Forced induction engine?
08-26-2004, 08:36 AM
#1
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Forced induction engine?
Is runner length/size a factor when working with a forced induction engine?
Here is my theory:
NA ram tuning = momentum of air the intake runner continues to fill the cylinder even though the piston has stopped at BDC or is rising on compression. Lets assume a very well tuned intake runner matched to a proper intake lobe builds 3 psi preassure at BDC.
Supercharging = You attempt to put 15 psi preassure into the cylinder, but it doesn’t at all get there due to restrictions in the port and valve. You may end up with 10 psi in the cylinder at BDC.
So, why not use ram tuning + supercharging to get 13 psi?
Supercharging does NOT render ram tuning ineffective. In fact, the opposite may be true.
Momentum of the air in the inlet port is what causes ram tuning, as described above.
Momentum = mass X velocity
Example:
Longer runner = more mass
Smaller runner = more velocity (but more restriction and less mass consequently)
Supercharging increases the density of the air in the runner. Higher density means more mass in the same runner and, thus, more momentum. Also, since the motor flows more air when supercharged, the velocity in the runner will be higher.
So, all things being equal, a supercharged motor has the capability of accomplishing more ram tuning than NA due to the higher density and velocity of the air in the runner. However, with a supercharger, you are trying to get 2 X more air through the same restrictive intake valve, so you still can't get it all in the cylinder.
Does anyone have any opposing theories or even better, any dyno numbers comparing different runner length/sizes with a forced induction engine?
Here is my theory:
NA ram tuning = momentum of air the intake runner continues to fill the cylinder even though the piston has stopped at BDC or is rising on compression. Lets assume a very well tuned intake runner matched to a proper intake lobe builds 3 psi preassure at BDC.
Supercharging = You attempt to put 15 psi preassure into the cylinder, but it doesn’t at all get there due to restrictions in the port and valve. You may end up with 10 psi in the cylinder at BDC.
So, why not use ram tuning + supercharging to get 13 psi?
Supercharging does NOT render ram tuning ineffective. In fact, the opposite may be true.
Momentum of the air in the inlet port is what causes ram tuning, as described above.
Momentum = mass X velocity
Example:
Longer runner = more mass
Smaller runner = more velocity (but more restriction and less mass consequently)
Supercharging increases the density of the air in the runner. Higher density means more mass in the same runner and, thus, more momentum. Also, since the motor flows more air when supercharged, the velocity in the runner will be higher.
So, all things being equal, a supercharged motor has the capability of accomplishing more ram tuning than NA due to the higher density and velocity of the air in the runner. However, with a supercharger, you are trying to get 2 X more air through the same restrictive intake valve, so you still can't get it all in the cylinder.
Does anyone have any opposing theories or even better, any dyno numbers comparing different runner length/sizes with a forced induction engine?
08-26-2004, 04:28 PM
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FI engines benefit greatly from short, fat runners. It is easier to flow more pressurized air like that.
The theory you are talking about has been applied to some cars already, the first that comes to mind is the VDI system used in 89-91 (Series 5) N/A RX-7's. It generated about a pound and a half of "boost" at high RPM's by redirecting the pressure waves from one rotor intake port to the other.
I don't see any purpose with even trying this on a supercharged car, I don't think it will work out like you think, plus you would have to design and fabricate a new manifold that would actually create the effect. Not an easy task.
The theory you are talking about has been applied to some cars already, the first that comes to mind is the VDI system used in 89-91 (Series 5) N/A RX-7's. It generated about a pound and a half of "boost" at high RPM's by redirecting the pressure waves from one rotor intake port to the other.
I don't see any purpose with even trying this on a supercharged car, I don't think it will work out like you think, plus you would have to design and fabricate a new manifold that would actually create the effect. Not an easy task.