The Duration Game

What Happens When We Add Duration to a 500 HP Big-Block Chevy

Words And Photos: Jeff Smith

It’s been said many times that the camshaft is the brain of the engine. It tells the valves when to open and how long to remain open. Those simple signals have a complex effect on how an engine performs and how much power it makes. While this may seem simplistic but minor changes in duration can have a huge effect on how an engine performs either on the street or the track.

To many enthusiasts, the camshaft and its specs can be a confusing jumble of numbers and specifications that are often bandied about in less-than-articulate bench racing sessions. For this story, we decided to concentrate on just camshaft duration so we can quickly drill down into this spec that has the most dramatic effect on engine performance – both good and bad.

Duration can be defined simply as the number of crankshaft degrees the lobe holds the valves open. The first thing we must remember is that the camshaft spins at half engine speed, which means the cam only turns one half revolution for every full spin of the crankshaft. Duration is doubly confusing because it can be expressed using two different specs, both of which are listed on a cam spec card that accompanies a new cam.

Advertised duration is defined as the number of degrees that produce lobe lift starting and ending from a specific measuring point specified by the manufacturer. Because it is difficult to identify exactly when the lifter begins its upward movement, manufacturers specify their chosen lift point. With COMP Cams, this measuring point is when the lifter achieves 0.006-inch of lift. So let’s say that a given lobe has an advertised duration of 280 degrees. This would be the duration (in crankshaft degrees) from 0.006-inch of lobe lift until the lobe reaches 0.006-inch on the closing point of the lobe.

The problem with comparing advertised duration numbers between different cam grinders is that not everybody uses 0.006-inch of tappet lift. They may use 0.004-inch for example. In this case, a measuring spec of 0.004-inch would add several degrees of duration to the advertised cam specs compared to a cam measured at 0.006-inch. This would mean that a cam measured at 0.006-inch with an advertised duration of 280 degrees might be 284 degrees (or more) if measured at 0.004-inch tappet lift. The only way to truly compare these cams would be to measure both at the same point.

For this reason, several decades ago Harney Crane recommended and started using 0.050-inch as a standardized point for duration specs. As you might expect, the duration listed at 0.050-inch and at 0.006-inch will be radically different. While this is true, there is a dedicated relationship between the two but we’ll save that for a different discussion. For the sake of accuracy, it is important to know the measuring point when discussing camshaft duration. There’s a huge difference between a cam with 250 degrees of advertised duration and one with 250 degrees of duration at 0.050-inch tappet lift. The difference can be between 50 and 60 degrees depending upon the lobe design.

If we had to narrow the effect that duration has on the engine to one parameter, it would be that duration has the greatest effect on the rpm points where peak torque and peak horsepower occur. This means that with a given engine combination, if we add several degrees of duration to the camshaft, this will tend to push the engine’s peak horsepower to a higher rpm. That’s the theory. Let’s say that we have a camshaft with intake duration of 224 degrees at 0.050 and we plan to step up to the next larger camshaft in that family of hydraulic roller cams. If the original cam produced its peak torque at 4,000 rpm, the next larger cam should push peak torque up to perhaps 4,200 rpm. Let’s stress that this is the theory. As we’ll see in our actual dyno test, engines don’t always perform the way the theory says they should.

By itself, shifting the peak torque to a higher rpm might not seem worth the effort. But the reason why big camshafts are so popular, (beyond their cool choppy idle sound) is because this added duration also pushes the peak horsepower point to a higher rpm. This happens because with a longer intake duration, the intake closing point is delayed, allowing more time (in degrees of duration) for the cylinder to fill with more air and fuel, making more horsepower. This is called the power band and is defined as the rpm spread between peak torque and peak horsepower. Most street engines offer a power band of 1,500 to 1,700 rpm. So if we bump the peak torque from 4,000 to 4,500 rpm and our engine has a 1,700 rpm power band, then peak horsepower will shift from 5,700 to close to 6,000 rpm. Of course, this assumes that the intake system, heads, and exhaust can support this rpm level and provide sufficient airflow to actually make more power.

Rather than just talk about the theory, we decided to test the effects of changing duration on a typical street big-block Chevy. For this story, we will be using a 0.030-inch over-bored 454 big-block Chevy displacing 460ci with JE 10.25:1 pistons, ported iron oval port heads with 2.19/1.88-inch valves, 1 7/8-inch primary tube headers, an Edelbrock Performer RPM dual pane intake manifold, and a Holley 850 cfm carburetor as our test engine. We will run three different COMP hydraulic roller camshafts through the engine (see our Cam Chart for the specs) with the only change between all three cams will be the intake and exhaust duration numbers. The three cams that we chose are all within the same Xtreme Energy family of lobes. This triumvirate employs the same additional 6 degrees of exhaust duration that helps with top-end power. The lobe separation angle (LSA) is also the same at 110 degrees for all three cams. There was, however, a slight increase in lift with the biggest of the three cams, but this should not dramatically affect the peak horsepower point.

We did our testing at Westech with help from Steve Brule’ using the company’s SuperFlow dyno. The engine was also equipped with a Milodon oil pan and pump filled with Lucas Hot Rod & Classic 10W30 high-zinc engine oil, and a Meziere electric water pump to keep the engine cool. Our first pull with the mild cam (TQ1 and HP1 on the chart) produced some amazing torque numbers from our mild Rat. With duration at a mere 224/230 degrees at 0.050, the Rat started with 551 lb-ft of torque at 3,000 rpm and its peak torque of 576 lb-ft occurred at a rather low 3,800 rpm. Peak horsepower was 510 hp at a conservative 5,500 rpm, producing a 1,700 rpm power band. This would be a great combination for a truck pulling a trailer and would also be excellent a heavy car with a tall rear gear. When the engine makes within 25 lb-ft of its max torque at just above idle speed, this makes for a fun street engine.

Swapping cams in the engine was a relatively easy process especially since we had equipped the engine with a COMP two-piece aluminum timing chain cover. This also helped with setting the end play on the cam, which we duplicated for all three roller cams. We decided that the first cam would be called Baby Bear and the middle and big cams would obviously be Momma and Poppa Bear. We decided to leave Goldilocks out of this. The Momma cam added six degrees to both the advertised and 0.050 numbers on both the intake and exhaust lobes with the same 110 LSA. The power results (TQ2 and HP2) made our evaluation interesting as it duplicated the same peak torque rpm point of 3,800 rpm. But the longer duration did extend the peak horsepower point another 200 to 5,800 rpm. Plus, we gained 17 horsepower from the additional rpm now with a peak horsepower of 527. That’s 1.14 hp per cubic inch (hp/ci).

The final test installed the biggest of the three cams (Poppa Bear) with 236/242 duration and a slight increase in lift on both sides of the cam. The theory says that a bigger cam should hurt power below peak torque while increasing the power up high – again essentially moving the torque curve upward in rpm. Comparing the torque of Baby Bear to Poppa Bear, the bigger cam sacrificed 23 lb-ft at 3,000 and essentially equaled the peak torque of the smallest cam at roughly the same rpm. But it did improve peak horsepower to 536hp – a gain of 26 peak horsepower over the smallest cam. The peak horsepower rpm remained the same as the Momma Bear cam.

If you study the power numbers and the graph, you’ll see that peak torque (measured in lb-ft) does not change because that is more a function of displacement and air flow than of cam timing. But what is much more apparent is how the numbers on either side of peak torque tend to rock or pivot around peak torque. So with the bigger cam, the low-speed power dropped while the high-speed power increased compared to the smaller cams. Looking at average torque, the middle cam was only 1 lb-ft of torque behind the big cam, so that would be another indication that this version would be the best choice compared to the other two cams – especially when you take into account the idle quality numbers. As you would expect, Baby Bear cam offered the best idle vacuum of 13 inches of vacuum at 900 rpm while Momma Bear delivered 11 inches while Poppa Bear barely squeaked in with roughly 8 inches all at the same idle rpm. A lower idle quality is one price you pay for a bigger cam.

Overall, our three cam test reinforced the theory, but there were some interesting effects that are definitely worth discussing. At first, it might be tempting to observe that the comparison of the Momma and Poppa Bear cams does not support the theory since the peak torque and the peak horsepower rpm did not change. But there were mitigating circumstances that we think affected these results. First, the engine was equipped with a dual plane intake manifold which tends to minimize upper horsepower improvements. Had we tested with a single plane intake, the peak horsepower results probably would have been more in line with the theory. Still, all three cams support the concept that with longer duration, peak horsepower will tend to move to a higher rpm point. In this case, since the engine is intended to be a torque beast on the street, the still conservative duration numbers kept peak power to roughly 6,000 rpm.

Just for grins, we plugged the Baby Bear power curve into our Quarter, Pro dragstrip simulation software and using a 3,700-pound El Camino or Chevelle with only a 3.08 gear and a TH400 trans, assuming no tire spin the car would run 11.30’s at 118 mph at 4,800 rpm through the finish line lights! So this is no slouch motor despite the fact that it barely makes 500 hp. This is where torque is your friend.

So we’ve seen that there are tradeoffs for adding duration and that for any given engine combination, there will be a cam that will deliver the most overall power for a given amount of duration and valve lift. Especially for a street engine, that cam may not necessarily be the one with the longest duration numbers.

Cam Specs

Camshaft Adv. Duration Duration -.050 Valve Lift LSA
XR276HR-10, PN 11-423-8
Intake 276 224 0.510 110
Exhaust 282 230 0.510
XR282HR-10, PN 11-432-8
Intake 282 230 0.510 110
Exhaust 288 236 0.520
XR288HR-10. PN 11-433-8
Intake 288 236 0.521 110
Exhaust 294 242 0.540

Power Graph

Power Chart

TQ1 HP1 TQ2 HP2 TQ3 HP3
3,000 551 315 540 309 528 302
3,200 557 339 551 335 540 329
3,400 567 367 566 366 557 361
3,600 573 393 576 395 569 390
3,800 576 417 581 421 576 417
4,000 574 437 579 441 577 439
4,200 568 454 574 459 573 459
4,400 560 469 569 477 570 478
4,600 551 483 560 490 565 495
4,800 543 496 552 504 558 510
5,000 528 503 541 515 547 521
5,200 511 506 526 521 534 528
5,400 495 509 507 521 516 530
5,600 478 510 491 524 499 532
5,800 455 503 477 527 485 536
6,000 427 488 452 516 464 530
AVG.* 533.5 450.8 541.8 459.2 542.8 461.4

Averages were calculated based on the entire curve with data taken every 100 rpm.

Parts List

Description PN Source Price
COMP XE-276HR-10 hydraulic roller cam 11-423-8 Summit Racing $290.07
COMP XE-282HR-10 hydraulic roller cam 11-432-8 Summit Racing $277.97
COMP XE-288HR-10 hydraulic roller cam 11-433-8 Summit Racing $290.97
COMP retro-fit hydraulic roller lifter set 854-16 Summit Racing $435.97
COMP 1.7:1 Ultra Pro Magnum rocker arms 1823-16 Summit Racing $407.97
COMP Hi-Tech roller timing set 3110 Summit Racing $50.97
COMP pushrods, 7.750-inch, order 8 8905-1 (8) Summit Racing $13.97
COMP pushrods, 8.850-inch, order 8 7910-1 (8) Summit Racing $12.97
COMP hydraulic roller dual valve springs 954-16 Summit Racing $132.97
COMP spring retainer, light weight tool steel 1732-16 Summit Racing $153.97
COMP super valve locks 612-16 Summit Racing $22.97
COMP valve spring seat 4782-16 Summit Racing $35.97
COMP 2-piece timing cover 212 Summit Racing $260.97
Milodon oil pan, low profile 30950 Summit Racing $255.95
Milodon oil pump pickup 18301 Summit Racing $49.95
Lucas Hot Rod & Classic 10w40 oil, 5Qt. 10683-1 Summit Racing $27.97

Sources

Aeromotive
aeromotiveinc.com

COMP Cams
compcams.com

Holley Performance Products
holley.com

Lucas Oil Products
lucasoil.com

Meziere Enterprises
meziere.com

Milodon
milodon.com

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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