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patman · 03-08-2018, 04:16 AM

Fwiw... Water injection is a thing, but it is ***NOT*** done by adding water to the fuel tank.

Some background:

The higher the octane, the *less* volatile the fuel, and the more predictable the burn.

What you want to happen is for the piston to come up in the cylinder, the spark plug ignites the air/fuel mixture at just the right instant, and the resulting combustion forces the piston back down again.

What can happen is preignition, which is the mixture lighting off too soon, before the piston is at the top of the cylinder. That’s the ‘rocks in a coffee can’ sound that means you have too low octane fuel, too much compression, or the spark timing is too soon in the piston cycle. And/or you have too much heat, causing hotspots (carbon buildup or sharp edges) that set the mixture off before the spark plug does.

The trouble is...performance is a fine line. Higher compression ratios and earlier spark timing usually yield higher power. Too much and you get preignition and engine damage.

Higher octane fuels can withstand higher compression ratios (including supercharging) predictably. Predictable burns mean you can push the ignition timing earlier and closer to that hairy edge.

So...water injection...is done by spraying a tiny bit of water into the engine (typically into the area between the air filter and the carb) that does two things: it cools the air/fuel mixture to lower the temps and reduce the chance of hotspots unintentionally igniting the mixture early, plus it has the side effect of literally steam-cleaning the combustion chamber to remove carbon buildup.

Ultimately that allows the use of higher compression and/or earlier ignition timing with fuel that would otherwise have preignited.

This was low-bucked in the 70’s using an extra windshield washer pump when the late-60s cars with high compression couldn’t take the lower octane fuels. Big RV with a big engine going up a hill pinging like crazy? Hit the extra washer button for a bit of relief to make it to the top of the hill. It’s also used to push the hairy edge of performance that tiny but farther.

03-08-2018, 04:16 AM	#25
patman Senior Member Join Date: Jun 2008 Location: NH Posts: 374 Thanks: 56 Thanked 152 Times in 76 Posts	Fwiw... Water injection is a thing, but it is *NOT* done by adding water to the fuel tank. Some background: The higher the octane, the less volatile the fuel, and the more predictable the burn. What you want to happen is for the piston to come up in the cylinder, the spark plug ignites the air/fuel mixture at just the right instant, and the resulting combustion forces the piston back down again. What can happen is preignition, which is the mixture lighting off too soon, before the piston is at the top of the cylinder. That’s the ‘rocks in a coffee can’ sound that means you have too low octane fuel, too much compression, or the spark timing is too soon in the piston cycle. And/or you have too much heat, causing hotspots (carbon buildup or sharp edges) that set the mixture off before the spark plug does. The trouble is...performance is a fine line. Higher compression ratios and earlier spark timing usually yield higher power. Too much and you get preignition and engine damage. Higher octane fuels can withstand higher compression ratios (including supercharging) predictably. Predictable burns mean you can push the ignition timing earlier and closer to that hairy edge. So...water injection...is done by spraying a tiny bit of water into the engine (typically into the area between the air filter and the carb) that does two things: it cools the air/fuel mixture to lower the temps and reduce the chance of hotspots unintentionally igniting the mixture early, plus it has the side effect of literally steam-cleaning the combustion chamber to remove carbon buildup. Ultimately that allows the use of higher compression and/or earlier ignition timing with fuel that would otherwise have preignited. This was low-bucked in the 70’s using an extra windshield washer pump when the late-60s cars with high compression couldn’t take the lower octane fuels. Big RV with a big engine going up a hill pinging like crazy? Hit the extra washer button for a bit of relief to make it to the top of the hill. It’s also used to push the hairy edge of performance that tiny but farther.