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Attitude Adjustment: Valve Lash for Power

Words and Photos: Richard Holdener

Whether you have a Ford, Chevy or Dodge, one of the key elements in improving the power output is the right camshaft. The cam is often described as the brain of the internal combustion engine, as it synchronizes the valve events that all but determine the character of the motor. Mild cam timing will obviously create a different personality than some high-strung, race cam, even if everything else on the motor remains constant. Of course, it helps when the cam timing is combined with both sufficient cylinder head flow and an optimized induction system that allows the motor to produce power in synch with the cam profile. Only working together can the combination maximize power, as it does no good to have a cam designed to make power at 5,000 rpm when the heads and intake want to rev to 7,500 rpm! Though the first step is always choosing the right cam, did you know there are additional considerations once you have made your choice? Things like advancing and retarding the cam during the installation cam increase or decrease the effectiveness of the combination. The same is true of simple things, like adjusting the valve lash.

Cam profiles come in two major varieties, including flat-tappet and roller profiles. Within each variety exists both hydraulic and solid versions. For this test, we will concentrate on the solid-lifter, flat-tappet cam profile, but the lash adjustments can also be applied solid roller cams. Why is it necessary to have lash in the first place, you ask? Unlike hydraulic lifters (flat-tappet or roller), the solid-lifter mechanisms are unable to compensate for the difference between the growth rates of the block, heads, pushrods, and valves. The lash must be set to allow for the differences in material growth rate, as well as for the design of the ramp rate of the cam. The lash must be neither insufficient nor excessive, as either can cause valve-train problems, irrespective of the attending changes in power production. For our dyno test, we were concentrating on the changes in power from lash adjustments that were well within what might be considered the normal range. This is especially the case with these older (factory) solid flat-tappet cam profiles, but as indicated previously, changes in lash can also be applied to solid roller cam profiles as well.

To illustrate just how much power adjusting the valve lash can be worth, we ran a pair of tests on two different motors. Test motor number one was a 350-inch small-block Chevy. The 350 featured a 4-bolt block, steel crank and factory (pink) rods swinging 11:1 forged pistons. The short block was topped off with a set of “Fuelie” heads, in this case with 492 casting numbers. Of course, the important part of our test was the valve train. The iron-headed small block received a dual-pattern, solid-lifter, flat-tappet cam offering a .459/.485 lift split, a 242/254-degree duration split and 116-degree lobe separation angle. This motor was built as a replica of a 1970 LT1, complete with a factory, aluminum dual-plane intake and four-barrel carburetor. For our test, we installed suitable valve springs that allowed us to safely rev the motor without fear of valve float. After adjusting the valve lash on the solid flat-tappet cam from .030 down to .020, we saw a sizable difference in power. The power output of the 350 increased above 4,000 rpm, by as much as 11 hp, but the torque below 4,000 dropped by 6-7 lb-ft. The tighter lash would certainly help improve trap speeds but might hurt overall drivability.

If it works on a Chevy, it must work on a Ford, right? To find out, we used another muscle car motor, this one a HiPo 289 small-block Ford. The Ford featured a 2.87-inch crank (even shorter than the 3.0-inch used in the 302). The 289 build also featured 5.155-inch factory rods (ARP bolts) connected to a set of forged, flat-top pistons. The pistons were designed to replicate the four valve reliefs employed on the 1965 HiPo 289 motors. Topping the short block was a set of HiPo (small chamber) 289 heads designed for use with an adjustable valve train.

Like the Chevy, the little Ford featured a solid-lifter, flat-tappet cam with .477 lift and 310 degrees of advertised duration (near 228 degrees measured at .050). The 289 was also equipped with a Shelby aluminum intake, 750 carb and long-tube, tri-Y headers. By altering the valve lash from .024 down to .010, we saw an even greater change in the power output of the 289 than the LT1 Chevy. The tighter lash once again improved the power output past 4,200 rpm in this case, while dropping torque down low. The lash improved power by as much as 24 hp, but dropped torque by as much as 19 lb-ft. Even more than the Chevy, the lash definitely changed the attitude of the 289 Ford.

Graph 1: Chevy 350-Effect of valve lash (.030 vs .020)

This test was run on a reproduction of a 1970 Chevy LT1 350. The small-block was equipped with a reproduction of the factory LT1 solid, flat-tappet cam. The dual-pattern cam offered a .459/.485 lift split, a 242/254 duration split and 116-degree lsa. Run with .030 lash, the 350 produced 346 hp at 5,300 rpm and 393 lb-ft of torque at 3,800 rpm. After adjusting the valve lash down to .020, the peak power improved to 353 hp at 5,400 rpm and 392 lb-ft of torque at 4,100 rpm. The tighter lash setting improved top-end power but traded low-speed torque production up to 4,000 rpm.

Graph 2: Ford 289-Effect of valve lash (.024 vs .010)

The power gains offered by a tighter lash setting were even more pronounced on the 289 Ford. The HiPo (Shelby) 289 was equipped with a factory reproduction cam that offered .477 lift and 310 degrees of advertised duration (roughly 228 degrees measured at .050). The solid-lifter (flat-tappet) cam was run at two lash settings, starting with .024. Run with .024 lash, the 289 produced 286 hp at 5,400 rpm and 309 lb-ft of torque at 3,900 rpm. After adjusting the lash down to .010 (very tight), the power output of the 289 increased to 302 hp at 5,800 rpm and 309 lb-ft of torque at 4,300 rpm. Like the Chevy, the tighter lash improved top-end power but cost low-speed torque below 4,000 rpm.