Valvetrain Geometry

[Earlie A]

I always assumed the goal of basic valve geometry was to work to have the downward force of The rocker directly perpendicular with the center of the valve stem decreasing side loading of any kind. And to be as stable as possible when achieving that, I know I'm probably dumbing that down but am I off on my hypothesis.

The mid-lift geometry arrangement will give the most vertical loading possible with the smallest sweep possible. Another point we have not discussed is that a longer fulcrum arm will also reduce sweep and side loading.

 

[Dart Frog]

The mid-lift geometry arrangement will give the most vertical loading possible with the smallest sweep possible. Another point we have not discussed is that a longer fulcrum arm will also reduce sweep and side loading.

That makes a lot of sense to me for what that's worth LOL

 

[Newbomb Turk]

The mid-lift geometry arrangement will give the most vertical loading possible with the smallest sweep possible. Another point we have not discussed is that a longer fulcrum arm will also reduce sweep and side loading.

Eventually you run out of room for a longer rocker.

We have a very tight box to work in.

 

[Bewy]

Turk,
To answer your question. Same lobe for all the tests; only prod length & rocker style was tested. Purpose of the test was to see how/if prod length changed valve lift.

 

[Earlie A]

Here's an old thread about broken HS rockers. Look at the picture in post #6. That's 3/8" wide bearings.
Broken rocker arms two weekends in a row

 

[gzig5]

Here's an old thread about broken HS rockers. Look at the picture in post #6. That's 3/8" wide bearings.
Broken rocker arms two weekends in a row

Not good.

 

[Woody500]

Here's some more thoughts on small block rocker shaft/rocker arm geometry. I'll only be showing geometry on the long (valve side) of the rocker arm. Geometry on the short (pushrod side) of the rocker arm is not being discussed. I'm working through some set-ups using several rocker options on ProMaxx, Speedmater and TF heads. This particular post and the attached drawings are based on using PRW stainless 1.6 rockers on a stock ProMaxx 171cc head. The valve is 2.02 by 5.010" long. Full valve lift for this example is 0.650".

The first drawing in the PDF file is just an overview. The second PDF is a zoomed in view of the stock rocker shaft location with the rocker arm shown in 4 different locations - closed, 1/2 lift (0.325"), fulcrum arm at 90 degrees to the valve, and full lift. In this example the full sweep of the roller tip across the valve is 0.069" which occurs between valve closed position and the fulcrum arm at 90 degrees.

The third drawing shows this particular set-up with the rocker shaft relocated to a position that 'corrects' the geometry according to mid-lift theory. There are two requirements here. The shaft location must be such that the rocker fulcrum arm is at 90 degrees to the valve, and the sweep of the roller tip must be centered on the valve tip. In this case raising the shaft 0.092" and moving the shaft horizontally 0.065" away from the valve is necessary. The resulting full sweep distance is reduced to 0.038" which is the least amount of sweep possible for this particular fulcrum arm length and a valve lift of 0.650". This mid-lift theory is what most people are referring to when talking about correct geometry.

Jesel seems to take a different approach to valvetrain geometry. If you go to the Jesel website and look under Tech Tips there is a description of their approach. Basically, they want most of the roller tip sweep to occur in the first 1/2 of valve lift when valve spring pressures are the lightest. This limits side loading of the valve when spring pressures are the highest. Jesel also prefers to center roller tip sweep in the second 1/2 of lift. This method is a little harder to nail down since the description is more vague than the mid-lift method. The fourth drawing shows a shaft location that gives a (somewhat) Jesel approach to the geometry using our sample head. By LOWERING the shaft 0.010" and moving the shaft 0.032" away from the valve a Jesel type geometry is achieved. This shows that the stock rocker shaft location is fairly close to giving a Jesel type geometry with the PRW rocker. The PRW rocker has a fulcrum length of 1.371". A Hughes 1.6 rocker has a fulcrum length of 1.344", or 0.027" shorter than the PRW. Using the Hughes rocker with the shaft in the stock location would be extremely close to a Jesel type set-up.

So what is 'correct geometry'? And how far off are stock iron heads and the ProMaxx/Speedmaster/Edelbrock variants?

If the PDF's don't work on your end I can take pictures and post them.

No PDFs work on this site for me, since their updates.
Just what you've said so far, has certainly given me much more than I knew from previous threads that mention but never explain any of the actual geometry updates. And makes choices easier if you want OOTB rockers to be closest possible to correct
What about Speedmaster(are they SS?) rockers. Do all mentioned have bushed fulcrums available?
Any aluminum rockers close?

 

[Earlie A]

No PDFs work on this site for me, since their updates.
Just what you've said so far, has certainly given me much more than I knew from previous threads that mention but never explain any of the actual geometry updates. And makes choices easier if you want OOTB rockers to be closest possible to correct
What about Speedmaster(are they SS?) rockers. Do all mentioned have bushed fulcrums available?
Any aluminum rockers close?

I’ll post some pictures when I get back to the shop. It may be later today but could be tomorrow.

 

[Earlie A]

Here's pictures of the PDFs and a picture of all the rockers I have at the shop. Top is Speedmaster aluminum with what looks to be split aluminum bushings. Next one down is stainless steel PRW with single (grooved) bronze bushing. Middle is Hughes aluminum with no bushing. Blue one is Mopar Performance (by Crane) aluminum with no bushing. Bottom is Harland Sharp aluminum with needle bearings. I do have a set of Speedmaster stainless steel but they are at NC Engine Builder's shop. If I remember correctly they look very similar to the PRW but they are not the same. I think they have the same split bushings as the red Speedmaster aluminum. I don't remember all the details but I do remember @NC Engine Builder did not like the Speedmaster stainless as much as the PRW stainless.

 

[Bewy]

One thing to note. In the drawing above, the fulcrum length is changing as the rocker sweeps through it's motion. Same on the p'rod side. That is why rocker geometry is a can of worms...

 

[Earlie A]

One thing to note. In the drawing above, the fulcrum length is changing as the rocker sweeps through it's motion. Same on the p'rod side. That is why rocker geometry is a can of worms...

View attachment 1716466128

I should have used different terminology since a fulcrum is really the point or axis of rotation. The centerline of the rocker shaft would be the fulcrum point or axis. The distance I termed 'fulcrum length' is a fixed distance. It is the distance between two centers, so by definition it does not change. You are correct that the 'effective' length of the lever arm does change as the rocker rotates. We are in agreement, just saying the same thing in different ways.

 

[RustyRatRod]

So reading through "all this" is the mid lift way the best or not?

 

[Earlie A]

It would seem to me that a mid-lift geometry on both sides of the rocker arm would be the 'best' place to start for most applications. This would mean the line that connects the roller tip center to the rocker shaft center is at 90 degrees to the valve at 1/2 lift and the line that connects the center of the pushrod ball tip to the center of the rocker shaft is at 90 degrees to the pushrod at 1/2 lift. This is shown in the picture below which is taken from the book 'Mid-Lift' by Jim Miller available on Amazon or on mid-lift.com.

This arrangement gives the least amount of sweep across the top of the valve and the least wasted motion (or the highest efficiency) of converting cam lift to valve lift. Both of those are good things.

As others have mentioned the mid-lift geometry may not be what race engine builders choose to use so it may not be 'best' in all cases.

 

[92b]

So reading through "all this" is the mid lift way the best or not?

I think the answer to that question is, it depends on the application and even then there may be some disagreement depending on where you place your priorities. For another opinion on rocker arm geometry check out Billy Godbolds book " High performance cams and valve trains" Chapter 2 discusses rocker arms. It's a great read. Billy breaks it down in text, pictures and diagrams to help understand. Sorry if I sound like a commercial.

 

[Earlie A]

So here's the interesting part of all of this. I've seen it posted many times here and on other forums that bad geometry breaks parts. It is usually suggested that the stock Mopar or Edelbrock style shaft location is too low. But we see that both Billy Godbold and Jesel recommend a shaft location that is lower than mid-lift and actually quite close to the Edelbrock style location (assuming the valve length recommended by Edelbrock is being used). There are a lot of opinions on what 'correct' geometry actually is. It sure seems to me like there is a fair bit of flexibility in setting up shaft locations and much of the parts breakage may result from other problems in the system.

 

[TT5.9mag]

The answer is it depends on the rocker being used. Most aftermarket heads use a shaft location that copies oem architecture and is (was) designed for a shoe (paddle, scrub) type rocker with its geometric point of rotation (which changes through the sweep) at the contact between rocker and valve tip. When you replace that with a roller tip rocker the point of rotation moves to the center of the wheel and now doesn’t change through the sweep, necessitating a different shaft location than oem to obtain the most beneficial geometry. In Mike’s tech page 2 he discusses why 1/2 lift method is now the accepted norm, and why “auto manufacturers and elite race teams” have abandoned the 2/3 rule of setting up geometry as it was a method of minimizing scrubbing with the focus being on longevity and wear not performance.

 

[Earlie A]

The answer is it depends on the rocker being used. Most aftermarket heads use a shaft location that copies oem architecture and is (was) designed for a shoe (paddle, scrub) type rocker with its geometric point of rotation (which changes through the sweep) at the contact between rocker and valve tip. When you replace that with a roller tip rocker the point of rotation moves to the center of the wheel and now doesn’t change through the sweep, necessitating a different shaft location than oem to obtain the most beneficial geometry. In Mike’s tech page 2 he discusses why 1/2 lift method is now the accepted norm, and why “auto manufacturers and elite race teams” have abandoned the 2/3 rule of setting up geometry as it was a method of minimizing scrubbing with the focus being on longevity and wear not performance.

Jesel still uses it something close to the 2/3 method. Billy Godbold's book is from 2023, correct? I'm not saying they are correct, but I am saying they are not antiquated ideas. Jim Miller's book on mid-lift was published in 2003.

 

[TT5.9mag]

Jesel still uses it something close to the 2/3 method. Billy Godbold's book is from 2023, correct? I'm not saying they are correct, but I am saying they are not antiquated ideas. Jim Miller's book on mid-lift was published in 2003.

Yea and i’m not disagreeing with you (or rocket scientist Billy Godbold) as clearly y’all are more intelligent than me. I’m just noting the reasons givin by Mike for what he does. I think it’s definitely a good reason to have a conversation.

 

[Earlie A]

Yea and i’m not disagreeing with you (or rocket scientist Billy Godbold) as clearly y’all are more intelligent than me. I’m just noting the reasons givin by Mike for what he does. I think it’s definitely a good reason to have a conversation.

I'm not disagreeing with you or Mike either. As 92b mentioned earlier, correct geometry depends upon where you place the priorities. What problem do you most want to solve?

 

[TT5.9mag]

I'm not disagreeing with you or Mike either. As 92b mentioned earlier, correct geometry depends upon where you place the priorities. What problem do you most want to solve?

Agree. And all are compromises at one end or the other. I like the idea of controlling the valve (acceleration, deceleration, on/off the seat, as well as slowing it down through peak lift) and I’m willing to give up some scrub width, or a little side loading to get it. Others may not.

 

[RustyRatRod]

So here's the interesting part of all of this. I've seen it posted many times here and on other forums that bad geometry breaks parts. It is usually suggested that the stock Mopar or Edelbrock style shaft location is too low. But we see that both Billy Godbold and Jesel recommend a shaft location that is lower than mid-lift and actually quite close to the Edelbrock style location (assuming the valve length recommended by Edelbrock is being used). There are a lot of opinions on what 'correct' geometry actually is. It sure seems to me like there is a fair bit of flexibility in setting up shaft locations and much of the parts breakage may result from other problems in the system.

Well, dang near every V8 Mopar made has bad valve train geometry and they didn't break. But neither did they push the boundaries regarding valve train.

 

[TT5.9mag]

Well, dang near every V8 Mopar made has bad valve train geometry and they didn't break. But neither did they push the boundaries regarding valve train.

With stock rockers they were pretty good.

 

[RustyRatRod]

I'm not disagreeing with you or Mike either. As 92b mentioned earlier, correct geometry depends upon where you place the priorities. What problem do you most want to solve?

Sometimes it don't need to be solved. As in running a stock or very mild valve train. Lots of these engines have run millions of miles with "bad" geometry.

 

[Earlie A]

Sometimes it don't need to be solved. As in running a stock or very mild valve train. Lots of these engines have run millions of miles with "bad" geometry.

RRR, you have hit on the very reason I posted this thread to begin with. For most applications the bad geometry may not be ideal, but it ain't going to break parts. And who is right when defining 'bad' and 'ideal'?

 

[RustyRatRod]

RRR, you have hit on the very reason I posted this thread to begin with. For most applications the bad geometry may not be ideal, but it ain't going to break parts. And who is right when defining 'bad' and 'ideal'?

Whoever is footin the bill, that's who. lol