Ive seen fmus with 12:1 ratio or 8:1 and 1:1, I never understood what they mean and i wanna know what they mean cause i wanna boost my z24 and i wanna get the right fmu ratio...........is there anyone out there that can tell me what all the ratios mean and give me alittle knowledge that is needed to know about them so i can buy the right one, and also opinions on brands and ratios..........thanks

A FMU is just a mechanical device that raises the fuel pressure in a relation to the boost level. So a 10:1 FMU will raise the fuel pressure 10PSI for every 1 pound of boost, and a 12:1 would raise the fuel pressure 12PSI for every one pound of boost. This is why an FMU isn't the best fuel management for forced induction. Because at high boost levels you have very high fuel pressure that is too high for your pump to handle and too much for your injectors to handle. If you have to use an FMU, I would look into an adjustable one that lets you adjust the ratio. They are a little more expensive but then at least you're not guessing what ratio would work best for you.

The ratio means how many psi of fuel pressure increase there is per pound of boost increase. For example, an 8:1 ratio FMU increases the fuel pressure 8 psi for every 1 pound of boost.

Assuming a stock cav is running about 40 psi of fuel pressure at idle...

@ 1 psi of boost, fuel pressure = 48 psi
@ 2 psi of boost, fuel pressure = 56 psi
@ 3 psi of boost, fuel pressure = 64 psi
@ 4 psi of boost, fuel pressure = 72 psi
@ 5 psi of boost, fuel pressure = 80 psi
@ 6 psi of boost, fuel pressure = 88 psi
@ 7 psi of boost, fuel pressure = 96 psi
@ 8 psi of boost, fuel pressure = 104 psi

Etc... So, as you can see, it can ramp up fast if you are running 15# of boost or so, more so that your pump, hose, or o-rings may handle. Also, the ratio you choose obviously effects tuning, a steeper ratio FMU will make you run richer. You need to pick one with a ratio to fit the amount of boost you plan to run and based on the fuel needs of your motor. Some companies make adjustable ratio FMU's, such as the Vortech Super FMU. There are a little pricey, but offer more flexibility for tuning. You can also install a air-bleed valve in-line with the vacuum reference hose leading to hte FMU. By screwing the air-bleeder valve in or out, you can fool the FMU into "seeing" less boost that there actually is, allowing for some play when trying to tune, if your FMU ratio is too steep. You can't fool the FMU into seeing "more" boost though, so it's a good idea to get an FMU that may be a little on the rich side, and then bleed a little boost off through the air-bleed valve until you get it where you want it.

FMU's are a bandaid approach to tuning, but it's a cheap and proven effective method that is hard to beat for the cheap cost. Tune on the rich side, then back off until you hit your target air:fuel ratio. Make sure to do this on a dyno with a wideband O2 sensor up the tailpipe, or on the street with a wideband if you have your own. We all know what happens if you go too lean! BOOM!