I think I've come up with a way to measure P/V without the pitfalls of a soft spring or the inherent inaccuracy of clay. Last night I degreed in my new Comp Cam 228/230 .588/.591 112+2. I have been worried about P/V since I'm running AS heads with the intake valves set very high.

Anyway what I did was measure the distance from the lip of the valve to the head deck, this measurementwas .125. To that I added the compressed OEM gasker thickness which is .052, for a total of .177.

I then zeroed out a dial indicator on the cam base circle on intake valve #1 lifter rim. I took measurements from the dial indicator at 4* BTDC, 0* TDC,5* ATDC, 7* ATDC, 8* ATDC, 10* ATDC, 14* ATDC, 17* ATDC. I multiplied those numbers by 1.70 the rocker arm ratio. That set of numbers was deducted from the .177 which is the valve depth in the head and gasket thickness. The resulting number was added to the depth the piston was measured in the hole or out of the hole with a feeler gauge. The resulting number if I've figured right is the actual P/V. How about some feed back?

Here are the P/V clearances that I calculated 4* BTDC .092, 0* TDC .066, 5* ATDC .046,7* ATDC .045, 10* ATDC .042,14* ATDC .047, 17* ATDC .059. From these numbers I think it's clear that I have to notch my pistons.

Does anyone disagree with my methodology? Is my analysis correct?

 

I don't think I'm following your methodology there. How are you measuring off the cam and correlating that to the piston?

You need to set up your dial indicator on the valve tip, using a light check spring and adjustable pushrod, and push the valve in by hand to measure your actual clearance to the piston top. Take your gasket compression into account if that is going to change. Measure your intake valve from about 5* BTDC to 15* ATDC and the exhausts from 15* BTDC to 5* ATDC.

Do you know what your heads were milled? Also, your .125" from the head deck to the valve edge tells you nothing since the valve travels at an angle. Set the head down on the work bench and measure off the valve tip until it hits the bench. Then add the gasket thickness and subtract the amount the piston comes out of the hole.

 

It appears this method will work. You have a set of measured data points that correspond to the where the components will be at those events when the motor is running. You could use a graph and plot those points to interpolate the distances between the p/v from your measurements.

 

1Fast, the heads are off the engine , that's the beauty of this set up, you are not having to deal with the soft spring and the inherent problem of establishing zero lash. What i am doing is setting my dial indicator at zero with the lifter on the base circle of the cam, as the lifter starts to travel on the lobe the dial indicator gets the reading which is multiplied by 1.7, the rocker arm ratio. The push rods, valves springs,and rocker arms are out of the equation.

 

That's fine as long as you keep in mind that the valve travels at an angle in relation to the piston. So, in other words, .500" of valve lift will only reduce the P-V clearance by .500" times the cosine of the valve angle. I don't know what the valve angle is for sure, but say it's 18*. That would mean .500" lift would bring the valve .476" closer to the piston.

That's why when I do it, I use a compressed gasket and check it with the head and valve train on the motor. Makes taking all these things into account automatic.

 

You make a very good point on valve angle, because the valve is in fact not travelling at a 90* angle to the piston. I'll go out to my shop and check this out.Thanks.

 

1fast, you are correct the valve angle is about 18* so that actual valve travel distance would be approximately 3% farther than if the valve were at a 90* angle. Consequently measurements taken at 90* would have to be made 3% longer for this method to be accurate. Good input.

 

Come on guys, somebody out there has to evaluate this.

 

Valve angle is15 degrees I believe.

 

I can't believe no sponsors have chimed in with their ideas as to whether this method is or is not viable. I find it hard to believe that some people may be blowing this idea off because it looks too complex. It's very simple accurate and logical and dummy proof. Comments!!!!!!!!!