mastrdrver

What are the drawbacks to having the valve open ATDC on the intake stroke? Reason I'm asking is that I know that the closing point is the most important timing event of all four, but I've had a hard time finding anything about opening the valve too late.

For relativeness, I've been working on putting my 408w together. Thanks to the current run up in pump prices, I'm thinking about going for 500 crank hp, instead of the 600 I was shooting for before to help with mileage. I figure I can put a 100, or so, shot of nitrous on it later if I need more power. Looking at things, it seems very easy to reach this goal with a mild cam, for the size of the motor. I'm just wondering what the drawbacks are if you open the intake later and close the exhaust soon while keeping the exhaust close and intake opening event frozen.

The setup consits of:

10.5:1 compression
Dart 225cc heads
Trickflow TFS-R box intake, ~10.5" runners
1 3/4 MAC long tubes into 3" exhaust with x-pipe

thanks

rskrause

You need overlap to get the intake charge flowing into the cylinder and you need time to get the valve open in preparation for the piston moving down the bore. The overlap starts the intake charge moving because of the inertia of the exhaust gasses still flowing out of the open exhaust valve, hopefully supplemented by a reflected negative pressure wave (when the primary length is appropriate for the desired rpm range). Race cams typically have 90-100 degrees of overlap, or more, for this reason. When selecting a cam, people look at duration, but really it makes as much or more sense to think in terms of how much overlap is needed as a starting point.

Rich

mastrdrver

Well Rich, the reason I want to cut down on overlap is to help gas mileage. I know the LS7, while a different engine, opens the intake way after the piston is already starting down the bore. I know there are multiple reasons for this, one to help emissions I'm sure, but I thought it would be a place to start when it came to my motor.

rskrause

I think you must be misunderstanding something. I don't think that any modern IC engine would have valve timing like that. Maybe the confusion comes from looking at specs at 0.050"? A VVT engine might also come close under some operating modes.

Rich

mastrdrver

You are correct. I've been talking about .050 specs so I can compare cams. So maybe I should go back and re ask the question correctly. What is the disadvantage of opening a valve closer to TDC (say less than 20 deg before) and opening it earlier (talking about seat duration here) and similar on the exhaust closing (as in closing it earlier)?