MAF meters and open-loop.ABout

[Daniel Houlton]

Jason_Leone at amat.com wrote:
>
> <<So maybe it is stopping at 5 volts and can't go any higher with more air
> flowing through it?>>
>
> You answered your own question. The Mass Air system is not ideal for
> forced-induction. In fact, it's pretty damn limiting. The MAF sensor can
> only put out 5V and that's it. It's really easy to max it out when you blow
> through more air, and even draw through more air. It just depends on the
> MAF housing diameter,intake plumbing of the vehicle, turbo characteristics,
> and the air flow velocity that cooling the MAF platinum wire.

I did some tests last night.  From what I remeber of monitoring the MAF
output pre-turbo, I think the highest voltage I got was around 4.1v and
that was always a spike when I nailed it.  About 3.8v was the steady
state WOT voltage.

I did the same tests last night with the turbo.  Under boost (max about
7 psi) and WOT, the highest reading I got was around 4.3v and again that
was a spike.  Also, the lean cut out I described before seemed to happen
right around 4.0v.  Under 5 lbs boost, the voltage would stay just under 
4.0v and going a little further with the throttle to trigger the WOT
switch would send it up to about 4.1v or so and it would go lean.  The
4.3v spike I got was hitting WOT (going from about 5 lbs boost to 7)
for about one second (before backing off because it was lean).

So, it appears that the MAF isn't necessarilly maxing out.  Maybe the
computer is?  i.e. 6 lbs of boost @ 5000 rpm is about 45 - 50 hp more
(mathematically anyways) than the computer expects it can ever produce.
Maybe it ignores the higher MAF readings because it knows it shouldn't   
ever read that high?


> about your Hitachi engine management system. You eventually reach the point
> at which the MAF system has to be replaced, or modified. You've reached
> that point, welcome to the club.

Glad to be here  :)


> You have a few options here:
>
> 1) You can get an aftermarket MAF sensor (from Split Second, etc.) and have
> a custom chip done (hey, go w/ larger injectors too)....or
 
Nope.  Too much and I'm not convinced the problem is my MAF.  Also, I
couldn't bring myself to pay $200 - $300 for a chip that isn't adjustable
for future tuning and mods.
 
>
> 2) You can use something like an HKS Vein Pressure Converter (which uses
> Speed Density, but sends a bogus MAF signal to your factory ECU)...or


>
> 3) Get serious about life, and get the real deal Speed Density based system
> (TEC II, DFI, etc.).

Can't bring myself to rip out the stock computer either :(  If it were a
sports car, project car, or race car, sure.  But this is a truck and my
(severely underpowered) daily driver.  The turbo is mostly for better
highway driveability (I can actually maintain 60 mph on a 2% grade now :)


> 4) Try lowering your boost

I'd rather not.  I'm not sure how I could either.  It's set up do deliver
full boost pressure to the wastegate actuator.  I could restrict it for
more boost, but how would I get lower boost?  Change the actuator or
hook a spring to it to help it open the wastegate?

>
> Concerning the A/F ratio:
>
> Have you done the necessary fuel requirement calculations to ensure your
> fuel supply is capable? Do you know the fuel pressure at the rail, the fuel
> pump delivery specs(and the Voltage at the pump), injector flow specs (at
> what test pressure), and the potential hp output at full boost? All these
> things need to be calculated in order for you to know if you have the fuel
> available....otherwise...you will run lean up top, and BOOOOOOM! Bye-bye
> pistons!

Er.  Sort of.  Stock my engine is rated at 120 hp and it uses 4, 20 lbs/hr
injectors (at 40 psi).  I have on order a rising rate FPR from cartech.net
which I'm hoping will raise pressure enough for those brief visits to 7 or
8 lbs boost which seems to be the limit without restricting the signal to
the wastegate actuator.  BTW, this will have to hit quite a high fuel
pressure.  Like 80 psi or so for brief periods.  My fuel hoses use spring
clamps.  Should I be replacing these with a more positive worm or screw
type clamp?

I've also considered an AIC (requires time or money to buy or make) or
just using an on/off VW type cold start injector with a Hobbs switch to
give it a fixed fuel boost at a certain boost pressure.

Another option is upgrading to larger injectors.  I want to get mine
cleaned and balanced anyways, and swapping to larger injectors at the same
time isn't too much more.  I figure I can go slightly larger (like 22 lb/hr)
and still keep the computer happy.  Maybe a little higher (like 24 lb/hr)
if I lower the base fuel pressure a little bit (from the stock 35 psi @
idle to 28 or 30 psi).  Any advice on this one?  If it'll work, it can
make the RR FPR *much* more effective and I won't have to boost fuel
pressure as high under load to get the proper fueling.  I could also
compensate for the larger injectors by modifying the MAF signal with a
couple pots (described at http://www.autospeed.com/A_0008/P_1/article.html)
I'd prefer not to buy one of the ~$300 units available to do this.

The fuel pump I don't know about.  I had planned on getting a fuel
pressure gauge to mount in the cab once I get the new FPR in.  Too
expensive for the AutoMeter electric ones though.  I may end up just
relying on the A/F ratio meter and maybe an EGT gauge instead or use one
of the cheaper digital Intellitronix ones.

BTW, I'm using an AutoMeter UltraLite white faced boost/vacuum gauge.  I
really like the look of these things.  Do they happen to make an exhaust
gas temp (EGT) gauge as well in this style?  What's the cost and part #?
I don't see one in Jegs or Summit or AutoMeter's web site, but I've heard
they do make one.

Also, I surprisingly am not noticing any detonation or knocking even when
I hit 6 or 7 psi boost and I'm still running cheap 87 octane and no IC
yet.  Stock compression ratio is 8.6:1  Timing still set to stock 12 deg
BTDC at idle and jumps (I think) to about 25 off idle.  Does this sound
right?  I expected it to be very obvious if it happens.  I'm concerned that
it may be happening and I just don't hear it.

So, the biggest thing I'm worried about is melting a piston.  The A/F
meter will tell me I'm lean, but it won't tell me I'm about to melt a
piston.  Is monitoring an EGT gauge the best way to prevent this?

thanks
--Dan