Timing Help on 383 LT4
Thread Starter
Safety Car
Joined: Jul 1999
Posts: 3,946
Likes: 63
From: Marianna FL
Timing Help on 383 LT4
Trying to get the low RPM/drivability improved on my 95. I have a little stumble under 2000 rpm when tipping into the throttle. The cam is a custom grind for the supercharger-234/244 about 600 lift and ground on a 114 c/l installed straight up. The engine has 9:1 static compression ratio. Initial runs were all showing about 10" vacuum at idle and I advanced the timing in the closed TPS table by 2 degrees (to 26 at 0-1800) and it looks like I increased idle vacuum to 12". I also increased the advance in "Main Spark Adv vs MAP vs RPM" 1 to 7 degrees from 70-100 kpa and 400 to 1800 RPM. I did not get a chance to drive it today, but it still seems to have the tip in stumble.
Any thoughts?
Is it possible my tip in stumble is more or less because the car doesn't like stoich at anything under 2000 RPM? At this point, its all light throttle so I am not entering PE.
Any thoughts?
Is it possible my tip in stumble is more or less because the car doesn't like stoich at anything under 2000 RPM? At this point, its all light throttle so I am not entering PE.
Melting Slicks
Joined: Sep 2010
Posts: 2,422
Likes: 202
Do you have scan logs?
What inj, Duty cycle, LT & ST counts, 02 mV, knock count?
for my custom grind 236/ 244 .580/.580 110 CL 11.5 static
your Main Spark table 400 - 800 smooth around 20.5 Kpa
Main Spark Table 1800 RPM should be about
65 Kpa 38 Deg 70 Kpa 36 Deg 75 Kpa 34 Deg
closed TPS i have 20 Deg 400 RPM 22 Deg 800 RPM 24 Deg 1200 RPM 28 Deg 1600 RPM 32 Deg 2000 RPM.
most of your tune will be in the VE table tweak in the MAF table.
Have you matched your inj in your constants table.
Most importantly what definition are you using EE or EEb
Look at your constants table do you have crazy readings for your cylinder volume & inj flow rate. Like mine @ 47 lbs EEb vs 59.51 in EE If you do your using the wrong definition and will be nearly impossible to tune!!
Great tip zeroing out the % Change RPM @ WOT
Math calculate the % change cool temp @ WOT example 44 Deg C 15.6 % Change 56 - 104 Deg C 14.5 % Change GM's calculation is way rich for protection.
You should re calculate the Open loop AFR vs cool temp as well again very rich
A wide band is a great tune saver. Dyno even better.
What inj, Duty cycle, LT & ST counts, 02 mV, knock count?
for my custom grind 236/ 244 .580/.580 110 CL 11.5 static
your Main Spark table 400 - 800 smooth around 20.5 Kpa
Main Spark Table 1800 RPM should be about
65 Kpa 38 Deg 70 Kpa 36 Deg 75 Kpa 34 Deg
closed TPS i have 20 Deg 400 RPM 22 Deg 800 RPM 24 Deg 1200 RPM 28 Deg 1600 RPM 32 Deg 2000 RPM.
most of your tune will be in the VE table tweak in the MAF table.
Have you matched your inj in your constants table.
Most importantly what definition are you using EE or EEb
Look at your constants table do you have crazy readings for your cylinder volume & inj flow rate. Like mine @ 47 lbs EEb vs 59.51 in EE If you do your using the wrong definition and will be nearly impossible to tune!!
Great tip zeroing out the % Change RPM @ WOT
Math calculate the % change cool temp @ WOT example 44 Deg C 15.6 % Change 56 - 104 Deg C 14.5 % Change GM's calculation is way rich for protection.
You should re calculate the Open loop AFR vs cool temp as well again very rich
A wide band is a great tune saver. Dyno even better.
Last edited by THE 383 admiral; Nov 25, 2014 at 11:28 PM.
Safety Car
Joined: Sep 2001
Posts: 4,154
Likes: 553
From: Glenbeulah Wi
Trying to get the low RPM/drivability improved on my 95. I have a little stumble under 2000 rpm when tipping into the throttle. The cam is a custom grind for the supercharger-234/244 about 600 lift and ground on a 114 c/l installed straight up. The engine has 9:1 static compression ratio. Initial runs were all showing about 10" vacuum at idle and I advanced the timing in the closed TPS table by 2 degrees (to 26 at 0-1800) and it looks like I increased idle vacuum to 12". I also increased the advance in "Main Spark Adv vs MAP vs RPM" 1 to 7 degrees from 70-100 kpa and 400 to 1800 RPM. I did not get a chance to drive it today, but it still seems to have the tip in stumble.
Any thoughts?
Is it possible my tip in stumble is more or less because the car doesn't like stoich at anything under 2000 RPM? At this point, its all light throttle so I am not entering PE.
Any thoughts?
Is it possible my tip in stumble is more or less because the car doesn't like stoich at anything under 2000 RPM? At this point, its all light throttle so I am not entering PE.
I would assume the stumble is a lean tip in. You should add more accelerator enrichment and see if the stumble goes away. If you has a WB sensor you could watch it at the stumble and see if the AFR goes lean. Short runner intakes are touchy on tip in throttle and usually require some tuning in this area after modification are performed to eliminate the stumble.
