[Gmecm] Two '747 problems
david at newcovenant.com
Fri Mar 11 22:46:18 CST 2005
I went to the 454 TBI (rated 670 cfm), because the stock 305 TBI (440
cfm) was air-starving the engine above 4500 rpm. The stock 440 TBI was
fine for the OEM setup with a 4500 rpm redline. But now with drastically
improved VE and a 6000 rpm redline, it just couldn't feed enough air or
fuel. The fuel pressure is controlled by a vacuum-modulated external
regulator, and is a steady 15 psi at idle, 22 psi at WOT. I'll admit the
670 cfm TBI is a bit large, but nobody makes a TBI in the more ideal 575
to 600 range.
The cam is 212/218 at 0.050, 270/264 advertised, with .495/.501 lift,
112 centerline, 108 valve center, 47 degrees overlap That's a lot of cam
for a 305, the max that CompCams would recommend for the car,
transmission, and rear gearing.
It has taken a lot of fine tuning on the 747 (As Bruce Plecan could
relate to, I've burned over 200 chips so far), but in closed loop it
idles at 700 rpm with 15 inches of vacuum. It has a definite lope, but
the idle only varies about plus or minus 30 rpm. It took a little
getting used to, but it is actually quite smooth around town, once I
learned its habits. The engine can be driven smoothly between 1200 and
2000 rpm. It cruises at 1950 rpm at 75 mph. The engine comes to life in
two stages, above 2000 rpm and then it really wakes up above 3300 rpm.
To coin a phrase popular in some tuning documents, I can light'em up at
will in first and second gears. I can get rubber on the 2-3 and 3-4
shifts, and have gotten rubber twice on the 4-5 shift (hot pavement).
When closed, the butterflies are adjusted according to Bruce's
recommendation of having most of the idle controlled by the butterflies.
At closed loop idle, the IAC steps vary between 5 and 10 steps. This
seems to work much better than closing the butterflies down, with IAC
around 30 steps at idle.
I set the base idle with the ESC disconnected. Base timing is at TDC,
because 9.3 compression combined with zero-gap piston rings made too
much compression for easy cranking with base timing at 4 or 6 degrees
BTDC, especially when hot. Once running, the idling area of the main
spark table is flattened at 16 degrees BTDC.
I came up with 16 degrees because I had observed before building the
engine that 16 degrees is where the stock calibration set the timing at
idle. Is this too much for idle? It just now occurs to me that it may be
too much advance at idle, because I just greatly improved the problem by
taking out all the cold compensation spark (I zeroed the whole cold
compensation spark table).
After taking out the cold compensation spark, it now starts nicely and
drops almost immediately to the target cold idle of 1100 rpm. However,
as it warms up it starts to oscillate when the temp gets above the
closed loop threshold, which is now set at 8 degrees C. (I saw that in
the bin of a chip from Edelbrock.) Perhaps I should raise the threshold
temp to a more stock-like 150 degrees?
It still straightens out and idles perfectly if I shut it off and
restart it after warm-up.
As for the stumble, I just took out all the knock retard and I lowered
the fuel's octane to 89, just to hear what was going on with the spark.
I can now hear a light knock in the same range and throttle as the flat
spot, 1600 to 1800 rpm under light acceleration. There is no knock when
in PE mode through that range. Could my flat spot be from a bit too much
advance in that range?
I'm fishing here. This strange cold startup behaviour has made me think
about a carb. I really don't want to do that.
From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On
Behalf Of JD
Sent: Friday, March 11, 2005 9:25 AM
To: gmecm at diy-efi.org
Subject: [Gmecm] Two '747 problems
In the "good old days", this would sound like too much carb for the
engine. Since the 454 Tbi is larger and the manifold most likely has
larger passages, do you think that could be the problem. Essentially you
have GM's largest TBI on it's smallest V8. When the throttle valve
opens, that's alot of air for a 5 liter to swallow. This may not explain
the idle as much as the stumble problem. If the butterflys are not
closing completely, it might explain the idle, too. Have you removed the
cover and set the base idle with the ESC bypass disconnected?
I'm new to the list and probably don't know what I'm talking about, but
as an "old" GM tech, that's where I would start.
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