Hey Guys,

I am fixing to install some PRC stage 1 LS6 heads milled to 59cc on my motor. Right now I have a MTI G1 cam that I am hoping will work (PTV clearance). I will check it with clay, I just wanted to first see if I could get an estimate on what it will be. To do this I created this excel sheet where I plugged in the info from the cam card. Right now I am calculating .110 intake valve opening @ TDC and I only have .130 free valve drop. Where am I going wrong, what am I not accounting for?

Thanks,
Pete

 

Keep in mind that minimum PTV clearance doesn't occur at TDC. Depending on the cam it can be anywhere from 20 degrees BTDC to 20 degrees ATDC. Typically minimum intake clearance will occur ATDC and minimum exhaust clearance will occur BTDC.

Here is the PTV 'map' of my engine that I used to flycut the pistons during my head/cam swap. I measured these with a degree wheel and dial indicator. 'Minimum' clearance points are indicated by the red boxes.

 

I did the same thing and found that the lowest point was 10 ATDC as well. Note also that using a dial indicator and using clay will typically produce slightly different results with the dial indicator showing smaller clearances. I did both ways as a cross-check.

 

I understand that the minimum value may not occur at TDC, although my sheet seems to show this, but the clearance I'm getting at TDC seems really small. What are you guys shooting for in terms of clearance, .060"?

 

Safe margins are above .080 intake and .100 exhaust.

many run less though.