Water Methanol AFR Calculation
 

Can anybody double check the math I have done here please?
I am not sure if this is the right way to calculate the ratio of AFR change when injecting 25 % by weight of water methanol. What would be the real AFR change?
I am injecting 510cc/min of W50/M50 by weight for 2040cc/min of petrol at peak power.

510/2040=0.25; 25%/2 = 12.5% or 255cc/min of injected methanol.
14.7 petrol stoich and 6.4 methanol stoich = 8.3 total AFR change; So 8.3x.125=1.04 AFR change?

Is the math good? Will injecting 12.5% methanol combine with petrol would give me an AFR change of 1.04?

So it looks like I would need to trim 7% petrol from my fuel map when I fully inject right?

Re: Water Methanol AFR Calculation
 

I'm not quite following one part of your calculation. I think you are off by twice as much. Here's how I do it, and it has been pretty close in practice:

Say for 2000cc of fuel injector, you jet at 25%(500cc of meth) and spray 100% meth. I don't use ideal stoich AFR, but power AFR, which is 12.5 for gas, and 5.5 for meth. So meth is about 44% of gas, for simplicity I tend to round down to 40%.

So if you jet at 25% and use 100% meth, that's like 10% more gas(40%). 10% more gas is roughly 1.0AFR in most situations.

Now for jetting at 20% and using 50/50 that's 10% methanol, which is about 4% more gas, or .3 to .4 of an AFR change.

On paper this may look funny, but we have been pretty accurate at predicting AFR change and providing proper jetting over the years.

NOTE: One thing to remember is the margin of error on whatever dyno you may be using. Some will have a issue with a .2 to .3 repeatability so many runs and proper load bearing dynos are needed to measure accurately.

Re: Water Methanol AFR Calculation
 

Ok, thanks, I had some numbers wrong in there.
So the right numbers are: I inject 12.5% meth x 0.44=5.5% more gas or around 0.5 AFR change.

I was close but not enough it seems since now it looks like I have to only trim 5.5% gasoline from my main fuel map at full load/boost.

Re: Water Methanol AFR Calculation
 

Charge cooling effect can increase air mass trapped in cylinder which would partially compensate for increased fuel value. It depends on accuracy of determining actual airflow into the engine.