There's alot of talk about airboxes periodically and I've made some measurements and done a little math and I'll just put it here for those who are interested to chew on. Here we go!...Cubic inches x rpm divided by 3456 = cfm requirements for an engine. That number gets smaller if you figure most engines are not operating at 100% volumetric efficiency. Supercharged engines are probably operating above 100% efficiency aren't they(?) But if you're not supercharged,here's what I see us all looking at. Your Viper may shut the fuel off at 6200, but chances are you'll be going thru the traps at quite a bit less than 6200rpm; so use a lower number for rpm when you're doing the formula. Next point, the stock NACA duct opening is about 15.8sq." by my measurement, enough to supply a non-supercharged , and certainly not 100% effecient 488ci engine in the 5600-6100rpm range. A Viper engine at 100% effeciency and 6000rpm needs 850cfm, at 80% effeciency - 680cfm! Quite a drop in CFM requirements isn't it? We all know the two valve Viper engine isn't the most sophisticated hunk of iron out there..it IS big and beautiful of course but it's operating at well less than 100%. 850cfm carbs use 2.25" throttle bore diameters by the way, which I calculate out to 15.9sq." total area. Now then, if the NACA duct is indeed allowing air to be sucked OUT of it as it's been implied in the forums, then that would create other problems. But as I see it the hood opening and those huge K&Ns (for sure) are big enough for 99% of the naturally aspirated Vipers. Supercharged engines may well need more inlet area. It's easy imagine this monster engine needing ungodly amounts of CFM , but it does look like the math speaks for itself. If I've missed something, do let me know. The more I learn the better: )
Mathematics, your airbox, and you..
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McGuireV10
Enthusiast
I've done quite a bit of research on NACA ducts lately (including "doing the math" on a wide variety of shapes, depths, ramp angles, divergence rates, and so on), and I'd be very surprised if the duct allows air to be sucked out.
I'm not saying it helps at all -- a NACA duct doesn't have anything useful to contribute to the intake problem at it's location in the Viper hood. However, there isn't anything about the Viper's duct that should make it prone to reducing air intake over a regular non-NACA-shaped intake at that location.
About the only thing that can make a NACA duct do more harm than good is to have excessively smooth, sloped walls (in the vertical plane). The sharper the wall edges, the better.
I can't comment on the rest, but if it's accurate, that's very interesting. Reminds me of various arguments I've read about what size throttle bodies are appropriate/required.
I'm not saying it helps at all -- a NACA duct doesn't have anything useful to contribute to the intake problem at it's location in the Viper hood. However, there isn't anything about the Viper's duct that should make it prone to reducing air intake over a regular non-NACA-shaped intake at that location.
About the only thing that can make a NACA duct do more harm than good is to have excessively smooth, sloped walls (in the vertical plane). The sharper the wall edges, the better.
I can't comment on the rest, but if it's accurate, that's very interesting. Reminds me of various arguments I've read about what size throttle bodies are appropriate/required.
GTSnake
Enthusiast
So are you saying that the NACA duct is not big enough and it's not getting enough air into the engine to maximize efficiency? If so, it would be great to have some aftermaket duct kit to expand the size of the opening for more flow...
jrkermode
Enthusiast
The pressure at the NACA duct opening is less than the pressure on the radiator. So air spilling up off the radiator into the airbox see's the NACA opening as a low pressure way out of the box. This condition applies to speeds up to about 140 MPH. At higher speeds conditions could be different. However, the Oreca car had the NACA duct sealed up and got its air from the grill.
When you stated the NACA opening is big enough, how did you decide what is an acceptable velocity for the incoming air? When I do air box math/layout I try to ensure the air box inlet gathers air faster than the engine can empty the box, so I get some ram air. For this calculation I use the average speed of the car in whatever event it is typically used.
When you stated the NACA opening is big enough, how did you decide what is an acceptable velocity for the incoming air? When I do air box math/layout I try to ensure the air box inlet gathers air faster than the engine can empty the box, so I get some ram air. For this calculation I use the average speed of the car in whatever event it is typically used.
Vipermed 97.01
Enthusiast
Funny you should bring this subject up http://vca2.viperclub.org/forums/showflat.php?Cat=&Board=UBB14&Number=291276&page=0&view=collapsed&sb=5&o=&fpart=1
Re: Mathematics, your airbox, and you..
Tom, I'm suggesting the duct appears to be large enough, unless you're running a turbocharger, supercharger etc. that would take engine efficiency to or above 100%. Figuring in speed may have its' merits, there was one Bonneville racer type formula that used speed and distance traveled, how much air per mile the engine would **** in etc, but oddly enough still came out at 850cfm. Ram air? Take it if you can get it (and hold it) for sure! If your connections aren't airtight,..if you haven't sealed the seam where the upper and lower front OEM airbox halves meet with something, you'll lose any ram you hope to gain. Then there's those 3 rain drains in the bottom of the water trough below the rain dam. They won't hold any air pressure either. Articles from HotRod or CarCraft that are 20yrs old still come to memory, incl one where a bolt-on ram air unit didn't do a thing until they taped up all the hose connections, the airbox lid to the airbox etc, then they picked up a tenth or more..It seems the opening appears to be large enough, the airbox itself has a huge amount of volume, and the filter elements, already oversized, once replaced with something like K&Ns that flow better are hard to be improved upon. Maybe that's why there's been alot of frustration posted when attempts both homemade and store bought to pick up HP behind the NACA duct don't yield results. I didn't invent the formulas : ) just thought it made for some interesting apparent realities concerning the CFM needs of an engine. There's alot of very quick drag cars running scoops with smallish openings that don't seem to match the the sometimes large and highly efficient engines beneath them at first thought. The velocity for the incoming air ? I don't have a clue..just how much will go through that opening at one atmosphere (I guess) is what the formula are based on(?). You guys know as much as I do now. Vipermed until someone puts some airboxes on a flowbench who's to say what's what? If your airbox has more volume (as it appears to) that may be important, alot of Japanese crotch-rockets are fitted with very large airboxes in recent years. I'm sure their factories test the dickens out of things and those large airboxes are there for a reason. If your filters manage the air better than the flat panel K&Ns(and they might) that's an improvement too. Beats me.. My comments have been more targeted at the much maligned NACA duct. I'm sure there's alot of room for improvement in what happens after the air is in the airbox.
Tom, I'm suggesting the duct appears to be large enough, unless you're running a turbocharger, supercharger etc. that would take engine efficiency to or above 100%. Figuring in speed may have its' merits, there was one Bonneville racer type formula that used speed and distance traveled, how much air per mile the engine would **** in etc, but oddly enough still came out at 850cfm. Ram air? Take it if you can get it (and hold it) for sure! If your connections aren't airtight,..if you haven't sealed the seam where the upper and lower front OEM airbox halves meet with something, you'll lose any ram you hope to gain. Then there's those 3 rain drains in the bottom of the water trough below the rain dam. They won't hold any air pressure either. Articles from HotRod or CarCraft that are 20yrs old still come to memory, incl one where a bolt-on ram air unit didn't do a thing until they taped up all the hose connections, the airbox lid to the airbox etc, then they picked up a tenth or more..It seems the opening appears to be large enough, the airbox itself has a huge amount of volume, and the filter elements, already oversized, once replaced with something like K&Ns that flow better are hard to be improved upon. Maybe that's why there's been alot of frustration posted when attempts both homemade and store bought to pick up HP behind the NACA duct don't yield results. I didn't invent the formulas : ) just thought it made for some interesting apparent realities concerning the CFM needs of an engine. There's alot of very quick drag cars running scoops with smallish openings that don't seem to match the the sometimes large and highly efficient engines beneath them at first thought. The velocity for the incoming air ? I don't have a clue..just how much will go through that opening at one atmosphere (I guess) is what the formula are based on(?). You guys know as much as I do now. Vipermed until someone puts some airboxes on a flowbench who's to say what's what? If your airbox has more volume (as it appears to) that may be important, alot of Japanese crotch-rockets are fitted with very large airboxes in recent years. I'm sure their factories test the dickens out of things and those large airboxes are there for a reason. If your filters manage the air better than the flat panel K&Ns(and they might) that's an improvement too. Beats me.. My comments have been more targeted at the much maligned NACA duct. I'm sure there's alot of room for improvement in what happens after the air is in the airbox.
monnieh
Viper Owner
Re: Mathematics, your airbox, and you..
I know Ram Air is basically bunk. You would probally go faster by taking out your spare. What I find valuable about the vipermed new air duct is that it is providing fresh cooler air to the airbox as opposed to it sucking in air from th engine compartment.
I ordered one of the introductory ones and will dyno next week and see if there is any hp gain. Although I think the dragstrip would provide a more accurate indidcation of wether the new air baffle gives more power.
I know Ram Air is basically bunk. You would probally go faster by taking out your spare. What I find valuable about the vipermed new air duct is that it is providing fresh cooler air to the airbox as opposed to it sucking in air from th engine compartment.
I ordered one of the introductory ones and will dyno next week and see if there is any hp gain. Although I think the dragstrip would provide a more accurate indidcation of wether the new air baffle gives more power.
Vipermed 97.01
Enthusiast
Re: Mathematics, your airbox, and you..
You are absolutely correct,air must be flowing at high speeds to get its peak #s,I do know this, that our vipermed airbox has removed detonation on some roe cars here on the board.and when i put the first prototype on my car what little load detonation i had remaining was gone.I then new that we had a winner
joe117
Enthusiast
Re: Mathematics, your airbox, and you..
This is true,
"alot of Japanese crotch-rockets are fitted with very large airboxes in recent years. I'm sure their factories test the dickens out of things and those large airboxes are there for a reason."
Many of the superbike engines are using a tuned cavity intake chamber. Lots of power to be gained in that science.
For your Viper, in my opinion, for what it's worth, I don't think there is anything worthwhile left to do, after tubes and filters, and you don't get that much from them.
An intake box and fancy tubes can make your engine look nice. It may be worth doing for that reason. You may pick up some air that's a little cooler, that will help.
All in all, air intake mods are not going to give you sotp power.
This is true,
"alot of Japanese crotch-rockets are fitted with very large airboxes in recent years. I'm sure their factories test the dickens out of things and those large airboxes are there for a reason."
Many of the superbike engines are using a tuned cavity intake chamber. Lots of power to be gained in that science.
For your Viper, in my opinion, for what it's worth, I don't think there is anything worthwhile left to do, after tubes and filters, and you don't get that much from them.
An intake box and fancy tubes can make your engine look nice. It may be worth doing for that reason. You may pick up some air that's a little cooler, that will help.
All in all, air intake mods are not going to give you sotp power.
Matt M PA
Enthusiast
Re: Mathematics, your airbox, and you..
As I understand it, RamAir is just about worthless on a fuel-injected engine. A friend of mine drag races every weekend with a rail, and he explained to me that many of the racers are trying to get fuel injection "approved" so that they can clean up the aerodynamics of the cars and get rid of those big scoops.
As I understand it, RamAir is just about worthless on a fuel-injected engine. A friend of mine drag races every weekend with a rail, and he explained to me that many of the racers are trying to get fuel injection "approved" so that they can clean up the aerodynamics of the cars and get rid of those big scoops.
McGuireV10
Enthusiast
I wasn't aware of the radiator spill issue, that's very interesting. I thought you meant the duct itself was losing part of the intake charge (which is what happens when you shorten or significantly smooth/round-off a NACA duct's walls).
I'm also not saying whether the duct is or isn't big enough -- I intended to convey that the NACA-style doesn't add or detract to the duct at it's location in the Viper hood. NACA ducts are designed to scavenge air by creating low pressure regions in a boundary-layer airflow, and sitting out there in front, there isn't really any laminar flow to scavenge from -- air just rams straight in -- which is why the SRT-10 went back to a regular duct.
(Which is unfortunate -- the NACA duct still looks cooler, and on a car like this, looks count.)
Also, I read somewhere that ORECA closed off the NACA duct due to some kind of rule about air intakes. The radiator leakage thing makes sense, though.
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