So, I just re-read the P730 thread again. Ran across the stuff about the fuel pressure being adjustable by vacuum.

Question: WHY?? Does the ECM not have enough range in terms of injector firing times to be able to handle idle to WOT at a fixed pressure?

It seems like it would be SO much easier to tune if you could depend on the fuel pressure being static at all time.

It seems to me that it would remove an unnecessary, fairly unpredictable variable. It also seems to me that the more control you can keep in the ECM, the better.

I'm asking this question trying to educate myself, i have not burnt any PROM's (YET!!)...so maybe it's not that big of a deal??

 

When you get into tuning the VE Tables, you will find there is a static relationship between the Load, VE Tables and vacuum (that affects the ultimate fuel pressure to the injectors).

Basically, if you have a particularVE Value @ load/RPM, unless you change the engine or fuel pressure - it will remain constant.

The vacuum reducing fuel pressure actually helps with idle because if it didn't, the injector pulse width would need to shorter, and high impedence injectors (like ours) can only go so short on the PW.

 

In addition to what Glenn said, there was quite the discussion on this subject on the DIY EFI site some time ago & the gist of it was that an injector "flow" works essentially off of a differential pressure (makes sense really). When you are @ WOT a port EFI setup has higher pressure (basically atmospheric) @ the output of the injector, & when you are @ idle (High Vacuum, the other extream) the delta P is much larger. in other words, as the throttle opens, the Fuel pressure has to increase proportionately to keep the fuel flow the same for a given flow rate.

When you have a TBI, the injector is essentially exposed to the same presure across it in all conditions so it does not need a rising rate of fuel pressure.

HTH's
BW