I am trying to evaluate the possibility of PTV clearance issues with some cams by looking at the valve timing events. I just want to make sure I understand this. Is the following statement true?

The smaller the number for IVC (BTDC) and EVC (ATDC), the less likely it is that PTV clearance will be too small.

Thank you for your help.

 

Anyone?

 

I think you should be more concerned with duration, lsa and advance for PTV clearance. To the best of my knowledge there's a direct relationship between duration and IVC and EVC though. The more duration you have, the more overlap you have so that'll dicatate when your valves close. To answer your question though, I think your half right. It seems like the first part of your statement is correct and the EVC isn't correct.

 

Yeah, this stuff makes my head hurt. I thought that if there was going to be PTV clearance issues with the exhaust valve, it would occur when the valve is closing. I thought the later it closed, the closer the piston would come to the valve. Now that I think about it, it seems to me that when looking at PTV clearance, we should only care about what happens when the piston is near top dead center. If this is indeed the case, then my original statement is wrong, and one should look at IVO and EVC. Then, my revised statement would be the smaller the number for IVO (BTDC) and EVC (ATDC), the less likely it is that PTV clearance will be too small. This would mean that you disagree with me completely. Yes, I am making progress now.

I found the following information @ www.webcamshafts.com.

1. Max Lift or Nose
2. Flank
3. Opening Clearance Ramp
4. Closing Clearance Ramp
5. Base Circle
6. Exhaust Opening Timing Figure
7. Exhaust Closing Timing Figure
8. Intake Opening Timing Figure
9. Intake Closing Timing Figure
10. Lobe Separation



So, position 8 is IVO, and position 7 is EVC. IVO and EVC respectively are the distances in degrees that the intake valve open and exhaust valve close from TDC, or when the piston is highest in the cylinder. When IVO and EVC are smaller, for a given lobe, then the lobes are further apart, indicating a larger LSA. Now, this would make sense, because I thought increasing LSA increases PTV clearance. This is just me thinking out loud. It seems to make sense to me now, but I was wrong with my first statement, so I wonder if I am still not looking at this correctly.

 

I'm almost positive this is correct. Look at when you see the exhaust valve open, and then close. When it's about to close and the intake starts to open, this is the area you need to take your measurements for PTV clearance. This is called the "overlap period". At no other point in the four stroke cycle do the valves get closer to the piston.

I believe you are also correct here.

Here's a good site to read about this stuff: http://www.fordmuscle.com/fundamenta...ve/index.shtml

I'm pretty sure it does. A LSA of 114 will net more clearance than a LSA of 112.

I might have been wrong in my last post now that I think about it. Maybe someone else will chime in and correct me if I'm wrong.

What I wish was if someone would find out if for a given amount of duration added, there could be a rule of thumb of clearance that can be subtracted. Ex: for every 2 degrees of duration, you lose .05 of clearance. I guess advance and LSA would somehow need to be included though...

 

Even better would be a PTV clearance calculator; something along the lines of a good compression ratio calculator. Plug in specs for your pistons, heads, gasket, cam, etc, and it spits out a PTV clearance. Of course, you would still need to measure at install, but this would be very helpful during cam selection for those who do not want to fly cut, like me.

Man, I wish some other people would step in and comment.