Originally Posted By 451Mopar
Originally Posted By madscientist
Who cares what's going on a BDC? The exhaust is open (usually) by then.

Using your example, the short rod would be that much QUICKER around TDC which affects intake centerline.


You lost me here? At BDC intake lobe the exhaust better not be open?
Yes, a shorter rod has less "dwell" around TDC, but this would be the overlap area, not intake centerline.

You can have different cam grinds with various duration and overlap, but the intake lobe (when degreed in) centerline usually falls within a small window when the piston position is around 3/4 stroke length from TDC. I was just curious how this came to be, and if there was significance to the piston position?

I know it is sort of like looking at the cam design backwards, starting with the lobe centerline(s) in relation to piston position, then selecting the opening or closing point and doubling it to get duration. For example, you don't see intake lobes ground with an opening point way before TDC, without a really late closing point (high duration, but normal ICL).
Example1 - 292 duration 108cl, intake opens @ 38 degrees BTDC, and Closes at 74 ABDC

If the closing point was much sooner (lower duration, with same early opening) than we would see intake lobes installed in the 90-100 degree range.
Example2 - Intake opens @ 38 degrees, but closes @ 58 degrees ABDC, results in the lobe center moving to 100 ICL, with only 276 degrees duration.

Example3 - the exhaust duration on both cams is 292, and we want equal exhaust overlap of 38 degrees ATDC, then the first example had the LSA of 108 (cam installed straight up), and the second example has a LSA of 104 (cam ICL = 100, so advanced 4 degrees.)



I think I get what your asking now, I hope.

You are stuck with LSA's and ICL's exactly because of what you have pointed out. They are a function of duration and opening and closing events. And the fact that all the lobes are on the same cam.

You can only open the intake valve so, fast and so early because the piston is hanging around TDC. Once the piston is going down the hole, you can let the intake valve literally. chase the piston down the hole. Newer lobes generally open later, open faster, get to max lift longer, stay ther longer and close faster than lobes from 30,40,50 years ago. You decrease the seat to seat duration, while gaining duration at .050 and even more importantly, at .200. That's why intake valve to piston clearance can be as close as piston to head clearance. If the parts are correct, you should NEVER bend an intake valve.

The exhaust is different. It is opening as the piston is coming UP the bore. The piston is chasing the valve back to the seat.

Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston