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Thread: Setting base timing for performance cam

  1. #1
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    Setting base timing for performance cam

    I am still gathering what I need for parts and information to swap a TBI into my carbed engine. I have an '83 305 with a mild Summit K-1102 cam (.420/.442, 262/272, 112*), stiffer springs, Edelbrock non-egr performer intake. Using an HEI distributor I have found that I need 20* base timing for best performance (no detination). At the factory 8* the engine will barely run. Summit recommends at least 4* more timing for this cam.

    I know the computer controller dizzy is supposed to be set at 0*, but I wonder if that will work ok with my engine. Is it possible or even advisable to add more base timing to make up for non-stock cams?

    Last question (for now :): Will 18inHg vacuum at idle work with the stock computer or am I going to have to get a custom chip or self learning computer?

  2. #2
    Super Moderator dave w's Avatar
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    I'm located south of you in Vancouver WA. I usually start with the factory TBI settings, and make adjustments from those settings. I've installed TBI conversions with a cam spec similar to the one posted above. The factory TBI settings will work, but ultimately a custom chip will deliver the performance the cam is capable of doing. I can, or other board members here can, program a custom chip for you. If you have the budget for a self learning computer, that is a good option. Most of us board members here take pride in our hard learned abilities to save on the budget and do our own chip. Sometimes it's a matter of time vs. money, the self learning computer can save time!

    dave w

  3. #3
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    Dave W,

    Thanks for the reply. All good to learn. My wife and I differ on what my budget is, but it is rather small at the moment :)

    I will end up needing a custom chip to remove the EGR at a minimum. It's good to know a stock chip will let it run, even if not optimally. Now to actually find one... I have a 350 PROM in my computer at the moment.

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    Super Moderator dave w's Avatar
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    One of the important parameters (fuel) of the 305 and 350 chips will have the same setting. The important thing to do is make sure the 305 has injectors from a 305 if using a 350 chip for a 305 engine. Do you happen to know what the injector numbers are in your throttle body? Post back the injector numbers if you have, then we can figure out if you have 305 injectors. Usually the first 3 numbers on the injector are "523". Maybe you can get your engine running with what you have on hand?

    dave w

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    I will see what it there, but the throttle body and computer I bought are off a 350. The computer is a 7747. I have not opened it up to see what's inside it yet.

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    Super Moderator dave w's Avatar
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    350 injectors / stock 350 chip on a 305 would be bad! 350 injectors / stock 305 chip on a 305 would be bad! 305 injectors / stock 350 chip / stock 305 chip, would be OK, but not great because of the cam specs.

    dave w

  7. #7
    Super Moderator Six_Shooter's Avatar
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    If you need 20* of base timing to run right, you have some mechanical issue.

    Either the outer ring has slipped on the balancer, or you have the wrong pointer. The engine should run and idle with 0* timing, maybe not real well, but should run.

    I'm trying to think of anything being mechanically out internally of the engine that could cause this, and I'm not coming up with anything off the top of my head right now.
    The man who says something is impossible, is usually interrupted by the man doing it.

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    RIP EagleMark's Avatar
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    Sounds like that was with HEI dist on carb engine... he's still gathering parts for TBI.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
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  9. #9
    Super Moderator Six_Shooter's Avatar
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    Quote Originally Posted by EagleMark View Post
    Sounds like that was with HEI dist on carb engine... he's still gathering parts for TBI.
    When's the last time you worked on ANY engine that required 20* of base timing to run. ;)

    Even when I've worked on carbed engines, the most base timing I would use was around 12*, but would run fine all the way down to 4*, 0* on some.
    The man who says something is impossible, is usually interrupted by the man doing it.

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    RIP EagleMark's Avatar
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    Quote Originally Posted by Six_Shooter View Post
    When's the last time you worked on ANY engine that required 20* of base timing to run. ;)

    Even when I've worked on carbed engines, the most base timing I would use was around 12*, but would run fine all the way down to 4*, 0* on some.
    Some HEI without vacuum only add 12 degrees through centrifugal advance and depending on weights none at idle. So 20+ is not uncommon on carb with a big cam engine.

    TBI base I always use 0 because it effects injector timing, but in the cal usually 23 for a good idle... it has to drop off idle...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
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  11. #11
    Super Moderator Six_Shooter's Avatar
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    I've worked with a lot of HEI and have NEVER needed more than 10* base.

    What vehicles only add 12* of total timing?
    The man who says something is impossible, is usually interrupted by the man doing it.

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    RIP EagleMark's Avatar
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    Aftermarket without vacuum...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  13. #13
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    Thanks for all the replies.

    It is quite common to run more timing on a carbed SBC. Granted 20* initial is about maxed. I am honestly surprised that I don't get detination with no EGR. The engine is a low compression L08. The book says 8*, but the cam I installed said to add 4* which makes 12. When I ran 12 the engine did run, but ran very rich, and yes it would still run down to 0* and below, but very badly. At 12* I could not lean it out enough to run well. The plugs were covered in unburnt fuel. Thinking that my balancer had slipped I degreed the crank and verified that my marks are 100% on the money.


    I added a spring kit and vacum kit to the dizzy. Experimentation led me to use 20* base. I could give it even more initial using the high vacuum method, but that gave too much at the top end. I am running about 52* total timing and the engine loves it. Like the old saying goes, give the engine what it wants, not what you think it needs.

  14. #14
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    Computer: 7747 with AFBZ 3557 prom

    Injectors: GM5235206 RPD - 55 lbs/hr 350 injectors, yes?

  15. #15
    Super Moderator dave w's Avatar
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    Quote Originally Posted by srgould41 View Post
    Computer: 7747 with AFBZ 3557 prom

    Injectors: GM5235206 RPD - 55 lbs/hr 350 injectors, yes?
    I agree, 350 PROM and 350 Injectors. Send me a private message if you would like a quote for a custom chip.

    dave w

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