i think i read it here awile back but can't find , anybody remember it ? Or have a link.. thanks

 

It's best to use a mechanical lifter even if the cam is designed for hydraulic lifters. You don't know if lifter bleed down will affect the readings. Set the lash at zero.

Set up a dial indicator on the rocker arm pushrod socket in line with the pushrod and a second dial indicator on the valve retainer parallel to the valve stem axis.

Take dial indicator readings through two complete revolutions - about every ten degrees. You don't have to use a degree wheel, but if you do you will get enough data to make a nice lift crank angle diagram. It's best to take multiple sets of data on each valve and do several valves. If the data is consistent and makes sense, your measurements are probably okay

At any point in the lift cycle the ratio or valve lift to lifter rise is the instantaneous ratio. It is not constant. For example, OE SB rockers start at about 1.37:1 at the lash point and peak at about 1.44 with a lobe heigh of about 0.3". The ratio increases with increasing lift.

If you have a mechanical lifter cam make sure you get data for both clearace ramps. This will allow you to determine the height of the top of the clearance ramp and using the actual rocker ratio at this point you can compute the best clearance. Most cam manufacturers, including GM just multiply the top of the ramp by the "advertised" rocker ratio, so recommended clearances are too loose. This means the valve clearance will not be taken up until above the ramp when velocity increases and jerk is near peak. This will shock load the valve train, and cause valve bounce at high revs and more rapid seat recession than if the valves are opening and close at not greater than ramp velocity where acceleration and jerk are zero.

I'm still looking for data on BB OE rocker behavior.

Duke

 

Thanks, was going to make a old hyd. lifter solid to test with

 

Lots of interesting ideas here: http://www.mid-lift.com/

 

vary interesting ,Thanks

 

Duke:

Is the reason the rocker ratio increases with increasing lift due to the rocker to valve stem angle never reaching 90 degrees? (I guess you might have to even look at the push rod angle to rocker angle as well.) At 90 degrees you would reach your maximum rates.

You might take a peak at the thread "Pushrod Length....can this be right?" I'd love to hear your opinion on that issue as it ties in with this thread.

 

It has to do with the overall rocker geometry, and the instantaneous fulcrum point may also move as the system goes through its range of motion.

It you set up a test fixture, you would find that the ratio increases as it does on the actual engine. Then if you kept lifting the pushrod the ratio would probably peak and then go down again.

I couldn't make any sense out of the other thread.

Duke

 

I guess that was the gist of the ramblings in the other thread, do you run your push rod length so that you have maximum ratio? This would occur when the rocker to valve stem angle is 90 degrees. The catch is that may not center the tip of the rocker on the valve stem.

 

I was curious about the OE rocker geometry, so I measured them. I don't try to "optimize" as long as they are reasonable, which is the case unless the heads and block are shaved too much.

Keep in mind that ANY rocker arm will have a ratio that varies throughout the operating range, and high ratio rockers don't offer much in return for their cost.

The key to obtaining high specific output is head flow, then you select valve timing to achieve the target torque bandwidth that matches the induction and exhaust system configurations.

It's called "system engineering" - getting everything to work synegistically so the whole is greater than the sum of the parts, and avoiding components that fight each other like high overlap cams with a high restriction exhaust system.

Duke

 

system

 

Duke:

Thanks for the response. I guess I'm trying to find out what angle the chevy engineers intended the rocker to sit relative to the valve stem. Is it 90 degrees at mid lift?

Certainly the least variation in rocker ratio will be achieved at 90 degrees, the least lateral stress on the valve stem will be achieved at 90 degrees, and the highest rocker ratio will be achieved at 90 degrees.

I'm gathering from you guys with lots of engine building experience this is not a big issue. Adjust the push rod length to center the roller tip on the valve stem and let it go.

 

i'm also try to keep my street engine's even on the ratio's,,, if they are
1.42 , i want them all to be,,, if i get 8 that are 1.48 they are going on the ex.