Fuel management unit question

[84FordVan]

What exactly does the ratio of an FMU mean? Say I get a 12:1 FMU, does that mean it will attempt to maintain a 12:1 AFR by increasing fuel pressure? Or should I go for an adjustable rising rate fuel pressure regulator with a set ratio of 1:1, which would be less expensive anyway. Thanks in advance!

 

[Austin]

a 12:1 fmu means that it raises the fuel presure 12 psi for every 1 psi of boost

12:1 is usally overkill

 

[84FordVan]

Thanks, I'll go with the adjustable pressure regulator then.

 

[Austin]

you can get smaller fmu's and some of them you can change the guts out them to change the ratio

 

[84FordVan]

I have seen down to 6:1 FMU's, does anything lower exist, say a 1:1?

 

[StrangeRanger]

1:1 would defeat the entire purpose of an FMU which is to increase the fuel supply disproportionately as boost increases. Vortech makes FMUs with ratios as low as 3:1 for systems which have already been fitted with oversized injectors

http://www.vortechsuperchargers.com/product.php?p=31

 

[JonL]

1:1 is what factory fuel regulators are; I believe. This is so that the pressure difference accross the injector, or between the intake manfold and the fuel rail stays constant. Most adjustable fuel regulators are 1:1 as well.

 

[Emerald 74 4X4]

I think most factory ones will increase only to a certain point. Since they are not made for pressures much above atmospheric, I think they limit out. I think that's why there are slot of aftermarket ones that regulate from 70 to 100 psi. Correct me if I'm wrong.

 

[StrangeRanger]

Stock regulators are not designed to work with boost they are only designed to respond to changes in manifold vacuum. They may or may not work at low levels of boost to maintain a constant pressure drop across the injector but they will not provide the enrichment that a boosted engine normally requires to run safely.

BTW the increase in flow through an injector is proportional to the square root of the increase in pressure drop. somewhere on Vortech's site they have a detailed explanation of the math and how their units work.