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Opel Rallier since 1977
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Discussion Starter #1
One standard of comparing cams is to measure the lobe duration at .050" lift at the lifter. This is all fine and good but one has to be aware that this is not the whole story of how the valve operates (which is what we actually care about), and it has to be understood that a .050" duration number for a hydraulic cam does not end with the same actual valve lift for a solid cam with the same .050" duration number. This is being brought up here, as it is one of the factors that is contributing to the trials and tribulations of one of our members, JayhawkJesse. The complaint is that a cam with a given duration at .050" is not giving the low RPM torque response that is desired. (At least that may be the problem.)

As I read his build thread form several years back, the cam duration of the low 220's range was from a recommendation, when the initial build was being kicked around. That was supposed to give a good low RPM torque response. But one thing that I think was missed at the time was if this recommended low 220's duration at .050" was for a solid cam grind or for a hydraulic. The cam that was eventually made for this setup is a hydraulic.

Let's look at what the actual valve lifts will be for a solid vs hydrualic cam with the same duration at 222 degrees. We'll use the theoretical 1.5 rocker ratio here, just for discussion purposes.
  • Hydraulic: The lifters will squish down a few thousandths of an inch as they take up the valve spring load, but generally, they don't compress much at all. So, at the cam angles that give .050" lilft on a test jig, the valve lift will be approximately (.050" lift - .002" 'lifter squish') x 1.5 rocker ratio = .072" valve lift.
  • Solid: As the lift rises, it first has to take up all the valve lash. If we use the typical hot lash from the old Isky catalog of .018", then then lifter has to rise .012" inch before the valve starts to move. So that leaves another .038" of lifter lift to actual move the valve before the .050" lift is reached. This is a valve lift of .038" x 1.5 = .057"
Now the difference between .072" and .057" valve lift does not sound like a lot, right? But what we need to look at is how much the effective duration has been changed, because that is typically the biggest factor in determining the engine's RPM behavior. In other words the solid cam is acting like a shorter duration cam (which will give better low RPM torque), when the effects of valve lash on the valve opening action are taken into account. So how much shorter?

We need to figure out the cam angle change on the solid cam that needs to be reached to get to that .072" valve lift of the hydrualic cam, the solid cam need to open the valve .015" more to get to that same valve lift, or another .010" at the lifter after we take out the rocker ratio. From one set of detailed cam data, the lobe velocity is approximately .002" per degree on the opening flank in this lift range, so that .010" lift difference at the lifter requires the cam to rotate an added 5 degrees, which shortens the effective cam duration by 5 degrees. Since this occurs one both the opening and closing flanks, the net total change in effective duration at the valve is -10 degrees for the solid cam.

In other words, with these particular cam profile numbers, a solid cam effectively acts like a 10 degree smaller duration cam at the valve than a hydraulic cam with the same measured .050" lift at the lifter. This difference will vary with the detials of the cam profile; it might be as little a difference as 6 degrees with a faster lift rate on the cam lobe around .050". But either way 6-8-10-12 degrees difference is a big deal. It is full step change in cam size.

So, you can't take a .050" cam spec for a solid cam and directly translate it to a hydraulic cam grind and expect the same performance. Due to the lack of valve lash, the valve with a hydrualic grind cam will open sooner and close later and have a longer duration at the valve, and thus poorer low RPM performance. Now the cam grinder may be able to compensate for this IF HE KNOWS that you are translating from one type to the other, but unless we explain all the details, then he won't know.

And we all have to be 100% clear when we give a cam duration number whether it is for solid or hydraulic lifters. I am typically thinking in terms of hydaulics and rarely remember to mention that, so need to do better myself in this regard.
 

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Thanks for the informative post!
 

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Opel Rallier since 1977
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Discussion Starter #3
This stuff is pretty important if you want to select cams for low RPM torque (where the exhaust scavenging effect is weak or none).

One associated thing to note is how advertised durations are measured on hydraulic versus solid lifter cams. In the US, the most common standard for hydraulic lifter cams is to specify the advertised duration at .005" or .006". But for solid lifter cams, this is measured at lifter lifts of .015" to .020". This is all to reflect the effect of valve lash on the 'effective' opening and closing angles of the valves.

Since advertised duration is the proper duration factor to use in computing DCR (dynamic compression ratio) and since DCR is a very good indicator of low RPM torque, then it is another reason that the difference between solid vs. hydraulic durations is important to know and why you should understand how to take the duration numbers from one type and apply it to the other.

And once again, this is all IF you are dealing with an application where low RPM torque is important. If you are designing for an application where the engine is always at mid to high RPM's, where the exhaust scavenging is working well, then this becomes far less important.
 

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Very good all around information Mark! Someday I’ll understand it all LOL.
I have always get muddied up reading the racing posts and confusing what’s good up top with what I’m mostly doing driving (under 4,000 rpms).

A little bit off topic but what are the advantages of buying a new cam vs. taking your old one in to get a re grind to the exact specs you want, split lift/duration etc.?
Most of the discussion’s on the forum are about buying a new cam so I’m thinking there must be something better about them but IDK what it is.
I’m out of my depth here but the only thing I can think of is that you can’t get a .490” lift from a stock cam?
 

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Opel Rallier since 1977
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Discussion Starter #5
I think this has been discussed before, so there is good info out there on this. But if you are re-grinding a cam and wanting a lot of extra lift, then you have to grind down the base circle to make the lifters sit lower, and thus make the lobe peaks relatively higher, as you cannot 'add' material to the existing lobes peaks. If you lower the position of the lifter a lot, then you do a few things:
  • Impact the rocker angles at closed and open and mid-lift; this effects the actual lift rates of the valves and if it gets out of hand, it can make the valvetrain less stable (like with more internal spring vibrations)
  • The lifters as they sit in the bores with valves closed relative to the oil passages gets changed, so you have to be sure the timing of the oil pressure does not get messed up, or the oil flow gets blocked to lifters 'down the line' in the oil passage
  • The relationship of the rocker adjustments to the threaded and unthreaded parts of the rocker studs gets changed, and if the lifters set too low, then the adjusters will hit the end of the threading or a shoulder of the stud, before you can reach the proper solid lifter lash, or hydraulic lifter preload adjustment. THEN you have a big problem...
 

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Cool stuff Mark, thanks!
 

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I think this has been discussed before, so there is good info out there on this. But if you are re-grinding a cam and wanting a lot of extra lift, then you have to grind down the base circle to make the lifters sit lower, and thus make the lobe peaks relatively higher, as you cannot 'add' material to the existing lobes peaks. If you lower the position of the lifter a lot, then you do a few things:
  • Impact the rocker angles at closed and open and mid-lift; this effects the actual lift rates of the valves and if it gets out of hand, it can make the valvetrain less stable (like with more internal spring vibrations)
  • The lifters as they sit in the bores with valves closed relative to the oil passages gets changed, so you have to be sure the timing of the oil pressure does not get messed up, or the oil flow gets blocked to lifters 'down the line' in the oil passage
  • The relationship of the rocker adjustments to the threaded and unthreaded parts of the rocker studs gets changed, and if the lifters set too low, then the adjusters will hit the end of the threading or a shoulder of the stud, before you can reach the proper solid lifter lash, or hydraulic lifter preload adjustment. THEN you have a big problem...
Great explanation, thank you for being so thorough.
So it would be critical to find out how much they take off the base circle, do you have an opinion on how much is too much for each hydraulic and solid?
 

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Opel Rallier since 1977
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Discussion Starter #8
I've never put any real thought into it, Tom. I did have the valves set into the head shallower than normal on the last head build and it ended up with thread interference on aftermarket adjusters on the studs. I'll have to dig to see how much the valve depth was from stock; that might a be a bit of a clue on one of the possible issues.
 
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