Better than methanol injection: Isopropyl Alcohol aka 'rubbing alcohol'

When it comes to turbocharging vehicles, you are only limited by the strength of your motor and the supporting systems ability to supply enough fuel without knock. More fuel, more air, more power. However, running it lean is how torque is built; but how do you effectively run lean without knocking? By injecting a chemical agent to act as a cooling medium into the boost stream prior to the combustion chamber.

By filling these areas with the right mixture of the right chemical; cooling and power gains are to be had by chemically increasing the fuel octane. But what do you inject? Most choose methanol as an easy solution due to its extensive cooling capacity and additional gain of energy. But, did you know that there is something better than methanol to inject? That's right - isopropyl alcohol, aka "rubbing alcohol". It is the same stuff that you find at the pharmacy used to sterilize wounds, tools, etc. It is also the same solvent used to remove glue deposits from vinyl graphics! Why on earth would we inject it into our motor? I'll tell you why!

Isopropyl alcohol (iPrOH) or C3H8O, according to wikipedia, "is a a colorless, flammable chemical compound with a strong odor. It is the simplest example of a secondary alcohol, where the alcohol carbon is attached to two other carbons sometimes shown as (CH3)2CHOH. It is a structural isomer of propanol."

Another noteworthy excerpt comes from ,"Isopropanol is a clear and flammable liquid at room temperature with odour resembles that of a mixture of ethanol and acetone; completely miscible with water, ethanol, acetone, chloroform, and benzene." That being said, notice the part where it says flammable and compatible with water, ethanol, and benzene; all found in gasoline! So it mixes well, but how flammable is it compared to that of methanol? Check out the videos below and watch for yourself.

Combustion of methanol alcohol

Confined fuel [iPrOH] air mixture burning

Did you see that difference in thrust between isopropyl and methanol? Isopropyl explodes with much more force and pressure for a shorter period of time creating less heat and a quicker combustion process. Now what happens if we add gasoline vapor to the mixture? We will get an even larger boom, but a cooler flame with more pressure. Put that under 20+ pounds of pressure from a turbocharger, and you've got yourself one hell of an explosive recipe. Tune the timing, and you are off running your car on 20% IPROH! So has anyone done it yet? Yes!

The WRX runs on a mixture of four 910cc injectors flowing 93 octane pump gas and one 800cc nozzle spraying a mixture of 70% isopropyl alcohol and 30% distilled water at a varied rate based on boost starting at 16 psi and sloping to maximum 22.5 psi on a 200 psi pump at 70% duty cycle. The TD06-20G turbocharger puts out 640 CFM and maintains above 60% efficiency well into 30+ psi. By adding the isopropyl alcohol injection, it cools the intake charge and adds more power than regular methanol injection.

So how is isopropyl alcohol injection better than methanol injection? Lets take a look at methanol. According to wikipedia, "Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH (often abbreviated MeOH). It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol (drinking alcohol).[4] At room temperature, it is a polar liquid, and is used as an antifreeze, solvent, fuel, and as a denaturant for ethanol. It is also used for producing biodiesel via transesterification reaction." What is the difference? Take a look at the chemical structure!

Energy in kj/g

As you can see, Isopropyl has 17% more energy content than that of methanol. Why? I'm no scientist but look at all of those hydrogen molecules! Because of their high heat tolerance of (H2) 28.836 J·mol−1·K−1, isopropyl has double that of methanol. Isopropyl has 8 atoms of hydrogen @ 28.8 while methanol only contains 4 at 52. That's a difference of 22.4 J·mol−1·K−1 in favor of isopropyl just based on hydrogen molecules alone. They use hydrogen to cool nuclear reactors, so why not use it to cool our intake charge? Exactly!

What about the rest of the molecule? Isopropyl alcohol as it is in pure liquid form upon first injection, can hold an impressive 154 J/(mol K) at 20°C-25°C. Methanol while in liquid form at upon first injection can only hold 79.5 J/(mol K) at 20°C-25°C. No matter how you look at it, from liquid to gaseous state during the duration of the isopropyl injection; it always comes out looking better than methanol.

If isopropyl alcohol can absorb more heat due to increased presence of hydrogen, costs less money to purchase because it is non-volatile and can be purchased anywhere, and contains more energy than methanol - why not use it? Not only is it cooler, evaporate quicker, and change from liquid to gas with less resistance; but isopropyl contains more energy as well. As I learned from working with "3gsucks", why fill that volume with anything but energy? It makes sense. Energy content isn't the only test that Isopropyl alcohol outperforms methanol. Lets take a look at the scientific tables below!

As you can see, Isopropyl alcohol outperforms in nearly every aspect required by turbocharged injection. What do we want out of our injection system? More power, higher octane, increased resistance to knock, lubrication, and the bottom line; more boost. By the sheer molar mass being nearly double that of methanol, isopropyl alcohol brings more bang for buck. Isopropyl alcohol also boasts a higher boiling point, and a lower melting point. Not shown is the all important autoignition point, which for iso is again better; 399C vs. methanol's 395C. Flashpoint for methanol is at 11-12C, and iso is 13C.

So what does this mean? By injecting isopropyl alcohol instead of methanol, you can get more power, additional cooling, and a healthier running engine at higher psi. By cooling the intake charge with both isopropyl alcohol injection, additional fuel by running slightly larger fuel injectors, maintaining a 22% alcohol/fuel equilibrium, I'm able to run 22.5 psi without knock, and still maintains 10.0 AFR!

Is isopropyl alcohol injection for you? Maybe, but you've got to be willing to sit down and do the science. If you blow your car up after reading this - that is on you. I have been injecting isopropyl alcohol for about half a year now, and have been running upwards of 20 PSI the whole time with no problems, and tons of fun! I suggest you at least try it. I did, and it worked out great.

2002 WRX running 20g and isopropyl alcohol injection @ 22.5 psi

As you can see, the vehicle runs quite effectively. But what about those flames? Thanks to Pete Stop Motor Company, we shot a flame video inside of their showroom. Check it out below and notice the color of flame; looks similar to that of the bottle above!

SubyPAL's WRX spitting isopropyl afterfire


  1. This comment has been removed by the author.

    1. You might want to try to be clear when you say Propanol. They are speaking of 2-Propanol or Isopropanol. Propanol usually refers to the N-propanol or 1-propanol. This could really cause a heck of a lot of confusion.

  2. This comment has been removed by the author.

  3. I've been using isopropyl alcohol with nitrous in my mustang because of this website. Does it have to be 70%, or can it be 91%? Snow performance said "a longer burn would be evidence of a higher octane", but this isopropyl burns faster in the YouTube experiment. I'm not sure what to do now...

  4. I've been using isopropyl alcohol with nitrous in my mustang because of this website. Does it have to be 70%, or can it be 91%? Snow performance said "a longer burn would be evidence of a higher octane", but this isopropyl burns faster in the YouTube experiment. I'm not sure what to do now...

  5. The autoignition temperature of methanol isn't 395C, it's 470C. iso-octane has an autoignition temperature around 395 though.

    Technically isopropyl alcohol should have the _least_ octane boosting effect of iso/ethanol/methanol, the thing that might make isopropyl alcohol better to work with is the fact it behaves more like normal petrol (you don't need to add as much volume for the ideal "mixture"). Higher energy densities being good is a bit misleading though since regular gasoline should be around 46.4 kJ/g compared to the 24.04 in iso and 19.7 in methanol (fun fact, ethanol has 26.4, so even more than iso).

    using mol K as a way of determining how much energy the substance will need to vaporize per volume (which is what we need) is a misunderstanding, it takes more molecules of liquid methanol to fill any given volume at the same pressure. A more relevant measurement is how many joules per cubic centimeters the fluid will absorb before it vaporizes, this is 926.9 J/cm3 for methanol, 726.1 for ethanol and 593.7 for isopropyl alcohol.

    Higher octane usually means _lower_ energy densities so we have the _ability_ to harshen the conditions overall (like running leaner mixtures, higher compression ratios or air compression).

    This is not to say iso-propanol is a bad fuel additive either, it has a higher octane rating than regular petrol, it's less corrosive at its purest form (but all these alcohols should get diluted to about 2-3% water naturally when exposed anyways), and its much easier to obtain in some places. But honestly just getting some bigger injectors, run E85 (if that exist where you live) and remap the injection to work with a turbo will be much cheaper and easier than messing around with water injection these days.