Air box rule

[MikeB]

So after what happened to the Acura, does this rule need to be looked at? Did they have an unfair advantage because of the leak?

[Nick Koske]

They could've had a advantage...as they would get more air, therefore more power. This rule is the basis on how the ACO determines engine performance. The ACO says, "here's a certain amount of air based on you class, engine, and weight...makes as much power as you can with it...good luck." To have a leak in the air box gives an engine the oppurtunity to make more power. Whether is did or not who knows. The ECUs give the engines the ability to make use of any extra air, because depending on the speed of the car there is more or less air available. This is why you see the restrictors on the P cars in the air flow...it gives the engine a certain amount of forced air induction.

Leaks in the airbox are unacceptable and all of the other cars passed the test.

[HORNDAWG]

Originally Posted by MikeB

So after what happened to the Acura, does this rule need to be looked at? Did they have an unfair advantage because of the leak?

No, the rule is fine. It is, unfortunately, a bad break (no pun intended) that the airbox failed on the Fernandez Acura. The dq is a high price to pay to discover a design flaw but I am sure it will be addressed even if only to build a Sebring specific box for next year to battle the pounding that the track metes out.

Nick Koske; They could've had a advantage...as they would get more air, therefore more power.

Can this be proven? The air going through the restrictor body becomes charged by the velocity created when the car is moving at speed by the shape of the restrictor orifice and size of the airbox which is designed to maintain said charge to be utilized by the engine mapping at the specific pressure. This scenario could of hurt the performance of the engine just as well as enhance it by allowing the charge pressure to drop by pushing out through the crack or have been wholly neutral in nature and only affected the idle of the engine as there is no charge in the box at 0 mph! (?)

But it is a rule and a good one in my view to keep people from cheating. That said, I in no way what so ever think that Fernandez or Acura planned this to occur and did not cheat!!!!

[Nick Koske]

Originally Posted by HORNDAWG

Can this be proven? The air going through the restrictor body becomes charged by the velocity created when the car is moving at speed by the shape of the restrictor orifice and size of the airbox which is designed to maintain said charge to be utilized by the engine mapping at the specific pressure. This scenario could of hurt the performance of the engine just as well as enhance it by allowing the charge pressure to drop by pushing out through the crack or have been wholly neutral in nature and only affected the idle of the engine as there is no charge in the box at 0 mph! (?)

Good point...I suppose without knowing whether or not there's a negative pressure in the air box it's hard to make the determination whether or not there's an advantage. I would have to say that at WOT (wide open throttle) with the car's velocity at zero there has to be a negative pressure in the air box (that's just physics, air/fluid flow happens across a pressure differentials). Now it's possible that at some time during acceleration there becomes a switch to positive pressure, due to the velocity of the air entering the restrictor. If the Air box pressure was ever positive then a leak would be detrimental from the power aspect. I would have to guess there would be an advantage on slow corners when the velocity of the air stream presented to the restrictor are lowest and the air box most likely has a negative pressure. Whether or not the air box pressure becomes positive and at what speed it may or may not occur is unknown, at least to me.

But when you hear stories like the C6.R could make 900ish HP to try and compete with a LMP1, I've got to think these engines are choked down quite a bit and always have a negative air box pressure.

By no means do I think Fernandez racing tried to cheat...they knew they'd have to pass the test after the race. Just an unfortunate casualty of the track know as Sebring.

[HORNDAWG]

Originally Posted by Nick Koske

Good point...I suppose without knowing whether or not there's a negative pressure in the air box it's hard to make the determination whether or not there's an advantage. I would have to say that at WOT (wide open throttle) with the car's velocity at zero there has to be a negative pressure in the air box (that's just physics, air/fluid flow happens across a pressure differentials). Now it's possible that at some time during acceleration there becomes a switch to positive pressure, due to the velocity of the air entering the restrictor. If the Air box pressure was ever positive then a leak would be detrimental from the power aspect. I would have to guess there would be an advantage on slow corners when the velocity of the air stream presented to the restrictor are lowest and the air box most likely has a negative pressure. Whether or not the air box pressure becomes positive and at what speed it may or may not occur is unknown, at least to me.

But when you hear stories like the C6.R could make 900ish HP to try and compete with a LMP1, I've got to think these engines are choked down quite a bit and always have a negative air box pressure.

By no means do I think Fernandez racing tried to cheat...they knew they'd have to pass the test after the race. Just an unfortunate casualty of the track know as Sebring.

Even with the massive power that can be developed with the LS7 at full song when the engine is restricted to meet the rules you want to have as positive a charge on the air inlet as possible within said rules to create the best air fuel mix you can get to optimize the combustion burn. Everything is optimized to the max cfm that can be had through the restrictor in mapping the ecu.

[Nick Koske]

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.

[HORNDAWG]

Also having to be considered is air temp, air density, and absolute pressure at the converging inlet of the bell mouth which is not wholly dictated by velocity alone. As the diameter and gradient of the inlet structure play large roles in the calculation of critical flow through the throat of the restristor by creating upstream pressure by nature of its shape.

[Nick Koske]

Originally Posted by HORNDAWG

Also having to be considered is air temp, air density, and absolute pressure at the converging inlet of the bell mouth which is not wholly dictated by velocity alone. As the diameter and gradient of the inlet structure play large roles in the calculation of critical flow through the throat of the restristor by creating upstream pressure by nature of its shape.

Right...but I don't think that will account for a 70% increase in the amount of air allowed through the restrictor to remove the pressure differential. If the engine has the capacity to 18900 lpm, then the air box needs to have 18900 lpm introduced to it...otherwise there is a vacuum in the air box. It doesn't seem possible to introduce 18900 lpm into the air box through the given restrictors and normal speeds. Sure there's a ton more variables that need to be taken into account to get an exact number, but generally if you're 70% off there's no way to massage the numbers to make it work.

[HORNDAWG]

Originally Posted by Nick Koske

Right...but I don't think that will account for a 70% increase in the amount of air allowed through the restrictor to remove the pressure differential. If the engine has the capacity to 18900 lpm, then the air box needs to have 18900 lpm introduced to it...otherwise there is a vacuum in the air box. It doesn't seem possible to introduce 18900 lpm into the air box through the given restrictors and normal speeds. Sure there's a ton more variables that need to be taken into account to get an exact number, but generally if you're 70% off there's no way to massage the numbers to make it work.

And just how much lift and duration is there in the cam and what type of cam to crank timing is there ie how much of the total volume is actually being utilized and therefore needs to be filled ie volumetric efficency? It is not as straight forward as you are making it out to be and there are many factors that NEED to be included! I will grant that somewhere in the intake there has to be a vacuum state in order to charge the cylinders (naturally aspirated), but where that occurs is the question. And whether or not the crack was sufficiently large enough to leak under any conditions other than under total blockage of the restrictor throat!