Re: Cam Installed... Help!!!

From: Andre Mauboussin (andrem@webcombo.net)
Date: Fri Oct 01 1999 - 20:31:06 EDT


Thanks Bob. Right now, I have 14inches of vacuum at idle. I just lowered my idle
speed with the set screw on the TB and I reconnected my Jacobs ignition. I'll see
how it runs now.

Andre

Bob Tom wrote:

> At 04:58 PM 10/1/99 -0500, you wrote:
> >The new work includes the following:
> >Stage I ported heads
> >Hughes cam
> >B&F StageII TB
> >I had a friend hook it up to his scan tool yesterday. Apparently, the
> computer
> >is looking for a 600rpm idle speed, but the truck is currently idling around
> >750. The computer is trying to lean out the mixture to slow the idle speed
> down.
> >It also indicated the timing is being retarded at idle speed. <snip>
>
> Andre
>
> >From just general knowledge of factory EFI systems that I have read, it's not
> uncommon that a computer would have to be recalibrated if the engine's
> breathing
> is modified too much (no idea how much is too much). The factory computer
> (in your
> case, MP PCM) is set up with some leeways but if it's reading in something
> beyond
> these preset calibrations, it just attempts to correct them the best it can.
>
> In my case, the addition of the Magnum R/T cam and, a little while later
> the B&F
> stage 1 tb, has actually drop my idle rpm to just a tick above 500 (as
> indicated
> on an aftermarket tach). This is with a '96 MP PCM. Your 750 rpm idle
> does seem
> quite a bit high.
>
> Poor airflow computer calibrations are more likely to occur on speed
> density engines
> where a MAP sensor only estimates airflow whereas, with a MAF system,
> airflow is
> actually measured. MAP sensors require at least 10 inches of engine vacuum
> (the more
> the better) at idle for proper operation. As a general rule, cams with
> 110-degree or
> wider lobe displacement angles, and durations under 220 degrees at .050" lift
> (sometimes 250) should be selected. Some radical cams result in such low
> vacuum
> pressure that normal accessories can't be run.
>
> Good luck.
>
> Bob, Burlington, Ontario
> '97 FR CC Sport, 5.2L, 3.55 SG, auto., 15.268 @ 88.86



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