Trying to decide...They will be non-self-aligning. I will be getting guideplates.

 

Once you getta big block

 

Short answer is when the loads the valvetrain sees demand it.

Bret will probably give a more specific answer based upon your individual engine. All the valvetrain bits need to be on the same page. IOW, they need to work together.

Good to meet you yesterday.

 

It would be an unusual small block that would need them, as was suggested. I had a long discussion with Manton about pushrods for an engine a buddy is building (SBF). He is using springs with ~250lbs on the seat and over 600# over the nose. Manton recommended an 11/32" x 0.083" pushrod.

Rich Krause

 

Nice to meet you too!

It sounds like a 5/16" might be the way to go...I don't see any need really for a 3/8"...

 

5/16 High Quality ones will work in most cases.

You could write a novel on this, but the short answer is when the bigger one makes more HP.

The simple explanation of PR and how big/stiff they need to be is dependant on a few things.

1. Pushrod Length - This is something you see more and more pro teams doing in PR motors, making the PRs shorter. The best way to do this is move the cam up in the block. Dodge and Toyota have about the highest cam placement in NASCAR motors today. The new DRCE blocks in Pro Stock move the cam up even higher in the block. This is all done because a shorter a pushrod is the less deflection it has in it.

2. Spring Pressure - higher the spring pressure the higher the loads on the PR are at seated and over the nose. Deflection of a PR is related to spring pressures, the higher you go the more it bends.

3. Rocker Arm Ratio - Here is another thing and almost nobody ever thinks about it. The Rocker Arm multiplies the lift on the camshaft when you go in the direction of cam, pushrod, RA, Valve. The side effect of this is when you go back the other way it multiplies the load the spring makes. So starting w/ a 1.5:1 rocker and going to a 1.7:1 rocker means that you will have 13% more force on the PR at all times, seated and over the nose. One reason why LS1's beat up lifters faster than LT1's

4. Camshaft Profile - Most importantly is the camshaft ramp. The more aggressive the ramp the more intense the actions trying to instantly bend the pushrod are. The faster you open the valve the more deflection that the pushrod will see at that moment.

5. Lifter Type - Solids and Hyd will act differently on a PR, but the SR setup is usually harder on PR and will cause more deflection. Think of a Town Car's ride (Hyd) to a Go Kart (Solid) for an easy example.

6. Rocker Arm/ Rocker Stud Stiffness - The less deflection that the other parts allow means that more loads will transfer to the PR meaning it will deflect more.


Most people think PR diameter is directly related to spring pressure. That's a pretty simple rule of thumb, but as you can see most of the valvetrain as something to do with it.

The big thing here is that with deflection the PR shortens up and applies less lift to the valve. It also doesn't replicate the cam's implied motion for the valve, therefore in most cases causing the valve area to be less, therefore less air/fuel goes into the motor = less HP. The 3rd bad thing that comes from deflection is that the valvetrain geometry gets away from optimum, causing odd loads on the parts evolved.

The higher the motor turns, the faster the PR has to go thru its paces. Even though it's doing it at half speed at 6,000rpm that puppy is doing its little dance at 50 times a second. It doesn't really get a chance to slow its dance down, so the only thing you can do is stop the dance from starting. Then throw on top of that resonant frequencies that each spring has and it starts playing all sorts of fun games with the PR too.

Basically you can't have enough PR stiffness.

This is not a easy thing to explain face to face, since it's so long and drawn out and it can get boring, but basically that's the best way I can describe it in my Smokeyese.

Bret

 

I thought i was the only person that ever thought about this. Earlier i was worried about my lifters when changing rockers and i called up Comp and they didnt seem to care what size rockers were on the car. The guy kept telling me that it didnt matter what size the rockers were. He only cared about open pressure.

But wouldnt LS1s eat up lifters IF they had the same spring pressure, but most LS1s use lighter springs. They eat them when people use "comp's cam experts" who tell them stock lifters are good to 380 open pressure, regardless of rocker ratios.

380 open pressure w/ 1.8 rockers is roughly the same as 460 open pressure on 1.5 rockers, so it makes quite a difference.