I got a good question for you cam guys
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by MadBill
Hey, SS!
Besides being a fine judge of character, per your kind words in the dynamic compression thread, you also gave the exact explanation I would have re the limits of lifter velocity and acceleration.
Re the bonus question: How do you get more lift? The same way as when you want to jump higher, you pole-vault! It's risky and tricky, but with the right stuff, at high RPM the pushrod goes 'sproing' as the acceleration turns negative approaching max lift and the valve soars 50 or 100 thous. higher than the lobe lift times RAR dictates. Then, if all goes well, the components comer back together and gently touch down on the closing flank, just like a ski jumper. (and, just like a skier that out-jumps the hill, if the landing is on the base circle, bad stuff ensues....)
Besides being a fine judge of character, per your kind words in the dynamic compression thread, you also gave the exact explanation I would have re the limits of lifter velocity and acceleration.
Re the bonus question: How do you get more lift? The same way as when you want to jump higher, you pole-vault! It's risky and tricky, but with the right stuff, at high RPM the pushrod goes 'sproing' as the acceleration turns negative approaching max lift and the valve soars 50 or 100 thous. higher than the lobe lift times RAR dictates. Then, if all goes well, the components comer back together and gently touch down on the closing flank, just like a ski jumper. (and, just like a skier that out-jumps the hill, if the landing is on the base circle, bad stuff ensues....)
I thought it might be time to get into controlled lofting. I was just checking the waters.
Now, assuming controlled lofting only begins when the rpm gets to a cetain point, will the amount of loft increase with increasing rpm above that point?
How do we limit the loft to a maximum value consistently?
Do we need to use rev kits to keep the lifters on the cam during "valve flight"?
Can you really do this for an endurance engine that turns over a million revs in anger during a race? They are not all in the lofting range, but at short tracks like Martinsville they are there for a lot of time.
Another thought on getting the launch: if the springs could only control the valve to say 9000 and you went to 9500, wouldn't that give you loft without (or in addition to) the pole vaulting pushrod? IOW, keep the pushrod stiff so it only contributes a little to the loft and use the inertia of the valve to get the rest.
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by stevewix
you must be referring to lifter side-loading. my fault, i didn't read your post; when it popped into my head it seemed trivial so i downplayed it. sorry about the confusion.
Sometimes the pushrod is the least stiff part of the valve train. Theoretically pushrods are loaded only axially, but in practice they flex or bend elastically (but do not take a set) for a number of reasons. Once they start the flexing due to load, they are probably the second weakest spring in the system. Valve springs are the first. Of course every thing is a spring, some are just stiffer. A decent rocker arm has a spring rate over 50,000 lb/in, but it's still a spring. Even 500 lbs of load (spring plus inertia) would deflect that rocker .010.
That may be one of the reasons we only see about 1.75-1.8 max RAR in OEM pushrod engines, but more in some race engines. Maybe not.
Didn't NASCAR & NHRA outlaw materials like Titanium for pushrods? Ever think how you'd make an Ti pushrod? It isn't just the cost of the material that is the problem, and you don't find lots of Ti tubing in pushrod sizes for sale.
Last edited by Old SStroker; Oct 24, 2005 at 12:47 PM. Reason: Clarity.
Originally Posted by LIL SS
A lot of this is covered in these different Cam Threads:
https://ls1tech.com/forums/advanced-engineering-tech/396086-camshaft-discussion-part-ii.html
https://ls1tech.com/forums/advanced-engineering-tech/394934-camshaft-discussion-part-iii.html
All though there is a lot of text, It will go over a lot of details such as cam lobe shape. A quick brief explanation:
Lobe A) Comp Lobe # 3728
285* duration @ .006
236* duration @ .050
157* duration @ .200
.602 lift with a 1.7 rocker
Lobe B) Comp Lobe # 6580
261* duration @ .006
235* duration @ .050
153* duration @ .200
.595 lift with a 1.7 rocker
Lobe A has +24* duration at .006
Lobe A has +1* duration at .050
Lobe A has +4* duration at .200
Lobe A has .007 more total lift.
What lobe is more aggressive?
https://ls1tech.com/forums/advanced-engineering-tech/396086-camshaft-discussion-part-ii.html
https://ls1tech.com/forums/advanced-engineering-tech/394934-camshaft-discussion-part-iii.html
All though there is a lot of text, It will go over a lot of details such as cam lobe shape. A quick brief explanation:
Lobe A) Comp Lobe # 3728
285* duration @ .006
236* duration @ .050
157* duration @ .200
.602 lift with a 1.7 rocker
Lobe B) Comp Lobe # 6580
261* duration @ .006
235* duration @ .050
153* duration @ .200
.595 lift with a 1.7 rocker
Lobe A has +24* duration at .006
Lobe A has +1* duration at .050
Lobe A has +4* duration at .200
Lobe A has .007 more total lift.
What lobe is more aggressive?
it helps if the comparison is apples to apples.
6580 has a 1.99-1.95" journal size along with a .904 flat tappet on top of it, and the the 3728 has a 55mm journal and a hyd roller tappet on top of it.
On top of that you are quoting the .020" duration numbers instread of the .006 numbers for the 6580.
FWIW the 3728 is more aggressive due to the roller lifter and the journal size alone and also by the specs.
Apples to Apples here guys. You can't learn anything with all the variables in the system being different, because you can't compare them.
Bret
TECH Resident
Joined: Dec 2004
Posts: 881
Likes: 1
The rocker question depends on other issues. Is there room for a high ratio or will the pushrod size be so short as to adversely effect geometry. Higher ratio requires less movement of the heavy lifter and pushrod.
With flat tappets, how aggressive you can get depends on your lifter diameter. And bigger lifters are heavier. So, in a class limited to .842 flat tappet lifters (stock small block Chevy), you need the big rocker to make up for the lifter. Fords (.875) and Mopar (.904) have an edge, although a possible weight problem.
With a roller lifter, some of that goes away. You can make a roller very aggressive. So your advantage would mainly be in the reduced movement of the pushrod and lifter.
Answer, talk to your grinder and get their advice. It should be ground to get the most within your class rules, maintenance (how often you want to change springs and valves), durability (and lose one to failure), and power requirements.
With flat tappets, how aggressive you can get depends on your lifter diameter. And bigger lifters are heavier. So, in a class limited to .842 flat tappet lifters (stock small block Chevy), you need the big rocker to make up for the lifter. Fords (.875) and Mopar (.904) have an edge, although a possible weight problem.
With a roller lifter, some of that goes away. You can make a roller very aggressive. So your advantage would mainly be in the reduced movement of the pushrod and lifter.
Answer, talk to your grinder and get their advice. It should be ground to get the most within your class rules, maintenance (how often you want to change springs and valves), durability (and lose one to failure), and power requirements.
