[Gmecm] Adjusting 7730/$8D for larger cam

Fri Feb 2 03:44:23 UTC 2007

Mike,
That's what's going on.  I did try playing with the MAP signal by hooking a
vacuum tester up to the sensor, but it was hard to see if it made a
difference, as the idle speed fluctuated some, and of course the MAP didn't
change in response.

I haven't done any datalogging since I started playing with this particular
bin - it's one I burned to correct the cylinder volume and injector size
constants that were off in my original tune.  It's better now, but still
rich.  Looks like I need to take a look at the BLM's soon.  I just need to
do that on a day with a good north wind so I don't fill the house with fumes
again...

John,
the cam is a Crane HMV272, 216* intake, 228* exhaust, on a 112* LCA.  I
advanced the cam 2* as I said, so I think I *COULD* get a decent vacuum
signal from this cam.  No heated O2 at this point, just the plain old 1-wire
GM unit.  I will be using my LM-1 at some point, but just trying to get
sorted enough to hit the road for a bit now.  Idle speed is 800-850,
depending on engine temp, and it does a good job of holding that, now that
my IAC is working like it should.  58kPa at idle would be awesome, as I
REALLY don't want to recam this critter if I can tune the problem away.

Thanks for the input, guys!

Clair

----- Original Message ----- 
From: "Mike V" <efi at dyakron.com>
To: <gmecm at diy-efi.org>
Sent: Thursday, February 01, 2007 9:32 PM
Subject: Re: [Gmecm] Adjusting 7730/$8D for larger cam

> I suck at explanations, but give me credit for effort.
> Sounds like you have a good idea of what's happening.
> The ECM is seeing less vacuum at idle speed than expected.
> With a MAP system, that is usually interperated as a load
> on the engine, and fuel is added.  The pig-rich problem
> you describe is fairly common after a larger cam is installed.
> If the fuel table (with its RPM vs MAP values) is
> suggesting a given amount of fuel, and the O2 sensor is
> seeing the over-rich condition, the block learn value
> will be lower than 128.  Mine was below 100 after a
> cam change in my Syclone V6..  As you take fuel
> out by reducing the values at the lower end of the
> table, you will see the block learn numbers go back up.
> If the value goes above 128 by very much, you went
> too far on the lean-quest and need to add
> some fuel back in.  Some folks have band-aided this
> cam-swap situation by using a micro switch to determine
> when the throttle is close, and then jury-rig the MAP value
> with a pot or resistor, thus lieing to the ECM about the
>  idle vacuum.   I prefer the insanity that comes with
> trying to tweak the tables.  I hope I didn't confuse the issue.
> BTW, I have not worked with the $8D code, so others
> may have better suggestions.
> MV
>
> > Now that my engine is running pretty reasonably, it's apparent that it's
> > PIG
> > RICH at idle. I'm about 99% sure this is due to the lower-than-I-hoped
> > idle
> > vacuum reading. When running with a carb, this engine managed about
13-14"
> > of vacuum (44-47kPa if I'm doing the conversion right), now it's 10-12"
> > (34-40kPa). The tune I received from my chip supplier supposedly wanted
> > 15"
> > at idle (51kPa). Since I'm not reaching this level, even after advancing
> > the
> > cam 2*, I think I need to change my idle vacuum level.
> >
> > Going through TPRT, it doesn't appear that this is any kind of one-point
> > setting, and is probably in the tables. Is it reasonable to adjust the
> > table
> > points on the idle end down to where I need them? It might be that I get
> > better vacuum if the mixture is leaner, but I'm thinking that the rich
mix
> > is due to the apparent load on the engine, not a quirk in the VE table
> > (yet).
> >
> > Any wisdom would be greatly appreciated...
> >
> > Clair
> > _________________
> > 67 Cuda Vert (?/833)
> > 68 Sport Fury Vert (NOM 440/727)
> > 69 Valiant 100 (340/833/7730EFI)
> > 74 Dart Swinger (440/727)
> >
> >
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