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Thread: Hi, new guy here doing a TBI conversion on a Ford truck engine.

  1. #1
    Carb and Points!
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    Hi, new guy here doing a TBI conversion on a Ford truck engine.

    Hi all, I have an old Ford school bus camper conversion that I'm swapping over to efi. It's a 330ci v8 with a worn out Holley 2bbl and points distributor. I have dabbled with some efi stuff before and got the (crazy?) idea to swap this over to a GM tbi system. The ford 330 truck engine shares nothing in common with the factory ford EFI engines so this seemed like the best bet for something simple and reliable. Scavenging all the components from the junkyard was pretty cheap. I think the hardest part will be installing the GM distributor guts into my ford distributor housing but really even that looks pretty straightforward.

    I grabbed everything out of a 94 with a 7427 BJYL. One thing I'm having a hard time with is how do the values in the main spark table interact with the main spark bias? The note in the xdf says how the table already compensates for this but I'm not quite understanding what that means in a practical sense. I see "Initial Spark Advance" is set to 0 in the calibration which matches what the initial timing at the distributor was set to at the factory in 1994. "Main Spark Bias" is set to 9.8. The main spark table shows 22.1* at 2000rpm at max vacuum. Now if we assume we were at a steady 2000rpm cruise, so no PE advance, engine at normal operating temperature and EGR & knock systems disabled and zeroed out as far as I can see the only spark table at play should be the main spark table. If I threw my timing light on it at that point , would I see 22.1* or would I see 22.1* + 9.8* = 31.9*? I want to make sure I'm reading this the right way, the last engine I built the old fashioned way ran 16* initial timing + 22* mechanical advance all in @ 2500rpm + 18* vacuum advance = 56* total at cruising speed. Comparatively speaking what I'm seeing in the bin seems super low so I keep questioning myself.

  2. #2
    Super Moderator dave w's Avatar
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    Gearhead-efi link to 16197427 information: http://www.gearhead-efi.com/Fuel-Inj...Information-0D

    I recently helped out with a 1971 International 1210 Pickup TBI conversion using the 16197427 computer and an EFI distributor from Performance Distributors. Maybe Performance Distributors can help with an EFI distributor for the 330 Ford? https://performancedistributors.com/...1-289-302-cid/

    The EFI distributor uses the GM 7 pin ignition module, without vacuum advance.

    dave w

  3. #3
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    Hello and welcome to the forum.

    There are a couple of easy answers to provide, and possibly some discussion points as well. I'm going to change the order of some of the comment/questions.

    I see "Initial Spark Advance" is set to 0 in the calibration which matches what the initial timing at the distributor was set to at the factory in 1994.
    This is a good place to start. Spark advance in the GM computer is essentially spark retard from the previous cylinder. Here's a rough summary of how it works. The distributor sends a reference pulse to the computer. The computer begins counting time. When a specific amount of time has elapsed so the crank should be at the desired amount of degrees before TDC the computer sends a signal back to the distributor. In order to deliver accurate timing the computer must know several different values including the number of degrees between distributor pulses and the number of degrees the distributor is advanced BTDC. In the ecm the math for spark advance includes a formula similar to this: Delivered timing = desired timing - distributor angle. So if the distributor is 6 degrees advance then the computer has to add only 4 more degrees to get the desired timing.

    One thing I'm having a hard time with is how do the values in the main spark table interact with the main spark bias?
    A number of desired advance values are obtained through lookup then combined. Main spark bias is subtracted from the desired advance. In your example if the desired advance in the table is 22.1 degrees, the main spark bias of 9.8 deg would be subtracted from 22.1 to equal 11.8 degrees advance. The main spark bias value is a little bit of a trick designed by the programmers. The spark tables do not accept negative numbers. If spark timing is desired to be less than zero degrees there is no way to enter the desired timing values directly into the table. But if the main spark bias is factored into the table values, you are able to build a spark map that delivers negative spark advance. Entering zero degrees into a main table cell with a main spark bias of 9.8 degrees produces desired spark advance of 0 - 9.8 = -9.8 degrees.

    If I threw my timing light on it at that point , would I see 22.1* or would I see 22.1* + 9.8* = 31.9*?
    Well... this is where it gets a little tricky. Many of the Tunerpro xdf files are made so the value you see in the main spark table represents the answer to the equation "main spark value - main spark bias = ?" IOW you don't have to do the math in your head. So if you are looking at 22.1 in the main spark table and no other spark adders are active you should expect to see 22.1 at the crank.

    However, it is common for delivered timing to be a few degrees different than what's showing on the scantool. There is a massively long thread here which dives into this although at this time I am unable to locate it.

  4. #4
    Carb and Points!
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    Dave, thanks for the tip. If I have issues with my distributor conversion I'll keep those guys in mind!

    1project2many, thanks for the explanation. I guess what's still confusing me is if 22.1 is what I would see on the balancer with a timing light in that scenario why is it that when I change Main Spark Bias from 9.8 to 0 I see the main spark map has automatically increased by 9.8 across the board?

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    It might help to think of this as a math problem. The computer subtracts the main spark bias from the table numbers. When you change the main bias it affects the entire main table.

    This is highly simplified, but for this discussion it will work.

    Table number - main bias = crank

    22.1 - 9.8 = 12.3
    22.1 - 0 = 22.1

    22.1 is 9.8 more advanced than 12.3

    If you want to set the main spark bias to zero you will need to add the same amount as MSB to the main table. I believe Tunerpro allows you to select an entire table then add a specific amount to it.

  6. #6
    Carb and Points!
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    Thanks that does help. It seems a bit counterintuitive that sometimes we are working with the raw data and other times there's some data manipulation going on in a table that looks like it's strictly input values. I understand why the author did it that way but it's not something I've seen in other tuning software I've played with.

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    Yes, there are many eccentricities in the code that can trip us up. I believe this was done to save time. By performing more of the spark math with 8 bit instructions the processor gets through the program in less time. There are other cool tricks as well. The conversion of time between reference pulses is to RPM using the number of cylinders constant makes it possible to use the same code on 8, 6, 5, 4, 3, and 2 cylinder engines.

    What wasn't answered is why there is less timing in these engines than in your older engine. As a trend the auto makers have tried to reduce spark advance. Early advance tends to produce higher NOx numbers. NOx is harder to clean up than HC. TBI engines often do run better with a bit more advance but I don't know of anyone who as attempted to get the emissions results from adding advance to one. Also note that TBI may do a better job of metering fuel so the mixture in the chamber is more consistent than with a carbureted engine. I've converted engines from carb to TBI and sometimes I have to reduce spark advance during the tuning process.

    Ultimately your engine will want what it wants. If you read the data correctly you will make it happy.

  8. #8
    Carb and Points!
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    Before I rip the ford engine distributor apart I'm going to run it to map out the advance curve. I have a hand vacuum pump so mapping out the mechanical and vacuum advance separately should be easy. I'm curious how it'll run with the GM timing curve. I suspect much better at idle and low rpm where the mechanical distributors give up so much timing so the thing will crank without fighting the starter. I'll play with the upper part of the curve later.

    This engine has a governor on it limiting the max rpm to 4000. It sits under the starter limiting air flow through the venturis. I wonder what the GM ecm would think of a situation where there's high vacuum and tps showing throttle opening high! I was planning on scrapping the governor and using the ecm to duplicate the function but it's a fun thought.

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