I've been reading alot about how Speed Density works. Basically, it uses the ideal gas law to calculate the density of the air going into the cylinder:

density = Pressure / R / Temperature

where:
Pressure is linearly proportional to the voltage from the MAP sensor
R is the ideal gas constant
Temperature is measured by the IAT sensor

Then, since not all the air is trapped in the cylinder, the computer multiplies "density" by VE%, which is programmed into a table. Note that the density equation and R are not accessible or adjustable. The computer then calculates the injector pulse-width required to reach the desired A/F ratio.

Now, assume the MAF is disconnected and the closed loop is disabled. If you were to simply replace the 1 bar MAP sensor with a 2 bar, then you would have half the voltage for any given pressure. Half the voltage means that the density formula will yield half the actual density and the computer will supply half the fuel (also adjusted by VE, so it will actually be less than half). So, to fix this situation, you half the injector constant, which doubles the pulse width, then condense the VE table to the left half.

I understand that there are some other tables that are affected, but could it be this simple to get the motor running reasonably well?

Mod's, sorry for the double post.

Mike

 

Mike,
I am not up to speed on the factory computer but with a Fast,which is speed density there were no problems tuning it.
Are ya tring to get the same results with a factory computer on the cheep?

 

I'm trying to avoid:
- the expense of a stand-alone.
- hacking up the harness.
- having to run 2 computers (stock for gauges, hvac, security, etc.)

If I go aftermarket, it will be MSII.

 

Interesting topic. This would also be helpful to those who still run on the street and need the OBDII tests for inspection.

 

I belive his has been brought up in the pcm section. Alvin has been doing this on the 93's for a few years now.

 

I totally understand.

 

That can still be accomplished with a piggybacked aftermarket ECU.

 

yes you can, and it works been doing it for years.

 

No doubt it can.

 

Just throwing it out there. I thought by your post that you assumed you were out of luck with OBD2 emissions testing if you went to an aftermarket ecu.

 

Ya, I didn't really think about it. Did you ever run your car on the stock PCM, or did you go to the BS3 from the start?

 

Can we get more details about piggybacking an aftermarket ECU and sucessfully passing an OBD 2 emission scan type test?
Thanks.

 

Anybody?

 

So, when you speak of cutting the injector constant in half, do you mean if I am running 42# injectors I would enter a value for @21# injectors? I noticed in your tune you sent me to check out that your value was for ~30#/hr. I know you have to be running a much larger injector, correct?

I hope Im not coming across as retarded, but Im new to this whole tuning area. Sorry if this is a dumb question.

Eric Bazan

 

Yes, I am running 84's. I really should have a 42 lb injector constant, but his is how I got the tune from dookie and it worked relatively well so I left it that way. My VE numbers are pretty low, so putting in a higher injector constant would mean I would need more conventional higher VE numbers.

Mike