Jeff Prock
Open flow testing:
Fuel Jet Diameter X Fuel Jet Diameter X .7854 X Number of Jets = Flow test jet area
(Flow test jet area / .7854)^.5 = Flow jet size in thousandths (not holley jet size)
For example: .030x.030x.7854x8=.005655 , .005655 /.7854 = .0072 , square root of .0072 = .085
So to test the flowing fuel pressure of a .030 fuel jet on an 8 jet fogger system, place a .085 jet or a #76 holley main jet (lookup size on table below) in the fuel feed bypass for the nitrous system and set the fuel pressure to the desired spec (usually about 6 lbs). What you have done is simulated the total fuel flow for an 8 jet fogger system with a single jet.
Holley Jet Holley Jet Holley Jet Holley Jet Holley Jet Holley Jet
Jet # Size Jet # Size Jet # Size Jet # Size Jet # Size Jet # Size
40 .040 51 .050 61 .060 71 .076 81 .093 91 .105
41 .041 52 .052 62 .061 72 .079 82 .093 92 .105
42 .042 53 .052 63 .062 73 .079 83 .094 93 .105
43 .043 54 .053 64 .064 74 .081 84 .099 94 .108
44 .044 55 .054 65 .065 75 .082 85 .100 95 .118
45 .045 56 .055 66 .066 76 .084 86 .101 96 .118
47 .047 57 .056 67 .068 77 .086 87 .103 97 .125
48 .048 58 .057 68 .069 78 .089 88 .104 98 .125
49 .048 59 .058 69 .070 79 .091 89 .104 99 .125
50 .049 60 .060 70 .073 80 .093 90 .104 100 .128
A fuel system using a .110 jet restriction (8 x .040 fuel jets) , a 7 psi pressure differential with .74 fuel specific gravity and a flow coefficient of .7 will flow .765 gpm. This equals 285 lbs/hr (gas weighs about 6.2 lbs gallon) This will support 500hp @ .57 BSFC. I have heard that a target .7 bsfc is a good baseline, at this fuel flow rate this will support 400 hp.
Fuel flow through orifice -
Q = Ao Co (( 2 x Pd / p)^.5)
Where:
Q - Flow rate in cubic meters per second
Ao - Area of orifice (fuel jet) in square meters
Co- Flow Coefficient of orifice (use .75 as an approximation of Reynolds formula)
Pd - Pressure Differential between inlet and outlet of orifice (6-7lbs)
p - fuel specific gravity (kg/ cu meter - multiply spec gravity by 1000 for metric equiv)
Or Why the villagers scurry when the dam breaks!
by Dave Koehler - www.koehlerinjection.com (link above)
Sooner or later, a nitrous user will come across this phrase in reference to tuning his nitrous system. It is exotic sounding and usually spoken in a voice that makes it sound like some big secret nitrous incantation. Blue Magic, maybe? For nitrous newbies this conjures up visions of test benches costing thousands of dollars. Fortunately open flow testing is a little more down to earth and requires some basic high school math and a financial investment costing less than a single nitrous bottle.
Open Flow Testing is the act of simulating the fuel flow pressure with the nitrous system on but without filling the engine up with fuel with the engine off.
Now, the word flowing gets tossed around a lot because that is what somebody called it years ago and it just stuck. While fuel is indeed "flowing" what we are really simulating is the pressure on the output side of the solenoid when the nitrous system is activated. Here in the shop we can measure actual fuel flow with high dollar sensors which is even more accurate but this is not always possible or easy to do in the field.
Why is this open pressure important? When the solenoid opens there will ALWAYS be a pressure drop. How much is dictated by the size of the pump , type of regulator used and the jet/plate/nozzles that are trying to be filled.
Picture the movie with the dam breaking. All that water built up behind the dam creates pressure on the dam. The dam breaks and the water runs down the valley sending the villagers scurrying. Now, all of a sudden there is a whole lot of flow going on at the dam but the pressure built up behind the dam is significantly reduced or gone and the down stream pressure is a best guess. This is what we need to know. What is the flow pressure in the valley now that the dam has burst? Once we know what this pressure is or is not we can make fuel delivery system changes to ensure that the desired fuel pressure is present when the nitrous system is activated.
When you buy a nitrous kit you normally get an instruction sheet that states what fuel pressure is desired with the nitrous system. What is sometimes not clear in the instructions is that this pressure is to be with the system ON. What the pressure is with the system OFF is unimportant in most cases.
Well, ok. How do we know what the pressure is without burning up or flooding out the engine? Open flow testing works on any plate or nozzle based system, street or strip, and you can do single or multiple stages. Testing will allow you to confidently dial in your systems according to the information supplied by the nitrous manufacturers or your nitrous guru. Once you have a baseline you can go back and make those small changes with confidence at home or at the track. This testing can also help pinpoint a fuel pump or regulator problem. The beauty of all this is it is done with the engine OFF. There is no need to wear the car out trying to guess at the running pressure.
Follow along as I break this down this great mystery. "Open" refers to the fuel solenoid being turned on and fuel "flowing" through it. This is what we are going to "test".
A good commercial open flow test tool consists of an on/off mechanical valve to simulate the solenoid opening and closing, a pressure gauge capable of reading in tenths of a pound, a jet holding fixture, along with associated hoses, instructions, etc.
Ok, let's get down to work and set up the test. First, do not be foolish. You will be working with a flammable fuel. Use all the safety precautions you ever learned or heard of when doing this test. Note that we are only testing the fuel side. Disconnect the fuel line from your regulator to the fuel solenoid and connect the input hose of the tester to this port on the regulator. This is normally a # 6 line. Put the output hose of the tester in the fuel tank or bucket. The next step is to choose a jet for the test. If you have a single plate this is simple. The flow test jet will be the same size as the jet in the plate. IE: .082 fuel jet =. 082 flow jet.
It gets more interesting with dual plates, multiple stages or foggers. I will explain a nozzle system with eight nozzles. The same math will work for 2 plates (jets), or whatever amount of jets you have a figure.
Note that some simpler commercial kits use carburetor jets for the flow jet. I do not agree with this as it confuses the racer/tuner since the carb jet numbers do not match up to the actual diameter of the hole. The other reason I use a special made jet is that if you are testing a large or multiple stage system there is not a carb jet large enough to simulate the flow. I will be discussing flow jets with flow numbers that match the diameter of the hole.
Class time. You will need your calculator.
To figure the flow jet size for multiple jets do the following math:
Diam. X Diam. X 0.7854 X N= Flow test jet area
Where N is the number of jets (i.e. 8 foggers) and Diam. is the inside diameter of the fuel jets.
As an example we will compute 8.032 nozzle jets:
0.032 X 0.032 X 0.7854 X 8 = 0.0064366 total area
Now that we have the total area we need to compute what diameter flow jet the total area equals. And it goes like this:
Flow Jet Diameter = SqRt (AREA / .7854)
0.0064366 / 0.7854 = 0.0081953
SqRt (0.0081953) (just hit the square root key) = 0.0905278
Rounded off = 0.091 flow jet
Pretty neat, huh? Now that we have the flow jet installed we can test. With the on/off valve closed and the output hose in the tank or bucket, turn on the gauge and zero it. Turn on the nitrous fuel pump and note the pressure reading. Open the on/off valve and note your open flow reading. Lot of fuel going through there isn't it? If not, the open flow test kit just paid for itself!! If the pressure is not where it's supposed to be adjust the regulator accordingly.
Here is where it makes nitrous more fun. Less work at the track!! Take all the jets you tune with and do the same test with all of them. You should be able to map what changes have to be made according to what jet to use. Take your black book to the track with this information and save a bunch of hurry up work at the track.
A Yours is a very common problem with the racers who use the Olds style heads. The problem is that with the valve angle and the small combustion area in the head, they are way too efficient! The Olds head makes a ton of power on a nitrous engine and that is why they are commonly used. There are a few guidelines to follow, such as timing retard and plug depth into the head.An Olds headed engine wants less timing than a Chevy of Pontiac headed engine, so change your timing program first thing. You did not say in your letter how much you are retarding, but I am sure you use the typical program with a nitrous engine of 27 or 28 degrees at idle, then 10 or so degrees out when the car leaves the starting line, then the balance out when the second stage comes on, down to 14 or 16 degrees total.You will need to look at your program and get more timing out on the leave, say 14 degrees, and then the balance out when the second stage comes on, down to probably 10 or 12 total degrees. Work with this a bit to see what makes your engine happy.The second thing to look at is indexing your spark plugs into the cylinder heads so they don't go too deeply into the combustion chamber and, if possible, all ground strap electrodes should stand the same height vertically. This can be done with plug washers and it seems to make a big difference in stopping detonation.
We run a 632-inch and a 706-inch engine. The 632 has dual foggers and the 90- degree soft plume nozzles. The 706 has tubes in the plenum and the B nozzle fogger. I have talked to NOS numerous times and keep getting different answers each time.
We are lifting ring lands on both motors. We have tuned these motors every way that we know of. On both motors we take 4 degrees retard for each 100 hp of nitrous. We run Sunoco nitrous fuel. Both motors run aluminum heads and EPD sheet metal intakes.
The 632 has Pontiac heads and the 706 has Olds heads. We run an MSD 7AL2 ignition. We have not been able to run the second system on either motor because the problem happens on the first system.
The 706 uses the tubes in the plenum as the first system. The last time we ran the 706 it burned every other spark plug and also lifted the ring lands on all 8 pistons.
Normally the motors will not hurt the spark plugs. We have a Racepak computer. Our cylinder temps run around 1280 to 1320 degrees.
A
Yours is a very common problem with nitrous race engines. If you are using the correct piston in your engines for a nitrous combination then your tune-up is too rich on the fuel side. What happens in a case like this is that you backside your piston much like a Top Fuel car does. What is actually happening is that unburned fuel lays in the piston ring land area and at some time during the run explodes, deforming the ring land or lands and pinches or breaks rings also. This, of course, lets oil come by the rings and start the real problems.
The newer nitrous piston design has a much thicker top groove that is down farther on the piston and is much tougher as far as this type of fuel backfire in the piston, so you may slightly pinch a second ring and not do engine damage before you can find and repair the problem.
This is where the leakdown test comes in so handy. You mentioned the soft plume nozzles are in one motor and manifold spray bars in the other which indicates that the rich problem is a fact in your tune up. The soft plume nozzle by virtue of it's ability to break fuel up into very small droplets always runs very rich, and spray bars through the manifold do the same because you normally run higher fuel pressures to the bars.
If I can make a suggestion, you should put both motors on nozzles for the first stages. You will run a reverse jetting pattern such as 38 nitrous, 34 fuel with fuel pressure at 5 3/4 to 6 lbs. This will give you a baseline tune-up which will get you the ability to get some good plug readings and help to step into a second stage.
Let me know if I can be of more help on this.
Wady A. Haman