No Subject

[Alan Cordeiro <cordeiro@avlna.com>]

>Wow, you guys love to pick a fight!!!! So all those experts who know better
>than me lets see the correct way to do it posted on the list.
>
>For all the others, remember that the boost sensor will only measure 15 PSI.
>If you remove the boost limit, you will remove the overboost protection. The
>dynamic impedance of a zener diode (hands up everyone who understands that)
>will allow the voltage to rise to 4.5v, just below the cutoff and thats as
>close as you can get without problems and without a lot of electronic
>engineering.
>

At the signal frequency we are dealing with ( 1 to 5 hz. ) the dynamic 
impedance will not be noticed at all. 

I have to speak from the only example I have, the MAC-11 in my '86 5kTQ, 
the sensor puts out 2.5 volts per bar absolute. With a stronger spring
my cutoff was between 1.5 and 1.6, on the dash guage, and I will not
swear to it's accuracy, but just telling what I get to see.

I have selected ( trial & error ) a spring with a 1.7 bar boost limit.
Occasionally I see 1.8 after prolonged accelerations. Yes the guage
"works" still.

My voltage divide ratio is such that it reduces boosts above 1.0 by
30 to 35 %, just giving me a cutoff if I go above 1.8 for a bit.
My engine will begin to ping above 1.7, but I rarely get there,
and will not stay there long.

Below 1.0 bar, the circuit has no effect. I am using a very low leakage
voltage regulator ( Linear Technology LT1004CS8-2.5, or Burr Brown 
REF1004-2.5, both of which consume only 0.02 mA or less ) in conjunction
with a voltage divider ( 1000 ohms:1500 ohms ). The voltage divide
function does not come into play until the input voltage from the
sensor exceeds 2.5 volts ( i.e. 1.0 bar absolute ), and thereafter it
reduces the input by about 30-35% for all higher values. I connected the
dash meter directly to the sensor output through a 1000 ohm resistor
so that it continues to get "true" reading.

The CPU will see a distorted picture of the manifold boost pressure after
the turbo boost comes on, but marginally distorted, seems to work
fairly well in my setup.

The ideal situation I see would be to limit boost to 1.6 bar, and hence
not have such a severe error on the timing advance. This will limit the
extra power, but be easier on your engine, turbo, intercooler, air hoses,
diaphragm etc. Of course we need to find the "right" spring to get this 
effect.

If we wanted, I guess we could do a bulk order from someone like
Midwest Springs, get stainless steel springs for less than $10 each
that would "let off" at 1.6 bar. A 20% reduction in sensor output
would then not cause too much of a timing error.

Those that want more power, "SAFELY", would go with the vendor of
their choice, SuperChips, IA,  TAP etc.

>I don't care what else you do, if you raise the boost cut level, you are in
>danger of losing the overboost protection, because the actual limit is so
>close to the limitations of the pressure sensor (4.65v boost limit, 5.0v
>sensor limit), so go ahead and remove the limit and the overboost
>protection. At least you know its gone and can make allowances for it.

The final limit is from the A/D converter, 0 to 5 volts max range,
on the ECU, which "reads" the input pressure for the CPU to see.

I think the CMOS op-amps come close to rail, perhaps 4.8 volts.
>From what I saw, the boost cutoff limit is about 3.8 volts anyhow....

>I love experts
>

Especially when they go on and on about things they think they
know, but have no idea about.......OK OK.

Alan Cordeiro