Technical Cam Regrinding: Why the Pointy Lobes?

I'm trying to understand why reground cams I've seen for Studebaker V8's seem to grind away most of the lobe leaving a small, almost pointed lobe.

It seems to me that if you set your criteria as:
1) preserve as much existing lobe material as possible, and
2) drop the baseline,
you will end up with a cam with a reasonable looking lobe.

I think it is safe to say
A) if you stretch a properly designed lobe in the degrees direction, you will have a good profile with reduced accel, jerk, and higher derivatives. However, this lobe would be beyond the existing lobe.
B) if you stretch in the lift direction, you will have a "good" profile with increased derivatives. This lobe would be entirely within the existing lobe - meaning, a small increase in duration is also possible theoretically.

SECTIONED PROFILE: If I drop the BL 0.100 inches AND simply "cut" the profile at the zero accel point and translate the lower profile along the slope at the cut, I would pick up about 20* of duration on each side of the lobe - 40* increase total.

Of course this would result in a lobe design where there was about .100" of lift at zero accel which would ruin all the derivatives. But then how much of the lobe would be removed for the required smoothing? (my guess is not that much)

In other words, I don't see why you have to resort to grinding most of the lobe away leaving this pointy lobe that is going to have a very short life.
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I would expect there are hand numbers for what can be done, lift and duration-wise, given a specific drop in base line.

 

My only guess would be that the reground lobe has the same profile, but due to its miniaturization it appears to be pointier. I realize that's pure speculation and not the answer.


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Do you have before and after pictures of said cam? The "pointiness" of a cam is directly related to duration, so if you're looking at a stock regrind the lobe will be a lot more "pointy" than if it's reground with longer duration.

 

I'll have to see if I have a "pointy" laying around - I do have a new stock cam for comparison.

I know that if you change the LSA (Lobe Separation Angle) from stock, you really start losing lobe material. In fact, since the OE is 112*, I wouldn't recommend changing it.

I think the problem is people specify a lift, duration, and LSA and don't really care too much how sharp the lobe is. I look at it the other way, decent lobe profile, what are your options.

Intuitively, "extending" the OE lobe to a lower base line sees like you should be able to pick up some duration w/o killing lobe material. A cheater cam with factory lift and duration but increased accels produces a fatter, "squarer" lobe. While these race cams are hard on valve trains, some aspect of this could be used to preserve lobe.

 

Somebody please school us about the regrinding process !

 

A very reputable cam regrinder told me that as a general rule, it's really hard to move the LSA more than a couple degrees on a regrind.

 

I found YouTube videos - here is one -

 

http://www.jalopyjournal.com/forum/threads/how-the-hell-does-regrinding-a-cam-work.36558/


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Mgtstumpy, if that's your drawing, change 'intake closes' to 'opens' on the overlap.

 

I thought Google was trustworthy

 

Back in the day we used to send our factory cams in to be reground for racing, but they would always spray weld the lobes up before regrinding because they didn't have cam blanks for everything.

If they didn't build them up, the lifter edge would gouge in to the ramp because the ramp acceleration was too much for a now (relatively) smaller lifter diameter. Thus the need for larger diameter lifters and or mushroom lifters.

Grinding .100" off the heel of the lobe results in a very pointed lobe and a lot of messed up geometry that needs to be rethought.

For instance, the lifter doesn't want to follow the pointier nose very well at higher rpms and causes what is called "lofting". (Briggs flatheads in go-karts and NASCAR intentionally did this all the time for various cam lift limitation rules!)

Then you have to remember the pushrod length, valve stem length, spring length, keeper and collar diameter and rocker arm geometry, then there is piston to valve clearance at certain degrees of lift, etc. At best, regrinding a stock cam without spray welding it up is a compromise.

Is it all worth it, or is it better to just find the right camshaft profile that works well with all the correct geometry built in? You'll have to decide that for yourself.

 

Several cam grinders, including Comp, still have lines of lobes that are intentionally designed to loft the lifter.

 

Bogus drawing.......
The whole idea behind regrinding is to preserve the nose of the cam, not cut it the same as the base or otherwise you will have the same profile, just on a smaller lobe....
Material is cut from the base, thus moving the lobe centerline so that the nose 'appears' longer in comparison to the base circle. This gives you the new lift.
Yes, some material will be removed from the nose in order to make a clean uniform lobe and when a large duration profile is specified then the nose is flattened a bit to provide the 'fatter' profile but, of course, then more material has to be removed from the base to get the lift back up....a bit if a juggling act all dictated by the 'master'.
As for moving the LSA, just how small do you want the lobe....?
Yes, lobes are routinely welded and reground, especially on cores that are obsolete. The only issues is cost.
Can you weld up all of the lobes and then use the stock pushrods? Sure, if nothing else was cut, turned, machined or otherwise changed from the oem measurements but, as soon as you use a fatter head gasket, surface the heads or the block, do a valve job, dress the rocker tips or use a different lifter then the pre-load number changes.
If you're dealing with a solid lifter package then it is a bit less disruptive.


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The regrind would look more like this.

[ATTACH

 

Man, my daughter draws better than you! Hmmmm, come to think of it, she draws better than me too!

 

Thanks buddy!

 

Keep your day job Vicky!

 

wow.....this is deep.....like under my valley pan....fer sure going to use some of this - thanks IT is educational....

 

I'm razzing Vicky about his artistic ability, but really,his drawing gives a pretty good picture of how it's done.

 

Do you know how much I have to draw? All day

 

I moved it and found another. I like mine better.
New one is a bit more lobe ish and to scale but harder to see the difference.

Grind the base circle but leave the lobe alone.
The lifter is close to the center of cam at base and lifter travels that much further at lobe. Easy peasy !!!

Lots of Internet pics are wrong

 

Taking drawing lessons from Homer huh!

 

Don't mess with the ramp

 

I liked your original drawing better Vicky, had a more traditional feel to it!

 

Back for your viewing pleasure.

 

^^^^^ Rorschach results are in.

 

First, Merry Christmas.
And second, that drawing looks like something you might see in a Gynecological book.

 

No those drawings are much different !
Did you spell gynecological on your own or get help from from spell check?

 

No, I'm just "one fart smucker"!