ntsqd
Heretic Car Camper
Extracting this from: http://www.expeditionportal.com/forum/showthread.php?p=388959#post388959
Dang that was a lot of cutting & pasting! I hope that I got all the responses & questions lined up right.
High Pressure vs. Low Pressure zones for air induction
- Thread starter ntsqd
- Start date
Dang that was a lot of cutting & pasting! I hope that I got all the responses & questions lined up right.
ntsqd
Heretic Car Camper
My take on it is this:
High Pressure (HP) air will "bleed" off anywhere that it can. Generically the center-bottom of the windshield will have the highest pressure, and the pressure will taper off going in any direction towards an edge that allows rearward flow.
If the rear-facing snorkle scoop is in front of some leading surface, then there will likely be a HP zone behind it.
If there isn't anything behind the scoop then there may or may not be a HP zone there.
The old school way to test this sort of thing is to tape short yarn tufts in rows along the surface in question, and then go for a drive. A simple thing would be to tape some to the edge of the scoop. If they 'fly' straight back then you know that there isn't any HP behind it. If they blow about randomly or even curl around and enter the scoop then you know that there is a HP zone behind the scoop.
Note that you may see different results at different speeds.
High Pressure (HP) air will "bleed" off anywhere that it can. Generically the center-bottom of the windshield will have the highest pressure, and the pressure will taper off going in any direction towards an edge that allows rearward flow.
If the rear-facing snorkle scoop is in front of some leading surface, then there will likely be a HP zone behind it.
If there isn't anything behind the scoop then there may or may not be a HP zone there.
The old school way to test this sort of thing is to tape short yarn tufts in rows along the surface in question, and then go for a drive. A simple thing would be to tape some to the edge of the scoop. If they 'fly' straight back then you know that there isn't any HP behind it. If they blow about randomly or even curl around and enter the scoop then you know that there is a HP zone behind the scoop.
Note that you may see different results at different speeds.
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redthies
Renaissance Redneck
So, playing devils advocate... if the h/p air is behind the snorkel, why do all the performance / drag cars have their ram-air, supercharger etc openings facing forward? I can see both sides of this argument, but if I ride in the back of a pick-up, I get ALOT more wind in my face facing forward over the cab than I do facing backward.:snorkel:
ntsqd
Heretic Car Camper
It is not just that they are facing forwards, it is also their location that is critical. The high dollar Pro Stock teams spend time in a wind tunnel finding the most ideal (highest pressure - lowest drag) location for that opening.
Have a look at a Winston/Nextel/****** Cup Car's engine air induction point. Those are centered at the base of the windshield. Their rules require a stock hood profile, so no part of the induction system can protrude up through the hood. Essentially they have recreated the Chevy Cowl Induction hood arrangement (rear facing opening) without using a hood bulge.
Pro-Stock drag cars aren't hindered by such a 'flat' hood rule, but they are required to have the intake covered - no exposed intake systems. Because of this the opening in the scoop can be placed far enough above hood (& usually quite a ways forward of the carbs) that it receives undisturbed air. They would actually get higher pressure air to the engine from the base of the windshield if they did not have the hood scoop, but the loss of horsepower due to the loss of intake runner length and size from not running the hood scoop is currently unacceptable.
Have a look at a Winston/Nextel/****** Cup Car's engine air induction point. Those are centered at the base of the windshield. Their rules require a stock hood profile, so no part of the induction system can protrude up through the hood. Essentially they have recreated the Chevy Cowl Induction hood arrangement (rear facing opening) without using a hood bulge.
Pro-Stock drag cars aren't hindered by such a 'flat' hood rule, but they are required to have the intake covered - no exposed intake systems. Because of this the opening in the scoop can be placed far enough above hood (& usually quite a ways forward of the carbs) that it receives undisturbed air. They would actually get higher pressure air to the engine from the base of the windshield if they did not have the hood scoop, but the loss of horsepower due to the loss of intake runner length and size from not running the hood scoop is currently unacceptable.
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BlueBomber
Adventurer
OK, i get the whole "cowl induction" thing now. thanks for the help.
SimonInAustralia
Observer
Yep, cowl induction hoods only work due to the opening being at the base if the windshield, where there is a high pressure zone created by the vertical windshield.
It is most likely that there will not be a similar vertical surface behind the snorkel ram when you turn it backwards, as they are usually positioned to the side of the car body/windscreen, so it would create a low pressure zone for the snorkel to try and breath through, not the high pressure zone like the cowl induction hood works with to get more air into the engine with reduced turbulance compared to a front facing scoop.
You see it turned backwards on those comp rigs to prevent as much dust as possible going into the snorkel, not for better air intake.
It is most likely that there will not be a similar vertical surface behind the snorkel ram when you turn it backwards, as they are usually positioned to the side of the car body/windscreen, so it would create a low pressure zone for the snorkel to try and breath through, not the high pressure zone like the cowl induction hood works with to get more air into the engine with reduced turbulance compared to a front facing scoop.
You see it turned backwards on those comp rigs to prevent as much dust as possible going into the snorkel, not for better air intake.
R_Lefebvre
Expedition Leader
In the absence of any concrete evidence either way, I'm going with my gut. I point it forward normally, and turn it back only because of dust, snow or very heavy rain. I did have it facing forward while running around at a winter rally, and pretty quickly started building up snow inside, so I turned it around.
toy_tek
Adventurer
In my purely non-professional opinion I think the question is a bit moot. The engine will pull in the required amount of air in regardless of the direction of the intake. I'd be surprised to see any real test data suggest otherwise. Has anyone actually ever reported a loss of engine performance purely attributed to a reversed snorkel? As pointed out on another board, a similar comparison might be to stick your head out the window at 60mph and breathe facing forward. Then turn your head around to face backward and see if its any harder to breathe.
As to whether the backside of a rearward turned snorkel might experience high or low pressure, I found this data pertaining to a side view mirror, which is probably close enough in size and shape for comparative purposes. Page 3 clearly shows turbulence and low pressure at the back side.
Mirror Aerodynamics
Also notice the front side, where the pressure builds up. It is only 1-2" from the surface.
Once again, non-scientifically speaking, my opinion is that this "low" pressure area is not significant enough to actually cause any disruption in air availability as to what the engine will require. And I'll further that by saying proximity to the windscreen would be a non-issue as well. However, I would welcome test data that would prove this one way or another.
After all, we aren't talking about ultra-tuned race engines at 150mph+.
As to whether the backside of a rearward turned snorkel might experience high or low pressure, I found this data pertaining to a side view mirror, which is probably close enough in size and shape for comparative purposes. Page 3 clearly shows turbulence and low pressure at the back side.
Mirror Aerodynamics
Also notice the front side, where the pressure builds up. It is only 1-2" from the surface.
Once again, non-scientifically speaking, my opinion is that this "low" pressure area is not significant enough to actually cause any disruption in air availability as to what the engine will require. And I'll further that by saying proximity to the windscreen would be a non-issue as well. However, I would welcome test data that would prove this one way or another.
After all, we aren't talking about ultra-tuned race engines at 150mph+.
JIMBO
Expedition Leader
:wings: Ford used "RAM INDUCTION" for its 427 side-oiler V8 in 1964
Chevy intruduced "Cowl induction" in its 1968, Z/28 Camaro, based on Windshield lower area "high pressure" and "boundry layer" hood effect
Chrystler introduced "shaker" hood scoops in its 426 "hemi" in 1970
All of these added 10 to 20 HP, but only way above 80 mph
Batteries not included !!
:bike_rider::wings::safari-rig::safari-rig:
Chevy intruduced "Cowl induction" in its 1968, Z/28 Camaro, based on Windshield lower area "high pressure" and "boundry layer" hood effect
Chrystler introduced "shaker" hood scoops in its 426 "hemi" in 1970
All of these added 10 to 20 HP, but only way above 80 mph
Batteries not included !!
:bike_rider::wings::safari-rig::safari-rig:
R_Lefebvre
Expedition Leader
Of course the engine will take in what it needs, and it would be very difficult to measure a performance difference. However, it does likely make a difference for fuel economy, for the same reasons a dirty air filter will.
toy_tek
Adventurer
This is contradictory. If you agree the engine is receiving the required airflow as RPM dictates, then why would fuel economy be compromised?
A dirty air filter restricts airflow. My opinion is that a reverse snorkel head doesn't. Therefore I don't see how it would effect fuel economy. The only difference is that you eliminate the "ram-air" effect, which, given our general engine performance bracket (non-competition), might be a negligible advantage at best.
:smiley_drive:
ntsqd
Heretic Car Camper
If the engine has to work harder to pull in the volume that it needs (& they are only rarely able to get 100% of displacement) then you take a hit on Volumetric Efficiency. VE plays a large role in how the engine performs overall. Virtually anything involved with the intake or exhaust tracts affects VE in one direction or another.
The whole thing is definitely directly speed sensitive. At overland speeds it probably isn't worth worrying about for power or mileage. At highway speeds it could be worth considering depending on the specifics of the vehicle and the snorkel.
I think that wee need an LM002 to test snorkel direction on!
The whole thing is definitely directly speed sensitive. At overland speeds it probably isn't worth worrying about for power or mileage. At highway speeds it could be worth considering depending on the specifics of the vehicle and the snorkel.
I think that wee need an LM002 to test snorkel direction on!
R_Lefebvre
Expedition Leader
The VE thing is commonly brought up as the reason, but it's not really accurate. The VE of an engine is much more drastically reduced by the restriction of the throttle butterfly when it's not wide open. 
So, say you're cruising at a given speed, requiring about 20% power, maybe requiring a throttle position around 20%. You do something to restrict the intake, you still need 20% power, but now you need to open up your throttle to 25% to achieve the same pressure in the manifold. VE is the same in both cases.
Then why the MPG difference? The engine map determines "load demand" by throttle position. The engine mapping will switch from closed loop "stoich" operation to open loop "rich" operation at a fixed throttle position. Maybe 40%. The more upstream restriction you have, the more throttle you need, the ECU thinks you have a higher power demand, and you spend more time in a richer high-power running mode.
The effect with fuel injection is pretty small. You spend some small amount more time in the open-loop mode than you would with a lower upstream restriction.
With carbs, I'm sure we all know that a clogged filter, or backwards snorkel, would act like a "choke", causing the engine to recieve a richer mixture. These are where your big losses occur. The air filter industry quotes these high figures to this day, trying to get you to buy filters more often, even though EFI engines don't suffer as much.
A Ram Air effect operates in the other direct. You're pushing on the throttle, allowing you to close it more often, resulting in less open-loop running. Again, the effect is small on EFI.
So, say you're cruising at a given speed, requiring about 20% power, maybe requiring a throttle position around 20%. You do something to restrict the intake, you still need 20% power, but now you need to open up your throttle to 25% to achieve the same pressure in the manifold. VE is the same in both cases.Then why the MPG difference? The engine map determines "load demand" by throttle position. The engine mapping will switch from closed loop "stoich" operation to open loop "rich" operation at a fixed throttle position. Maybe 40%. The more upstream restriction you have, the more throttle you need, the ECU thinks you have a higher power demand, and you spend more time in a richer high-power running mode.
The effect with fuel injection is pretty small. You spend some small amount more time in the open-loop mode than you would with a lower upstream restriction.
With carbs, I'm sure we all know that a clogged filter, or backwards snorkel, would act like a "choke", causing the engine to recieve a richer mixture. These are where your big losses occur. The air filter industry quotes these high figures to this day, trying to get you to buy filters more often, even though EFI engines don't suffer as much.
A Ram Air effect operates in the other direct. You're pushing on the throttle, allowing you to close it more often, resulting in less open-loop running. Again, the effect is small on EFI.
ntsqd
Heretic Car Camper
Not so much engine VE, but total system VE. My SOP estimate was that the difference would be in the single digit percentages to maybe the very low double digit percentages. Over a long distance that would start to add up. From here to the grocery store and back, not so much.
Reducing the flow potential, as pulling intake air from even a slight vacuum would do, makes the engine work harder regardless of it's fuel system design though what Rob says about the differences between carb and EFI is interesting.
Reducing the flow potential, as pulling intake air from even a slight vacuum would do, makes the engine work harder regardless of it's fuel system design though what Rob says about the differences between carb and EFI is interesting.
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