Tom Wilson
August 12, 2004
01. Horse Sense: Changing rocker arms is a quick and easy power solution-so was our test. When another, more complicated, test fell through due to a lack of parts, swapping rocker ratios quickly and easily provided a power boost. It might do the same for you trackside someday as well.

It's no secret that adding duration and valve lift helps make power. But besides changing camshafts, there is another way to modify valve events-change the rocker-arm ratio.

Ford V-8 rocker ratios-at least in small-block pushrod engines-are 1.6:1 as designed and sold by Ford. What this means is the lobe lift ground into a camshaft is multiplied by 1.6 to arrive at the valve lift. Thus, a cam lobe that measures 0.353 inch at its greatest height (the lobe lift), will open the valve 0.565 inch (0.353 x 1.6 = 0.565).

02. These 1.6 Hi-Tech stainless steel rocker arms are similar to the ones we've been running on our engine since day one. Comp's premium rockers, these high-strength, high-durability pieces are light yet feature exceptional fatigue strength compared to aluminum rockers. Rebuildable and available for every popular Ford pushrod V-8, these rockers are everything except inexpensive.

Furthermore, the valvetrain industry supports hot rodders by providing alternatives-in the case of small-block Fords, rocker arms with a 1.7:1 ratio. It doesn't take long to figure out valve lift increases with the "bigger" ratio, in our example going from 0.565 inch with stock-type 1.6 rockers to 0.600 inch with 1.7 aftermarket ratio rockers.

No Free Lunch
So, why doesn't everyone simply fit 1.7 rockers and make more power? Well, there's nothing but tradeoffs inside engines, and with rocker-arm ratio you're cashing in some valve-guide longevity for a bit more horsepower when fitting higher-ratio rocker arms. Ford has designed its small-block valvetrains to work with 1.6 rockers-interestingly, many big-block engines use 1.7 ratios in stock form, and it's worth noting small-block Chevys roll out the door with 1.5 rockers. But when the engineers calculated these ratios, they balanced the inevitable side load a rocker arm exerts against the valve guide and made their choice. When you move from a stock 1.6 ratio to a faster 1.7 ratio, valve-guide wear can be expected to increase. For a drag engine or maybe even a dedicated road-racing engine this is no big deal, but for street duty where the miles pile up and no one wants to be rebuilding cylinder heads on a yearly basis, valve-guide wear can be an issue.

03. & 04. Because they are durable enough and definitely less expensive, most enthusiasts opt for aluminum roller rockers, such as this 1.6 version shown with a 1.6 stainless version. Obviously the aluminum rocker is bulkier, which might pose a spring interference problem should large valve-springs be used. The aluminum rockers also must use a smaller roller trunnion to keep a maximum of aluminum mass for strength. These are both Comp rockers, and both are fine choices. Interestingly, Comp says the steel rocker can be lighter than the aluminum version at the valve end because of reduced material there. The difference has to be academic for all but ultra-rpm engines.

High-lift rockers are also tougher on the valvespring because they accelerate the valve. And there is always the possibility of crashing the valves into the pistons with more lift. This point is less of an issue with street engines and their mild compression ratios and valve events than it is with race engines, but it can happen with some aftermarket cylinder heads with unusually shaped combustion chambers.

Furthermore, there are often better ways of getting more valve lift. The clearest example is simply using a larger-lift cam. A higher-lift cam with the appropriate opening and closing ramps provides the benefits of higher lift with less trauma on the valve guides, springs, retainers, and so on.

And not to toss the entire idea of high-lift rockers under the bus, but extra valve lift is often not the best way of gaining power via the valvetrain. Given the option of increasing duration or lift, engine builders typically choose duration because it effectively provides more valve opening and resultant airflow than lift. That is gained via different camshaft lobes.

All that said, higher-ratio rockers have some advantages. They are less expensive and easier to install than a new cam. They can be tuned on a cylinder-by-cylinder basis, and they can be easily installed after the cylinder head, cam, and intake combination have been fitted to the engine and run for a while. That allows some fine-tuning on an existing engine where redesigning another camshaft is not practical.

4.

Two Schools of Thought
Tuners seem to fall into two camps when it comes to rocker arms. One says higher-lift rockers provide a way to even the power outputs of end cylinders, which often are lazier than the center cylinders at higher rpm due to longer runner lengths in the intake manifold. Not an issue with modern EFI intakes, this thinking dates from the carbureted school, where a single four-barrel intake inevitably offers shorter center runners and longer end runners.

The other school of thought says so what if some cylinders run better than others. If higher-lift rockers make more power, why not run them on all the cylinders and gain the maximum engine output, rather than the smoothest?

We belong to the second, max-power school of thought, so when we fitted our 347 Coast High Performance dyno mule to the Westech 901 SuperFlow engine dyno, we had a four-step plan on fitting 1.6 and 1.7 ratio rocker arms.

08. We had no qualms about mixing steel and aluminum rockers as this view of the aluminum 1.7 intake rockers and 1.6 steel exhaust rockers shows. Not normally done, there is no reason why an engine couldn't be run this way.

Using all Comp Cams hardware, from an XE282HR hydraulic roller camshaft to the roller rockers, we began with 1.6 ratio rockers on all valves, then tried 1.7 rockers on the intake valves while leaving the 1.6 rockers on the exhausts. Next we installed 1.7 rockers on the exhausts, so all valves were using 1.7 rockers, and then we put 1.6 rockers back on the intakes so we could try the 1.7 exhaust/1.6 intake combination. And just for the record, when we were finished testing, we fitted 1.6 rockers back to the exhaust before putting the engine back into storage because that's the normal Ford configuration and no doubt the way we'd want the engine next time we reached for it.

On the Dyno
The results are easy enough to summarize, if not a little scary for us at first, because nothing we did made any changes until the end. That is, there was no power increase or decrease until we got to the 1.7 exhaust, 1.6 intake combination, which made 10 more horsepower. This is both logical and puzzling at the same time.

09. To try the 1.7 rockers on all valves, we simply added the remaining 1.7 aluminum rockers to the exhaust valves.

The logical part is, the Z304 cylinder heads on our test engine are typical of many Ford small-block heads-they breathe proportionally better on the intake side than they do on the exhaust side. That means the exhaust port usually responds better to improvements, and savvy Ford tuners typically begin their power quests with the exhaust ports of the cylinder heads. This is the reason why many Ford camshafts are ground, with more lift and duration for the exhaust valves than the intakes.

The other consideration is, how much lift did the 1.7 rockers provide us, and was that extra valve lift in an area where the cylinder head could use it? Without a flow-bench test of the head, we had no guess going into the test, but some button pushing on the calculator, along with a bit of empirical knowledge of Ford cylinder heads, would have given us a good guess. Consider the following chart of valve lifts and rocker arm ratios. Using the lobe lift from our XE282HR Comp Cams hydraulic roller cam, we see valve lift reaches to the 0.600-inch level.

  Lobe LiftValve Lift
1.6 1.7
Intake 0.353 0.5650.600
Exhaust 0.358 0.5740.608

10. Of the four configurations tested, this combination of 1.6 intake and 1.7 exhaust rockers is the only one that made more power.

Now, 0.600 inch of valve lift may be wonderful stuff on a Super Street Outlaw engine, but for our unported, medium-performance Z304 cylinder heads, this is simply too much. The airflow is becoming turbulent and "stalls" in the high 0.500-inch range on nearly all streetable in-line-valve, small-block Ford heads, especially without custom porting. So adding much more than 0.550 inch of valve lift usually proves not worth the effort. In our case, it seems the intake port was unresponsive to the extra lift, but the exhaust port had more flow left in it, so it yielded a bit more power when the valve was opened farther.

The puzzling aspect is why the 1.7 rockers on all valves did not yield some extra power. Clearly they are a benefit when fitted just to the exhaust valves, so at least the exhaust valves should have been making some power. And if the 1.7 rockers did nothing when fitted to just the intake valves-they didn't make or lose power when just the intakes wore 1.7 rockers-then why didn't the 1.7 rockers on all valves turn up some power?

We're confident it isn't a testing issue, as we made multiple runs and averaged the data. We're also familiar with this dyno and whatever quirks it may have. There's nothing in the test procedures that would mask a power increase of this magnitude, and there is plenty of back-up data incorporated in our findings to weed out any anomalies. Yet the 1.7 rockers on all valves didn't help a whit, while 1.7 rockers on just the exhaust were worth at least 4 hp.

Perhaps the more lift on the intakes resulted in less efficient in-cylinder motion of the mixture, but we can't say.

The practical point is, this is the sort of tuning that can be done only on an individual engine. There are so many variables in the combustion-chamber shape, manifold tuning, and so on that although the flow bench will give some direction, only trying each combination on the dyno will tell for sure-which is why we do these things, we suppose.

Dyno Results
1.6 Rockers Intake, 1.7 Rockers Intake, 1.7 Rockers Intake, 1.6 Rockers Intake,
1.6 Rockers Exhaust1.6 Rockers Exhaust1.7 Rockers Exhaust1.7 Rockers Exhaust
RPM TORQUEPOWER TORQUEPOWER TORQUEPOWERTORQUEPOWER
3,000 398 227 390 223 400 229 396 226
3,100 399 236 393 232 400 236 396 234
3,200 402 245 398 243 402 245 399 244
3,300 405 255 401 252 404 254 403 254
3,400 406 263 403 261 406 263 405 263
3,500 407 271 405 270 407 271 406 271
3,600 409 280 407 288 408 280 408 280
3,700 411 290 411 298 410 289 411 290
3,800 414 300 414 308 414 300 414 300
3,900 417 310 416 317 417 310 417 310
4,000 419 319 420 328 419 320 419 320
4,100 422 330 421 337 422 330 423 331
4,200 424 340 423 347 425 340 424 339
4,300 425 348 426 357 427 350 426 349
4,400 428 359 430 368 430 361 429 360
4,500 431 370 429 377 433 371 431 370
4,600 432 379 430 385 434 380 432 379
4,700 431 386 427 390 432 387 431 386
4,800 428 392 425 397 430 393 429 393
4,900 426 398 423 403 429 400 428 400
5,000 426 406 419 408 428 408 427 407
5,100 423 411 418 415 425 413 426 414
5,200 420 416 415 420 422 418 424 420
5,300 417 421 414 426 419 423 421 425
5,400 415 427 411 431 416 429 418 430
5,500 410 430 405 433 413 433 415 436
5,600 405 432 400 434 407 434 411 439
5,700 400 435 394 436 400 435 406 441
5,800 395 436 388 437 396 438 400 442
5,900 390 439 383 439 391 439 395 445
6,000 387 443 379 440 386 442 390 447
6,100 384 446 374 442 379 441 386 449
6,200 380 449 368 442 375 443 382 451
6,300 371 445 n/a n/a 370 444 376 452

There's nothing like testing, testing, testing, when looking for that last bit of power. Of the four possible combinations of rocker arms, all four were practically identical in the lower rpm ranges, two were virtual repeats of each other, one showed the slimmest of losses, and one showed a definite improvement.

We began with 1.6 rockers on all valves, as this is the standard small-block Ford arrangement. A careful examination of the numbers shows the second pairing of 1.7 rockers on the intake and 1.6 rockers on the exhaust was the unhappiest combination. It's close to the baseline, but there is a barely there yet still distinct trend toward less power as the numbers are down about 1 hp wherever you look.

Putting 1.7 rockers on all valves didn't help or hurt. These runs looked like backups of all the 1.6 rocker arrangements.

Finally, 1.6 rockers on the intake and 1.7 rockers on the exhaust showed an unmistakable improvement, especially the higher the rpm went. At the very top, this combination was worth at least 4 hp. This is nothing that you'll feel, but one of those building blocks that makes one engine that's seemingly the same as another run better.

Clearly this engine combination is looking for some improvement on the exhaust side of the combustion cycle. We're also guessing there is some combustion efficiency or other difficult-to-determine phenomena aiding the 1.6 intake/1.7 exhaust combination, as the 1.7 all-around combination didn't show an improvement.

All testing was done on a SuperFlow 901 using 300 rpm/sec sweeps. All data shown is corrected, torque is given in lb-ft, and all data shown here is an average of multiple runs. 5.0

Sources

Comp Cams
Memphis, TN 38118
800-999-0853
http://www.compcams.com