does any mod...

[Ameer Antar]

this is a loaded question! well, first a look at CIS will help. The A/F 
ratio is not calculated by the ECU. In fact, the air-flow sensor is what 
determines the A/F ratio for the most part. The higher the air-flow plate 
is, the more fuel is allowed to enter the injectors. The angle of the cone 
around the sensor plate dictates how the plate moves for a given amount of 
air. This cone is calibrated for a stoich 14.7:1 A/F ratio. The more air 
flows, the more fuel...all this is so the A/F ratio remains the same.

Now the ECU takes info from the O2 sensor and adjusts the mixture really 
thru the air-flow sensor plate. This is done by the CIS freq. valve 
adjusting the fuel pressure which sits on the plunger which is attached to 
the air-flow sensor plate. This adjustment of pressure atop the plunger 
adjusts how easily the air-flow sensor plate is able to move. If you take 
off the air-flow sensor intake boot, you can feel pressure behind the 
plate, so that there is more resistance to air flow. Otherwise, too much 
fuel would be metered to the injectors. So the ECU really doesn't decide 
the actual A/F ratio, it only has a small adjustment to center it around 
whatever is metered by the air-flow through the sensor. So the ECU will not 
limit the movement of the air-flow sensor plate beyond 1.4 bar, it will 
continue to compare to the o2 sensor readings and shoot for the stoich A/F 
ratio.  The only real limit is the limit of air-flow sensor plate. Once it 
reaches its maximum level, the fuel dist. can no longer provide any more 
fuel and the A/F ratio will steadily increase as more and more air enters 
the engine. But it's been said that stock CIS can handle up to 2.0 bar.

In a modified vehicle, at 1.4 or 1.6 bar the A/F ratio should still shoot 
for the stoich 14.7:1 ratio. The WOT switch allows for a richer mixture not 
b/c the engine needs it to survive, but to give the car the max. amount of 
oomph at extreme conditions. If you look at a graph of A/F raitios, the 
max. power in an engine is a bit above 14.7:1, while max. economy is a bit 
below 14.7. Stoich is a compromise between those two conditions, and WOT is 
the only time you want the max. power, w/ disregard for economy. It's OK to 
use stoich A/F ratio at 1.6 bar b/c the engine will be fine...no need to 
increase D.C. But beyond that you begin to run into the problem that the 
engine is now running under very high compression, esp. at high boost. High 
compression is a problem b/c it maximizes the amount of energy released 
from combustion, causing excess heat and therefore pinging. The pinging is 
not caused by the wrong A/F ratio, but high compression. To combat this, 
what many tuners do is make the mixture richer to cool the combustion. They 
also use water injection to cool it. Another method is lowering normally 
aspirated compression [by changing pistons or thicker head gaskets] and 
using big intercoolers to lower air temps. But this is only a concern for 
very high boost.

A chip should do:
-shift 1.6 bar fuel cutoff to some max. value [1.8-2.0 bar]
-add additional timing entries for higher boost
-adjust WOT boost value to some max. level [1.6-1.9 bar]
-adjust any entries where timing is fully retarded beyond stock boost level

other than that, air-flow sensor plate will adjust the mixture 
appropriately as long as boost is kept within limits.

-ameer


At 01:21 PM 12/4/2000 , you wrote:
>modify A/F ratio (thru duty cycle, I think it is the only way) at high
>boost?
>Do those chips do anything except changing cutt-off pressure value and
>boost map?
>
>Thanks.
>
>Konstantin Bogach.
>200tq 89