Yellow Bullet Forums banner

Valve Train/Cam Design

27K views 306 replies 38 participants last post by  qtrpounder 
#1 ·
Not wanting to clutter up the 360ci thread anymore, but thinking system design, lobe vs. ratio ect... is a GREAT topic to discuss....................

Notice I put valve train BEFORE cam design! :cool:

In my mind, other than the core size and lifter placement, the cam/lobe is the LAST thing to design in the system.................?

Thoughts??????????
 
#4 ·
In my mind, other than the core size and lifter placement, the cam/lobe is the LAST thing to design in the system.................?

Thoughts??????????
It would make sense then to optimize the design, material, and weights of the components based on desired fatigue life. Part of the fatigue analysis for the valves would include an optimum seating velocity for the expected life of the valve. This would give you one component of your future lobe design....
 
#5 ·
Copied this from the 360 thread, as I think it is a good direction!?!
With this "multiplied" pressure on the heaviest parts in the system, along with reducing their velocities for a given valve event/lift curve............would the spring (and/or pressure) need to remain "constant" between the two desings?

IMO no. You would need less spring pressure due to the lower velocities on that side of the system. My question is, if you lower the spring pressure, do you still have enough to control the valve.

I think thats why valvetrain design is way more complex then just picking out a lobe and a spring. In theory i would think you want just enough spring to control the valve motion and just enough spring to control the lifter/pushrod to achieve minimum deflection and friction. Reaching this balace is what makes a great valvetrain IMO.
Thoughts?
 
#8 ·
There's got to be a trade off somewhere, where more rocker ratio has diminishing returns. I'd think getting as much work done as possible at the cam and lifter would be where to start, and from there, get the rest with rocker ratio.
 
#14 ·
It would seem that way, but 150# with a 2:1 rocker would essentially be the same as 200# with 1.5:1. The lifter needs X amount of pressure to stay on the lobe, whether it's from a 2:1 rocker, or 1.5:1. By using rocker ratio you can lighten the spring (mass) and your lifter acceleration on the lobe can be less and more easily controlled. That's one school of thought.
 
#22 ·
One problem is that there is no magical design. Everything discussed here is subject to the design of the whole system in terms of component weights, geometry, and rpm ....... just to name a few.

We can't say this way is better than that way as a mandate.... because for what application, design of system, and rpm?

Very complicated issue.
 
#30 ·
One problem is that there is no magical design. Everything discussed here is subject to the design of the whole system in terms of component weights, geometry, and rpm ....... just to name a few.

We can't say this way is better than that way as a mandate.... because for what application, design of system, and rpm?

Very complicated issue.
this...
 
#24 ·
I think the term 'high ratio' is subjective. Why would 1.5 be considered low and 1.9 be considered high? Why isn't 1.8 or 1.9 considered moderate? Because General Motors only came with 1.7 on a bbc in 1965?

I don't think we should get caught up in a number like that and calling it high, or low.

If I want a particular lift at the valve per deg.... you may not get there but with a particular ratio of multiplication.... it just is what it is.
 
#28 ·
I think the term 'high ratio' is subjective. Why would 1.5 be considered low and 1.9 be considered high? Why isn't 1.8 or 1.9 considered moderate? Because General Motors only came with 1.7 on a bbc in 1965?

I don't think we should get caught up in a number like that and calling it high, or low.

If I want a particular lift at the valve per deg.... you may not get there but with a particular ratio of multiplication.... it just is what it is.
If we could get the cam and lifter for a pushrod engine to do what we wanted the valve to do, would you rather see a 1:1 rocker, or still use some ratio?
 
#31 ·
I have always considered that ratio works in your favor on the lifter side.... because the spring has the ratio in its favor to keep the lifter on the lobe. It's multiplying the spring against the lifter....

Again, just a thought that I have had about it.
 
#33 ·
I have always considered that ratio works in your favor on the lifter side.... because the spring has the ratio in its favor to keep the lifter on the lobe. It's multiplying the spring against the lifter....

Again, just a thought that I have had about it.
On the lifter side, how does it know the difference? Pressure is pressure as loing as there's enough to keep the lifter following the lobe.
I thought about pushrod cup frictional losses but again...on the lifter side, pressure is pressure. Now the distance traveled may have an effect. Higher ratio, less distance traveled on the lifter side and the lobe doesn't have to be as intense...which in turn requires less spring pressure yet, but less mechanical advantage on the lifter side means more "work" involved @ the rocker. It really is an interesting dynamic.
 
#42 ·
I dont think its that simlple. If you used a 1.5 and a 2.0 rocker arm you would need a completely different lobe profile to achieve the same vave motion. Hard to do an A-B test on that. We are also under the physical design dimensions on the blocks we are using. If we could use a 4 inch cam journal things may be different.
 
#53 · (Edited)
OK,...so if increasing the rocker ratio and compatible cam lobe redesign equals decreased spring requirements, it seems that the decreased spring requirement would be coming off the lifter side of the equation. Assuming that line of thought is correct, and without having to dig out my old books, there must be a percentage balance (in general) of what portion of the spring pressure is dedicated to lifter side vs valve side?

Next thought is, I've always been told (by pushrod designers) that increased pushrod weight (increasing size) has minimal effect on spring requirements..??
Hope this makes sense.

Blessings..............Ron
 
#54 ·
Next thought is, I've always been told (by pushrod designers) that increased pushrod weight (increasing size) has minimal effect on spring requirements..??
Hope this makes sense.

Blessings..............Ron
Seen just the opposite..

Just 10gr on a 11000 rpm comp engine was bad news.
 
#62 ·
This may be a dumb question but does the camshaft grinding equipment that we have today have the percision to grind a lobe for a higher ratio rocker 2.2-2.3+?

Reason im asking is the rocker just multiply's the motion of the cam lobe. It actually multiply's any action on that side of the rocker (pushrod/lifter movement). The more the ratio the more it multiply's, so i would think the lobe would have to be ground extremely percise and the pushrod / lifter extremely rigid in order for the valve motion to be stable with a 2.2-2.3 + ratio.
 
#67 ·
Ok, so if the goal was 1.00-1.050 lift with a 65mm cam core, and 9500 rpms, would you use more lobe or more rocker ratio?. So far my plan is 1.8 rocker ratio and custom .575 lobe. Durability and a stable valvetrain is the key here since 40 psi boost will be used. I'm trying to learn just like everyone else, so go easy.
 
#75 ·
So I have an honest, loaded question. Do you think there is a difference in endurance vs. an drag engine in the ratio vs lobe? I mean RPM is RPM, but much different application than just going up and down.

I ask because I don't know anyone running big ratio rockers on a big lift (.980+) application. In fact I just got some rockers for a buddy that picked up a S-O-T-A. engine and they were 1.85/1.8.
 
#77 ·
So I have an honest, loaded question. Do you think there is a difference in endurance vs. an drag engine in the ratio vs lobe? I mean RPM is RPM, but much different application than just going up and down.
Hell ya there's a differance................a drag engine doesn't have to live for 500 consecutive miles!!! LOL
You can really thrash some limited run stuff, but to make good power, it needs to be controled........to an extent. Power levels will tollerate a certain amount of instability before you start losing, at the sacrifce of endurance of course. But the more efficient we can move the valve, the more we can manipulate the pattern to suit what the engine combination wants, not just what we are capable of giving it.
At lower power/rpm levels, it's not that big of a deal, but you still sacrifice durability if you aren't the best you can be. In the upper classes, as you well know, you need to have EVERYTHING together to do what is needed!


I ask because I don't know anyone running big ratio rockers on a big lift (.980+) application. In fact I just got some rockers for a buddy that picked up a S-O-T-A. engine and they were 1.85/1.8.
1.8-1.85 is a fairly big ratio compared to a standard 1.5 SBC, but a stock BBC is 1.7. So by todays standards, it isn't huge, but 10-15 years ago, that would be getting after it pretty good ratio wise!

I don't believe high ratio's are the end all (well, to a point mabye but...LOL) but from a pure design stand point, I haven't seen a good reason why the opposite would be true. Why not use more of the spring to control the valve, instead of all the other mass in the system?

That is why I started this.................let's talk about why a bigger lobe is better, or max rocker ratio for a given design is best.
 
#98 ·
Can't we just make the balance point of the lobe centered on the axis of rotation? Just leaves more on the base circle side and offsets the lobe weight. They wouldnt really have a "base" circle anymore the lifters would always be moving also. Just thinking here.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top