Does anyone have any data or info on air scoops, the pressure they create, and maybe jetting info when using them? For example how much pressure does a hilborn style scoop put down a carb at 50mph or 100 mph. I had vented my crankcase into my scoop and not into the air cleaner base.....and did not account for the pressure it would see at 120mph. I pushed the dipstick out of the pan and a bunch of oil....dumb mistake.
when we ran our front motor car , i was told that the scoop did not pack untill about 100 mph . we ran about 150 mph , so it helped on the top end .. just what we were told then ... steve
I have absolutely NO firsthand knowledge of this at all, BUT- a motorcycle racer I know took a Buell Blast to Bonneville a couple years ago, had issues with exactly what you're talking about. He built an intake that pointed the carb dead into the wind,and topped it with a velocity stack. Ran great right up to 89mph, and would go NO further. An old timer stopped by the pit later as they were trying to figure out WTF!? and told them to block the direct air flow. Off came the velocity stack, on went a makeshift air box. I'd be lying if I said I remembered their speed, but it was 100+.
The Mopar Direct-Connection race book had several pages on the subject. There was also some input several years ago from Warren Johnson in the back of Nat'l Dragster many years ago. Maybe a 'net search will help.
Noted expert Jere Jobe did extensive testing with scoops on 97s and found that they perform much better when the boat scoops face towards the driver.
I guy I know who runs at bonneville with a naturally aspirated engine gets 2 psi of boost in the intake manifold at 270 mph!
glen, get a hold of sumner/1fatgmc over at the RRT. he has some knowledge of scoop design and their affects on speed. he learned it during the building of hooley's bonneville stude. later jim
It really simplyfied way to look at it: Imagine that the front area of your scoop is 1x2dm=0,02m2 The volume this scoop will scoop up is the scoop area multiplied by the speed you travel. (this is really simplyfied) 5 liter motor at crusing @ 2000rpm (30% fill rate?) = 90m3/h 5 liter motor at full throttle @ 7000rpm (100% fill rate?) = 1050m3/h If your going faster than the intake speed you get boost. 90m3/h divided by 0,02m2 = 4,5 km/h 1050m3/h divided by 0,02m2 = 52,5 km/h Your likely to get boost inside you scoop. Depending on your scocp size of course. And dependeing on your scoop position your likely to get more air that is being pushed off the front of your car, and a lot of turbulence fugging up your vacuum signal.
Weasels comment on the aluminum scoops on 97's et al is true. Some friends running a multi-carbed flatmotor roadster at Santa Maria dragstrip back in the day had gasoline coming out of the forward facing scoops and hitting the driver in the face - no windshield. Turned the scoops around and problem solved. Kinda counter-intuitive and it was a new one on all of us. Tunglegubbin, explanation much appreciated. I do know that internal combustion aircraft have a forward facing - and small - intake air scoop that gives a bit of boost at 150-200 knots (180-230 mph). Glen, you must be getting some good pressure in there if you blew the dipstick off. Would be interesting to see the readings if you stuck a combination vacuum/pressure gauge onto an intake manifold bib. Dash mounted ones are available and the older good quality test instruments have vacuum/pressure reading capabilities. Far as cold air vs hot underhood air, I understand there's a one HP gain for every 10 degree temperature drop in the intake tract you can get vis a vis underhood air. Common temps under the hood in summer can exceed 300* F. A small seat of the pants example was a 90* F day at a small country airport and I tried to take off with the carburetor heat on. After about a 300' roll, the engine wasn't pulling like it should have and I was about to abort when I realized what I'd done. With cold inlet air coming in the engine picked up and the takeoff was completed with no problems. Guessing here, but I estimate the engine lost about 20% power with carb heat on. With the Cessna 150C rated 100 HP that's a helluva big cut. Carb heat is intake air that comes off a sealed box that picks up heated air from the exhaust manifold. Getting back to Glens engine puking oil, are you running a PCV setup or breathers? Either ones ok, mostly just curious....
We put a Ramchargers style scoop on a 440 powered '64 Plymouth coupe and sealed it to the air cleaner housing. The pressure screwed up the carb jetting and it ran lousy at any higher speeds. Vented the pressure off by knocking holes in the outer air cleaner housing, problem solved. The extra pressure isn't worth the jetting headaches, since the boost is unpredictable. A minor side wind can remove all the pressure, for instance.
It was a combination of both since the pressure is minimal it is my setup. I T'd my two breathers into one -6 outlet into the bottom of the scoop, it was too much restriction. I never had this problem until I made this connection. The stick is also in the pan not the block. Now I know why those come with a 3/4" fitting. this pic shows the wrong way.......I changed it back last night to the 3/4" fittings.
My guess is that that size -6 isn't the problem, it's where it's run to. In my opinion, you shoud either try to suck a vacuum in a motor, or vent it very very well. You are kind of stuck between those two right now due to building a little pressure in the scoop. I imagine that if you ran that line into the carb spacer it would solve your problems.
all of you can call me crazy as a shit house rat , but if you are trying to vent the motor and the venting leads to the back of the scoop where all the excess collected air is going , dont you think that instead of evacuating the motor it is jamming all the excess air into it ? also I was thinking how a manual vacume works ... if you put the lines running more to the front of the scoop before the carb and put them at 35 or so degrees inline and back to the carb then you will have achieved a vacume at speed when the air rushes to the carb inside it will pass over the holes and create vacume and suck the air venting the case .. Its kinda how a sand blaster works to get the sand sucked up .. just 2 cents from a fresh head ..
Going directly from the breather to the carb riser which is manifold vacuum without a PCV valve will create fuel/air mixture problems - way lean. With all the dinking around I've done with my PCV problems - which involves oil drainback capacity in the valley along with a poorly baffled PCV valve I'm guessing that running a close to stock PCV setup on the street and a race-only style breather setup at the track may be a good way to go with Glens engine.
Why do all the top drag cars, top fuel, funny car,pro stock and the likes have forward facing scoops or injectors. I have the same style scoop I plan on running on my 41, but not sure of the flow inside the scoop as compared to a K&N setup.
the general engineering rule of thumb is that gases are treated as "incompressible" below about 0.3 Mach velocities. in an engine that would be velocity down the intake (pumping plus external velocity) not chassis speed over the ground. 0.3 Mach (about 230 mph) is where pressure increase is approaching 5% over zero-velocity. a cup held out the window pushes hard on your hand, but the pressure in the cup isn't that high. the high-velocity air piles up and spills over the cup, just as it would if you held it in flowing water. and there's zero chance of it creating enough pressure to push oil up the dipstick. does the crank have oil scrapers? more likely would be the sustained high speed/high rpm wound all the oil in the crankcase into a huge ball around the crank.
A single-6 line isn't even close to being big enuff to vent your crankcase. Unless there is also another issue, the pair of 3/4 lines ought to solve it. Good luck.
No one's mentioned the OBVOIUS. For engines that run worse with scoops than without (like the bike and other cars above !). It's the tuners problem, NOT the scoop (if designed well). More air...NEEDS...MORE FUEL to compensate for the additional air ! And the scoop I had on my old Pro Gas Anglia, "enlarged" (or was "packed") by about 60 or 70 feet out, whatever mph that was. Mike
Old thread, but the subject of scoops is still the same... A perfectly designed scoop that converts all the energy in the "moving air" (actually, the car moving and static air, but it's easier to think of the air as moving) can create a pressure increase of about 5% at 200mph, and it changes with the cube of the speed - in other words it's no more than about 1,25% at 100mph. And remember, that's with a perfectly designed scoop that catches the air with a relatively small opening, slows it down by gradually increasing the size into a more or less large chamber that the engine is fed from. It is worth knowing that it is very easy to create a low pressure by using air moving at high speed - it's the high speed of the air moving through a carburettor that causes the vacuum that sucks the fuel from the float bowl for example, basically any time you have air moving at high speed past an object there will be areas of low pressure. Increasing pressure using high speed is rather difficult by comparison, and as I said first, the amount of pressure achieveable at "streetcar speeds" is very limited. You usually do get some increase in pressure in front of the car simply because of the car has to push air out of the way - not a huge amount of pressure, but better than nothing. Thanks to that it can be a good idea to feed the engine air from the area just in front of the windshield - but air flowing past the opening can also create a low pressure, so not all attempts are successful. Most of the so called scoops meant for street cars, especially the short small ones that fits directly on the carburettor is a complete waste of time. They sit in the turbulent & low air speed zone more or less behind the radiator, so they don't really have any quickly moving air available to catch and therefor can't convert that speed to pressure. Even if they did sit very high up in undisturbed air many of them are simply completely incorrectly designed. As I mentioned, the goal is to lead the air in through a relatively small opening, then gradually increase the size making the air slow down, trading speed into pressure, eventually ending up in a "large" chamber with slow moving air. Most "scoops" have a big opening, continue with the same size or even a reducing size, and then going more or less directly to the carburettor. Such a funnel can not catch more air and force it into the carb, instead, alot of air enters the opening but most of it just goes back out again when it can't go into the engine. This causes turbulence and most likely reduced air flow into the engine. Such a scoop catches the heavy stuff that can't change direction and go back out, i.e. bugs and rain, it does nothing postive for engine performance. Hot rod & Classic car people tend to like small scoops, placed relatively low more or less straight on the carburettor. Functionally, those are usually completely useless... so if you're one of those guys, go get something you like the look of, and try to avoid anything with a forward facing opening that goes directly down into the carb, or a big forward facing opening that funnels down into the carb. If you really want the increased power, get a high scoop with a relatively small forward facing opening that gradually opens up into a large chamber over the carbs. If you like SOME extra power, but not a scoop that feeds your engine rain, dirt and bugs, you still want the relatively large chamber over the carb, but feed it from behind, take the air just in front of the windshield.
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