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Nick Koske · 03-26-2008, 02:06 PM

As long as the engine sucks air through the restrictor the pressure in the airbox is negative, again just physics. There may be a speed that that doesn't happen, but probably not at WOT.

Lets say the LS7 is capable of 900HP unrestricted...for the sake of the argument. What makes that power is a certain volume/mass of air and a certain amount of fuel...in each piston (mass of air + amount of fuel = power). Now if you tell me that a restricted LS7 makes a max of 590HP (from corvette racing site)...so what's changing...they are not limiting fuel, they don't have to...they are limiting the volume/mass of the air, that's all that can get in through the restrictor. So if an engine un-restricted is capable of producing 900HP, is making a amount of power less than the 900HP it seems to me that there would always be a negative pressure in the air box. Why...because the unrestricted engine would be able to get the air it wants, the restricted engine is unable to do that because it trying to suck air through a 'straw'. In my eyes the only way they could get a positive air box pressure on the restricted engine is to not allow the throttle to open the full amount which they may actually tune, but it seems to me that you may as well open it the throttle as big as it gets to allow the engine to suck the amount of air it can.

Positive charge...negative charge...doesn't really matter (obviously positive is better hence turbo and super chargers)...they are shooting for the maximum amount of air by mass. That is allowed by opening the throttle the whole way and letting the piston, on it's down stroke, suck the air through the intake, from the airbox, through the restrictor.

Q=AV (perfect orifice equation, fine for this demo, since I don't know the shape of the ACO mandated restrictor)
Q=Volumetric Air Flow
A=Area of Restrictor (2 x 29.9mm dia per ACO, I think they use two, .001404m^2)
V=Velocity of Air Flow (assume car at 150mph, 4023m/min)
Q =.001404m^2 * 4023m/min = 5648 liters per min travel through the restrictor

Now you have a 7 liter engine that makes 590 HP at 5400 RPM. A V8 has a cylinder fire every 90 degrees, so four times in one rotation. So you can say that the engine exhausts half of the 7 liters every rotation. So an unrestricted, perfect flow LS7 will have a 3.5 lpm flow through it for every rotation.

3.5 l * 5400 RPM = 18900 liters per min.

So even at 150 MPH with 5648 lpm flow through the restrictor the engine still wants 18900 lpm. So there has to be a negative air pressure in the air box at 150mph @WOT. The LS7 seems to be always sucking

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03-26-2008, 02:06 PM	#6 (permalink)
Nick Koske Senior Member Join Date: Oct 2007 Location: Bay Area, CA Posts: 329 Blog Entries: 4	As long as the engine sucks air through the restrictor the pressure in the airbox is negative, again just physics. There may be a speed that that doesn't happen, but probably not at WOT. Lets say the LS7 is capable of 900HP unrestricted...for the sake of the argument. What makes that power is a certain volume/mass of air and a certain amount of fuel...in each piston (mass of air + amount of fuel = power). Now if you tell me that a restricted LS7 makes a max of 590HP (from corvette racing site)...so what's changing...they are not limiting fuel, they don't have to...they are limiting the volume/mass of the air, that's all that can get in through the restrictor. So if an engine un-restricted is capable of producing 900HP, is making a amount of power less than the 900HP it seems to me that there would always be a negative pressure in the air box. Why...because the unrestricted engine would be able to get the air it wants, the restricted engine is unable to do that because it trying to suck air through a 'straw'. In my eyes the only way they could get a positive air box pressure on the restricted engine is to not allow the throttle to open the full amount which they may actually tune, but it seems to me that you may as well open it the throttle as big as it gets to allow the engine to suck the amount of air it can. Positive charge...negative charge...doesn't really matter (obviously positive is better hence turbo and super chargers)...they are shooting for the maximum amount of air by mass. That is allowed by opening the throttle the whole way and letting the piston, on it's down stroke, suck the air through the intake, from the airbox, through the restrictor. Q=AV (perfect orifice equation, fine for this demo, since I don't know the shape of the ACO mandated restrictor) Q=Volumetric Air Flow A=Area of Restrictor (2 x 29.9mm dia per ACO, I think they use two, .001404m^2) V=Velocity of Air Flow (assume car at 150mph, 4023m/min) Q =.001404m^2 * 4023m/min = 5648 liters per min travel through the restrictor Now you have a 7 liter engine that makes 590 HP at 5400 RPM. A V8 has a cylinder fire every 90 degrees, so four times in one rotation. So you can say that the engine exhausts half of the 7 liters every rotation. So an unrestricted, perfect flow LS7 will have a 3.5 lpm flow through it for every rotation. 3.5 l * 5400 RPM = 18900 liters per min. So even at 150 MPH with 5648 lpm flow through the restrictor the engine still wants 18900 lpm. So there has to be a negative air pressure in the air box at 150mph @WOT. The LS7 seems to be always sucking.