5.0 Mustang & Super FordsHow To Engine
High-Lift Rocker Arms: Lift and Invigorate
Playing with rocker ratios is an old hot rodder's trick-here's how we found more horsepower on our 347
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).
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.
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.
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.
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.
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 Lift||Valve Lift|
|1.6 Rockers Intake,||1.7 Rockers Intake,||1.7 Rockers Intake,||1.6 Rockers Intake,|
|1.6 Rockers Exhaust||1.6 Rockers Exhaust||1.7 Rockers Exhaust||1.7 Rockers Exhaust|