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Reid Vapor Pressure and Drum Preserve explanation by Jim of Dyno Tech Research

Using compressed nitrogen gas to properly store and dispense drummed racing gasoline and other liquids.

Failure to properly store racing gasoline can result in the loss of the gasoline’s “front ends”. This can result in a subsequent lean condition in the engine’s combustion chambers leading to reduced HP, and possibly detonation and preignition in otherwise properly tuned engines! Racing gasoline should be stored in sealed and pressurized containers to preserve the critical front ends of the fuel!

Racing gasoline is not a pure substance as are water and methanol, but is a mixture of hundreds of petroleum hydrocarbon molecules. Those petroleum hydrocarbon molecules each have precise boiling points—from approximately 50 degrees F to 400 degrees F. The molecules that have the lowest boiling temperatures are referred to as the gasoline’s “front ends”. These important front end components are the first part of the gasoline to begin vaporizing in an engine’s cool intake tract, and are critical in creating optimal vaporization especially at low operating temperatures. And, being the first to vaporize, the front ends are the first to ignite and create the heat necessary to quickly vaporize the heavier molecules, which will then be able to burn in the combustion chambers. Front ends are also the first to boil off and “escape” into the atmosphere from racing gasoline improperly stored at room temperature (or higher) in unpressurized containers.

But how can those important front ends escape from resealed drums? We used to think that replacing the threaded cap back tightly in the bung in our drum of racing gasoline was an adequate means of keeping the remainder of the stored race gas “fresh”. How wrong we were! Racing gasoline stored in sealed containers (drums or jugs) acts very much like carbonated Cola stored in two liter plastic bottles. Every time the Cola bottle is opened to pour off a glassful of beverage to consume, some carbonation (CO2) is released to the atmosphere as a “whoosh”. Then when the bottle is resealed, more of the CO2 boils off to rebuild pressure in the growing headspace in the bottle. A full bottle of Cola has very little headspace to fill with pressurized CO2 gas, but as the contents are consumed, that headspace volume increases, requiring ever more dissolved CO2 to be released to rebuild the pressure. And each time the pressure is released from the bottle, the CO2 remaining in the stored Cola is reduced as a % of what it originally had. Eventually, the remaining Cola’s carbonation is diminished to the point of being unpleasant. It’s now “flat”, or “dead” like stored racing gasoline that’s been subjected to similar abuse.

When you first open your “sealed” 50 gallon drum of racing gasoline, you are rewarded with a pleasant, fragrant “woosh” of boiled-off “front ends” escaping into the atmosphere from the tiny headspace in the full drum, above the gasoline, very slightly reducing the gasoline’s actual RVP (Reid Vapor Pressure = the head pressure created by the front ends that boil off from the gasoline in a sealed container at 100 F). But all is good—the first ten gallons you siphoned out of the drum is dandily fresh and loaded with most of the front end molecules that were blended into the gasoline at the refinery. Your engine is happy, making maximum horsepower at the 13/1 ratio of air to fuel (measured by weight) you tuned it to, most of which vaporizes and burns in the combustion chamber. But now the headspace in the drum above the stored racing gasoline has grown by the air volume left by the 10 gallons of siphoned-off racing gasoline. Now, lots more light ends can boil off to rebuild the pressure in the resealed drum, further reducing the actual RVP of the 40 gallons left in the drum. Then next time we crack open the drum to siphon out more racing gasoline, the “whoosh” releases even more light end gases into the atmosphere, further reducing the RVP of what’s left. Now headspace is even greater, and the loss of light ends is multiplied each time the drum is opened and resealed. Besides the loss of critical front ends, the siphoned off fuel is replaced with air containing various amounts of water that can combine with the remaining race gasoline! By the time we’re down to the last 10 or 20 gallons in the “sealed” drum, the light ends are essentially gone, and the race gasoline left is STALE and even perhaps contaminated with water! Now, the engine that’s been tuned perfectly with fresh fuel at 13/1 A/F now may be having perhaps only 80% of the stale fuel vaporizing in the combustion chambers and the remaining 20% vaporizing and burning in the hot exhaust system! So that 13/1 is now burning at an actual 16/1—still firing cleanly (the explosive limit of air/gasoline ranges from 10/1 to 17/1) but way too lean for max HP, and creating much higher combustion chamber temperatures that can create detonation and preignition and/ or piston seizures! But the EGT’s are low—how can that be? All the fuel that finally vaporizes on the way out of the combustion chamber just cools the exhaust temperature probes, then may finally burn as it combines with the unused oxygen in the exhaust system! And, your wideband A/F ratio meter’s O2 sensor in the exhaust pipe still shows 13/1. You’re being fooled by stale race gasoline as your engine suffers from at best, reduced HP and at worst, seized piston(s)!

What to do?

The most practical means to safely store and dispense racing gasoline is to pressurize the drum with just enough nitrogen gas to prevent the front ends of the gasoline from evaporating at the actual stored temperature of the gasoline. If a particular blend of racing gasoline has a published RVP rating (100 degrees F) of, say, 6 psi then it should be pressurized to approximately 3 psi (@half of RVP) at typical 70 degrees F storage. Then, the 3 psi nitrogen gas pressure can be used to dispense gasoline as needed from a valved siphon hose from the bung to the bottom of the drum. The displaced fuel is replaced with inert, dry nitrogen gas eliminating any possibility of water contamination! It only takes a few dollars worth of nitrogen gas to preserve and dispense 50 gallons of racing gasoline.

Carbon steel racing gasoline storage drums are typically hydrostatically tested—50 gallon size to 36.8 psi and 30 gallon size to 44.1 psi.

The Drum Preserve system uses a specially manufactured low-pressure solid brass nitrogen regulator that is designed to deliver maximum nitrogen pressure well below the drums’ hydrostatic test pressure. And, each billet aluminum Drum-Storage bung has a 10 psi pressure relief valve to help provide protection against accidental over-pressurization of the drum.