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Ask Sarah Forst

 Ask Sarah: I Need More Track Time From My Water Injection System!

By Sarah Forst

Dear Sarah

I have a 1996 Mazda Miata with a Jackson Racing MP45 supercharger and an ECU controlled Aquamist water/methanol injection system.  The water tank I use is about 3/4 gallon in size, so when I fill it up with straight windshield washer fluid (no additives, just water and methanol) I'm running the tank dry well before my 30 min. track session is complete.  This leads the ECU to cut boost in a not-so-subtle fashion and leads me to lose up to 10 minutes of track time.  While a bigger tank is the obvious answer, I'm not thrilled with the idea of taking up even more room in the already space-challenged trunk of the Miata (with no trailer, I still carry everything I need for the day in the car).  My question is: Could I gain more track time by simply running a higher ratio of water to methanol?  I read in Mike's article that more water made less power, but more methanol burned faster.  If this would work, I'd be happy to lose a little power to get in a full track session!


Drew

Water injection
Adding a turbo or supercharger to your engine can drastically increase the pressures and temperatures within the combustion chamber.  Water or water/methanol injection helps absorb some of that heat so you can boost without having to retard your timing.

A wise tuner once said "with great boost comes great responsibility."  All things being equal, increased boost pressure causes a denser air mixture, which leads to higher peak cylinder pressures and increased engine temperatures.  To reduce these temperatures but maintain the same level of boost, you can retard the timing.  This obviously isn't the best option for power.  Here comes water injection to save the day, and maybe your engine!

Water or water/methanol injection basically accomplishes the same thing (knock suppression and controlling temperatures) but in different ways.  Water injection works more like a coolant to reduce in-cylinder temperatures.  It has a latent heat value of 540 calories per gram, versus 264 calories per gram for methanol.  This means it is much more effective at absorbing heat than methanol is.  You'd have to inject about twice as much methanol by mass to pull out the same amount of heat during combustion than if you were injecting straight water.  Water injection doesn't increase octane value, but it enhances it by letting you run higher boost pressure on pump gas without experiencing pre-ignition or having to retard the timing to compensate.  Since engines equipped with knock sensors pull out the timing when they sense knock, you might be losing out on some power without even knowing it!  There is a point of diminishing returns though- too much water without enough boost, compression, or timing to produce complete combustion could promote misfires. 

If you don't include some form of fail safe in your water injection system, make sure you tune your car without the injection to run a 10.5-11:1 air/fuel ratio to reduce the chances of detonation.

This is one advantage of adding methanol to the water injection.  Methanol evaporates more quickly as it absorbs heat, which helps distribute the injected water more evenly.  This quicker evaporation also means it absorbs more heat in the cold pipe and intake manifold, producing lower temperatures which lead to a denser (more volume: more power) charge of air into the cylinder.  Since methanol and water evaporate and in the case of methanol, ignite at different points, this also helps with more complete combustion. 

Got a difficult tech question? Email Sarah at asksarah@motoiq.com

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Comments
jere
jerelink
Sunday, October 24, 2010 6:11 AM
Thanks for covering some of the mixtures of water to methanol. Is there a brand of windshield washer fluid that you can recommend? Or is it just what ever blue stuff at the gas station?

What do you think about ditching the intercooler altogether and injecting pre-turbo for cooling at the same time of injecting post turbo for det suppression?

Mrad
Mradlink
Tuesday, November 02, 2010 2:56 AM
Good Call on the watching out for freezing in the winter. Here in the northeast it's about that time of year again. Great article and thanks for the info, I know boosted powertrains are always talked about --except what if you just have a high compression aircooled motor (porsche)? How much of a diffrence will the water injection make with keeping that cool running as well?
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