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#4484652 - 08/10/17 12:19 PM Curious about knock retard- GM 3800
TmanP Offline


Registered: 10/30/15
Posts: 439
Loc: MN
I went for a drive with an OBD scanner app this morning. I was curious to find out what my knock retard % was.
I'd say 80-90% of the time, it was 0 degrees. Under medium-light acceleration, it read about 2-3 degrees. Under moderate acceleration, it read about 4-5 degrees. I did two near-WOT runs and saw that it read in the 3-6 degrees range with a brief spike to 7-8 degrees (at about 5000 RPM). From what I've read, the 3800 tends to increase timing in some areas, necessitating knock retard sometimes. (Again, from what I've read, the naturally aspirated Series 3 3800 [L22] has the most timing advance of all). It seems to be quite normal, but I'm curious whether anyone who knows the 3800 can confirm this. The owners' manual says to use an octane of 87 or higher, and I've used primarily 87 E10. Premium (91) is 30-50 cents more expensive (30 cents is 91 with ethanol, 50 cents is 91 without ethanol or 93 with ethanol). Any thoughts?
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#4484674 - 08/10/17 12:37 PM Re: Curious about knock retard- GM 3800 [Re: TmanP]
L_Sludger Offline


Registered: 10/04/09
Posts: 3947
Loc: Ohio
Why don't you try higher octane and check the WOT retardation then? You have a test setup just begging for experimentation.
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#4484681 - 08/10/17 12:46 PM Re: Curious about knock retard- GM 3800 [Re: TmanP]
Trav Offline


Registered: 11/20/06
Posts: 19355
Loc: MA, Mittelfranken.de
It can run on 87 but it will pull timing, I use 91 or 93 in them, the fuel economy seems to improve slightly also partially offsetting the price difference.
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#4484709 - 08/10/17 01:11 PM Re: Curious about knock retard- GM 3800 [Re: TmanP]
TmanP Offline


Registered: 10/30/15
Posts: 439
Loc: MN
It is Naturally Aspirated- NOT supercharged. Does that make a difference??
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#4484795 - 08/10/17 02:11 PM Re: Curious about knock retard- GM 3800 [Re: TmanP]
DriveHard Offline


Registered: 11/24/03
Posts: 1090
Loc: Middle of Iowa
Originally Posted By: TmanP
It is Naturally Aspirated- NOT supercharged. Does that make a difference??


Not really...GM pushes the timing for maximum efficiency and power. Run a couple tanks of premium and I bet you see that retard go to zero under almost all conditions. My high compression engine truck pulls timing even with premium. The only time it won't pull timing is when I run E85.
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