Give the man a cheroot! Actually the question wasn't for you. MB.

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.

 

I was concerned about the higher loads on the pushrod and lifter from the multiplication. Yes the pushrod side of the system moves less, so there are fewer inertia loads from the lifter and pushrod themselves, but the valve, retainer, and a portion of the roacker arm and spring are still moving the same amount and causing inertia loads (F=Ma), so the pushrod and lifter get more loads with high ratio than with low ratio rockers.

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.

 

I was gonna add that part on the extra loads the pushrod and rocker see from the multiplication of the higher rocker ratio... the old man beat me too it.

 

LOL,

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

 

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.