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Thread: tuning with a vac gauge?
          
   
   

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  1. #1
    threearmsinjune is offline CHR Member Visit my Photo Gallery
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    tuning with a vac gauge?

     



    I read the thread from earlier this month concerning ignition timing using a vacuum gauge and light. My question concerns the degrees of ignition advance. So here is the situation:
    I finally got the SBC 350 back into my work truck. I put a small cam in it and brought the CR up a bit, but it is still intended to be just a solid reliable everyday driver of a 3/4 ton 4WD. I ran into some cooling issues last weekend, vapor lock maybe? So finally this week I have the thing fairly worked out and take it down the road. Timed at 0 deg it is an absolute waste of a truck. It is like a kid tripping over his shoelaces! So I ditched the factory specs and decided to try the vacuum gauge. I figure that it would be a direct reading of how the engine combo wants to be happy with itself. The vac topped out at 19" while advancing the ignition and I retarded it back down to 17". Then I checked the timing light and came up with 17 degrees advance. The truck runs much better but still doesn't start stretching its legs until after 35-40MPH. It also idles rather slowly in drive at the light. I am assuming that there is low vacuum at this point, most likely due to the cam timing and overlap.
    Does this sound typical? I am questioning the cam timing, that maybe I should time it so that the intake closes sooner. This would increase cylinder pressure for more low end torque and also the load-idle vacuum as a side effect. I really want to get the most out of this engine but it is definitely just a working class truck.

    btw. cam is comp 12-249-4 112deg/on 108 CL installed straight up. http://www.compcams.com/Technical/Se...umber=12-249-4 Static CR 9.5:1
    Last edited by threearmsinjune; 03-31-2007 at 01:33 PM.

  2. #2
    thesals's Avatar
    thesals is offline CHR Member Visit my Photo Gallery
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    what kind of rear gear are you running and if its an auto what stall speed is your torque converter? you've got it down right with the vacuum you should be getting it to as high as possible at idle with the proper idle rpm, around 650-700.... thats not really a big cam so i doubt its cam specs that are throwing you off.... she should start making power at about 1000 rpms..... i would look more into the carb, rearend and torque converter now to really bring it to life.... if you've got a 2.73 gear or something you'll never have any torque out of your motor.... i'd run at least 3.73 but i'm lookin closer to a 4.10.... with maybe just a mild torque converter maybe a 1,200 rpm stall speed
    just because your car is faster, doesn't mean i cant outdrive you... give me a curvy mountain road and i'll beat you any day

  3. #3
    southerner's Avatar
    southerner is offline CHR Member/Contributor Visit my Photo Gallery
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    got a timing card ? I would look at the specs and then maybe advance the cam by 4 degrees, this will give some more low rpm torque.

    looking at your max vac, you should get at least 20. And set at 17 is borderline with late ignition timimg. I would shoot for 18 to 18.5. And if you dont get any ping leave it at that.

    This guage you are using is it a normal vacume vacume guage or a compound meter ?
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  4. #4
    Dave Severson is offline CHR Member/Contributor Visit my Photo Gallery
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    What timing specs are you showing, initial or total? Are you sure the balancer ring hasn't slipped, and that the balancer ring and the timing tab are correct for each other???? IMO, a timing light is still the best way to time an engine, if the components are correct, that is.
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  5. #5
    threearmsinjune is offline CHR Member Visit my Photo Gallery
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    I am still using the factory TBI and manifold. I found an Edelbrock performer for the TBI setup. I would like to look at this and possibly shifting the cam earlier 4 degrees. I would then be interested in a computer chip system. Once the engine is living up to its (mild) potential, then I want to look at the rear gearing. At the moment I am not sure of the rear gearing or the torque converter. They are factory......I bought this truck with a shot motor for a work truck and it has blossomed from there. I am using it to learn the process of performance. I have a background in performance motorcycles and this automotive performance is a new thing for me. I guess I am saying that I have a decent base knowledge but the hands on is still coming along. So all the experience you can share is great. Do I sound like I am on the right track?

    Timing specs were initial using a rollback light. The components are mild and probably not quite matched but I believe them to be close for my application.
    I like the idea of the vacuum gauge. It is a direct measurement that I can read in numbers. I can see the dial increase to its max and decrease as the timing is advanced to far. I can then repeatedly and predictably retard the timing back down a measureable reading on the gauge. The timing light doesn't really allow me to visibly monitor the engine like the vac. gauge. I still have to set the timing either to a predetermined point or keep advancing through trial and error until I notice ping/decrease in performance and back it off a few degrees.
    Last edited by threearmsinjune; 03-31-2007 at 06:48 PM.

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