Don Terrill’s Speed-Talk

Doing some valve train upgrades to my 422 sbc . New valves ,springs, retainers ,pushrods. Going to 3/8" pushrods from 5/16", will there be a big advantage using .120 wall over .080 wall pushrods on the 3/8" pushrods . Springs are 260lbs on the seat and 650lbs at max lift . Max gross lift is .710 on the intake side . Dan

As a general rule of thumb,the thicker the better.
Guy

6.50camaro wrote: ↑Tue Jun 23, 2020 2:29 pm Doing some valve train upgrades to my 422 sbc . New valves ,springs, retainers ,pushrods. Going to 3/8" pushrods from 5/16", will there be a big advantage using .120 wall over .080 wall pushrods on the 3/8" pushrods . Springs are 260lbs on the seat and 650lbs at max lift . Max gross lift is .710 on the intake side . Dan

How much pressure did the old springs have when you used 5/16 pushrods?

The diff between a .120 and a .080 wall 3/8 pushrod is the .120 wall is 9% more resistant to bending, but it is also much heavier.

For comparison sake, an .080 wall 3/8 pushrod is almost twice as stiff as a .120 wall 5/16 pushrod so unless your spring pressures went way up, the .080 wall 3/8 pushrod might work for you.

Blackford , the old springs were 220# on the seat and 580# at max lift . The old push rods were 5/16 x .080 but they also were 6.550" long I was using Crower Hi-Seat roller lifters. New pushrods are in the 8.000" range for use with normal seat height lifter and a lot lighter weight roller lifter with a .900 base circle cam and a plus .200 valve lenght . I would have thought the .040 thicker wall would be more than 9% stiffer ,but I never researched that . Dan

6.50camaro wrote: ↑Tue Jun 23, 2020 3:55 pm Blackford , the old springs were 220# on the seat and 580# at max lift . The old push rods were 5/16 x .080 but they also were 6.550" long I was using Crower Hi-Seat roller lifters. New pushrods are in the 8.000" range for use with normal seat height lifter and a lot lighter weight roller lifter with a .900 base circle cam and a plus .200 valve lenght . I would have thought the .040 thicker wall would be more than 9% stiffer ,but I never researched that . Dan

I assumed length was the same or close. Your new pushrods are going to be a bit longer which will make them more prone to buckling, but .080 3/8 pushrods will probably still be OK. They are 2x stiffer than the .080 5/16 pushrods you used before. If you're not concerned about pushrod weight, use the .120 wall pushrods.

In a cross section, the farther away the material is from the centerline, the more the impact on stiffness. The .120 wall pushrods have more material, but the added material is closer to the cross sectional centerline so it has a smaller impact. That's why a 3/8 pushrod is twice as stiff as a 5/16 pushrod when they both have .080 wall thickness.

blackford wrote: ↑Tue Jun 23, 2020 5:37 pm If you're not concerned about pushrod weight, use the .120 wall pushrods.

Pushrod weight should be a non-issue in this situation. Weight on the "lifter side" of the rocker arm is not as critical as the weight on the "valve-side" of the rocker arm. This is the reason as to why we utilize weight saving items such as Titanium retainers, valves, etc.
Therefore,increases in pushrod diameter and wall thickness will result in a significantly stronger pushrod and the increase in pushrod weight is not detrimental.

Good rule of thumb on selecting pushrods:

Run the largest pushrod that you can physically fit in the engine
Diameter before thickness. When you can no longer increase in diameter, increase wall thickness.
Pushrod weight has little effect on valve train, but stiffness does. The stiffer the better.

The relative inertia of a pushrod's weight (and everything else on the cam side) vs. inertia of the valve etc. is inverse to the square of the rocker arm ratio.
1.5:1 rocker: valve side weight × 44.4%

panic wrote: ↑Wed Jun 24, 2020 9:57 am The relative inertia of a pushrod's weight (and everything else on the cam side) vs. inertia of the valve etc. is inverse to the square of the rocker arm ratio.
1.5:1 rocker: valve side weight × 44.4%

Makes sense during the valve closing event since the rocker ratio creates a mechanical advantage on the cam side components.

Sure, it's a mech advantage pushing it back, doesn't warrant being wasteful[mass and/or $] about it.

I quess I could weigh the old Crower hi seat roller lifters and the 5/16 pushrod and compare against the new lite weight std seat lifters with the 3/8 pushrod just to get a comparison to see if there is added mass or less mass on that side of the pivot . Not that that is going to matter because I am going to use 3/8 pushrods and will probably go with the Manley .135 wall pushrods . I know I said .120 wall in the title of the post but I just double checked and found i had a little brain fade at the time I posted this . Dan

Useful Information:

I'm not aware of any engine out there yet that hasn't made a gain of some sort by going to a larger, thicker pushrod. Not always a lot, but there are gains. Never seen one go backwards and lose power as a result though....Even spintron testing has proven this....pushrods deflect a lot more than most think, even the big, thick, heavy ones....the more you can reduce that, the better off your valve train is...it's just one piece of the puzzle though....

last set of AFR heads i bought instructions said use at least .120 thick pushrods. found that funny for mild heads with hydraulic roller. i think they are smarter than me

It has nothing to do with weight, That is why MadBill and I use .187 wall 3/8 pushrods.