Landracing Forum
Tech Information => Technical Discussion => Topic started by: Tharon on June 04, 2014, 07:18:57 AM
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Hi all, new to Landracing.com and first time forum user, so please be gentle. I have some questions on forward facing hood scoops.
I run a Holden commodore (Caprice in USA) at Lake Gairdner in Australia.
E/Pro, 260 ci V8, 1000cfm throttle body, EFI (MoTec M48)
I have read the article in Hotrod magazine regarding the results on this subject and I am looking at using the Harwood #3182 which was one that they tested. The manufacturer list these scoops as an "open back".
If this literally is an open back, what would be the advantage, I thought the high speed of the air into the scoop at the front would negate the purpose of having the back open a,s it would be a greater force than the low pressure at the base of the windshield.
Yes, I am hoping to benefit from ramming the the air
Sorry if this is a repost of an older subject, I am just after info before I make a purchase and I thought you guys would be best on the subject.
Thanks in advance for any input.
Tharon.
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If you want the benefit of ram air at speed you need a sealed system... If the air is just passing through a scoop it is not sealed. If it has a pick up at both the front and the back it seems to me that the one with most pressure would just push the air out the other side.
We used to pick up air from both sides of the Lakester, but our data showed we were not increasing the air pressure in the air box. We changed to just one source and air pressure was developed in the box.
I am not familiar with sedan hood scoops, but I would only pick from the front or the back.... just my opinion.
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My 2 cents: http://www.engineprofessional.com/
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Scoops are Simple? :roll:
Good read, Woody.
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.........Yes, I am hoping to benefit from ramming the the air..
Besides picking up some rammed air effect, if the back is closed and the scoop is sealed as mentioned by SS, you can pickup better aero by having the right size scoop opening.
If the scoops frontal area is sized so that most of the air hitting the opening is going into the engine then the air flow around the opening will give you better Cd there. If the opening is too large, as that one probably is, the air the engine can't use is piling up at the entrance to the scoop and becoming turbulent there and then going past the scoop in a manner that is making the Cd worst.
Drag cars can produce a lot of power at low speeds and get it to the ground with their tire/track combination so they need a larger opening to get the air for the power at low speeds. We can't use high power usually at lower speeds so don't need that large opening in general. At high speeds where we can use the power the small scoop opening can 'scoop up' a lot of air in the distance the car travels for each 2 revolutions of the engine's crank.
I have more info here and it is these guys, John Burk and Tom Burkland, that turned on the light for me :-)....
http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html (http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html)
I also have a spreadsheet for figuring the scoop inlet size here...
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html (http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html)
As a couple examples here is the old scoop on Hooley's Stude that worked very well and got him a record at 249+ with a roots blown 400 ci SBC...
(http://purplesagetradingpost.com/sumner/hooley/2005-Bville-10.jpg)
Notice the size of the scoop on Rick and Jack's 360+ mph streamliner below...
(http://yacoucci.com./gallery/d/2952-2/103_0308_IMG.jpg)
(http://yacoucci.com./gallery/d/2943-2/103_0305_IMG.jpg)
http://yacoucci.com./gallery/v/Neb/album84/103_0305_IMG.jpg.html (http://yacoucci.com./gallery/v/Neb/album84/103_0305_IMG.jpg.html)
Sumner
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Thanks for your thoughts guys.
Stainless i had exactly the same thoughts about "open back"and one cancelling the other out.
I just didn't know if i may have been missing something or maybe they are just sold with open front and back so the purchaser can do as they wish.
I will get to study everyone's links in the coming days.
Thanks heaps..
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I don't get to say this much but... Mine is too big! My scoop that is. I did the math on Sumner's formula and for a D engine my scoop is way oversize. Will be thinking about a good way to extend the current scoop while reducing the inlet like I see on Hooley's Studebaker. Another project.
BR
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I started with one of Harwood's old Formula scoop we ended up whacking it all up ---- what used to be the rear became the front---( we just came forward on the taper until we liked the diameter, radiused the inside and tapered the top and sides to fair as we wanted--- but we were on a lakester did not have to deal with bothersome things like windshields and such
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.........Yes, I am hoping to benefit from ramming the the air..
Besides picking up some rammed air effect, if the back is closed and the scoop is sealed as mentioned by SS, you can pickup better aero by having the right size scoop opening.
If the scoops frontal area is sized so that most of the air hitting the opening is going into the engine then the air flow around the opening will give you better Cd there. If the opening is too large, as that one probably is, the air the engine can't use is piling up at the entrance to the scoop and becoming turbulent there and then going past the scoop in a manner that is making the Cd worst.
Drag cars can produce a lot of power at low speeds and get it to the ground with their tire/track combination so they need a larger opening to get the air for the power at low speeds. We can't use high power usually at lower speeds so don't need that large opening in general. At high speeds where we can use the power the small scoop opening can 'scoop up' a lot of air in the distance the car travels for each 2 revolutions of the engine's crank.
I have more info here and it is these guys, John Burk and Tom Burkland, that turned on the light for me :-)....
http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html (http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html)
I also have a spreadsheet for figuring the scoop inlet size here...
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html (http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html)
As a couple examples here is the old scoop on Hooley's Stude that worked very well and got him a record at 249+ with a roots blown 400 ci SBC...
(http://purplesagetradingpost.com/sumner/hooley/2005-Bville-10.jpg)
Notice the size of the scoop on Rick and Jack's 360+ mph streamliner below...
(http://yacoucci.com./gallery/d/2952-2/103_0308_IMG.jpg)
(http://yacoucci.com./gallery/d/2943-2/103_0305_IMG.jpg)
http://yacoucci.com./gallery/v/Neb/album84/103_0305_IMG.jpg.html (http://yacoucci.com./gallery/v/Neb/album84/103_0305_IMG.jpg.html)
Sumner
Sumner...Not sure what your comparing, Jacks 122.99 max G motor 360 mph hood scoop looks bigger than Hooley's 400 cu.in. scoop.
I would never put that small a scoop on the 222 Camaro.
JL222 :cheers:
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.... Sumner...Not sure what your comparing, Jacks 122.99 max G motor 360 mph hood scoop looks bigger than Hooley's 400 cu.in. scoop.
His is smaller, it was the perspective of the picture. Here is a better one of his scoop...
(http://yacoucci.com./gallery/d/2816-2/102_0241_IMG.jpg)
.....I would never put that small a scoop on the 222 Camaro.
JL222 :cheers:
Nor would I recommend one that size. Not sure if you ran through the formula or not but besides how far the car travels in 2 engine revolutions (figured by gearing and tire size) the other two factors are displacement and if it is naturally aspirated or blown. A motor running 14-15 lbs. of boost compared to na is going to need about twice the scoop opening and if you were running 18-30 #'s 3 times the scoop opening.
For instance with Hooley's old motor, 400 ci, with 28 inch tires and 2.47 rear the scoop size with a 10% oversize would be:
na = 6.5 sq. in.
8# boost = 10 sq. in.
15# boost = 13 sq. in.
28# boost = 19 sq. in.
Since we ran an overdrive 4th gear our real final ratio was about 2.22. That would of resulted in openings smaller than above since the car went further each 2 revolutions but I used the 2.47 which also oversize the scoop some. We also never had much luck with the roots blower over about 8-10 #'s since the air got too hot with no intercooler and I think we made less HP down at the 4-5 as a result vs. running less boost. We also ran the car extremely rich, in the low 10's which I felt kept the motor together. Other than forgetting to turn the cooling water on never hurt the motor on a lot of runs to the 5 mile and even then no piston damage.
The 572 that is in the car now won't be run over 14-15 #'s with the present turbos so the opening for it is sized at about 24 sq. in. (3 X 8 inches) even though the formula says 17 sq. in.. This gives us some leeway if we got better turbos at some point.
Sparky makes a good point in that this is more critical with a streamliner or lakester if there is nothing behind the scoop except hopefully clean air. Our present scoop...
(http://purplesagetradingpost.com/sumner/Hooley%202013/wing--87.jpg)
... has the whole car behind it sitting down there at the front of the car :-).
Good luck to you guys this year,
Sum
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This must be one of the most interesting threads.
I need to ask a question.
A turbo charger spinning at full boost will take in a certain amount of air but will a scoop increase that volume?.
What I'm getting at is will more air forced in just stack up at the intake?.
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The pressure inside the scoop rises with forward speed. This increase in pressure raises the air density. More air molecules + more fuel molecules = more HP. When you add the engine it becomes a flow limiter. It can only move so many CFM. Turbos just move more and stuff it inside the engine. What you are really interested in is the mass flow - more molecules! When you size the scoop inlet to CFM with the column of air hypothesis what happens is the engine will reduce the air pressure inside the scoop, you will have a lower air density and you WILL lose HP especially at lower mph! The CFM matches just fine [at the design speed only] but you will have less molecules of air in each cubic foot. If the opening is too big then the engine can't keep up and you get spillage out of the scoop and higher drag. Several other interacting factors come into play! Scoops are Simple - NOT! In some cases the lower HP may have aided traction in the launch but it will cost mph on the far end. Most scoops tend to increase HP some but do you want 50%, 75% or 100% of the potential? :? That's just my three cents! :-D
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Woody and the rest of you: It's a hypothetical question from a novice in this stuff, but how 'bout a scoop with variable bits? Like the scoop's open front could change for different speeds, or maybe a spill/waste gate at the rear to let out excess air (out the back?) when the scoop is bringing in too much air and creating the spill drag?
Has it been done - or at least studied?
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Yes, it has been studied and applied....
Check out the inlets of a lot of jet fighters...
The inlets have to manage any supersonic shock waves that could get into the compressor section.
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Okay - the technology exists. Has it been tried on race cars and bikes? Has it been tried on land speed vehicles?
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There is rumor that F-I guys are doing a version of it by where the hold their helmet in front of the intake scoop.
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....When you size the scoop inlet to CFM with the column of air hypothesis what happens is the engine will reduce the air pressure inside the scoop, you will have a lower air density and you WILL lose HP especially at lower mph! The CFM matches just fine [at the design speed only] but you will have less molecules of air in each cubic foot. If the opening is too big then the engine can't keep up and you get spillage out of the scoop and higher drag. Several other interacting factors come into play! Scoops are Simple - NOT! In some cases the lower HP may have aided traction in the launch but it will cost mph on the far end. Most scoops tend to increase HP some but do you want 50%, 75% or 100% of the potential? :? That's just my three cents! :-D
Adding to Woody's thoughts I'd personally not size the scoop for an "exact" size CFM wise in high gear. I'd size it for the gear where you are able to run WOT and need maximum HP. For a small motor car the need for a larger scoop opening might be in one of the lower gears where the car is needing to make maximum HP.
In our case so far that situation isn't until we are running our 1 to 1 3rd gear. Even then our scoops have been sized over 10% larger than needed using the CFM required formula that I posted. Still when you look at the scoops we have used they will look quite small compared to what people are use to looking at when watching cars at the drags where they can use HP at much lower speeds.
I also wouldn't size the scoop the same way for one of the mile tracks as you would for the salt as there again you can use the HP in lower gears with the better traction so you don't want to restrict that HP as Woody mentioned.
Size the opening for the first gear where you can use full engine HP with the exception if that is going to hinder drag in the final gear to the point where you would give up some HP at a slower speed to help with drag and the HP needed to overcome it at your terminal speed.
An example of some of this is Speed Demon's inlet opening size...
(https://dl.dropboxusercontent.com/u/2766640/SD/42_1.jpg)
.... not that large for a car that can make up to 2500 HP, but the car is not to WOT until 380 mph and probably not making maximum HP until past that if at all.
So far here we are talking about the scoop opening only. Once the air gets into the scoop you have another whole bunch of theories coming into play. That is Woody's domain :-). Go back and look at the Nish streamliner over the years and you will see a lot of different shaped scoops with very different internal volumes past the inlet. A couple were huge in size past the inlet.
...I need to ask a question.
A turbo charger spinning at full boost will take in a certain amount of air but will a scoop increase that volume?.
What I'm getting at is will more air forced in just stack up at the intake?.
It will stack up if the opening is too large and have to spill around it. You can get a charge effect but I don't think I've ever seen this over 2 lbs.. Still even if it is 1 lb. and you are blown that is just 1 less pound the blower is going to have to make so you will have lower air temps after the compressor which lowers the load on the intercooler.
Sum
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If you can get your hands on issues 162 and 164 (May and July of 2014) there are a couple of articles on air scoop design written by Marco deLuca and Angelo Camerini, both former heads of aerodynamics at Ferrari and BMW. You probably will need to read a couple of times but lots of good info and they also discuss inlets into turbos, which do benefit from a properly designed inlet. Pretty good stuff.
Rex
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If you can get your hands on issues 162 and 164 (May and July of 2014) .....
Issues of what Rex? Thanks,
Sumner
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Race Tech magazine
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Ducts vs scoops?.
Danny Thompson is going to use ducts.
I like his idea better for my application.
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Sorry Sum, Tortoise is right it is "Race Tech" magazine.
Thanks,Tortoise!
Rex
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To Slim's point: The basic function of a scoop is to collect air. If it has no outlet you get max pressure [and spillage]. If you add a "leak" aka the engine then the air has a place to go but the pressure will go down. If you add another leak in the back of the scoop you just stole some air from the first leak and the pressure will go even lower! :-(
I will be at the LA Roadster Show Sunday if any of youse guys are around! :cheers:
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Okay, Woody -- you're part way to answering my query.
Now that we've established that a port at the rear (or someplace) of a scoop will reduce the pressure inside of it -- how about if we are careful to adjust the size of that port, while on the fly, to maintain just enough positive pressure to realise a gain - and to NOT have excess spilling back out the front and creating a pressure/drag? At low speeds it'd be closed, and as the speed increases it would open in conjunction with increasing speed (probably as measured by a sensor or two or three, inside and outside the scoop and also perhaps programmed to stay within some certain and preset parameters) to maintain just the right balance between wasted air and too little air in the scoop (vacuum)?
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OK..what size scoop for 4500 cfm. 29in tires 1.85 gearing 500 cu. in.? :-D
IF I stick my fist in the inlet tubing while idling the 222 Camaro it sucks my arm in. If I dared to whack the throttle I think it would spit me out the exhaust :-o
JL222 :cheers:
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. . . as the speed increases it would open in conjunction with increasing speed (probably as measured by a sensor or two or three, inside and outside the scoop and also perhaps programmed to stay within some certain and preset parameters) to maintain just the right balance between wasted air and too little air in the scoop (vacuum)?
At design top speed, you'd have a larger inlet, higher drag than a system with an internal airflow just sufficient to pressurize the engine. Much simpler, and lower drag at design speed, would be an inlet enlarging at low speed, (or a secondary inlet, like a musclecar "air grabber" scoop,) closing as top speed approaches.
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OK..what size scoop for 4500 cfm. 29in tires 1.85 gearing 500 cu. in.? :-D
IF I stick my fist in the inlet tubing while idling the 222 Camaro it sucks my arm in. If I dared to whack the throttle I think it would spit me out the exhaust :-o
JL222 :cheers:
What boost level so I don't have to make a new spreadsheet using the cfm figure :-).
With 30 lbs. boost and the other info (not the cfm) and with a 10% oversize about 18 square inches, about 5 inch dia. if round. If an oval scoop 3 inches high in the middle with rounded ends (1.5 inch radius) the middle section between the rounded ends would be 3.6 inches long so the width of the scoop would be 6.6 inches.
20% over size would be about 20 sq. inches. I'd probably go with 20-22 sq. inches if it was my car, which it isn't :-D,
Sum
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..... (or a secondary inlet, like a musclecar "air grabber" scoop,) closing as top speed approaches.
If you need the air at the lower speeds I'd like that approach,
Sum
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May have mentioned this before . On the very front of streamlined bodies there is an area that sees full impact pressure
at all speeds . A small area sees 100% and a larger area sees 90% etc . The pressure is already there so inlet doesn't need to be sized and works in all gears . The vertical peak of my nose has a larger area with near peak pressure .
http://www.motorsportsinnovations.com/Bvile-pics/bs%20nose%203-07%20(1)_small.jpg
Ducting would be a problem if the engine wasn't in front .
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May have mentioned this before . On the very front of streamlined bodies there is an area that sees full impact pressure
at all speeds . A small area sees 100% and a larger area sees 90% etc . The pressure is already there so inlet doesn't need to be sized and works in all gears . The vertical peak of my nose has a larger area with near peak pressure .
http://www.motorsportsinnovations.com/Bvile-pics/bs%20nose%203-07%20(1)_small.jpg
Ducting would be a problem if the engine wasn't in front .
Sam took that approach with E-Z-Hook ....
(http://images.motorcycle-usa.com/PhotoGallerys/large/Sam-EZ-Hook-DL.jpg)
(http://images.motorcycle-usa.com/PhotoGallerys/large/65033IMG_9203.jpg)
I looked at it for my lakester and would love to do it but like you said getting the air ducted past me is a problem,
Sum
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Should have added , not sure that a nose inlet would work on other than a NACA type streamlined profile . The pressure area would be tiny on a pointed nose and maybe lower pressure and turbulent on a more rounded shape .
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I still want to know if, IN THEORY, my idea could work. I don't care if it will be used on a car or bike -- just wonder if there's a way to get around the problem of spilled air making drag - while still giving the engine at least as much as it can use - if not more (presurised). That was my original question - theoretical.
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I still want to know if, IN THEORY, my idea could work. I don't care if it will be used on a car or bike -- just wonder if there's a way to get around the problem of spilled air making drag - while still giving the engine at least as much as it can use - if not more (presurised). That was my original question - theoretical.
Work as in reducing drag? Maybe, maybe not. If the air you are releasing out the back is turbulent or creates a turbulence there then you probably aren't gaining much.
If you have a scoop that is aero correct for little drag on the back side of it...
(http://www.myrideisme.com/Blog/wp-content/uploads/2009/12/n17r98b.jpg)
.... like on Tom's car above then I think you would be in a loosing situation by loosing that shape and putting on a scoop that is up in the air with an opening back on it. Just my opinion :-).
Only way you could really tell would be in a wind tunnel or Woody doing analysis on the exact scoop shape your are purposing attached to the car or bike in question. I like the option of a second scoop, a flap or other type opening that could be used at slower speeds and closed at higher speeds like John suggested.
Sum
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OK..what size scoop for 4500 cfm. 29in tires 1.85 gearing 500 cu. in.? :-D
IF I stick my fist in the inlet tubing while idling the 222 Camaro it sucks my arm in. If I dared to whack the throttle I think it would spit me out the exhaust :-o
JL222 :cheers:
What boost level so I don't have to make a new spreadsheet using the cfm figure :-).
With 30 lbs. boost and the other info (not the cfm) and with a 10% oversize about 18 square inches, about 5 inch dia. if round. If an oval scoop 3 inches high in the middle with rounded ends (1.5 inch radius) the middle section between the rounded ends would be 3.6 inches long so the width of the scoop would be 6.6 inches.
20% over size would be about 20 sq. inches. I'd probably go with 20-22 sq. inches if it was my car, which it isn't :-D,
Sum
The f3-139 is capable of 45+ lbs boost and the inlet is 6in as is all our ducting =28.27 sq in we oblonged the 6 in tube and flared it out to fit a Harwood scoop which is 3 X 15 with rounded ends about 43 sq in,
Not going smaller for an unknown drag reduction after posting a time of 286 mph at the 21/4 and 294mph and indicated 318mph just short of the end of the ist timed mile and 2 miles to go.
A spread sheet based on cfm, boost and temp might help. There's low temp boost and hi temp boost with a lot more density in the low. Also supercharging compacts and shrinks the air and it seems that a good supply is needed before the blower,
The Rolls Royce 2 stage Merlins used a much bigger 1st stage impeller to pump into the smaller 2nd stage impeller as the second stage air is compressed.
JL222 :cheers:
PS. I like your inlet placing on Hooleys Stude.
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.. The f3-139 is capable of 45+ lbs boost and the inlet is 6in as is all our ducting =28.27 sq in we oblonged the 6 in tube and flared it out to fit a Harwood scoop which is 3 X 15 with rounded ends about 43 sq in,....
45 lbs. would be 26 sq. in. with a 20% over cushion, so not far off of you 28 sq. in. but if your 6 inch inlet has been oblonged it isn't 28+ sq. in. any longer, but less.
You guys running at the July meet or Speed Week? Hope to see you there,
Sum
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Hi Guys,
One thing lacking in this discussion (specifically regarding naturally aspirated engines) is clean airflow, turbulence at carb or injector inlet. Ducting and plenum design can be critical. Several years ago I started a thread on plenum design and never really got much information.
Running a D/Street Roadster I started out using a cutdown Pro Stock hood scoop and ran about 170 MPH. My goal was to eliminate the drag of the hood scoop. I had a one core radiator build and "designed" a bell intake and duct with an expanding volume plenum.
The first run at Bonneville that year we lost 25 MPH. SO MUCH FOR MY GREAT DESIGN! Went to the CARQUEST in Wendover and bought two 11"x 2" air cleaners and ran them into the wind. Next run was right back to 170 MPH.
Just food for thought.
Tom
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Hi Guys,
One thing lacking in this discussion (specifically regarding naturally aspirated engines) is clean airflow, turbulence at carb or injector inlet. Ducting and plenum design can be critical. Several years ago I started a thread on plenum design and never really got much information.
Running a D/Street Roadster I started out using a cutdown Pro Stock hood scoop and ran about 170 MPH. My goal was to eliminate the drag of the hood scoop. I had a one core radiator build and "designed" a bell intake and duct with an expanding volume plenum.
The first run at Bonneville that year we lost 25 MPH. SO MUCH FOR MY GREAT DESIGN! Went to the CARQUEST in Wendover and bought two 11"x 2" air cleaners and ran them into the wind. Next run was right back to 170 MPH.
Just food for thought.
Tom
Good info :cheers:
We have tried to keep our scoop inlets in clean air. If you don't have wind tunnel data I'd personally keep the bottom of the inlet 6 inches or more above the surface under it. I'd be affraid to try and design a scoop very close to the body like Speed Demon but I'd bet a lot that they have wind tunnel or CFD data to show that it works there and of course the speeds to back that up.
Likewise with scoop off the sides of the body and naca ducts. Don't just buy a generic one and stick it on and hope. You really need to know what the air is doing by these openings and in them as well. One person who currently has a world record and uses a naca duct for inlet for a blown motor said he would never use one in the same place for a na motor unless he had a lot of data. A turbo or other compressor can make up for some poor scoop and inlet design since it is drawing air in. That doesn't mean good design before it isn't important but that it will make up for some mistakes that would hinder one with a na motor.
As you mentioned what happens between the inlet opening and the carb or throttle body is also very important, especially with a carb that needs the booster signals. We are running a carb on Hooley's car and ....
(http://purplesagetradingpost.com/sumner/Hooley%202013/carb%20hat-8.jpg)
(http://purplesagetradingpost.com/sumner/Hooley%202013/carb%20hat-11.jpg)
(http://purplesagetradingpost.com/sumner/Hooley%202013/carb%20hat-25.jpg)
...Gordon at Superior Air Flow recommended that we add the spacer I made above to the bottom of his carb hat to help with the air going into the carb. We took his advice and did that....
http://purplesagetradingpost.com/sumner/Hooley%202013/13%20-%20hooley-construction-2013-7.html (http://purplesagetradingpost.com/sumner/Hooley%202013/13%20-%20hooley-construction-2013-7.html)
At some point the car might be switched to throttle body and EFI. I think that is more forgiving than a carb since you aren't relying on the throttle body to get the right signals to mix the fuel correctly the injectors will do that. The carb was a cheaper and much easier thing to do first. With EFI I think we would of been crazy to of run the car without dyno time like we did with the carb,
Sum
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I feel the need to put my two cents worth in on air scoop design. We need to consider many things to design an air inlet system that potentially may provide additional air inlet pressure to the engine. The thing that the inlet system should be designed to do is to convert the kinetic energy of the high velocity incoming air into pressure. Again we need to remember the basic rules laid down by Daniel Bernoulli, and that is that air moving fast will be at a lower pressure than air traveling slowly, so what we need to do is get the incoming air slowed down and that is accomplished by increasing the cross sectional area of the ducting that the air is traveling through and providing it with a large volume in which the air will be at a low velocity and higher pressure. Normally aspirated engines will typically be more responsive to this type of inlet because if you could raise the plenum pressure by only 1 psi you would potentially increasing the engine air inlet by almost 7% which should provide an almost equal horse power increase. (This is assuming that you are at STP (Standared Temperature and Pressure) conditions with pressure being 14.7 psi)Turbo/super charged engines see less of an increase because a 1 psi inlet to the turbo/super charger on a engine that is running at say 20 psi boost above atmospheric is only going to see a 2.9% potential increase and the more blower boost the less the percentage of increase will be from a well designed inlet. The increase in cross section area should start immediately after the inlet, the angle that the sections increases needs to be low enough to prevent the incoming air from separating from the duct surface and any attempts to change the direction of the air flow need to be gradual. Runs of constant diameter or cross section will begin to act as a choke as the boundary layer thickens and reduces the area that the high velocity air flow passes through. This is especially true with rectangular shaped cross sections as the thickening of the boundary layer along the long sides of the rectangle can greatly reduce the effective area rapidly. You also need to pay attention to the edge radius of the inlet itself,most of us typically use some sort of formed radius or a piece of formed round tubing to make the inlet radius better is to make the radius similar in cross section to the leading edge of a wing section, which can greatly affect the efficiency of the inlet. (Pain in the a$$ to make!)
A note on John's(JL222) super charged hemi, for 4500 cfm at 300 mph you need a cross section area of 24.5 sq. inches which is a tube with an inter diameter of 5.6 inches so a 6 inch I.D tube would be about 15% over sized at that speed so it is probably a good fit. If you are not concerned about "air spillage" due to the inlet tract being to large I could not see a reason to increase the inlet size to 7 or even 8 inches in diameter, one thing that is happening with a turbo/supercharger like yours with the blower inlet facing forward it that the air that is traveling through the duct stops as soon as it hits the back of the duct ,i.e. pressure recovery, and that is where your inlet is so it should benefit from additional inlet pressure.
Regarding SSS's thought about bleeding air out of the plenum, what is happening is that the bleeding increases the air velocity in the duct and plenum and therefore reduces the pressure recovery. This typically can increase overall aero drag due to the increased velocity in the air inlet system and also if the vented air is not discharged into a low pressure it could, as Sum pointed out, actually increase drag.
Probably the best example of a variable inlet configuration is the SR-71, the large "spear point" that you see protruding from the front of the engine inlet is moved froward as the planes speed increases this does two things, it reduces the area of the inlet and also increased the volume of the plenum in front of the engines inlets which keeps the inlet velocity below sonic.
Rex
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Probably the best example of a variable inlet configuration is the SR-71, the large "spear point" that you see protruding from the front of the engine inlet is moved froward as the planes speed increases this does two things, it reduces the area of the inlet and also increased the volume of the plenum in front of the engines inlets which keeps the inlet velocity below sonic.
Rex
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Might a more blunt/rounded "spear point" work better at the speeds most current LSR vehicles run if such a system were applied to them?
Ed
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Been on the road so here is some more food for thought!
I think you can see the relationship between mph and scoop size better if it is graphed. The % between the mph is the change in area. The inlet area would have to change a lot from 50 mph to 250 mph. You can see why the drag guys want a bigger hole for the first 60 feet! And they are accounting for the drag hit at the far end, too! Could be done but you have to ask is it worth it and can you package it, operate it and still meet the rule book? :?
The RAM air chart is the theoretical psi for a CLOSED scoop. Turn the engine on and the density inside the scoop WILL drop with a column of air opening! The CFM numbers will match just fine at the inlet but the air WILL be thinner! IMHO, the hardest part is getting the pressure balance at the boosters or a compressor face! :x
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Woody, I get the air balance @ the boosters issue but go into your thoughts about balance on the compressor face.
In my simple world I just want to recover as much altitude as I can. So if I'm at 29.95 at home & 25.50 at Bonneville I would like to get the ~4" hg back. This is to keep my turbo pressure ratio as close to the dyno test as possible. If the we run at a higher altitude with no pressure recovery the turbine will spin faster to make up for the higher pressure ratio and will cause more back pressure. Some of us fight excessive system back pressure for added performance.
Your thoughts.
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....In my simple world I just want to recover as much altitude as I can. So if I'm at 29.95 at home & 25.50 at Bonneville I would like to get the ~4" hg back. This is to keep my turbo pressure ratio as close to the dyno test as possible. If the we run at a higher altitude with no pressure recovery the turbine will spin faster to make up for the higher pressure ratio and will cause more back pressure. ...
I'll also be interested in what Woody has to say.
I ran some numbers through BW's Matchbot using the turbos we have on the Stude but for a 500 ci motor, vs. the 572 in the car, at 6500 rpm.
Altitude.. temp.. lbs. boost..Compressor Pressure Ratio..HP..Compressor Outlet Temp..Exh. Manifold Pres
Sea level.....90.......16#......................2.16..............1385...............296 F.......................18#
4600...........90.......16#......................2.33..............1297...............319 F.......................18#
4600...........90.......18#......................2.49..............1384...............339 F.......................20#
From sea level to 4600 feet you loose about 100 HP at the same boost and the air temps out of the compressor go up about 23 degrees. To get that back you need to run 2 more lbs. of boost 18 vs. 16 and that raises the compressor outlet temps 43 deg. over sea level. Also as you mentioned the Pressure Ratio has gone from 2.16 to 2.33 and then 2.49.
So making up any of the altitude loss is going to be big in the terms of the compressor not having to work so hard to make up for it and the intercooler also doesn't have to cool hotter air.
Our present setup on the Studebaker sucks and we know it, in that we have no plenum area before the compressor inlets. This might be fixed this year but I kind of doubt it at this time. The only saving grace for us is that we are not trying to run anywhere near what the combo should be capable of if everything was maximized.
How about it Woody, would feeding the air from the scoop inlet into a crude box ....
(http://purplesagetradingpost.com/sumner/Hooley%202013/turbo%20mounts-3.jpg)
...before the two turbos help in the area above where the "Y" is now. They would each draw out of opposite sides of the box. Would this help our pressure recovery? We would also make the track from the inlet to the box a constantly expanding volume. We did that on the scoop for the old motor with the inlet track expanding from the inlet to the bug catcher on the roots blower, no plenum though.
One thing we all find is that sometimes what is best either can't be done in time if you ever want to run or it just won't fit in the car.... :-D :cry:
Sum