DavidNJ

In the real world, NA intake ports operate with the pressures and temperatures of a flow bench. The biggest difference is that the fuel is usually injected in the manifold and is not represented in a dry flow bench. Still, much less of an issue in an injected car than a carburated car. Intake flow velocity generally mirrors the valve lift.

In the real world when the exhaust valve opens 2500°F gases at 100psi differencial open to the port. Supersonic flow is created around the valve. Flow is pretty much done in the first 60-90° of opening, well before max valve lift.

However, they are both typically bench tested with the same pressures, 28 in of water, and temperatures. In another forum it appears that some exhaust flow stuff doesn't show up at 28". That an exhaust port that seems worse at 28" seems better at 50" or 70", and more importantly, better on the dyno and better on the track.

So, has anyone flowed exhaust ports with higher pressures? This is most interesting for the new LS7 head which supposedly doesn't flow as well in the exhaust port at 28" as some other heads.

hammertime

iTrader: (1)

Very interesting. I've often wondered how much this is taken into consideration. Yes, the ports flow less than the intake at 28" - but as described, the real world operating conditions are much different. One would think that the 28" numbers might give a better indication of how well a cylinder is evacuated when used with a well-tuned exhaust. High velocity exhaust gases should establish enough inertia to continue flowing more residual gases out of a cylinder, even though the 28" flow numbers aren't the best. I wonder what the results might look like if low-lift numbers were compared at a higher depression?