Bruce ( Shrinker )

Fuel and Air Systems

  1. Bewy

    Bewy Well-Known Member

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    Hysteric,
    With all due respect to Bruce, here is what I know [ not surmising ] from over 50 yrs of working on/building engines. And I don't think Bruce 'gets' it as far as idle timing goes. He is not alone...
    When cam duration/overlap increases, idle timing [ I.T. ] needs to increase. Other factors that will require marginal increases in I.T. are low CR, large ports, sluggish intake manifold.
    And I don't agree with Bruce that Chrys 'are a bit different to other US products'. They all have combustion chambers, cams, valves, etc. The only thing that will differ is the amount of I.T. for each engine/combo.
    In my opinion, Bruce has made a simple scenario quite complicated. Simple, in the sense, that if you increase I.T. from 10* to 40* with no other changes, & idle rpm increases 300 rpm, then what you have is a winner. It is very obvious in this scenario that the engine is making more power with more timing & no downside. The engine also runs cooler, indicating efficient use of fuel, air & timing. I have done the above many times, although a 300 rpm increase is unusual, usually 50-200 rpm increase. Once optimum I.T. is determined & set, I can reduce the idle rpm, often to lower rpm than it had with less I.T., & it idles smoother with more vacuum. The tip in response is also always improved, & I have cured off idle flat spots just by getting the I.T. correct. There is a reason racers lock their dists for max timing...
    It is most important to set I.T. first before drilling holes in t/blades for bypass air, & then finding the engine idles/responds a lot better with a heap more I.T., but now it idles too fast & you have to fill some holes that didn't need drilling...
    Chrysler missed the boat on manifold sourced vac adv. Others did not like the GM divisions. Not sure about Ford.
    Chevs I believe idled with about 24*, a combination of init + MVA.
    Pontiacs until 1968 left showroom floors idling with 26*, 6* init + 20* MVA. These engines had 10.75:1 CR & mild cams, with intake duration under 200* @ 050. When you consider these parameters, suddenly 35-40* I.T. for a modified cammed engine doesn't sound that much...
    My GTO with Webers idles with 48*.
     
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    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      Glad it seemed to work for you but its not universal.
      What is universal is the underlying physics of combustion, compression, and so forth.

      Just for the record.
      1965 Cheverolet L-78 with 4150 list 3124
      65l78_9_4cb76915662fccb81e408f6ad568bf5e10224fb9-jpg.jpg

      This appears to be an original 4150 Holley List 2818
      47682672d1319133625-numbers-for-an-original-holley-2818-choke-housing-2818rh3-jpg.jpg

      As shown in this diagram
      b-engine-ign-05-vacuum-advance-lines-gif.gif

      I don't have a Pontiac FSM. Would love to see that. Seriously.
      I do see in the tune up specs that the v8s are to idle with 6* BTC at 500 and 600 rpm. That's pretty much what we'ld want for an idle rpm. Slip the clutch and if it has any torque to speak of away we go. :)

      Kaiser's 230 Tornado used manifold vacuum port for its vac advance. What does that prove? On its own. Nothing.
      Ford and VW used a venturi based advance system for several years. What does that prove? IDK. That they could skin the cat a different way. cool-gif.gif
       
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      • TT5.9mag

        TT5.9mag Two atmospheres are better than one

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        Thank you.
         
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        • Bewy

          Bewy Well-Known Member

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          Mattax,
          It works for everybody, not just me. It works on Chrysler engines. It will also work for you...

          But let me hasten to add this: if you are using a locked dist, or some other means that provides a LOT of init adv, then, you probably do not need MVA. Or just adding a few degrees might be optimum. And you can add too much. Unfortunately, I have NEVER seen a detailed procedure to dial in MVA. And here in lies the problem, with no detailed procedure that is correct [ the Accel, Crane instructions are useless ] MVA is often incorrectly dialled in. When it doesn't work, the word goes out. 'MVA doesn't work'. An analogy is Bill gets a wheel alignment & afterwards the car pulls to the left & he says 'Wheel alignments do not work'. I developed my own method for dialling in MVA, it works EVERY time & will on your engine, subject to the above.

          Don't know to load documents onto these sites, but if you PM an email address, happy to email the MVA info on Pontiacs & other useful info.
           
        • carby tuning techniques

          carby tuning techniques Active Member

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          yes i agree.
           
        • carby tuning techniques

          carby tuning techniques Active Member

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          That is a very nice looking engine bay. very nice.
           
        • carby tuning techniques

          carby tuning techniques Active Member

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          I like that.
          For me thats the fun of tuning,is working out what works for the car at hand and seeing that smile on their face when they drive it.
           
        • Bewy

          Bewy Well-Known Member

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          Greg,
          Thanks for the comments. While I could find plenty of comments from people on MVA more knowledgeable than me, I think David Vizard's comment in the Nov 04 issue of Pop Hot Rod magazine takes the cake:

          ' At idle & low speed operation, the amount of advance reqd to most effectively to utilize the air & fuel entering the engine can be as much as 50-55 degrees. this is handled by the vac adv: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the timer to listen up & listen up good. A functional vac adv is the single most effective camshaft tamer you can get. By taking the time to hook up the vac adv to a manifold vacuum source you can get a big cam to idle as it it were about 20* less than it really is. Conversely, if you are looking for a decent idle the use of vac adv will allow you to use a cam of, at the very least, 5* more duration/overlap than would otherwise be the case.'
          [ Italics are mine ]
           
        • carby tuning techniques

          carby tuning techniques Active Member

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          load sensing spark advance is extremely important in achieving great driving standards.
          When i tune engines i make sure that the spark strength is as strong as i can get it,this includes changing coils and ignition types,alternators (which i monitor as we drive - a lot of the time they get changed) etc..its such an important part of creating a strong flame kernal in the cylinder.
          I always change plugs and dress them so that they fire as good as i can get them to fire.I change the gap to suit the vaporization of the carby,if i get the carby good i can then try bigger gaps.Bigger gaps engage more fuel molecules and if the amp energy is strong over the duration of the firing then all sorts of running standards change.If the way that the carby gasses the fuel is poor then the gaps become very critical.
          Once i have got these aspects in place i then use a timing curve that suits.
          Vacuum advance is a big part of it all.
          Its generally in that order that i start the process.This works good for me.
           
        • Hysteric

          Hysteric Well-Known Member

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          That makes a lot of sense especially from a practical perspective .

          Thanks again.
           
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