Re: GMECM Digest V2 #78

[Jody Shapiro <jshapiro@netgate.net>]

> From: ECMnut@aol.com

> octane fuel reduce the severity?  As he also mentioned pulling 
> a bunch of timing out of it as you near 100KPA is a good idea.  
> If it reduces the knock spike, you'd better not "deafen" your sensor.
> BTW, its not unusual for a fastburn style chamber to require less
> than 20 deg of timing under boost.

Dunno how this applies to LT1's, but on my stock LT1 w/ 7psi of boost from
the stock-pullied Vortech, I typically ran 1-1.5degree/psi of knock
retard via Crane HI6-TR ignition box.  So at 7psi boost, I had ~26psi of
actual spark advance.

> From: dhardin@ovec.com
> 
> 20 degrees of timing sounds high.  My old 68 GTO (10.75:1 CR with 87 octane,
> stock engine) pulls the timing down to about 5 or 6 degrees under load with a
> max of about 20 total at light load and still pings.  I'd go by the ping don't
> ignore it.

10.75 compression with a stock cam and 87 octane gas.  That just sounds
like a recipe for detonation - no wonder you had to run so little timing
advance. :)  Do your exhaust manifolds glow red much? ;)

> From: Roger Heflin  <rah@horizon.hit.net>

> shifts, both things that physically rattle things around.  The is what
> most false knock is, something moving around the engine that makes
> noise, I have heard that timing gear sets will make the car computer
> think it alwasy has knock.

My 300mi-old 383 LT1 has the Lt4 extreme-duty timing set in it and when
running around with no blower belt on I get no knock retard activity at
all.  I was worried about that timing set since others have apparently had
trouble with it sometimes.  So far so good...

> From: "Jim Wilson" <Wilson.James4@worldnet.att.net>

>    As far as the bottom end is concerned its a  complete Lunati Stroker w/
> 6" rods so as far as rod to stroke ratio it is the same as stock.

Close:  5.7/3.48 = 1.64  6.0/3.75 = 1.60

>      In regards to this knock retard problem I have tried a 6# pulley and
> there was no change in the K R. What I get on the Tech 1 is 1 frame of Knock
> Signal at the 7 degree spike then the retard will go down but never approch

What happens when you pull the blower belt?  If you're truly picking up
false knock from something else mechanical in the engine (headers hitting
the frame, noisy roller rockers, etc) the false knock will still be
present.

If the knock retard activity is only there under boost, I'd do just what
you're already doing.  Highest unleaded gas you can get (Sunoco has a 104
octane unleaded) and lots of timing retard.  I'd be really surprised if
you're still having true detonation problems with lots of timing retard,
high octane fuel, and the 9.4:1 compression on an LT1 at only 6psi of
boost.

There are guys running 9.7-9.9:1 compression LT1's with 14psi of boost on
100 octane fuel and little-to-no timing retard with no knock retard
activity.  Do you have a really small cam in the motor that's possibly
building up a ton of cylinder pressure?

-Jody

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