A study on Cummins pushrods...... Regardless of the pushrod you want to use, find out what the buckling strength is and compare that to the data for a given HP level to make sure you have the right part for your particular application. Data, not opinions, should transcend any brand mongering. Many a buyer has been duped, being sold something that does not address the needs in his application, by somebody who does not understand what they are selling. Some products are made up to address a certain price point, not the needs of a certain application. This because sellers know that in many cases a person will buy based on price point, not technical merit. To this end, a .080" wall thickness will be a very minimal gain in rigidity over a factory piece. It's price point and it's effectiveness are equal....minimal
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, connecting rods, wrist pins etc., to reduce mass excessively would be to get a part that either doesn't live long, or a part that reduces performance. This is where pushrods fall. I hear 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 from no deflection. The pushrods in street driven diesels are much less weight than nascar pushrods that spin more than double the rpm. Research supports a more rigid design meaning more mass. When running double the horsepower they were designed for, factory pushrods deflect before they open the valve, which reduces lift, and retards cam timing, which in turn reduces airflow. The exhaust pushrod has to overcome phenominal pressure from cylinder pressure that acts on the valve face and is multiplied by the rocker arm and then applied to the pushrod. Some nascar engines use .437"o.d./.120" wall and others .500"o.d./.168" wall and spin over 10,000 rpm. A heavier pushrod has more benefit than a light weight, less rigid pushrod, especially since it is on the low velocity side of the rocker arm.
Some people say that performance pushrods are not necessary, the numbers say otherwise.
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, motorsport supply, Hamilton 3/8, Hamilton 7/16
factory, X, motorsport supply, 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- 1794 lbs , 741 lbs., 1982 lbs, 2064 lbs, 4035 lbs
Buckling weight is a way to define rigidity and resistance to bending in a pushrod. Now that you know what the buckling force is on the various pushrods on the market, let's look at how that affects your engine factoring for your rpm, horsepower, displacement and rpm you make your power used in the calculations.
The following equations are for average cylinder pressure not peak which would be much higher close to TDCC. Average cylinder pressure is more useable than peak cylinder pressure as the exhaust valve is opening nowhere close to TDC. 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. Also a lot of trucks I have seen peak at around 2600rpm-3300rpm but, the calculations are assuming 2,600rpm.
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 1.69 rocker ratio.
275 hp=273psi x 2.65sq in= 728 lbs +220lbs spring press. X 1.34 exh. rocker ratio is 1271 lbs.
504hp= 2102 lbs force on pushrod
630= 2553lbs on pushrod
1000=3306 lbs on pushrod
1200= 3909 lbs on pushrod
Factory pushods are rated at 1794 lbs, Motorsport supply 1982 lbs. Hamilton 3/8" 2064 and Hamilton 7/16" 4035. Over 500 rear wheel horsepower and you at least need the 3/8 .095" wall. If you are 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 can reduce spool and power. If you are a low budget street racer you will not notice much difference but if you are racing or sled pulling, there is a benefit.
As for a cummins pushrod that has a wall thickness of .080", buckling strength will be around ~1850lbs.
The only way to really compare pushrods is to look at buckling force, the type and hardness of the ends, and the surfae area of the ends. Take the buckling force, then compare that to the horsepower/cylinder pressure for your application and then factor for your surface area of valves. If a person uses boost pressure to dictate what pushrod to use, that is a pretty good indication that they have no idea what they are selling. I have seen 40psi produce 200hp and I have seen 40psi produce 1,000hp.
In the end it is up to you the consumer to judge your needs and the remedy.
