Hood Scoop

[32hotrod]

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 ??

[robfrey]

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?

[stratman59]

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

[stratman59]

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

[32hotrod]

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

[Stan Back]

Please clue me into what the vent holes do (or don't do).

Stan

[Cajun Kid]

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

[stratman59]

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

[Rex Schimmer]

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

[Harold Bettes]

Rex,

Very nice reply and description of Daniel's piddlin', too! 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.

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.

Now you can return to doing something more productive than reading my meandering.

Regards to All,
HB2

[Rex Schimmer]

Harold,
Really looking forward to reading your book!!!!

Rex

[fredvance]

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

[stratman59]

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

[Rex Schimmer]

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

[fredvance]

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. Robbie thanks for the link I will look at it.

  Fred