I'll be firing my motor up soon and beging tuning my PROM. I now have the LT4 HOT cam installed. I recently read that it will change the vacuum in my motor (among other things). I also read that I should change my VE table values to compensate for this cam because it could cause the motor to stumble, blow smoke, or quit.

Here's where I'm confused, please let me know where I'm wrong (I'm new at this):
At idle the hot cam will produce less vacuum (and more kPa) than a stock cam.
More fuel is added because of the increased kPa.
O2 recognizes a rich condition.
BLM is decreasing.
VE value is adjusted by (VE value) - (VE value) x (BLM value/128)
A/F ratio should be nearing 14.7
All is good?

So why should I change the VE values if the ECM can compensate for changes taking place? Does it take too long for that adjustment to be made, and by that time the motor already died?

 

Good try but that's not the case ...

At idle the hot cam will produce less vacuum (and more kPa).
The car now idles in a DIFFERENT portion of the VE map
Fuel is subtracted in this region of the map because the cam is less volumetrically efficient at these lower RPM/kPa readings
O2 recognizes a rich condition.
BLM is decreasing.
VE value is adjusted by (VE value) - (VE value) x (BLM value/128)
A/F ratio should be nearing 14.7
All is good?

Now it is

1) In Open loop NO ADJUSTMENTS ARE MADE. That means that you will be dumping way too much fuel into the engine for possibly several minutes when you first start the car. Ouch.
2) If BLMs bottom out to 108 then the computer can adjust no further and you will still be running rich.

Tim

 

Doh! Forgot about the (now) different position on the VE table. Thanks!

I guess the appropriate question to ask at this point is, should I start
this motor before making some adjustments to the VE table values? I'd have
to guess at the values though...and that isn't good. Is the LT4 HOT cam
radical enough to not allow my motor to (safely) run on a stock calibration
in the PROM?