That would be like disabling one of the intake valves in a 4 valve per cylinder piston engine. Yes it will still run but the power would be down somewhat. Just not a good idea.
On Mon, Oct 12, 2009 at 4:24 PM, jrhopkins
<jrhopkins@windstream.net> wrote:
has anyone used blocked off the center two apertures and used the front and rear intake manifolds only? i do not understand the need for the center split intake for aircraft use.
The center ports are smaller (the primary ports in the center iron) are part of a system designed to use a carburetter. That means that high velocity was needed to keep the energy high and the fuel air mixture from forming droplets and adding to HC.
So the smaller barrels of the 4 barrel carb open first to operate the engine on the high velocity runners to those ports. Most of the time the engine ran only on those ports. So the effect is that the carb looks small and the fuel mileage is fair, and HC (unburned Hydrocarbons) are within requirements.
The key is velocity. The smaller the tube per cubic foot of mixture per second, the higher the velocity. So the center ports and runners are small. Since only about 20 HP is needed to go 60 MPH, most of the engines life is run out on the small ports.
When more power is demanded, low manifold pressure opens the secondaries in the carb via a vacuum diaphragm and the big ports in the front and rear irons are called into use. The fuel economy idea is out the window and in a properly tuned car the engine RPM will be high enough to provide fair velocity in the big runners and ports. So the engine will accelerate to the best power RPM (red line) to deliver the engines rated HP.
In the fuel injected engine this feature is still in use so you see the throttle bodies multiple inlets.
Same idea.
In aircraft use, there is little use for 20 HP beyond a slow taxi. Take off power would be everything you can come up with HP wise. So all 4 ports and runners are needed. Once in flight, close to 50 HP would be needed just to maintain level slow flight, so there is just no application where the plane would fly on just the center ports. As the plane climbs the available HP goes down based on air density changes,
so in most cases, the throttle is left at wide open or very close to that. Further power reduction is by leaning to maintain the ideal fuel air mixture, or even leaning further to well over lean, or Lean Of Peak.
That is Lean Of Peak EGT (the rotary does that very well), So power goes down as well as fuel consumption and EGT. The throttle will be left wide open.
Closing off the center ports would be the same as not opening the throttle by some amount.
If you want to build in more power, the 84-85 12A has the biggest runners and ports in the center iron.
Lynn E. Hanover