Solid Cam With "Forgiving" Lash Ramp Design?

[DaveMcLain]

I just finished a Chrysler Slant 6 engine build for a customer. It is a totally stock engine for a '67 Dodge pickup and just for kicks I did a plot of the old original cam and a new stock replacement which for Stan's database measured:

(stock cam) .389 lift 193 duration at .050 intake and exhaust on a 110 lobe sep

(replacement cam) .407 lift 198 duration intake, .415 204 duration exhaust on a 109 lobe separation.

What I found interesting are how the ramps are designed on all the lobes on both the opening and closing sides..




Here I zoomed way in on the opening ramp of the exhaust lobe on the replacement camshaft. See how there is a VERY long constant velocity section of the ramp which goes from about 60 degrees in the graph all the way over to about 98 degrees during which the velocity in inches per degree only changes from .000426 to .000432. Lift over that distance changes from .00479 to .01285. So given the engine's 1.5 ratio rocker it seems like the lash could be set anywhere from about .007 to .020 with no real change in contact velocity. In other words the cam would run as quietly at .020 as it would at .007... Did OEM manufacturers design cams this way so that it could run a very long time and even be way out of adjustment yet the contact velocity would still remain very low?

It seems like this idea wouldn't be terribly useful in a performance application where it is assumed that the lash will be checked more often and kept within a reasonable range of adjustment. What are the effects of this sort of design on performance?

[Stan Weiss]

Dave,
I am not sure why they did cams that way at that time. This is the Chevy 30 30 cam and look at its constant velocity ramp.

Stan

ab-cam-vel-chevy-30-30.gif

[CamKing]

I still use constant velocity ramps for almost all my cam designs.
The point is to set the hot lash at the end of the CV ramp, then when the engine is cold, the tighter cold lash will be at the same velocity.

[groberts101]

sorry for an apparently dumb question here guys.. want to understand what I'm seeing.. what is the blue line?

[CamKing]

Dark blue is lobe lift(actually lifter rize)
Green is Velocity(of lifter motion)
light blue is acceleration(of lifter motion)

[PackardV8]

I still use constant velocity ramps for almost all my cam designs.
The point is to set the hot lash at the end of the CV ramp, then when the engine is cold, the tighter cold lash will be at the same velocity.

Learn something every day. I thought I'd been told the long slow ramps were because the valve springs back when wouldn't handle the "high intensity" we routinely use today.

[F-BIRD'88]

The old valve springs were easy on the cam lifter life.

Less is better, longer.

[CamKing]

I still use constant velocity ramps for almost all my cam designs.
The point is to set the hot lash at the end of the CV ramp, then when the engine is cold, the tighter cold lash will be at the same velocity.

Learn something every day. I thought I'd been told the long slow ramps were because the valve springs back when wouldn't handle the "high intensity" we routinely use today.

You're confusing lash ramps with opening and closing ramps.

The lash ramp is the part below the lash point. What some people call the opening(or closing) ramp, is the part above the lash point.
It's the part above the lash point that was much slower on the old cam designs. This allowed them to run less spring pressure, but also made the cam much longer at the seat duration.

[lada ok]

I wonder if it was some sort of safety feature, a hang over from the flat head days.
Of course lots of home mech existed with varying amounts of expertise, many would look for the valve just starting to open, then back her off a bit, that long plateau gives you quite a bit of lee way.
I well remember putting a ' Ford 10 special ' cam in my little ol' flat head, ... the beggars that did the cam never told me to open the valve stem clearance up to 0.014" from the std 0.006 " ( from memory ).... 20 seconds of run time and just about all the EX heads disappeared out the exhaust pipe ... pretty expensive lesson for a young school boy !

[pdq67]

I remember asking my good buddy Doc when the last time the valve cover had been off his '70 Duster?

He asked why I wanted to know so I told him.

Anyway he said that it never had been off in 175,000 miles or so and the little slant-6 engine purred like a kitten. Sucker didn't hardly use any oil at all to boot.

pdq67

[DaveMcLain]

I wonder if it was some sort of safety feature, a hang over from the flat head days.
Of course lots of home mech existed with varying amounts of expertise, many would look for the valve just starting to open, then back her off a bit, that long plateau gives you quite a bit of lee way.
I well remember putting a ' Ford 10 special ' cam in my little ol' flat head, ... the beggars that did the cam never told me to open the valve stem clearance up to 0.014" from the std 0.006 " ( from memory ).... 20 seconds of run time and just about all the EX heads disappeared out the exhaust pipe ... pretty expensive lesson for a young school boy !

Generally speaking most flathead engines have pretty radical cam action.



Here is a graph showing the Slant 6 cam in light blue for lift, red for velocity vs a stock Ford 8N tractor cam(flathead) in green and dark blue. At only .006 lift the 8N cam is already at about double the velocity that the Slant 6 design has in its constant velocity area. Even taking into account the 1.5 rocker ratio of the Slant 6 the Ford cam is still hitting it pretty hard and doesn't seem to allow much for variations in lash.

Another interesting factor is how much of the lifter the Slant 6 design actually uses... .748 so the velocity never gets very high. I would say that it's just because the duration is so short and if it did use more of the lifter the accelerations plus and minus would be too severe. Question is then how much duration is really needed to make a Chrysler .906 lifter any advantage? I'm thinking something around 260 at .050.

[Stan Weiss]

I wonder if it was some sort of safety feature, a hang over from the flat head days.
Of course lots of home mech existed with varying amounts of expertise, many would look for the valve just starting to open, then back her off a bit, that long plateau gives you quite a bit of lee way.
I well remember putting a ' Ford 10 special ' cam in my little ol' flat head, ... the beggars that did the cam never told me to open the valve stem clearance up to 0.014" from the std 0.006 " ( from memory ).... 20 seconds of run time and just about all the EX heads disappeared out the exhaust pipe ... pretty expensive lesson for a young school boy !

Generally speaking most flathead engines have pretty radical cam action.



Here is a graph showing the Slant 6 cam in light blue for lift, red for velocity vs a stock Ford 8N tractor cam(flathead) in green and dark blue. At only .006 lift the 8N cam is already at about double the velocity that the Slant 6 design has in its constant velocity area. Even taking into account the 1.5 rocker ratio of the Slant 6 the Ford cam is still hitting it pretty hard and doesn't seem to allow much for variations in lash.

Another interesting factor is how much of the lifter the Slant 6 design actually uses... .748 so the velocity never gets very high. I would say that it's just because the duration is so short and if it did use more of the lifter the accelerations plus and minus would be too severe. Question is then how much duration is really needed to make a Chrysler .906 lifter any advantage? I'm thinking something around 260 at .050.

Dave,
I believe that you are looking at this the wrong way. If I have a fixed lift the larger the lifter diameter the less duration I would need if I use max velocity. The other way around if my duration is fixed I can get more lift from the larger diameter lifter before I get to max velocity.

Stan

[lada ok]

The engineers chose the follower dia for bore wear and longevity ( big is good ,bigger is better ) ?? The resultant contact area was what it was.
good graphs Dave : my mistake trying to compare a 3 arc cam to mathematically derived curve

What year is that 30 - 30 Stan ? looking at that graph of yours, you'd almost think that they designed the cam , then realized they didn't need the duration, so simply extended the ramp, or is it a universal pattern ? ... plenty of room there to increase lift, vel, dur , without changing bugger all ?

Why did the little ' Ford 10 special ' need 0.014 " & not the original 0.006 " on the EX

[PackardV8]

What year is that 30 - 30 Stan ? looking at that graph of yours, you'd almost think that they designed the cam , then realized they didn't need the duration, so simply extended the ramp, or is it a universal pattern ? ... plenty of room there to increase lift, vel, dur , without changing bugger all ?

The infamous "Duntov 30-30", so named because the .030" clearance was noisy, was just about the first Chevrolet V8 factory high-performance cam; it also sold for $30 across the counter. Legend has it designed by/copied from the great Ed Windfield.

[DaveMcLain]

I wonder if it was some sort of safety feature, a hang over from the flat head days.
Of course lots of home mech existed with varying amounts of expertise, many would look for the valve just starting to open, then back her off a bit, that long plateau gives you quite a bit of lee way.
I well remember putting a ' Ford 10 special ' cam in my little ol' flat head, ... the beggars that did the cam never told me to open the valve stem clearance up to 0.014" from the std 0.006 " ( from memory ).... 20 seconds of run time and just about all the EX heads disappeared out the exhaust pipe ... pretty expensive lesson for a young school boy !

Generally speaking most flathead engines have pretty radical cam action.



Here is a graph showing the Slant 6 cam in light blue for lift, red for velocity vs a stock Ford 8N tractor cam(flathead) in green and dark blue. At only .006 lift the 8N cam is already at about double the velocity that the Slant 6 design has in its constant velocity area. Even taking into account the 1.5 rocker ratio of the Slant 6 the Ford cam is still hitting it pretty hard and doesn't seem to allow much for variations in lash.

Another interesting factor is how much of the lifter the Slant 6 design actually uses... .748 so the velocity never gets very high. I would say that it's just because the duration is so short and if it did use more of the lifter the accelerations plus and minus would be too severe. Question is then how much duration is really needed to make a Chrysler .906 lifter any advantage? I'm thinking something around 260 at .050.

Dave,
I believe that you are looking at this the wrong way. If I have a fixed lift the larger the lifter diameter the less duration I would need if I use max velocity. The other way around if my duration is fixed I can get more lift from the larger diameter lifter before I get to max velocity.

Stan

With a flat tappet while the max velocity is limited by the lifter diameter the acceleration is not but the amount of acceleration that can be used is limited by what is practical for a spring as well as the rest of the valvetrain. This means that a larger lifter is only a benefit once the duration gets to X amount; at say .050 what is that amount for a .906 as used in this engine?