Engine Knock - Too Lean or Too Much Timing?
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Pro
Joined: Jun 2002
Posts: 544
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From: La Jolla California
Engine Knock - Too Lean or Too Much Timing?
I've been trying to adjust the timing to get rid of all my engine knock. What I am doing to test this is I get going about 30 in 3rd gear (TH-350) and then just floor it. If I'm too advanced I hear it knock and I just retard the timing until the knock stops. Currently I have 29* of mechanical advance (vacuum disconnected) all in by 3200 RPM (stiffest springs) and no knock. If I advance the total or go w/ a quicker advance I get knock.
What I'm wondering is if the carb (Holley 750 DP) is just going too lean when the secondaries just dump open causing the engine to knock. It seems when I ease into the pedal, or downshift and then floor it, I don't hear this knock even with the timing advanced more.
My engine specs are as follows:
383 LT1 (modern) running 11.7:1 compression
Holley 750 DP (stock jets)
GM Carbed Dual Plane Intake
Comp Cams CC306 w/ 1.6 RR (duration @50 230i/244e w/ lift .544i/.576e)
Aluminum Heads "Stage II" ported (flows 265i/190e @.50 w/ 2.02/1.6 valves Hooker LT Headers
Dual Flowmaster Exhaust
3.55 rear end
2500 stall
TH-350 trans
160 deg thermostat
MSD 6AL
Meziere electric water pump
It seems I can run around 34* timing w/ a quicker advance if I'm not flooring it while cruising in 3rd gear. Is it possible that I'm getting knock still but just can't hear it over the engine?
Does this sound like too much timing or too lean a fuel mixture?
Thanks!
What I'm wondering is if the carb (Holley 750 DP) is just going too lean when the secondaries just dump open causing the engine to knock. It seems when I ease into the pedal, or downshift and then floor it, I don't hear this knock even with the timing advanced more.
My engine specs are as follows:
383 LT1 (modern) running 11.7:1 compression
Holley 750 DP (stock jets)
GM Carbed Dual Plane Intake
Comp Cams CC306 w/ 1.6 RR (duration @50 230i/244e w/ lift .544i/.576e)
Aluminum Heads "Stage II" ported (flows 265i/190e @.50 w/ 2.02/1.6 valves Hooker LT Headers
Dual Flowmaster Exhaust
3.55 rear end
2500 stall
TH-350 trans
160 deg thermostat
MSD 6AL
Meziere electric water pump
It seems I can run around 34* timing w/ a quicker advance if I'm not flooring it while cruising in 3rd gear. Is it possible that I'm getting knock still but just can't hear it over the engine?
Does this sound like too much timing or too lean a fuel mixture?
Thanks!
Racer
Joined: Sep 2001
Posts: 354
Likes: 0
From: Prineville Oregon
Re: Engine Knock - Too Lean or Too Much Timing? (Langadorf)
You never stated what fuel you are running. With pump gas and around 92 Octane your maximum compression even with aluminum heads shouldn't be more than about 10.5:1 so you'll probably need to add some octane booster, mix Av-gas, or mix racing fuel.
Your 750 DP should not be running lean, they are primarily a racing carb and are jetted rich if anything. Holley doesn't recommend them to be run on the street although a lot of people do. With an automatic, your low stall speed, and 3:55 gearing you'd probably accelerate quicker with a 750 vacuum operated secondary carb anyway. Sell the DP to someone with a stick or change your gearing to 3:90's-4:11's and change your converter to at least a 3500 stall to take advantage of the DP. With 230 in. - 244 ex. duration @ .050 your not running enough stall anyway...your power band will just be STARTING around 3000-3200 RPM
You can also try running a few heat ranges colder on your plugs to lessen the chance of detonation. Deen
Your 750 DP should not be running lean, they are primarily a racing carb and are jetted rich if anything. Holley doesn't recommend them to be run on the street although a lot of people do. With an automatic, your low stall speed, and 3:55 gearing you'd probably accelerate quicker with a 750 vacuum operated secondary carb anyway. Sell the DP to someone with a stick or change your gearing to 3:90's-4:11's and change your converter to at least a 3500 stall to take advantage of the DP. With 230 in. - 244 ex. duration @ .050 your not running enough stall anyway...your power band will just be STARTING around 3000-3200 RPM
You can also try running a few heat ranges colder on your plugs to lessen the chance of detonation. Deen
