Trying to ditch the FMU looking for insight

[AllTheGoodNamesAreGone]

background info and plans here..
http://www.camarov6.com/ubb/ultimate...;f=16;t=002631


walbro.. + 30# injectors + good tune

If it works out.. it shuld not only run better because its tuned.. but FMU's are pretty much junk.. I have been keeping an eye on mine with my wideband guage and fuel pressure guage.. and saying its inconsistant is an understatment..

where as if i had a fuel system that was up to par at a static pressure.. I could probably almost nail a prefect a/f ratio across the board.. and put less stress on my pump and injectors..

so.. i dont really see tooooo many negatives here..

insight.. advice.. anyone ever tried it?? also feal free to help out in that other thread..

[Tiago]

thats what everyone that can actually tune the PCM does. You should still add a 1:1 rising rate pressure regulator to it in place of the FMU to equalize the pressure the injectors see on either side.

[TurboProject]

Yea, if you could tune for boost then that would be ideal. Since I can't tune with the stock computer I built a megasquirt (www.megasquirt.info) and ditched my FMU and tuned with that with 36# injectors. It's working great and it's a lot more reliable, consistant, and smoother than the FMU. I need a wide band though so I can actually see what the crap is going on...just sort of seat-of-the-pants at this point. [img]tongue.gif[/img]
Yea, you will need a 1:1 pressure regulator to keep the same pressure differential btwn the intake pressure and the fuel pressure, but I think the stock one will do that. I have an adjustable one that bolts into the stock location on my fuel rail and it does that. You could set your fuel pump to run while the engine isn't running, feed a regulated down air pressure and see if the fuel pressure raises 1 psi w/ each psi of air pressure. I did it with mine and it worked.

[AllTheGoodNamesAreGone]

hrm.. thanks guys..

I will see about that 1:1 pressure adjsutment..

[njp98]

I'm also interested in this, how would the rising rate presure regulator help out and why 1:1 ratio. Does than mean 1 psi boost : 1 psi fuel, if so thats seems way to low. Also I though that the L36 computer couldn't sense anything over 0 psi and could only read vacuum. Could someone explain how this method would work?

[ August 12, 2004, 06:03 PM: Message edited by: njp98 ]

[TurboProject]

As far as the computer MAP only sensing up to 0 psi I don't see how you could get around not using a FMU. That's why I went with the seperate computer controller.

But if you could tune based on increased pulsewidth alone and can read boost pressure then you just need to keep the same pressure differential btwn the manifold pressure and the fuel pressure. This is so the injectors are always flowing the same amount per unit of time that they are open. If there was more pressure in the intake and you didn't up the fuel pressure by the same amount to counteract this then the injectors flow rate would go down because they are working against this extra pressure. This is why you want 1 psi of fuel pressure for 1 psi of boost. It keeps the injectors flowing the rate that the computer thinks they are flowing. The same thing happens even with a stock NA setup under vacumm. At WOT it's some pressure and then when you get vacumm by closeing the throttle both the manifold pressure and fuel pressure go down. But don't try and corrilate 1 inHG to 1 psi without converting cuz it's not the same scale.

If you can tune with the MAP for boost pressures then you just throw in some large injectors(to an extent) and have the pulse width real low at idle and then have it real long at high boost/high rpm.

The purpose of a FMU that raises fuel pressure more than boost is to get the injectors to flow more at the same pulse width for people who can't tune.

[ August 12, 2004, 06:40 PM: Message edited by: TurboProject ]

[TurboProject]

But as far as trying to tune with a MAP that only reads up to 0(only vacumm) I guess it would just take a lot of trial and error with a FMU with different ratios of raising fuel pressure and tuning pulse widths at different conditions. I would think your FMU would have to be real accurate and real consitant for it to work very well.

I don't really see any other way to get more fuel for more boost. Since your computer can only turn up the pulsewidths based on up to 0 psi and rpm, you might be able to tune the car for one boost setting based on the characteristics of your engine, but even then. What happens when you're in a high gear and the rpms are low and the boost is high? That would be a different condition than if the car was in a low gear with high boost at a higher rpm, and the stock computer would be blind to this fact as far as boost is concerned.

I'm definatly not an expert tuner though. I just know how my megasquirt works and it reads boost pressures, so if someone else knows a different approach to not being able to read boost pressures let us know.

[ August 12, 2004, 06:34 PM: Message edited by: TurboProject ]

[RGB]

I haven't tested this but I think the computer looks at the MAF 1st and only uses the MAP as double check. Our MAF tops out at 300? hp. 7? psi. Beyound that you need some other way to signal more fuel. So if you are going to have someone "tune" your computer wouldn't you want them to do it after installing a larger MAF (lt1?) and at least 36# injectors?

[TurboProject]

Oh, I forgot about the whole MAF thing...my 3.4 is MAP only so I don't even think about MAF [img]tongue.gif[/img]

If it takes a MAF swap then I'd think you wouldn't want to go through the expense of having someone tune your car if you're going to change something that could throw it all off.