27 DEC 2005

I've decided to try and use all of the factory components as much as possible. (MAT, CLT and MAP as well as the triggers).

Dealing with modifying the firmware is not something I want to tackle right now.


The first tooth (tooth-center) after the missing tooth will be 65 BTDC of cylinder 1/4 @ 120 degrees (Factory was 75 BTDC). Will use config.trigger_tooth=00, so the trigger will also be at 65 BTDC.


The firmware is same for 18-1 as for 36-1, but tooth_wheel=11 (decimal 17) is used (instead of tooth_wheel=23 # that is decimal 35)


21 DEC 2005

Thanks for the previous answers!

I have been researching the sensors for triggering on the L67.

It uses 3 Hall effect sensors, 2 for crank (18x and 3x) and 1 for cam.

The timing diagram for it looks like this:


It does not have any missing teeth for the crank but provides an 18x pulse count per revolution. The 3x crank signal is used for what GM calls 'fast-start'. It allows the ignition to sequence in no more than 120 degrees of cranking. The 3x rising edge is always 75 degrees before TDC.

Is there a way to make this work with the current v3.3?

The hall sensors are powered from a 10VDC source according to the GM service schematics. The output from the halls are 0-7 VDC which should be OK with the a zener input clamp.


I'd like to go with sequential injection, DIS/wasted spark without any OEM electronics.

Some additonal information is available at:

the type is C3I Type 1--Fast Start

Thanks again!


12 DEC 2005


A long post for a first, but I've been lurking for a few months...

A little about me:

I have been tinkering with cars since I was 16, rebuilding my first engine when I was 17 in a '65 Olds 442.

In the last twenty some years or so I have experimented on a hobby basis with electronic design, programming and various processors.

My oldest hacks include a memory expansion card for the VIc-20 and a SCSI HDD interface for the Amiga 500. (Sometimes it's hard to believe that I paid $300 (in 1988 dollars) for a 20 MEGAbyte SCSI disk drive)

I currently work in the technology industry as a C/C++ programmer.

My project:

I am preparing to install a GM L67 (3800 Superharged from a GTP) engine in my playtoy, an '88 Formula Fiero 5 speed.

After considering ODBI and OBDII options, I decided on GENBOARD 3.3, a VERY impressive piece of hardware!

I have a few questions before I plunk down some cash on one...

I noticed on the Wiki that the VEMS-GROUP is moving towards ver 4. Does this obsolete 3.3, or is that an entirely different product line?

[MembersPage/PhatBob] Version4 will be a new beast entirely. v3.3 is a working system with many units in daily drivers, I can't see it becoming obsolete, just superceeded by newer models.

What is the future for support of 3.X?

[MembersPage/PhatBob] Pretty much the same as it is now, at least for the next few years, its an 'open' support system so the knowledge base will remain current.

What hardware features of 3.3 are currently supported in the software?

Knock? Both banks?

EGT? Both banks?

[MembersPage/PhatBob] The system supports 2xEGT

WBO2? Both banks?

NB02? Both banks?

[MembersPage/PhatBob] No

SPI MMC data logging?

Mass Airflow (MAF) Sensor?

[MembersPage/PhatBob] We use speed/density which utilises a MAP rather than a MAF

If I choose to use NBO2 (factory sensors) is the code for this functional?

[MembersPage/PhatBob]You can use NBO2 sensors, but I'd advise against it, the price of the wideband sensors is such that struggling by without

It appears that A/D channels are available for EBP and FP, are they used with the current firmware?

[MembersPage/PhatBob] These will need to be soldered onto the board.

Is there any code to support emission controls (EGR, EVAP release, etc)?

[MembersPage/PhatBob]Are those solanoid controlled? If so we have 2xadditional configurable outputs that can be used to drive solanoids.

Is there a timeline to support unimplemented features?

[MembersPage/PhatBob] Only the one you set when you decide to implement them.

Is the VEMS version of MegaTune a fork of the orginal, or are the source trees common? It appears that firmware and MegaTune are released as a package?

[MembersPage/PhatBob] The difference is the vems3.ini file. The packaged software is there to make it easier for people to get their systems running without having to check for versions in several different places.

Is there a live repository of source code for the firmware and MegaTune? (CVS, SVN) The sourceforge one is of date with the code in the Wiki. (028)

[MembersPage/PhatBob] Check the SVN page: SubVersionSvn

I plan on doing some firmware hacking once I get the board...

[MembersPage/PhatBob] There's many that do, my advice would be to get it working on a car first as thats a pretty steep learning curve. It will give you a good feel of what works

What is the current resource allocation and utilization? Percentage FLASH Memory, RAM, ports, etc. (How much overhead for new functions is available? Assuming current release with default make)

Is there a 'recommended' matrix for un/seldom used pins in order to preserve compatibilty with existing fuctions?

I'd like to add:

1. MCP2515 CAN controller to eventually connect a (yet to be designed) intelligent LCD display of some kind. (Digital dashboard). I'll need an interrupt to interface. Is there one available on the '128 that is available and won't break compatibilty with existing firmware? It appears that a CS line is already in place (CS_CAN)

2. Removable media MMC datalogging via SPI. (Again a CS appears to be available (CS_FLASH). (Very simple FAT32, Single file, root only) || (RAW format, read out through 3.3 RS232?)

3. Shift light, and generic 'idiot lights' (TOO HOT, TOO LEAN, etc) , for use until I design a display. (Extend LBUS w/ another '259 if not enough lines? CS? What is the function of SS pin 10 on header?)

[MembersPage/PhatBob] That can be done with the current 259 outputs in some small measure. If you use the LCD it would be a simple case of turning on a warning light and writing a bit of code to display the error on the LCD.

4. Oil level input (Sensor factory installed on L67, it's there why not use it) (A/D channel allocations?)

[MembersPage/PhatBob] You may find that one of the spare A/D channels is ready to be used for this.

5. Possibly unimplemented features inquired about above.

Maybe with the addition of the VSS (Vehicle Speed Sensor) input it should be possible to calculate MPG? Add a Fuel level input and have instantaneous MPG, Distance to empty... (I think I saw an article on this recently, can't seem to find it right now, Circuit Cellar?)

Finally, What is the process for contributing code to the group?

[MembersPage/PhatBob] Check the SVN page.

Answers appreciated!