I've been nerding out on fire suppression and thought I'd share a few things i have picked up on. I'm not an expert but I did stay at a holiday inn express last night that was on fire...but fact-checking is most welcome.
History
AJ Foyt's team debuted the first on-board fire suppression system in 1967 at the Indy 500. He used Halon 1301, and it didn't take long for everyone else in racing to adopt the system using 1301. It was the standard for nearly 30 years in almost all forms of professional racing.
Halon 1301 had a big drawback though-that it has to be stored at HIGH pressure to keep it aqueous, which proved problematic in racing applications where heat and other harsh environment factors were causing accidental release failures. This is where Halon 1211 stepped in. With a much lower aqueous pressure and similar suppression effectiveness, it supplanted 1301. Here's the problem with 1211, it’s more toxic to breathe. In toxicology speak, it has a much lower LOAEL (lowest observed adverse effect level) concentration than 1301. Using the SFI 17.1 test procedure for enclosed driver compartments, a 5lb system will net you 12X the LOAEL for 1211, versus 1.7X for 1301. Once the SFI procedure was established and racing governing bodies adopted the standard (ie NASCAR) and saw toxicity ratios greater than 1.0 for both systems, Halons got BANNED. You may see that nearly all rule books today prohibit it.
DuPont, seeking to fill the void in the racing market, began promoting their FE-36 product, which is the trade name for the gas HFC-236fa. It passes the SFI 17.1 toxicity criteria with a ratio of 0.83, doesn't deplete the ozone like Halons (in the 1990s ozone depletion was another nail on the coffin for Halon), and best of all, it puts out fires just like Halons. DuPont's FM-200 (chemical name HFC-227ea) is also SFI tested and passed, and there isn't any meaningful information to distinguish it from FE-36. So what you will find are FE-36 and FM-200 are the DuPont products are on the short list of SFI 17.1 approved suppression systems.
3M is now recently in the racing market (last 12months) with their own gas C6F12O, trade name Novec 1230 which is now on the SFI approved list. Discerning if this gas is any better than the DuPont products is also difficult without really digging into the chemistry. The bottom line is there appears to be no citations of meaningful technical differences among FE-36, FM-200, and 1230, so I'm assuming that most racers will decide based on price points. If someone knows more than me on this please fill in the blanks.
Powder vs Foam vs Gas
This issue really has been the core of the discussion on the forums. I believe we can all agree that there is no bad choice. Even an ABC handheld extinguisher sometimes has its advantages over suppression systems, in that you can fight the fire from outside your car (or in a moment of racing comradery, your competitor's car).
Handheld Extinguisher
Here's a great story from my father, who owns child care centers and transports 14 kids at a time to and from school each day. He has a fire extinguisher mounted in the 15 passenger van on the leg of the first bench seat where the 'big kids' sit. One of the hooligans got bored during the drive one day, pulled the pin out and wanted to see what would happen if she kicked the handle with her foot. It was depressed for at most 2 seconds, but the powder quickly dispersed and FILLED the whole van in seconds. Visibility was 0 and so was breathability...the powder induces heavy coughing and discomfort. Fortunately he was going 5mph in front of a school at the time, came to a stop and everything turned out fine, but it could have been a lot worse. The bottom line is it is really is unwise to try to use an ABC powder extinguisher while you're still in the car. The powder is overwhelming, disorienting, and in most situations probably more of a hazard to your safety than a benefit.
Foam System
There are several SFI approved foaming suppression systems...and since they pass the 17.1 test you can trust that they will do their primary job: giving you time to get out of the car, and secondarily, hopefully stifling the fire sufficiently to allow the fire crew to get there. The distinct shortcoming of a foaming system has already been discussed by many of you-the coverage is not omnidirectional. It simply doesnt propagate like a gas; foam can't be relied on to get in the many hard-to-reach places in a car. The other two concerns are logistical: foam is corrosive...so there is appreciable cleanup necessary to prevent the foam from damaging your car, including the metal itself. In fact, the reason foam systems must be sent back for certification every 2 or so years is to ensure that the coating is preventing a corrosive reaction in the cylinder. The second issue is that foam is a surfactant; it's slippery. Foam on the track has been claimed to be a slipping hazard to the cleanup crew, and a difficult to clean up from the track surface. Whether or not your system wets the track is conjecture, but it is something to know in the decision process.
Gas System
The gas systems mentioned from 3M and DuPont are also on the SFI approved list; but have the reputation that the gases are suffocating. In essence yes, it is not sustainable to breathe the stuff; flourocarbons are bad for you. And I can't tell you from first hand experience what the sensation is to inhale these gases. However, the important takeaway that the suppliers emphasize is that the suppression gases are not replacing 100% of atmospheric air in the car with suppressant. The concentration of a 5lb or 10lb system fully discharged in the cabin is between 10% and 12%, which is all that is necessary to snuff the fire. The design concentration of the system is such that the occupants can remain alert and functional. The SFI criteria takes into account LOAEL levels, so you can expect that the system isn’t going to knock you out. You can see in the video above a validation of this, as the person walks in and out of the test room seemingly unfazed by the room full of the stuff he's breathing.
The design discharge rate of fire suppression systems is targeted between 0.25 and 0.5 lbs per second, which equates to 10 to 20 seconds depending on the number of nozzles used. Until learning of this, I had assumed that supression systems were an instantaneous "dump" of the tank. Also, another debunked misconception is that a 10 lb system does not inject 2x as much suppressant as a 5 lb into the cabin. For reasons of complying with LOAEL levels, the 10 lb system is designed to be plumbed into the trunk area, not to double down on the amount of gas in the cabin. At this point, it is important to consider the importance of a trunk firewall, as it has important implications on the gaseous concentration of the system.
Conclusion
If there's any conclusions I've reached for myself, is that there seems to be a significant racing precedent for using a gas system. I don't plan to hotbox myself to find out for sure, but think I am reassured that the gas systems are designed to keep occupants breathing. I think the real appeal of a foaming system is that the sprayed surfaces stay "wet" which could suppress the fire for a longer time. If fire crews take a while to reach the scene, this could play out to a real advantage in minimizing carnage. However, personally I am more compelled by the lower maintenance of gas suppression systems, and the omnidirectional coverage of a gas filling the car.
I hope this was helpful! Like I said I'm not an expert at this stuff, just fearful of fires and fascinated by the history, science, and design of suppresion systems. Please contribute if you see areas where I missed or perhaps even flat out wrong.
Link to SFI approved fire suppression systems:
http://www.sfifoundation.com/current171products4.pdf
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