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Thread: L98 Vette PCM options

  1. #1
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    L98 Vette PCM options

    To start I am working on a 1990 Corvette with the L98. Later this year I am going to swap the top end, heads intake and a matching cam, for LT1 parts. Now I know that they are not a direct bolt on but I got the heads stupid cheap and I figure that it is easier to have a pro shop weld 4 block offs in than trying to re-engineer the intake to fit an old bolt pattern and make it a wet intake. While I am at it I have a few must haves for tuning and a few wishes.

    Must:
    accept a wideband
    communicate with the ccm
    match or exceed the current tuning precision and adaptability

    wish:
    loose the distributor for CPP or DIS
    sequential fuel injection
    flash memory

    I can think of 3 options but I am up to any suggestions. First is to stick with the current ecm and accept I have to bore a precision hole for a distributor and not get the SFI to ease low speed tuning and efficiency. Second is Megasquirt, I am very familiar with the system but can not for the life of me figure out how to take a CAN signal and build a module to transmit the data over the ALDL line. The last and I figure closest to my desires is the LT1 PCM as it was used from 1992-1996 in the same cars. The issue here is to fool the pcm into thinking it has an opti attached.

    At this point I am still a few months out and am up for suggestions. Is is possible to reprogram the T1 PCM to expect something other than the opti pulse train? Thanks in advance.

  2. #2
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    You mentioned mega-squirt, look into speeduino. It’s an arduino based ECM and is much cheaper than mega-squirt.

    My personal opinion, based off your intentions to use lt1 heads and intake why not just spend $500 and pick up an entire lt1 engine or block. Yes it will add cost but it will reduce cost and complexity by not having to do block-offs. Could also sell the l98 to recoup some of the costs.

    This would give you the sequential you seek plus flash memory, and accept a wideband. also if you get a 94-95 you will be able to use the FREE tuning software combo of EEHack & Tunerpro.

    If the opti is your fear they are much more realiable then the internet makes them out to be if they are sealed and vented properly, there are also a couple options to eliminate the opti and run gen 3&4 style coil-on-plug style setups.

    Unsure if it will place nice with your current ccm but I’m not sure if I’d let that stop me from doing a complete lt1 swap either tho.

  3. #3
    Super Moderator dave w's Avatar
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    OBD2 computers (like the '0411) require both a cam and crank signal input. The SBC OBD2 cam signal input is a 2:1 ratio to the crank input. Generally, OBD2 computers have better aftermarket support than OBD1 computers.

    EFI Connection offers several options to convert opti spark to the OBD2 '0411 computer. https://www.eficonnection.com/home/c...efi-24x-lt1lt4 With some reverse engineering, an A La Carte option of leveraging EFI Connection components to the L98 could be a solution? Obviously reverse engineering is not a "Dirt Cheap" option. Obviously a hole in the intake = "Dirt Cheap Option"

    dave w

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    Quote Originally Posted by johnny_b View Post
    You mentioned mega-squirt, look into speeduino. It’s an arduino based ECM and is much cheaper than mega-squirt.

    My personal opinion, based off your intentions to use lt1 heads and intake why not just spend $500 and pick up an entire lt1 engine or block. Yes it will add cost but it will reduce cost and complexity by not having to do block-offs. Could also sell the l98 to recoup some of the costs.

    This would give you the sequential you seek plus flash memory, and accept a wideband. also if you get a 94-95 you will be able to use the FREE tuning software combo of EEHack & Tunerpro.

    If the opti is your fear they are much more realiable then the internet makes them out to be if they are sealed and vented properly, there are also a couple options to eliminate the opti and run gen 3&4 style coil-on-plug style setups.

    Unsure if it will place nice with your current ccm but I’m not sure if I’d let that stop me from doing a complete lt1 swap either tho.
    Thanks for the suggestion but finding and rebuilding the bottom of an LT1 would require pulling the engine and at the moment thats is not on the table because I don't have a garage or the hoist to do it with. I am not afraid of the Opti as I have an LT1 firebird that I have rebuilt and daily at his time. I did find some more info last night about the difference of the 2 data streams and I'm not sure Its feasible with my limited knowledge and tools to reprogram the LT1 pcm data stream to match one for an L98. My mention of the MS unit is because I have a MS3 with 3x unit just siting here. Used it on my LT1 firebird last and the opti timing worked but was picky.

    Quote Originally Posted by dave w View Post
    OBD2 computers (like the '0411) require both a cam and crank signal input. The SBC OBD2 cam signal input is a 2:1 ratio to the crank input. Generally, OBD2 computers have better aftermarket support than OBD1 computers.

    EFI Connection offers several options to convert opti spark to the OBD2 '0411 computer. https://www.eficonnection.com/home/c...efi-24x-lt1lt4 With some reverse engineering, an A La Carte option of leveraging EFI Connection components to the L98 could be a solution? Obviously reverse engineering is not a "Dirt Cheap" option. Obviously a hole in the intake = "Dirt Cheap Option"

    dave w
    I will look into EFI connection again to see if there are any items I can find to make this easier.

    Thanks for the help.

  5. #5
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    Dave W is right about the EFI connection option.

    I will add however that converting an LT1 to obd2 can cost as much or more then buying an entire used LT1 engine.

    but if you don’t have the tools and space to do an engine swap I guess that’s out.

    you could always piggyback the l98 ECM in addition to the LT1 pcm with the necessary inputs the l98 ECM needs to for the ccm functions you want and just not have it hooked to injectors etc
    Last edited by johnny_b; 01-05-2019 at 06:46 PM.

  6. #6
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    for PCM i would go Megasuirt or stock with Moates.net Ostrich 2.0.

    As for LT1 heads, i alread did that mod on 305 TPI, lots of work but it does the job.

    best regards

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    WAS hoh did you route the coolant? I am thinking of using the end ports to feed the remote thermostat.

    The idea of Megasquirt and a piggyback L98 ecm did cross my mind, may go that route. Is there any listing of all the features they have unlocked with the S_UAJP V5 build. Not against the old prom method just kinda like flash memory better.

  8. #8
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    well, i used stock LT1 heads exits in the back, tapped fittings and routed hoses to remote thermostat housing.
    as for ecm i was running MS2 with TPI adaptor board

    best regards

  9. #9
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    WASyl the more I dig into the $8D ecu the more I want to go back to MS and call it done. The one hinge point is I cant figure out if I will be seeing a system error on the dash of my corvette as the dash computer wont have a data stream to work with. The info provided is just for fuel economy and range so I can do without, just dont want to keep staring at SYS flashing.

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    never used MS on Vette but you can always use one spare port and program it to send valid signal

    best rgrads

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    First is to stick with the current ecm and accept I have to bore a precision hole for a distributor and not get the SFI to ease low speed tuning and efficiency.
    Why do you need to bore a precision hole for a distributor? Did you ask the ecm? Because as far as I'm aware it only knows pules in, wait, pulse out. If you could get an aftermarket module to replace the distributor or even a Northstar ignition the ecm would operate it just fine. (Side note: There's much less "precision" to the distributor hole than you may guess. Years ago there were a couple of folks what were converting LT1 manifolds to use with standard heads and the pictures of their work were, umm, amazing to say the least.)

    SFI to ease low speed tuning
    Why does SFI "ease low speed tuning?" This is completely baffling?? You have a device delivering fuel to an intake port. Whether the fuel is delivered in 1, 4, or 8 pulses it's still delivering fuel. And you still have the task of matching up the fuel delivery to the airflow using instrumentation and SOP gauges. If you think about it, carburetors are continuous flow devices so that should make them nearly impossible to work with.

    and efficiency
    Actually, this is done for emissions purposes. And there are some tradeoffs. You loose heating and vaporization in the intake port when you move away from batch fire. So typically fuel is injected at higher pressure through smaller holes in order to create smaller fuel droplets. This allows the heat generated during compression and ignition to substitute for time spent in the intake port. There is some advantage for OEM's trying to meet CAFE and emissions standards but i think it's safe to say your car will not suffer by using batch fire strategy.

    Sorry if I sound like I'm beating you up. Seems like folks can get stuck thinking along specific lines. The only ecm's I'm aware of that will fulfill all your requirements and wishes are 01228331, 16168001, 16163993, and 16196394. I believe they are made of unobtanium.

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    Quote Originally Posted by 1project2many View Post
    Why do you need to bore a precision hole for a distributor? Did you ask the ecm? Because as far as I'm aware it only knows pules in, wait, pulse out. If you could get an aftermarket module to replace the distributor or even a Northstar ignition the ecm would operate it just fine. (Side note: There's much less "precision" to the distributor hole than you may guess. Years ago there were a couple of folks what were converting LT1 manifolds to use with standard heads and the pictures of their work were, umm, amazing to say the least.)
    It was my understanding that without the 5 degree tilt and exact hole placement it will cause an oil leak on the manifold, strain the gear set and eventually fail the distributor and oil pump drive. This is not a top concern to avoid as I see there is very few ways around using the rear distributor as at minimum a sensor.

    Quote Originally Posted by 1project2many View Post
    Why does SFI "ease low speed tuning?" This is completely baffling?? You have a device delivering fuel to an intake port. Whether the fuel is delivered in 1, 4, or 8 pulses it's still delivering fuel. And you still have the task of matching up the fuel delivery to the airflow using instrumentation and SOP gauges. If you think about it, carburetors are continuous flow devices so that should make them nearly impossible to work with.
    Personal experience working with megasquirt and various GM engines has left me with the experience that at low RPM and low loads it is easier to achieve your desired AFR with sequential flow. Last engine I used both batch and sequential was the L32 V6 and it was night and day. The difference is if you are putting cold wet fuel on a closed valve or open valve, and how you will handle the inherent pooling. In short SFI at lower speeds like idle to cruise rpm the fuel has less tome to form into droplets and fall out of atomization.

    Quote Originally Posted by 1project2many View Post
    Actually, this is done for emissions purposes. And there are some tradeoffs. You loose heating and vaporization in the intake port when you move away from batch fire. So typically fuel is injected at higher pressure through smaller holes in order to create smaller fuel droplets. This allows the heat generated during compression and ignition to substitute for time spent in the intake port. There is some advantage for OEM's trying to meet CAFE and emissions standards but i think it's safe to say your car will not suffer by using batch fire strategy.
    While this is true the efficiency and benefit of SFI is during low speed and low load times allowing more precision to shoot at an open valve. Past roughly 2k-2500 and more than about 25% duty cycle it may as well be batch fire. This is why it was a want not a must have.

    Quote Originally Posted by 1project2many View Post
    Sorry if I sound like I'm beating you up. Seems like folks can get stuck thinking along specific lines. The only ecm's I'm aware of that will fulfill all your requirements and wishes are 01228331, 16168001, 16163993, and 16196394. I believe they are made of unobtanium.
    I always appreciate input that is constructive. I will continue to look up my options and surely do not feel SFI is a 100% must have, I like it but can tune around it.

  13. #13
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    Sounds like a cool project. Have you given any thought to using Holley EFI?

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    Quote Originally Posted by 1project2many View Post
    Why do you need to bore a precision hole for a distributor? Did you ask the ecm? Because as far as I'm aware it only knows pules in, wait, pulse out. If you could get an aftermarket module to replace the distributor or even a Northstar ignition the ecm would operate it just fine. (Side note: There's much less "precision" to the distributor hole than you may guess. Years ago there were a couple of folks what were converting LT1 manifolds to use with standard heads and the pictures of their work were, umm, amazing to say the least.)


    Why does SFI "ease low speed tuning?" This is completely baffling?? You have a device delivering fuel to an intake port. Whether the fuel is delivered in 1, 4, or 8 pulses it's still delivering fuel. And you still have the task of matching up the fuel delivery to the airflow using instrumentation and SOP gauges. If you think about it, carburetors are continuous flow devices so that should make them nearly impossible to work with.


    Actually, this is done for emissions purposes. And there are some tradeoffs. You loose heating and vaporization in the intake port when you move away from batch fire. So typically fuel is injected at higher pressure through smaller holes in order to create smaller fuel droplets. This allows the heat generated during compression and ignition to substitute for time spent in the intake port. There is some advantage for OEM's trying to meet CAFE and emissions standards but i think it's safe to say your car will not suffer by using batch fire strategy.

    Sorry if I sound like I'm beating you up. Seems like folks can get stuck thinking along specific lines. The only ecm's I'm aware of that will fulfill all your requirements and wishes are 01228331, 16168001, 16163993, and 16196394. I believe they are made of unobtanium.
    Actually at lower rpm SFI setups spray fuel on the back of a closed intake valve giving fuel time to heat on the back of the valve and in the port. Which leads to better atomization and better burn all around.

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    Quote Originally Posted by Queencityspeed View Post
    Sounds like a cool project. Have you given any thought to using Holley EFI?
    no I have not as I feel it would have the same stumbling block of Megasquirt with the ALDL communication. The only reason I seem stuck on Megasquirt is I have a MS3+3x setup that I pulled from another project.

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