LT1 PCM Tuning


Important safety tip: Programming your own PCM can lead to anything from huge horse power and torque gains to a completely destroyed engine. I've elected to share the information below to help advance the state of LT1 PCM tuning on the 'net.

It is your responsibility to learn how to tune your PCM safely and properly. I am NOT responsible for either your programming success or programming failures if you use any of the information or datafiles from this page. IE: If you blow stuff up, it's your problem not mine.


Notes:
- PCM flash image files created by LT1 Edit.
- ALDL cables are from AKM Electronics.
- Log files created by TTS Datamaster v3.4.1 software.

Recommended Links:
Jeff Stevens' PCM Tutorial
Christopher Bennight's LT1 Edit page


May 3rd, 2002
Purpose:
Initial light off based on a Hypertech tune.

Baseline LT1 PCM file.

Best guess for my 383 with:
50# injectors, Fuel pressure = 49PSI,
6 Speed with 3.73 rear gears.

Logfile - Initial cold start light off to closed loop.

Notes:
- Idle seems lean in open loop. - Need to raise idle to 1000RPM, the cam is very choppy.
- Tach reads +100 over what the PCM reads - why?
- Closed loop seems to be a bit rich (see histogram tables)???
- Block learns at idle not too far off, LJ's throttle body mod works :)
- Stuff to Review: KS counts, IAC #'s.
- No codes! Yay!
- Soooo much to learn!

May 4th, 2002

Purpose:

Changed idle to 1000RPM for all normal temperatures, in and out of gear.

4may-02.zip LT1 PCM file.

Logfile - Initial cold start light off to closed loop.

Logfile - Warm start light off to closed loop.

Notes:
- Tranny installed today, tnx Jody!
- This 8 second tune sums up today nicely as the clutch is not releasing properly, slave or master cylinder hosed?

The rest of Engelbert Humperdink's (does he make your bumper clink? ;)
rendition is here in full sappy stereo sound. I'm too lazy to find Tom Jones' better but still sappy version for you.

- Wrong backup light switch installed on T56 by shop, cable won't mate up, need to replace.
- No driving time due to clutch problem, the car is still on jackstands, drat.

May 18th, 2002

Purpose:

First and second driving logs - Off the jackstands!.

Logfile - First road test logs.

Notes:
- Mcleod Master Cylinder installed and works, Yay!
Tnx to Jody, LJ and Matt Gork! Street Twin ETA next week.
- AS&M TB sticking at ~1400RPM+-, blipping un-sticks it :-/
- Idle too hot at 1K, need to drop to 900RPM

Misc:
- Need to check for possible vacuum leak at IM and fix
exhaust leak LH side - Small oil leak still, trannt block offplate wet
- tighten Vortech fitting @ block, maybe RMS also? (ugh!)

June 25th, 2002

Purpose:

Did a vulcan mindmeld with LJ and this is our best guess pre-dyno tune. Adds a lot of fuel up high compared to the Hypertech tune. Backed the idle down to 900. LJ verified/sanity checked the timing too.

Notes:
Next tune will be worked up on a Dynojet w/a Horiba wideband O2 sensor stuffed in the tail pipe.

25jun-02-lj LT1 PCM file.


July 20th, 2002

Purpose:

On the dyno at last.

The main focus for this day is getting the fuel right. I had an O2 bung welded into the Borla pipe just behind the cat. We used the dyno's wide band O2 sensor to dial in the air:fuel ratio into the low 12.2:1 area. We also pulled out 2 degrees of timing up high.

We saw 8ish degrees of knock retard, most of which is likely "false knock". My knock sensor bypass circuit isn't working properly, so it's back to the drawing board on that device.

If you have the Dynojet viewer software you can look at the power and A:F ratio curves.

Final peak dyno #'s were 510HP and 469 #/Ft of torque, not too bad for a fresh out of the box motor and our first dyno tuning session.

Logfile - Dyno run test #7 for 20July-dyno4.zip.

Final dyno tune LT1 PCM file.

Dynojet data for final LT1 PCM file.

Notes:
Still need to tweak the injector offset to fatten things up at idle.
Knocks Sensor Switch doesn't :-/
I forgot to open the Brola up, I had the smallest hole plate in there the whole time. That oversight cost me at least 10HP, maybe up to 15, not enough to effect the fuel curve much, but definately annoying... :-/
July 28th, 2002

Purpose:

Add more fuel only at idle

The engine is running hot at idle, 210* or so. It immediately cools down once the car gets moving. The dyno O2 sensor showed the car is idling very lean.

A couple of things can cause this behavoir:

- Lean mixture
- Retarded timing

Given the timing curve is stock at that RPM and the O2 indicates very lean, I need to fatten up the idle fuel injector pulsewidths. This is done in the Injector Offset vs. Volts table.

This table adds additional pulsewidth to the injectors to compensate for the speed the injector opens at at various voltage levels. IE: An injector opens much faster at 12 volts than at 9 volts (when the engine is starting), so a fudge factor has to be used by the PCM to ensure proper amounts of fuel at any voltage.

I asked Jeff "LJ" Stevens for his advice on the best way to go about doing this, and this was his response:

1) Warm up the O2 sensors by turning ign on but not starting engine.
   After ign is on, but before starting engine, start a DataMaster capture.
   When O2 sensor output stops dropping (could take 5 min), start engine.
   Save the D.M. log.  Look at your batt voltage during cranking and the
   O2 readings during open loop time.

2) Dink with the injector-offset-vs-voltage cells in the neighborhood of
   your cranking voltage if the engine doesn't light quickly.  If you
   still have stock values there, add roughly 1/2 ms per iteration until
   you get a nice, sharp lightoff.  If you have my curve, back off 1/2 ms
   per iteration.

3) Assuming the post-start open loop O2s are still way low, add a fraction
   of a ms to the injector-offset-vs-voltage cells in the neighborhood of
   your idling battery voltage.  I'd try adding 0.2 ms for starters.
I will post the results of performing this procedure here soon.


January 3rd, 2003

Purpose:

Log road data for sharing with the BA Brain Trust

The first .zip file contains a cold light off to closed loop with some throttle blips in the garage, then a warm start to open loop.

The second .zip contains three files. The first is cold start and driving about 20 minutes in mixed city and highway conditions. A small amount of WOT in 3rd, 5th and 6th gear.

The next two files are more driving around town in heavy traffic, no freeway fun :-(

Cold and warm light offs and idle

Cold and warm light offs and driving

Reminder: the PCM code is unchanged from the Dyno Day tune above, I have made no changes to it since then.


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