When I was looking at basic cam design information, I got the impression that centering the overlap at TDC (i.e. the overlap period starts X* before TDC and ends at the same X* ATDC) was desirable, at least from a drivability standpoint. When I look at some of the popular off-the-shelf cams, I notice that the overlap period is biased towards BTDC, that is most of the overlap occurs befores TDC. I haven't observed many popular cams biased to ATDC.

Is there a reason to bias overlap to BTDC, or is that just a function of people trying to run big duration cams with relatively wide LSAs that require the cams to be advanced in order to keep the power band in a usable area? It seems like having the overlap biased towards BTDC would increase the reversion into the intake that makes MAF based tuning difficult.

It certainly seems that if having overlap centered at TDC is a benefit, cams with slightly smaller durations and tighter LSAs could achieve that.

 

The reason the overlap is biased toward BTDC, is because negative exhaust
pressure is used to help scavenge the chamber when both valves are open.

There is only a small window of opportunity in the cycle when this can happen. Due to propogation delays and pulse timing, valve events are
placed a little sooner to react with all of the drama occuring at high RPM.

We're dealing with milliseconds of time, and as RPM increasese the valve opening
events should be adjusted to synchronize with the pulses.

Just remember that everything is constantly changing at every RPM, load, etc. Inertia, frequency, and time is against us in a non-variable valve train.

What works well at 6500 RPM, doesn't work so well at 3400 RPM.

 

Also, if you have any exhaust backpressure at all with a big cam, you will kill the power because of reversion. Shifting the overlap to an advanced position will reduce the impact.