I was just wondering because right now I am considering just an adjustable ATI fmu instead of changing the injectors. I know my powerdyne is way too rich and may make me adjust the PE tables WAY too lean setting me up for some real hurting if it fails. I would like to set the fmu as low as I can (4:1) so it follows much closer. I have started to think a fmu might actually make it easier to tune.

Hal

 

Tuned with an FMU ... yes, but it was with a Vortech Super FMU, put it on the dyno with wide band O2 sensor, made a few passes and changed the ring inside the FMU to get to an 11.8 : 1 Air/Fuel ratio.

I did not touch the PE curve or the injector sizes.

It has been stable for a year now ... 5,000 km

Hope this helps

LWM

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95 A4, SLP catback, KBDD sfc, airfoil, Vortech S-trim, 30# SVO injectors, Autometer electric FP guage, boost guage, Hotchkis STB, AS&M headers, AFPR, Vortech Aftercooler, 3.125" blower pulley, 7" crank pulley, Vigilante 2,800 stall, MSD 6 BTM, SLP 35mm front bar, SLP 21mm rear bar, LT-1 editor ... , 437hp at the wheels. --- members.shaw.ca/mackenzl/LWMsZ-28/LWMsZ-28.html ... 01 vert in the making ...

 

Does the vortech one do anything much different then the ati (other then the ati uses a screw adjustment)? Is the vortech nonlinear or something?

Hal

 

As far as I know, the Vortech unit behaves much the same as any other adjustable unit out there, you only really need be concerned as you make more than 4psi of boost and then again as you approach the MAF's 471 gms/sec upper limit.

All the adjustments I did, were made to lower the maximum fuel pressure I was seeing at 10 psi of boost from 80psi initially to a final, 65psi at 10 psi of boost.

That was enough to get the Air/Fuel ratio to go from 10:1 down to 11.8:1 and that made an additional 50 hp or so.

And I hear tell that if you go to 12.5:1 there are a few more ponies to be had.

No rocket science there.

Hope this helps

LWM

 

Hey, great, thanks for the info!

I have a feeling that a fmu set very thinly (meaning that it adds like only 2 or so pounds of fuel pressure per pound of boost) would be the best way to go. That little pressure certainly wouldn't cause the fmu to fail and would much better follow the actual boost output then just a rpm setting.

But, if what some say is true, that the pcm somehow maps to a maf air volume table even in PE mode (15), then it makes little difference either way.

What did you mean by only having to worry if you go over 4#? Do you just mean you need something like an inline pump to get enough gas flow?

That last 10hp certainly isn't worth blowing your motor over, thats for sure.

Hal

[This message has been edited by Hal Fisher (edited August 20, 2002).]

 

For the first +/- 4 psi of boost the computer via the MAF sensor takes care of additional fuel needs.

An FMU set so as to add smaller amounts of fuel pressure according to demand (boost) always seemed to me to be the easy way out. I understand that the Carrol super fueler does this electronically, and presumabley more accurately, but, as you put it, a "thin" FMU has worked well for me.

For all I know, the computer may be working with me, keeping injector pulse widths around about where they need to be given the rather outsized air volumes I am sending its way ... these MAF enabled cars tend to take care of themselves to a remarkable degree.

So, yes, I'd say that the car computer is factoring in some of the excesses we are sending its way ... so at the end of the day, we "tune" ... it "fine tunes" ... or the other way around ... depends on how you look at it ... either way there is a lot of slop in the system that will help keep your car in "harmony with the A/F ratio" ...

Hope this helps

LWM