Landracing Forum
Tech Information => Technical Discussion => Topic started by: 32hotrod on November 22, 2009, 07:16:16 PM
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On our car (88 Mustang)a must due to engine height. But the one from last year was a quick n dirty deal. Basically a rear facing 8" high x 17" wide X 38 long tapered to the front box open at the rear
OK some ideas
Fiberglass similar to Harwood pro stock
Forward facing with smaller opening in the back
Or
Rear facing
Or revise the one from last year to a smaller opening 2 " off the windshield
Comments Ideas ??
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From what I understand there is a minimum hood to windshield angle for rear facing scoops need to work. I don't know what that angle is. Anybody else know?
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From what we have been told rear facing scoops don't work.
In the wind tunnel the air hit the front of the car and then then the top of the windshield not hitting the windshield at all. The aero 2 scoop(pro stock scoop) was best in the wind tunnel but the aero scoop was a close 2nd.
Just what I have been told I am not an expert by any means.
Robbie
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32hotrod,
Not trying to steal your post just looking for some of the same answers as you.
Here are the pics of our hood scoop its so tall because our good motor has a sheetmetal tunnel and the top of the carb will about 5 inches from the top of the scoop.
Will we be better out making the openning smaller or drilling holes in the back of the blocked off scoop.
Thanks for any comments.
Robbie
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Hi we settled on a Harwood aero II scoop with a K&N aircleaner mopunted facing foward . In talking with them they suggest 2 one inch holes directed to the engin bay. This is to vent air during the shut down.
Only time and a timeing slip will tell if it was the best idea
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Please clue me into what the vent holes do (or don't do).
Stan
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Please clue me into what the vent holes do (or don't do).
Stan
DO = Let air out
Don't DO = let air out
LOL
Charles
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We had a cowl type but it was to far from the windshield.
When turned it around we picked 14 mph but the scoop was to close to the top of the carb, so we went with aero scoop shown a few posts up. The scoop was designed to go to the windshield but if we put it on that way we would have to take the hood off do to the length of the cowl. Harwood said to block off the back of the scoop and drill two one inch holes in the isolator plate between the hood and the carb. we did that, but the problem is at about 140 the hood is picked up about one inch at the back. The question is do we drill holes in the back of the hood scoop to let air out or block off part the front opening to not let as much air in?
Thanks
Robbie
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Air scoops for land racing are have a much different function than a drag racing scoop. Drag race cars need big openings a low speeds to allow enough air into the engine so it can develop maximum hp and drag race cars are not usually restricted by horse power vs aero drag as a limit to their top speed. Completely different for a land racing car. The function of a land racing designed scoop is to provide the minimum aero drag at the cars top speed and to recover the energy of the incoming high velocity air convert it to pressure to improve engine performance. The size of a land racing scoop inlet at the cars maximum speed should be about 10-15% larger than the engine air volume required at the maximum engine speed. This is why you see the inlet size on a 500+ cu. in. engine on a car that will go 375 be only 2-3 inches in diameter. If the inlet size is to large then the excess air that is not taken into the engine is spilled out of the scoop inlet and this excess air will generate a vortexs that will disrupt the air flow around the scoop and can greatly increase aero drag. This is probably the function of the internal bleed holes that your scoop manufacturer is telling you to drill in the scoop. These additional holes will allow excess air to flow thru the scoop and not spill out the scoop inlet which would increase drag. The size of these holes can have some affect on the amount of pressure recover that your inlet/scoop/plemum system can generate. If they are to large then the velocity thru the inlet/scoop/plenum system will be to high, you will lose pressure recovery and also increase drag as there will be some sort of pressure drop for the air going thru the holes and this will be drag.
Back in the 1800s a guy named Daniel Bernoulli came up with a lot of ideas regarding pressure in a gas related to its velocity. The gist of Mr. Bernoulli's thinking is if the gas (air in our case) is at a high velocity then the pressure is lower than air at a lower velocity and he came up with the formula P1V1=P2V2 which means that the pressure of air at one speed is equal to the pressure of the air at a different speed. What this means to us is that if we can slow inlet air down enough the pressure will increase. So a good landspeed air scoop will start with a small inlet sized for the engine's air requirements at maximum speed and then increase in cross section to a very large cross section which will decrease the air velocity and increase its pressure. Pretty simple untill all of the details related to what is happening on the inside of the scoop are taken into consideration. That is what Land racing is all about, how we all approach the solutions to these details.
Have fun.
Rex
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Rex,
Very nice reply and description of Daniel's piddlin', too! :cheers: He did many interesting things and wrote his treatise Hydronamica in 1733 and was the first to actually measure blood pressure in the human body. He was a gearhead he just didn't know it. :-D :-o
There is much to be credited to the early icons of fluids. I have tried to include many of them and the history of their work in my latest book The Engine Airflow Handbook (HP Books) which should be in print about June of 2010. This is not intended to be a shameless plug, but I had never seen anything that covered much of the history of airflow and tried to do a decent job of listing those pioneers. :wink:
Now you can return to doing something more productive than reading my meandering. :roll:
Regards to All,
HB2 :-)
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Harold,
Really looking forward to reading your book!!!!
Rex
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Rex and Harold, is there a formula to figure out the size of the ram air scoops for my motorcycle? It is 1340cc( 82ci), 220 hp, and has run 219mph. I am not too good at math. It has 2 ram air inlets. Any help would greatly appreciated.
Fred
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Rex,
Thanks for your advice.It goes with what i was thinking, that making the opening smaller instead of letting the air out the back of the scoop.
Howard.
I will be looking for your book.
Fred,
http://purplesagetradingpost.com/sumner/bvillecar/bville-scoop%20info-1.html
I got this off Sumners site about sizing hood scoop openings
Thanks guys.
Robbie
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Fred,
I am going to assume that you red line your engine at 12000 rpm, so that means that it needs 82 cu. inches x 6000=492000 cu. inches of air per minute is required if you engine is at 100% volumetric eff. (Note: the 6000 comes from air is sucked into the engine every other revolution) 220 mph = 232,320 inches per minute. (220x5280x12/60). So now we have the cubic inches of air your engine needs per minute and we have the air velocity in inches per minute so if we divide the cubic inches/minute by the inches/minute we will come up with the the area of the required inlet; 492,000/232320=2.12 sq inches. Going with my rule of thumb of needing the inlet 15-20% larger this would be: 2.12x1.2=2.55 sq inches. If the inlet is going to be round then the diameter is: 2x the square root of (2.55/3.14169)
which is:1.80 inches diameter. Several things to remember here, this diameter is correct at 220 mph so it may actually (and probably will be) a restriction to your engine at speeds below 220 and if you are running a naked or partially stream lined motorcycle going with this small of an inlet may actually slow you down because it will restrict the engine until 220 mph and the aero advantage may not even be seen by a motor cylce that is not fully streamlined. Also this inlet diameter must not have any length to it, you must start expanding the inlet channel immediately after the inlet, the larger the crossection of the inlet the slower the air velocity and the more pressure recovery you will have. The inlet must also have a very generous radius at its leading edge, no sharp edge. My guess is that you are probably running a Hayabusa and if you are running the standard air inlet and air box they are actually quite good. You may see some marginal increase in power if you reduced the area of the two inlets but it may not be measurable.
Rex
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Thank you Rex I really appreciate that. You just saved me a lot of work, that may have made me slower. And we all hate to do a lot of hard work and go slower. :-P Robbie thanks for the link I will look at it.
Fred
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Rex,
Thanks for the formula.
Thats alot better than what i had.
Robbie
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Mr Schimmer and Mr Bettes, Boyle eclipsed Bernoulli by a few years with that formula -he published it in 1662. (One of the few things I retained from HS Physics.) And Charles added temperature, as I recall.
To add to Rex's description of scoop requirements, you can't WISH the air to go places.
You can MANAGE it as witnessed by the strange tabs, winglets and bumps the F1 people come up with. Like Larry McReynolds said a 'couple weeks ago "There's no free lunch in aero."
A simplistic scoop shape might be to picture the exhaust valve throat as the open airstream and the header flange as the "target" for your airflow. I.E the carburetor (s) or injector ram tubes.
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Rex,
Stan and I have had discussions about Air Scoops vs. Air Ducts. We both run Street Roadsters and the rules regarding Hood Scoops have changed in 2009 to limit the front of the Scoop to the front hood line. The rule was intended for other body types but we got swept up by it for this year at least.
With a non-taped Air Duct such as Stan runs, the expansion of the cross section is internal to the Duct. He also said that this should be just inside the inlet (opening quickly). Should that divergence be a certain angle in relation to the distance from the inlet to the carb, say 45 degrees, or 90 degrees to airflow at the inlet face, or some other angle?
I understand that in addition to pressure recovery the goal is also to reduce turbulance in the plenum.
Tom
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I was told the objective was calm air at 10,000 below sea level :-D
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As I recall a couple of years ago Tom Bryant did a lot of research on hood scoops for the Tom Thumb coupe.
It's probably on his web site that's listed under links on this site. Tom I am sure would be glad to talk with anyone on this. www.bryant automotive redding Ca.
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Glen,
Thanks for the info.
Robbie
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Tom,
I am not completely up to speed on the new air scoop rules for your roadsters class but the general rule is that the inlet should expand at around 7-9 degrees per side, so if you did a cone the included angle would be 14-18 degrees. The idea of course is to keep the air attached to the interior of the scoop and try to prevent turbulance, also if the air has to turn 90 deg from the scoop/plenum to the engine inlet then you should probably think about some sort of turning vanes to help the air to go into the engine. Lots of big time research is being done on air scoops by the F1 guys and they have been doing it for years so just keep the sides smooth, no sudden transitions, try to not have sharp corners and good luck.
Rex
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Thank you Rex!
That gives me a good idea where to start.
Tom
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Tom, remember unless the rule has changed the taper or anything you want must be on the inside of the part that protrudes in front of the grille shell. I believe the rule still states that the air device must not taper and that has always been the outside dimensions....JD
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Jimmy Six,
You are talking about roadsters or all cars.
Robbie
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Robbie,
JD and I were talking about a roadster application, but applies to cars of all types. Look at the SCTA Rulebook for definitions. Hood Scoops and Air Intakes/Ducts are two different animals and have different specifications.
Tom
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All cars -- but he's referring correctly to Air Intakes, not Hood Scoops.
Stan
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Tom --
I've got to learn how to type faster.
Stan
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How about a variable inlet like the SR-71 uses? :cheers:
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Floyd,
If I was doing a "cost is not object" project I would certainly do something like the SR-71 inlet. It would give you the best of both worlds, large inlet for low speeds and reduced area for high speeds and great aero. As one of my more favorite sayings goes "Only time and MONEY prevent me from doing this!""
Rex
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Rex,
I've got the front part of the Air Intake figured out thanks to your advice. What about the rear part? The object is to turn the air into the carb with the least amount of turbulence.
I'm running a single 4 Barrel with a 2" air horn that seals to the bottom surface of the hood. Should the back of the Intake/Plenum direct all the air flow into the carb (your "turning vane"), or have an opening that allows some bypass air?
I guess you will always have some reversion of air with a sealed system. Maybe I'm overthinking this too much.
It would sure be nice to have an Air Flow facility to see how the air reacts to different designs. Way beyond my budget and expertise.
Thanks for any advise!
Tom
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Tom,
As I have said the F1 teams are in a continous development program on their engine air inlet scoops and they can and probably do anything you might even consider that could affect the scoop efficiency.
As far as having a "vent" hole in the scoop I am against this approach for two reasons. 1. You are probably bleeding pressure (air) out of the inlet which is exactly why you went to all the work to build the thing in the first place. 2. A vent hole promptes air flow thru the scoop which will cause an increase on drag, now that would need to be balanced against the drag that could happen if you made the scoop inlet to large for the engine requirement and you would then have air, and probably turbulant air, bleeding out of the front of the scoop. Probably a good place for some testing at your "Air Flow facility".
I do think that you need some sort of guide vanes on the inside of the scoop to get the air pointed into the carb. Another thought is providing some sort of "dampening" material inside the scoop, like a good high flow air filter, to help reduce and any interior pressure pulsations. I have always wanted to try lining the large volume section of the scoop with some very porus foam, like fuel cell foam, to provide internal dampening. Another thing to test at your "Air Flow facility". Build a scoop with your best thoughts and then put a Magnallic pressure gage connected to the inside and see if you get good pressure recovery, make changes and see if you can get it better. For us guys without unlimited budgets the old "cut and try" method is probably our only way.
Good luck, Rex
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Something I have considered for a while. This is a "general" video (not a ton of scientific information, just an overview of how to accomplish it) of the idea that water can have improved laminar flow and how to accomplish it, air and water are not that much different.
I saw a very large scale professional installation at the Bellagio and the work they did to improve the flow of water for their fountains. In the back of my mind, I'm thinking there may be some benefit to looking at what they have done and how it may apply to air flow/intakes.
http://www.youtube.com/watch?v=OV-IazRk0sU (http://www.youtube.com/watch?v=OV-IazRk0sU)
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Here is an image of the air intake on a 1994 Formula 1 car showing the large increase in size after the scoop opening.
(http://atspeedimages.com/image.php/4fc9ed2e-f15a-11dc-d9a9-0019e3f8e432/branded/1994_ferrari_412_t1b_f1_airbox.jpg)
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Thanks Rex!
I'll definitely work on some type of turning device. Good idea on low density foam.
Tom
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I have seen few of the guys that build their own flowbench use bundles of drinking straws and or egg crate covers from fluorescent light fixtures to straighten out flow.
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I was thinking of making a full scale mockup with viewing ports using a leaf blower and a smoke generator to get some ideas. My redneck flow bench.
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You might look at Pro Stock intakes and how much room on sides of carbs also, you know they have the latest tech.
JL222
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...
... N.A.C.A. DUCT.
find a photo of my 85' t-bird and you will understand.
find a photo of the Kibler D-C / STR and you will understand.
( the one that did not have the 9 gallon fuel tank in front)