Is Pushrod Flex Limiting Power?
The simplest link could be your weakest.
Words: Brandon Flannery
Tim Cole of COMP Cams told a story of how his engine quit making power and he spent way too much time and money overlooking one simple component: His pushrods. Inspired, I asked him to write it down: “In the ’90s, I was struggling to get my Buick V-6 drag car to be competitive against the Chevy splayed valve headed motor. One Chevy in my NHRA class was faster, and I was bound and determined to catch up. I had spent countless hours and too much money in that quest….but I kept plugging. My motor would go up to one rpm and then flatten out. I chased and chased heads, manifold combinations, carburetors, gear ratios, and cam grinds — but to no avail.
There was a tech in our R&D department at COMP Cams who occasionally went racing with me. One day, he told me that I was not using a stiff enough pushrod, based on what Spintron testing was showing. He convinced me to try a new set of thick wall, bigger diameter pushrods.
We went out to Memphis Motorsports Park and tested. Data acquisition was used to monitor engine performance, as always. We made a baseline pass in the normal configuration, and the results were the same as always. (What do they say about the definition of insanity?) Then, we changed pushrods, rechecked valve lash, and went out for another run within the hour. The car picked up 5 mph, coupled with an .08 reduction in ET. Data showed rpm did not even attempt to flatten out! I had been fighting this pushrod flex syndrome for all this time. When the next opportunity arose, we set the national record and that Buick was not beaten for class again.”
Now, pushrods are very simple things. All they have to do is transfer elevation changes from the camshaft to the rocker arm. They become compressed as the lifter rises on the cam lobe, and again by spring pressure as the valve closes. This “pole vaulting” action and battle of forces does funny things as rpm increases. Once the valve train goes out of control, the motor loses its ability to accelerate, as in Tim’s case where it flattened out at a certain point.
The load and speed of the lifter changing direction creates tremendous bending forces that makes the pushrod deflect and cause problems. A pushrod under deflection stores energy as it bends and then releases that energy in an uncontrolled fashion. The pushrod is no longer a solid link transferring motion at precise moments. Instead it moves erratically, creating a wave of ill-timed events that shortens duration and controls the valve differently from the shape of the cam lobe.
The angularity differences between lifters and cups or adjusting screws on the rocker arms can also contribute to deflection, as will the additional load caused by increasing rocker ratios or spring pressure. These angles can cause side-loading, most of which is generated at the lower end of the pushrod. Deflection can result in harmonics that often cause the pushrod to “hula” around and rub the block and/or heads. Witness marks on heads have occurred over a half of an inch away from pushrods.
Since a pushrod is longest when it’s perfectly straight, any change also affects length. Events of the valvetrain are carefully orchestrated to gain maximum performance, and altering the push-rod length means less lift (or erratic lift as it springs back into shape) that alters the scheduled events. The rocker will not see full travel in relation to the lobe profile, and the valve will not open correctly.
This loss of control also affects duration (the time the valve is supposed to stay open.) Since the valve was supposed to open earlier than the time deflection is making it happen, and then snapping shut, duration is shortened. The valve will not be open long enough. This loss means the engine isn’t breathing to its full potential.
Another by-product of reduced duration is that by the time the valve begins to open, it’s opening “late” and the cam timing is being retarded. This means the charge isn’t filling and exiting the cylinder correctly and combustion isn’t as efficient. This is equivalent to moving the timing.
Deflection of the pushrods can also cause “lofting,” or valve float, especially in hydraulic applications where the lifters pump up and hang the valves open at high rpm. Open valves don’t build compression or create a one-way path for the intake and exhaust charges, translating into lost power or worse. At the other end of the cycle the release of energy causes valve bounce, resulting in reversion and possible valve failure.
Stock push rods are generally made from 1018 steel tubing. Because they are matched to their demands, they have a fairly low buckling weight and are not good beyond 320 psi of seat pressure, or 4,500-5,000 rpm. Performance applications use stiffer valve springs, more cam lift, and see higher rpm — all of which increase valvetrain loads. If overlooked, it’s easy to see why the pushrod is a major fail-point.
Exhaust-side pushrods also have to overcome extreme cylinder pressures, especially those that exceed the spring load when using power adders. The correct timing of this event is critical to performance.
The solution? A stronger pushrod. Deflection can be reduced by increasing a pushrod’s outside diameter or wall thickness.
Pushrods made from 4130 Chromoly also offer higher tensile strength — 4140 even more so. Deflection can also be reduced with dual-taper pushrods. A tapered pushrod will withstand harmonic frequencies to a higher rate than a solid, straight tube.
Guide plates that restrict motion are also popular, but need to be used with straight (non-tapered) pushrods hardened to withstand the wear caused by rubbing on the plates. Engine builders know that weight stalls rotating components, and they are hesitant to add any more to the valvetrain. This is why the pushrods may be overlooked. One thing to keep in mind: When saving weight comes at the expense of deflection, it’s not a savings.
Thankfully, the pushrod is on the “slow side” of the valvetrain. The additional pushrod mass is significantly outweighed by the benefit of having the correct duration and valvetrain stability. Weight savings on the faster valve side is far more critical and can show big improvements. Combining lightweight springs, retainers, valve locks, and rockers with a good pushrod can improve valvetrain accuracy and add horsepower.
It’s not uncommon for engines to need a smaller-duration cam after upgrading to better push-rods. Additional duration may have been used to compensate for the loss in deflection, often with compoundingly negative results. It becomes a case where “too much is still not enough.” Once the right pushrods are using of all the duration, the cam may prove to be too big.
We asked Kirk Peters of Lunati Cams for recommendations on proper pushrods for various applications. His first bit of advice was to make sure the rocker arm geometry was correct.
“Proper rocker arm geometry cannot be achieved without the correct length pushrods; therefore getting a pushrod length checker is very useful. Lunati and other companies offer them in different incremental styles starting from 5.800 inches and ending up at 11.800 inches.
He suggests a 5/16-inch, .080-inch wall for hydraulic flat tappets like Lunati’s Sportsman Series Pushrods and a .105-inch wall for those high performance applications where rules strictly mandate the use of a 5/16-inch pushrod. These are heat treated for strength and made from 4130 chromoly steel.
Solid flat tappets should step up to a minimum 3/8-inch, .080-inch wall, while a solid roller application pushrod should be a minimum 3/8-inch, 135-inch wall. Lunati’s Signature Series pushrods or COMP Cams Hi-Tech line gives you seamless carbon-nitrided tubing that is then heat treated to eliminate deflection and stress fractures.
Kirk says those who need heavy duty pushrods, like solid roller applications or extreme duty solid flat tappets like NHRA Stock Eliminator, should use a .165-inch wall, double-taper 7/16-inch pushrod.
“These extremely durable pushrods are double tapered and capped with 5/16-inch semi-spherical ball ends machined to 210 degrees to ensure ample clearance,” he adds. “They are made from chromoly and are case hardened through multiple heat treatments.”
Finally, the big boys. Many solid roller applications with big nitrous or big boosted blower motors with crazy cylinder pressures should run a fat ½-inch, .200-inch wall pushrod on the exhaust to help overcome those forces.
If you have a high-performance engine and haven’t put a lot of thought into the pushrods, or just bought what somebody recommended, there is a chance you are leaving power on the table. Bigger isn’t always better, but a little invested pushrod weight in the valvetrain might increase horsepower or open the path to getting the most efficient cam for your application.