I cut a hole in the hood and put a blower style scoop on my car hoping to increase the air flow - It didn't work and i think it actually lowered performance. After some discussion with Rob (Canadian Hot Rodder), I'm looking to do a custom ram air similar to his. Am I going to run into similar problems with air/fuel mix at high speeds??? I'm going to be using a Demon Jr. Carb.

Thanks in advance - John

The speed of Millers LSR run probably is much greater than any of us theorizes out of their own streeter - so its hard to get an idea of how his experience will match up with ours. Using an air filter as a "turbulence control" is going to have an effect all by itself.

To measure small air pressure effects on the mining machines, I used to use a "Slack Tube" that I got from the Detroit Diesel dealer. It is a long piece of clear 1/4 tubing fitted into a "U" tube holder (marked in inches of length) and of some required scientific length (18-24 inches?) and then the "U" (only) is stretched out straight up and nearly filled up with tinted distilled water. When the two ends of the tube are exposed to differing air pressures - the water will move to a different height in each tube. It reads the air pressure difference in "inches of water". I would expose one end to "atmoshere" (stick it out a hole or cracked window) and then place the other end where I wanted to check the difference.  I don't know if such a gadget would work on a moving car or not - you can't have air blowing directly over the end of the atmospheric tube, it will mess with the reading - and 'dead air" inside the car might not be atmospheric pressure either.

We are usually involved in measuring engine vacuum in "inches of Hg (Mercury)" but those units are too big for this sort of measurement. 

    Manometer

My Dad made one in '68 to help a neighbor of ours set-up the dual carbs on his Triumph Spitfire. It was just a double U tube made out of aquarium tubing stapled on a plywood leftover. It worked (I think) but I know the fellow eventually bought a made for the purpose (Uni-Sync?) tool to do the job. 

What I got for my British cars many years ago has cast into it "Synchro-test" but we always called it a "Uni-Syn". Close enough.

Forced air will lean the mixture and simply requires richening up the carb.  Of course it will be overly rich when air is not being forced in.  My roadster at 135 mph requires fatter than normal jetting to compensate for the increase in air pressure at the carb at these speeds.

Density is inversely proportional to the pressure.  1 atmosphere is 407 "WG (still air) so with a good hood scoop at 60 mph you would increase the density of the air going to the carb to (407 + 1.73)/407 or about  0.4%  which isn't much.  However at Ted's terminal speed of 135 MPH the density has increased by a little over 2%

BTW the manometer mentioned was probably a Dwyer Slack tube manometer,  here is their web site

http://www.dwyer-inst.com/Products/TOC.cfm?MainCategory_ID=294&iCurrentCategoryID=296

I'm not a scientist or engineer, but I understand the math.  I also get it that it would be difficult to measure air density changes with a change in velocity before the carb and while in motion, but do you think there could be an instrument that would have small enough increments to measure this change in the intake manifold (i.e. vacuum or boost)?

Location of the scoop is the quandary.  I cannot use headlight or radiator support openings for air induction.  I know it is not in a favorable position in the middle of the hood and rules will not allow me to raise it enough to perform like it would on a roadster or other small frontal area vehicle.

If I could measure the density change, I could develop confidence in one of the few changes I could make.  One suggestion by experienced racers would be to turn the scoop around and face the windshield where it is said to have greater pressure.  Another is to make the opening in the front of the scoop smaller than the body of the scoop (I've seen this on pro-stocker hoods, but I have no Idea why it would increase density due to increased velocity).

I know I could make one of these changes and use the performance as a guide, but that may not happen for some time.  Also the performance changes might be so small that other variables might have played a bigger role.  It may seem odd, but I only average about one run per meet.  There are other changes that have higher priority and I try to change only one thing per run (i.e. timing and gear ratios).

I am still on the steep side of the learning curve................Any thoughts from anyone would be appreciated (even you Roy!).

Charlie - I think something that measures in inches of water will be fine enough scale to get an idea what's happening. But I also think vibration might be more of a hazard to reading the "wet" instruments than anything else though. When I checked the mining machine pressurizers - we wanted .2-.5 inches of water minimum and that is plenty enough to knock you down when you open the door latch. We mounted the instrument at a central location and then ran tubing wherever we wanted the comparative readings. As long as the tube opening didn't have air actively blowing across the open end (larger tube cut at an angle and wrapped in gauze) - it worked as a static pressure source.

So if you mount a "comparator" style gauge under the dash on the passenger side - mount and baffle an interior pressure source for reference - and then run a "sensor" tube (or tubes) to the points where you want the comparison pressure reading ....photograph the guage during a live run?

Nascar guys have run off the plenum at the base of the windshield forever - they don't use a scoop, just an underhood inlet box sealed back to the plenum. Its probably externally insulated too - but I've never seen one up close.  

     Re Doug, Would seem as though the inertia of the air would have more effect than the formula gives. What am I missing?