Re: Cam Installed... Help!!!

From: Bob Tom (tigers@bserv.com)
Date: Fri Oct 01 1999 - 20:03:13 EDT


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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