Valvetrain Geometry

[Earlie A]

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.

 

[Bewy]

Correct geometry is in the eye of the beholder...the be-holder holding the rocker arm....

 

[gzig5]

Looking forward to going through that doc. I've got the same head/rocker setup and I need to correct the geometry. This should be helpful in visualizing some of the parameters. I don't think I'll be cutting the shaft mounts deeper but that is an interesting concept.

 

[Earlie A]

and I need to correct the geometry.

This statement is sort of why I made the post. Many people on here say it. I've said it in some posts. But how do we know what good is? How do we know what 'correct' is? There are multiple theories on how rocker geometry can be set up. Who's right?

 

[Killer6]

Preferred geometry is dependent on the amount of design lift, I believe Jesel has it correct for higher lift apps, I believe the 90°@1/2-lift is more appropriate for mid .500" & lower lifts. I'd like Member @B3RE to offer an opinion on the positional decision making involved with these variables.

 

[jbc426]

Read Mike's Tech Article over at B3 Racing. I used his custom spec'd T&D Rocker Arms and geometry correction kit on an RB motor. The valvetrain has never been so smooth, quiet and happy to rev as it was with his set-up. I highly recommend him.

 

[Earlie A]

Read Mike's Tech Article over at B3 Racing. I used his custom spec'd T&D Rocker Arms and geometry correction kit on an RB motor. The valvetrain has never been so smooth, quiet and happy to rev as it was with his set-up. I highly recommend him.

Mike makes very good arguments in those articles, but there are other opinions and approaches.

 

[Newbomb Turk]

Mike makes very good arguments in those articles, but there are other opinions and approaches.

You want the fastest speed off the seat, the slowest speed over the nose and the narrowest sweep pattern.

Why would anyone want the valve to be slow off the seat and fast over the nose where all the flow is?

 

[TT5.9mag]

Mike’s (@B3RE) tech articles are full of valuable info. Correct geometry is when there is as little wasted motion as possible from lifter to valve and the valve is accelerated off the seat, and set down on the seat at the most efficient time in the most efficient way.

 

[Earlie A]

Good input. T&D also seems to advocate the mid-lift theory. 'As little wasted motion as possible' and 'narrowest sweep pattern' both occur with mid-lift geometry. 'Fastest speed off the seat and slowest speed over the nose' do not. These would occur if the fulcrum arm were at 90 degrees to the valve with the valve closed, but I understand what you are saying. I think you are comparing the Jesel approach with the mid-lift approach.

Killer6's comment about the Jesel approach being used on high lift stuff may be valid. Obviously Jesel is doing something right.

 

[TT5.9mag]

Controlling the valve, with regards to acceleration on and off the seat, as well as slowing it down through peak lift is why there’s power in correct geometry. The narrow sweep, and where it’s located on the stem become secondary factors, and are results of getting the shaft location (more) correct. I’d rather see good control of the valve and a little wider sweep, a little off center, then give up the benefits of valve control to narrow the pattern or center it.

 

[Earlie A]

Controlling the valve, with regards to acceleration on and off the seat, as well as slowing it down through peak lift is why there’s power in correct geometry. The narrow sweep, and where it’s located on the stem become secondary factors, and are results of getting the shaft location (more) correct. I’d rather see good control of the valve and a little wider sweep, a little off center, then give up the benefits of valve control to narrow the pattern or center it.

Are you saying you don’t think mid-lift is always the best solution? More acceleration early and less velocity at high lift would place the shaft higher.

 

[TT5.9mag]

Are you saying you don’t think mid-lift is always the best solution? More acceleration early and less velocity at high lift would place the shaft higher.

I tend to think so. I used the mid lift method on my BBC with stud mounts and it’s been flawless but I did exactly zero testing to see if what I did was best. The shafts on mopars, I learned all I could and realized there were smarter people than me (once again Mike @B3RE) and I sent him measurements and let him tell me where to put stuff and why.

 

[TT5.9mag]

In case some haven’t read it I’ll throw up a link. Trust me it’s worth reading, over and over until it clicks.

B3 Racing Engines LLC - Mopar Rocker Arm Geometry Tech

 

[Bewy]

The joys of rocker theory/geometry....

 

[Earlie A]

Here's some good information on Jesel's approach from their catalog. Scroll down to page 28. They are showing a setup with 0.850" valve lift. In this example they set the rocker shaft 0.050" LOWER than the shaft would be if set up with the mid-lift (they call it half lift) theory. Lowering the shaft makes the valve slower off the seat and faster at full lift than a mid-lift set-up. So, Jesel is prioritizing a reduction in side loading and guide wear over valve speed and maximum lift potential.

If friction is ignored and a roller tip is perfectly centered over the valve stem throughout the lift cycle there is no side loading of the valve. Side loading is a result of the horizontal friction force between the roller tip and the valve stem and twisting forces caused by roller tip to valve centerline misalignment. If a valve stem is viewed as vertical, the horizontal friction force of the roller 'skidding' across the valve tip is directly proportional to valve spring pressure. The twisting forces would increase with increasing valve lift and with increasing roller tip to valve centerline misalignment. Of the two forces the horizontal friction force has the potential to do the most damage.

Killer6 probably has this right. The higher the valve is lifted (higher twisting forces) and the greater the spring pressure (higher friction forces), the more important a Jesel like approach to geometry becomes.

https://static1.squarespace.com/sta...29d891/1698970607081/Jesel+Catalog+Vol+14.pdf

 

[Newbomb Turk]

Here's some good information on Jesel's approach from their catalog. Scroll down to page 28. They are showing a setup with 0.850" valve lift. In this example they set the rocker shaft 0.050" LOWER than the shaft would be if set up with the mid-lift (they call it half lift) theory. Lowering the shaft makes the valve slower off the seat and faster at full lift than a mid-lift set-up. So, Jesel is prioritizing a reduction in side loading and guide wear over valve speed and maximum lift potential.

If friction is ignored and a roller tip is perfectly centered over the valve stem throughout the lift cycle there is no side loading of the valve. Side loading is a result of the horizontal friction force between the roller tip and the valve stem and twisting forces caused by roller tip to valve centerline misalignment. If a valve stem is viewed as vertical, the horizontal friction force of the roller 'skidding' across the valve tip is directly proportional to valve spring pressure. The twisting forces would increase with increasing valve lift and with increasing roller tip to valve centerline misalignment. Of the two forces the horizontal friction force has the potential to do the most damage.

Killer6 probably has this right. The higher the valve is lifted (higher twisting forces) and the greater the spring pressure (higher friction forces), the more important a Jesel like approach to geometry becomes.

https://static1.squarespace.com/sta...29d891/1698970607081/Jesel+Catalog+Vol+14.pdf

Most likely the lobes they would be using would be incredibly fast to begin with.

No matter what you still need to control the valves.

With the small cam bearings/core diameter you have flex and flex eventually breaks parts.

Most guys on here are running sub .700 net lift and steel valves so while valve speed (more of it) usually makes more power, it won’t if the lifter is getting lofted over the nose (unless that’s what you’re after) and it’s a sure way to kill a lifter or the lobe or both.

The same goes for the opening and closing side of the lobe. While smacking the crap out of the lifter to get things moving can let you delay valve opening it won’t matter if all you are doing is bending the pushrods.

On the closing side you can absolutely beat the seats right down in short order if the valve is hitting the seat and bouncing multiple times. I don’t think there is a valve that hits the seat once and doesn’t bounce. All the spintron video I’ve seen shows some bounce, but the number of times it bounces and the velocity and height it hits when it bounces can cause power loss and for sure seat/valve damage.

RPM is a factor in all of this. The more cycles per second the more chances of the valve train going chaotic. In that case you may have to decide if getting slower off the seat to control the valve train is more important than over the nose speed is.

I have an engine on the dyno right now that at 6000ish the valve train got unhappy. So we made the decision to change valve springs.

While there was a minimal increase in seat and over the nose pressure (in fact I think we went down about 10 on the seat and gained 20 over the nose (SFT cam) the wire diameter was bigger and the spring rate was slightly higher than the springs that were on it.

Now the engine will zip up to 7k without issue. It’s not making more power there but the valve train is under control better than it was.

I’ve seen pushrod flex drive the valve train into chaos. I’ve seen a change of retainer from Ti to steel get the valve train under control, which is a bit counterintuitive.

But, once you get the valve train excited it won’t stop being excited until the rpm is lowered enough to get things back in control.

The 5/16 bolts holding down the rocker shafts are a serious weak link. That’s why I’m not a big fan of using solid rollers on small blocks that are going much over 6500 and certainly 7k is the limit because the bolts don’t have the clamp load to keep the shafts from moving more than they already do. Even 3/8 is pretty small when you only have 5 bolts holding the shafts down and the centers are that far apart.

I suppose the upshot of all this is there is no one right answer to which way to set the geometry.

There are way too many variables to say this is how it is.

Here on this forum most guys are not running .800 plus net lift and rpm over 8k, so IMO if you can get the valve speed up off the seat and slow it down over the nose AND not drive the valve train into chaos that’s probably going to make the most power.

You can only fit a certain diameter pushrod in any given head so that’s a valve speed limit.

You can only get a certain spring load before either the cam/lifter won’t take any more spring load (a huge limit with SFT cams) before you start killing parts.

That’s another reason to dyno an engine. It’s pretty easy to see distress in the valve train.

 

[Newbomb Turk]

The joys of rocker theory/geometry....

View attachment 1716464542

View attachment 1716464543

Were two different lobes tested in the length test? And what was the lash number or was that at zero lash?

Also, I’ve never seen a Crane rocket that was billet. AFAIK they were all made from extruded bar stock.

In fact I think most aluminum rockers are still made from extruded bar stock. The extruding process compresses the grain structure and makes the rocker stronger.

 

[Earlie A]

The 5/16 bolts holding down the rocker shafts are a serious weak link. That’s why I’m not a big fan of using solid rollers on small blocks that are going much over 6500 and certainly 7k is the limit because the bolts don’t have the clamp load to keep the shafts from moving more than they already do. Even 3/8 is pretty small when you only have 5 bolts holding the shafts down and the centers are that far apart.

I'm having some of the same thoughts, especially when that 5/16 bolt goes into an aluminum head. Edelbrock and Speedmaster use a helicoil insert in the rocker shaft bolt holes. ProMaxx, AFR and Trick Flow do not. So with the Edelbrock and Speedmaster the tapped hole in the aluminum is probably 3/8". The helicoil reduces it to 5/16"

Take a look at the picture below. These are some different rocker shaft hold down caps. The two on the top are steel. The two on the bottom are aluminum. In my opinion the caps need to be steel and they need to be stout. A rigid cap will reduce the effective length of the shaft between centers. I can't see the aluminum helping very much. Even though the rocker stand under the shaft is aluminum, a wide, stout steel cap should add rigidity to the system. It would also change the resonant frequency of the rocker shaft. But just like in the example you posted with the steel vs Ti spring retainers, you never know if the frequency change will help or hurt. Rigidity seems like a good thing to me though.

 

[Icetech]

You want the fastest speed off the seat, the slowest speed over the nose and the narrowest sweep pattern.

Why would anyone want the valve to be slow off the seat and fast over the nose where all the flow is?

I can answer that! It's people that listen to **** like this and just want their valvetrain to be mellow.. and take it easy.....


Personally i want my valvetrain to be more like this

 

[Earlie A]

Put a solid roller under my hood and Ventura Highway in my 8-track please.

 

[Icetech]

Put a solid roller under my hood and Ventura Highway in my 8-track please.

 

[Icetech]

This is your camshaft on soft rock

 

[gzig5]

Were two different lobes tested in the length test? And what was the lash number or was that at zero lash?

Also, I’ve never seen a Crane rocket that was billet. AFAIK they were all made from extruded bar stock.

In fact I think most aluminum rockers are still made from extruded bar stock. The extruding process compresses the grain structure and makes the rocker stronger.

I can't swear to it, but I'm pretty sure that all round and rectangular aluminum stock, or "billet", are extruded. The only difference between bar stock from SpeedyMetals and the Crane rockers is the shape of the die the aluminum was pushed through, as long as the starting alloy is the same. Extruding to near net shape is a savings of machining time, but the general material properties would be the same unless there was some exotic heat treat cycle required. Billet aluminum is cast into a big block (billet) but then heated and extruded to the round, square, hollow, etc shape that the end user needs. It isn't like forged vs cast steel, where the grains structure and strength of a forging are significantly stronger.

Take a look at the picture below. These are some different rocker shaft hold down caps. The two on the top are steel. The two on the bottom are aluminum. In my opinion the caps need to be steel and they need to be stout. A rigid cap will reduce the effective length of the shaft between centers. I can't see the aluminum helping very much. Even though the rocker stand under the shaft is aluminum, a wide, stout steel cap should add rigidity to the system. It would also change the resonant frequency of the rocker shaft. But just like in the example you posted with the steel vs Ti spring retainers, you never know if the frequency change will help or hurt. Rigidity seems like a good thing to me though.

I don't disagree with you but then I also don't understand how the BB guys are using aluminum main caps as an upgrade in high HP builds.

Rigidity is usually but not always a good thing. I was a reliability engineer for ten years before retirement and did a lot of Long Term Reliability and HALT testing (Highly Accelerated Life Testing) on a variety of products. It was a lot of fun and we beat the **** out of everything and the engineers always complained that nothing could survive, but that was the point. Find the failure modes in a short amount of time. Anyway, you would be surprised what resonant frequency can do to a supposedly rigid design. Sometimes, adding compliance and altering the natural frequency is a better idea. There are always caveats though, and in a mechanical system like the SBM valvetrain, less flex is generally going to be better at the higher stress levels.

I've always wondered how much a thicker rocker shaft of the right heat treated material would increase stiffness of the system. With pushrod oiling, you could run a solid shaft no? Solid should be more resistant to flex between the hold downs with strong springs. On the other hand, I would have thought it's been tried before and if it isn't popular, maybe the difference is insignificant.

 

[Earlie A]

Rigidity is usually but not always a good thing. I was a reliability engineer for ten years before retirement and did a lot of Long Term Reliability and HALT testing (Highly Accelerated Life Testing) on a variety of products. It was a lot of fun and we beat the **** out of everything and the engineers always complained that nothing could survive, but that was the point. Find the failure modes in a short amount of time. Anyway, you would be surprised what resonant frequency can do to a supposedly rigid design. Sometimes, adding compliance and altering the natural frequency is a better idea. There are always caveats though, and in a mechanical system like the SBM valvetrain, less flex is generally going to be better at the higher stress levels.

I've always wondered how much a thicker rocker shaft of the right heat treated material would increase stiffness of the system. With pushrod oiling, you could run a solid shaft no? Solid should be more resistant to flex between the hold downs with strong springs. On the other hand, I would have thought it's been tried before and if it isn't popular, maybe the difference is insignificant.

I was just thinking about some of this same stuff as far as resonant frequency goes. More stiffness should raise the resonant frequency. More mass should lower it. Would take a lot of high level testing to find out which is best.

I've looked at some bending/stiffness properties of hollow shafts vs solid shafts vs smaller diameter shafts. Solid is certainly stronger than hollow but not by much. It's the OD and the distance between supports that has the most effect. That's part of the reason a rigid, wide steel cap makes the most sense to me. Reduce the span.

Reducing the diameter of the shaft gives a rocker arm manufacturer more flexibility to design a strong rocker arm. A 7/8 shaft and a 1.5"+ spring make for a thin rocker arm.