I am aware that valve size and port size are very important in n/a engines. To keep intake air velocity up for better cylinder filling at low to mid rpm. To keep from having a bog or dogish area in the engine. But does this apply to boosted engines. Do I need to build a boosted engine the same way I would a n/a engine or would larger valves and intake runners not affect the low end power because to air is being forced in under pressure instead of being drawn in under vacuum.
restrictive areas will increase pressure where non-restrctive areas will allow increased flow volume
So the more flow the better on boosted engines. I've over built one small block and I hated it. It took it til 3500 rpm to make any power at all. So under boost that won't happen.
So volume is more important than velocity on boosted engines because the supercharger is creating the velocity regardless of port size.
The rpm range where the engine makes its power is determined by the cam. How well it flows air in and out is determined by the size and shape of the ports and valves. Don't forget thet the exhaust system is also part of the overall airflow system.On a boosted engine, the blower or turbo is pushing air & fuel in. If the ports are restrictive the booster is less efficient because it has to work harder to move the air into the cylinders. Less restrictive ports allow the boost producer to work more efficiently and the engine as a whole to make more power.Air flow is no less important in a boosted engine than in a naturally aspirated engine. Velocity is important in any engine, it's just easier to achieve in a boosted engine.
That all makes since. I've never built a boosted engine before and I don't want to leave power on the table. Let's say I have two 350sbc. Both are built identically. Same compression. Same cam, same valve size, same intake, same brand of heads. Only difference being the intake runner size. One is 170 the other is 200. In a n/a engine the 170 runner would make a little more low end power while the 200 runner would lose a little of the low end but gain more top end. Correct. Now the same engines under the same amount of boost. Would the 170 runner still make better low end power or would the 200 make more power across the board.
On a n/a motor the velocity of flow, or the speed of the flow, would be greater with the 170 head than the 200 head regardless of engine speed. The 170 port would work better with an engine that makes peak power under 6500rpm. The 170 heads are better suited to lower revving engines under 385ci. The 200 heads are better suited for larger displacement engines or high winding smaller engines. To put a 200 port head on a 350ci engine that is only expected to turn 5500rpm is asking for sluggish performance. I know that is an extreme scenerio, it's just to make a point. I take from your UN that you are building a small big block? IF it were me and a 396, I'd go with the 200 port head.
No the 396 is already built. And slowvelle is the name my car got from my coworkers who all have 454 or bigger engines and they all blow my doors off at the track. But what can I expect out of such a small engine in a car that tips the scales at 4200 lbs.