want to do is pump up the compression to 10.0:1 and then add a very short–duration camshaft. This will radically increase the low-speed cylinder pressure, and the engine will detonate. One way to evaluate an engine's combination is to measure cranking cylinder pressure. Typically, a cranking pressure in excess of 200 psi could be excessive and cause detonation. Let's get into this a little deeper. By the way, you might have guessed by now that one of the best ways to increase that 1.1 hp/ci number to something closer to 1.25:1 is to increase the static compression ratio.
Short-duration camshafts open the intake valve later and close it sooner than a longer-duration cam. The most important point of the four intake and exhaust valve events is the intake closing point. For most long-duration performance cams, a later closing intake point allows the intake port more time at high rpm to fill the cylinder. Remember that at 6,000 rpm you have exactly half the amount of cylinder filling time in actual milliseconds to fill that cylinder compared to 3,000 rpm, even though the intake valve duration has not changed. Since we're talking about making more torque at around 3,000 rpm, we would want a shorter-duration intake lobe that will close the intake valve sooner. This might sound counterproductive, but the reality is that at lower engine speeds, the intake port has time to fill the cylinder. By closing the valve sooner, this captures more of the existing air and fuel in the cylinder. Remember that we cannot begin to compress the mixture with a rising piston until the intake valve closes.
Let's look at an example of a short-duration intake lobe. We chose a Comp High Energy 240H cam with 192 degrees of duration at 0.050 tappet lift. The Comp timing card says the intake valve opens at 9 degrees before top dead center (BTDC) and closes 50 degrees after bottom dead center (ABDC). These specs are for the intake lobe at the advertised duration figure of 240 degrees. We can calculate the duration by adding 9 plus 180 (the duration in degrees between TDC and BDC) plus 50 equals 239 degrees. Comp probably just rounded the figure off to 240 degrees for simplicity's sake. Let's compare this to an Xtreme Energy flat-tappet hydraulic 284-advertised duration cam with an intake opening point of 34 degrees BTDC and an intake closing of 69 degrees ABDC. Right away, you can see that the critical intake closing point on the longer-duration camshaft is a massive 19 degrees later than the short-duration camshaft. This means the longer-duration camshaft will have a much lower dynamic cylinder pressure at low engine speeds because the intake valve closes much later than it will with the short-duration lobe. This is also why longer-duration cams require more static compression in order to run decently at lower engine speeds because they are losing lots of cylinder pressure with that late-closing intake. You can measure this again by looking at the cranking cylinder pressure of an engine. Add a long-duration intake lobe and watch the cranking cylinder pressure drop. That's all due to the later-closing intake valve.
Conversely, you can probably also now see why too much static compression with a short-duration intake lobe can create excessive cylinder pressure at low engine speeds. This will almost always result in detonation. I've driven a small-block Chevy with 11:1 compression and a 190-degree short-duration hydraulic flat-tappet camshaft (it's a long story), and the half-throttle detonation sounded like eight rocks clattering in a three-pound Folger's coffee can*. This is something you definitely want to avoid.
*A long time ago, coffee came in cans. Back to the story…