The Secret Formula: Pounds Per Hour and Flow Numbers

The Secret Formula: Pounds Per Hour and Flow Numbers

When you’ve been around the nitrous industry long enough, you know the value of having your systems flowed to get exact numbers. Even the new guys who read about it in the forum quickly realize how valuable flow testing can be. Today we are going to go over nitrous system flow testing, and we’ll give you the formulas we use on our flow bench.


We often see these two terms get mixed up. Though they sound similar, they are vastly different. AFR stands for air fuel ratio, and is a measure of how much fuel your engine ingests versus how much air it ingests. NFR means nitrous to fuel ratio and is a measure of how much nitrous goes into your engine versus how much fuel goes into your engine with the nitrous. NFR is measured in pounds, for example 7:1 means 7lbs of nitrous to 1lb of fuel. The larger the ratios spread, the leaner the tuneup will be. And the lower the spread, the richer the tuneup will be. It is important to note that the ideal ratio will change between nitrous systems and applications. Here’s some examples of what NFRs we use in calculating flow:

12:1 Dry Direct Port and Dry Plate Systems

7:1  Wet Direct Port

8:1 Wet Plate Systems

Items Needed to Flow Your System

To flow your system, you’ll need a few important items. The first you’ll need is good, accurate scale. We use the ones we sell with our bottle filling stations, they are easy to read, accurate, and reliable. The other item you’ll need is a graduated cylinder. You’ll need the graduated cylinder to measure the fuel side when it’s time to flow it. These can be picked up online at relatively cheap prices. Also, it will help to have a piece of paper with a grid on it, or a  computer with a spreadsheet application like Excel or Google Sheets. Another item that you will need is a tuning jet kit. The reason you will need this is that when you flow the fuel side of your system, you may need to make several jet changes in order to attain the best NFR.

Flowing The Nitrous Side

When we flow the system, we are getting the pounds of nitrous it flows in a one hour period. But don’t worry, we won’t be flowing it for a full hour. We’ll choose a 5 or 10 second flow time interval and then use some math to get the pounds per hour. For this example, we’ll be using a .136 jet. After we take the system off the vehicle, get the bottle up to pressure (950PSI) and get the weight of the full bottle from the scale, we’ll be ready to start flowing. First, write down the beginning bottle weight so we’ll know what to subtract from. For example, let’s say the starting weight on our 10lb bottle is 9.85lbs. Now we will need to choose on what flow interval to use. We’ll use both here to show you an example of the math behind each way of doing it. Once you’re all set up, activate your nitrous system for 5 seconds then turn it off and write down the weight on the scale. For example, let’s say the ending weight was 7.41lbs. That means we flowed a total of 2.44lbs in 5 seconds. Now, to convert that into PPH we need to that 5 seconds into 1 minute:


Nitrous Outlet Stinger and Puck on Flow Bench

 12 (5 seconds into 1 minute) x 2.44 = 29.28lbs per minute

60 (number of minutes in an hour) x 29.28 (lbs flowed in one minute) = 1756.8 Pounds Per Hour

 To convert PPH to HP, divide the PPH number by 3.6.

1756.8 / 3.6 = 488HP

Another way to calculate the horsepower based on a 5 second flow is like this: 

2.44 (lbs of nitrous flowed in 5 seconds) x 2 =4.88

Then, remove the decimal to get the HP, which would be 488HP. 

The last option for calculating HP is based on a 10 second flow. In 10 seconds, it would flow 4.88lbs. In that case, we would remove the decimal and would leave us with 488HP. 

Before Flowing the Fuel Side

When flowing the fuel side, be sure to flow it with whatever fuel you are going to be using in the vehicle. This is because every fuel has a different specific gravity, thus it will have a different weight based on having the same volume of another liquid. For the fuel side, use the graduated cylinder to catch the fuel that your system flows, then measure it in ML on the graduated cylinder. For an example, we’ll use the specific gravity of VP C16 to show you how to calculate fuel flow. C16 has a specific gravity of .735 @ 60 degrees. If we figure it against a gallon of water (8.34 pounds at 60 degrees), this means that C16 weighs 6.1299 pounds per gallon (.735(SG of C16) x 8.34 (SG of Water)= 6.1299 pounds per gallon). Also, you’ll need the formula for converting ml to liters, and liters to gallons. To convert ml to liters, divide the ml by 1000 (240ml /1000 = .240L). To convert the liters to gallons, use this formula: .240L x .2642 = .0634 gallons. Then, you’ll need to convert the gallons to pounds. To do that, use this formula: .0634 x 6.1299 = .3886 pounds. The last of the formulas is calculating the PPH. To do that, use this formula:

.3886 x 6 x 60 = 139.896PPH.

Flowing the Fuel Side

Now that we know how much nitrous and fuel we are flowing, it’s time to figure out how far off we are. For this, I will be using a wet plate system as an example with a 8:1 NFR, and we’ll be flowing a .105 jet at 7PSI fuel pressure. In our nitrous flow test, we measured out 1756.8 PPH. To find out how much fuel we would need for this, use this formula:

1756.8 / 8 (8:1 NFR) = 219.6PPH.

Using this formula, we now know we need 219.6PPH of fuel for our wet plate system. To calculate the amount of fuel needed in a 5 second period, use this formula: 

2.44lbs (lbs of nitrous flowed in 5 seconds) / 8 (8:1 NFR) = .305  pounds of fuel in a 5 second period.

Now, it’s time to flow the fuel system and calculate the PPH. In our example, the fuel side flowed .305 pounds in a 5 second period. To convert that to PPH, use this formula: 

.305 x 12 (how many 5 second periods in 1 minute) x 60 (minutes in an hour) = 219.6PPH.

If you are flowing on a 10 second interval, you’d calculate it like this: 

4.88lbs (lbs of nitrous flowed in 10 seconds) / 8 (8:1 NFR) = .61 pounds 

Now, to determine how much that is in pounds per hour, use this formula: 

.61 x 6 (10 second periods in one minute) x 60 (minutes in an hour)= 219.6PPH.

Now we see everything is right where it needs to be!  Our flow sheet is taking shape now, and would look similar to this: 

Nitrous PPH N2O Jet HP Fuel PPH Fuel Jet Fuel Pressure NFR
1756.8 .136 488 219.6 .105 7PSI 8:1

Though it may look easy, flow testing a nitrous system can be a very labor and material intensive process. Commonly, we go through several bottles of nitrous getting a tune right for 1 system, and it takes hours so it may not be something for everyone to do. If you’re interested in getting your nitrous system flowed and getting specific tune-ups, give us a call here at Nitrous Outlet!