Thread: Help with the math please!!!

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Does anyone know the formula for determining total valve lift after installing 1:6 rockers? Also how do I determine if it is safe to use a higher ratio? These heads are good for 500 lift.

Valve lift right now is 212\222 @50 and 435\460.

Last question,

Is it worth the time and effort to install higher ratio rockers only?
Pros and Cons?

TOTAL valve lift is Lift at Cam times rocker ratio

For instance, a 0.320 lift at cam with a 1.5 rocker ratio equals 0.480 lift at valve

your 212 / 222 number is the duration and has nothing to do with the lift

your 435/460 is probably the lift with 1.5 rockers if this is a small block chevy engine ....making the cam lift 0.290 and 0.307. The first number is for the intake and the second for the exhaust.

If you have a calculator, you can multiply the cam lift times your rocker ratio.

Then, you should check each valve at the lift to make sure you dont have spring bind.

In order to determine theoretical valve lift, the rocker ratio is multiplied by the cam lift. For instance, if the cam lift is 0.290", then the theoretical lift is 0.290" times 1.5 or 0.435. If the same lobe is multiplied times 1.6, then the theoretical valve lift would be 0.464".

In order to find the 0.290" intake lift at the cam, I simply divided 0.435 by 1.5, so to find the exhaust lift at the cam, you'd divide 0.460" by 1.5 and find that the exhaust lobe lift at the cam is 0.3066". Then to determine the theoretical valve lift after installing 1.6 rockers on the exhaust side, multiply 0.3066 times 1.6 and you get 0.490".

I've changed to 1.6 rockers in engines in the past, but could not tell you definitively that there was a horsepower increase from the increased lift because I never dyno'd the motors before and after. I can tell you that you will experience a difference you can feel with a change from stamped rockers to full roller rockers. In some cases, they have been determined to free-up 20 to 30 hp by those with access to a dyno.

In other words, I felt a difference in the seat of my pants, I'm just not sure if it was the increased lift or the decreased friction or a little of both.

If you do change to 1.6 rockers, check valve to piston clearance, retainer to seal clearance, coil bind and bind at the rocker/stud interface if using stamped 1.6 rockers.

Sorry robot, I was apparently typing while you had already posted.

thanks robot and tech.


Could you elaborate a bit more on the spring bind and valve to piston tests?

To check for coil bind:
pick an exhaust rocker that is easily accessible.
turn the engine over until the valve is completely open and the spring is compressed.
with a feeler guage, be sure you can insert a .030 blade between the coils.
as long as you have at least .030 between the coils you should be OK.

VP clearance is a little more involved with the motor assembled. Buy 2 checking springs. These are light springs that will close the valve on its seat, but you can still push the valve open with your thumb. Remove a spark plug, screw a quick disconnect air fitting into the hole. Attach a hose with shop air to hold the valves on their seats while you remove the rockers, keepers, retainers and springs from the intake and exhaust valves on that cylinder. Assemble your checking springs, retainers, keepers and rockers back on the valves. Pull the retainers up hard with your hands so that you know they're locked on the keepers. Screw the rockers down until you have taken all the slack out of the pushrods. Disconnect the shop air.

The closest either of the valves will come to a crash with the piston is the exhaust valve on the exhaust stroke when the piston is chasing it back up the bore, although you'll want to check both the intake and exhaust.

Determine where the TDC timing tab location is on the timing cover. Some Chevys use a 12:00 O'Clock location, some 2:00, and some 2:30. We'll assume a 12:00 position.

Turn the motor in direction of rotation and watch for the exhaust valve to open, then begin closing. Watch for the TDC notch on the damper to come around to about 10:30 O'Clock. Press down on the exhaust rocker at the valve stem and record how much clearance you've got (how much further you can push the valve off its seat than the cam did). Now press down on the intake rocker and record the clearance. Rotate the motor to 11:00 and do it again, then 11:30, then noon, then 12:30, 1:00 and 1:30.

You'll want to take the measurements with a 6" dial indicator with the depth slide. Use the edge of the retainer, but be careful not to dislodge the retainer. Those springs are light and it won't take much to disassemble the whole mess. Also, press down on the rocker, not the retainer for the same reason.

If you determine that the timing pointer is at the 2:00 or 2:30 position, just start your checking about an hour and a half earlier than 2:00 or 2:30 and run through an hour and a half later.

If you have 0.090" clearance on the intake and 0.110" on the exhaust, you're good to go.

nice technique. Sure beats coating the piston top with clay, then pulling the head and measuing the dimples in the clay!

Thanks Techinspector,
It may take a couple of days for me to get my nerve up to attempting this. Or I may just have a shop take a swing at it.

I didnt know it would be that involved.

Tech,

If it's NOT your motor, there is a faster way to check piston to valve clearance:

1. take car to mall parking lot
2. start engine
3. hold accelerator to floor
4. check valve to piston clearance
5. either drive home or call wrecker

Also known as "the parking lot attendant test"