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The pushrod is a very important and often overlooked component in any high horsepower build. It doesn't have to do a much, just transmit lift from the lifter to the rocker arm, and bend or deflect as little as possible. Reducing mass in an engine is almost always a good thing however there are a few places, in order to work correctly under high stress, that require a super rigid part. The problem is that more rigidity almost always means more mass. In these applications that require a super rigid part, pushrods, rods, wrist pins etc., to reduce mass would be to get a part that either doesn't live long, or a part that reduces performance. This is where pushrods fall. There are ways to get a strong part with better design and better materials without gaining mass in some instances. But that mindset too often means one-off designs and exotic materials, and that means a lot of money. In the pushrod world there is a small group of materials that work best for the given mass. Mainly performance pushrods are made from a form of chromoly. Ours are no different being made of 4130 chromoly with hardened 8620 pressed in ends. I hear a some concern about pushrods and weight, but I can assure you the negatives of additional mass are more than out weighed by the benefits of having the valve open at the correct time, having the valve stay open longer and the additional lift gained fom no deflection(bending). The pushrods in street driven cummins are just a fraction of the weight of nascar pushrods that spin more that double the rpm's of a cummins. Nascar teams spend millions on engine design and research supports a more rigid design with more mass. Hamilton pushrods are manufactured by the same company that builds for many of the top teams internationally in all forms of racing. We have spent many hours designing and comparing with every "performance" diesel pushrod on the market.The main reason we have uprgraded pushrods is that the factory pieces deflect before they open the valve, which reduces lift, and retards cam timing, which in turn reduces airflow. The main culprit is the exhaust pushrod. It has to overcome phenominal cylinder pressure, especially on trucks over 500 hp. Check out some of the info I have collected you make any conclusions.
Lets look at wall thickness, O.D., material, and load buckling weight(using Uhlers equation for column strength in steel columns with moderate sideloading) first, then design. This buckling factor is the approx. weight in lbs needed to buckle a steel column. In the first picture from left to right factory,brand x, brand y, Hamilton 3/8, Hamilton 7/16
factory, X, Y, HD3/8 , HD7/16
wall thickness-.075", .041", .083", .095", .168"
O.D- .375", .313", .375", .379", .437
material- mild steel, chromoly, chromoly, 4130chr, 4130chr
Buckle weight- 1794lbs , 741lbs., 1982lbs, 2064lbs, 4035lbs
This buckling weight is the true measure of strength in a pushrod. Also the common nascar p.r. is .437" with .120" wall, some even use .500" .168" and spin over 10,000 rpm. Mass should be of no concern in this particular instance, especially when it is on the low velocity side of the rocker arm.
The following equations are for average cylinder pressure not peak which would be much higher for short moments. At the points in which the pushrods will be affected average is more precise such as exhaust opening. Also for horsepower the equation calls for BHP which is at the flywheel. For this I assume any reading obtained at the rear wheels will have a 16% loss in the driveline. For the equation to see how your cylinder pressure lines up look at the second number whiich is a derivative of the BHP. Also a lot of the trucks I have seen peak at around 2600rpm 792,000xBHP/C.I.x rpm=avg cyl pressure
325hp at crank = 273 at the wheels= 275psi 600 hp at crank=504 at wheels= 509psi. 750=630hp at the wheels=636psi 1000=840hpat the wheels=848psi 1200=1008hp at the wheels=1018 psi
now lets figure load on the pushrod factoring 2.65 sq. in. of valve face surface area, to calculate for a 12v find the surface area of your valves and then use either a 1.7 or 1.8 rocker ratio.
275 hp=273psi x2.65sq in=728lbs +220lbs spring press. X1.34 exh. rocker ratio is 1271lbs.
504hp= 2102 lbs force on pushrod
630= 2553lbs on pushrod
1000=3306 lbs on pushrod
1200= 3909 lbs on pushrod
Now here is where the rubber meets the road. Factory pushods are rated at 1794 lbs, Brandx 1982 lbs. Hamilton 3/8" 2064 and Hamilton 7/16" 4035. over 500rearwheel horsepower you at least need the 3/8 .095 wall over 630Rwhp you really need to jump to the 7/16" .120" wall. If you don't the pushrod deflects first then opens the valve. It delays the valve opening which reduces spool and power. If you are a low budget street racer you will not notice much difference in spoolup or major power but if you are racing or sled pulling and want to make sure you don't snuff that big charger at the big end, use a better pushrod to open the exhaust on time at full power and full cylinder pressure. It will help the chargers stay lit by not retarding the exh opening.