Engine Vacuum vs RPM?

Small Block Mopar Engine

  1. Caco2120

    Caco2120 Well-Known Member

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    Morning team FABO,

    I was under the impression that manifold vacuum dropped lower and lower with increasing Load...which usually goes hand in hand with RPM. (WOT=0 MAP)

    I recently saw someone's dyno results claiming that the Vacuum increased with RPM?

    like 10 at idle, and 18" at 2500...

    am I incorrect in thinking they had this backwards? if not, teach me something :)

    Maybe they were recording the numbers with the engine at zero load, but increasing RPM?
     
  2. nm9stheham

    nm9stheham Well-Known Member

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    If it was under a low load, then the carb would be mostly closed and the vacuum could go up. So your hypothesis on what they were doing may be right.

    The reference to vacuum dropping under load is that it drops more and more with increasing load. With a fixed load, then vacuum may go up with RPM. (Even at WOT it ought to go up with RPM, even though it will be near zero.)
     
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    • qkcuda

      qkcuda Well-Known Member

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      Mine idles around 8.5 inches of vacuum, but pulls about 17 inches when cruising at low throttle opening. High overlap cams don't create good vacuum at idle, but it makes less difference at higher rpm.

      Wideband.jpg
       
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      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        That's about right. A fixed load would be a smaller and smaller percentage of the engine's power as the rpms move up to the peak.

        Vacuum is an excellent indicator of load relative to engine power. That's why it can be used as a reference for high load enrichment (step up or power valve), and adjusting timing for light load low density conditions (vacuum advance). Engines with computer based feedback fuel delivery use a MAP sensors fot the same purpose.
         
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        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          Qkcuda's example is good.
          Engines set up for racing using higher overlap cams tend to have lower vacuum at idle.
          Even the little load from an automatic transmission engagement is a big chunk of the available power at 600 - 700 rpm, if it will even idle that low. But once its off idle, under light load, vacuum goes up quickly.

          Even at WOT, there can be a slight difference in MAP depending on which gear your in. I never trust 1st gear for making decisions on jetting or timing. 3rd under full load is what I prefer at the strip or on a dyno. Dyno lets me see the top, but the strip is real.

          MAP is never really zero as I'm sure you know. So logging it through WOT can be useful. Obviously if you're running a two barrel or a restricted class the Vac will increase more noticibly than a big carb where it may not be measurable at all.
          On the other hand vacuum can drop at the top end if the combustion isn't dialed in either due to fuel distribution or something else. In other words power is going up but the engine is not pulling as hard so vacuum drops. Interesting, huh! Not exactly what I wanted to hear. But it is what it is. I've made some changes since then so maybe I've fixed it. Won't know 'til I get it back on the dyno or have the logger at the track.

          For example here's a pair of chassis dyno runs on primary barrels only.
          Roller chassis dyno is providing something like max load when the throttle is max. More load can be added but this is close enough for most purposes.
          Vacuum is Dark blue.
          Its a full automatic so we have to squeeze the throttle to get it to third without having it downshift.

          upload_2019-2-12_9-52-32.png

          here's the same engine on all 4 barrels.
          Looking at the vac drop around 1:10, it appears that something is causing the engine to perform a little off when the secondaries were opening. Whether its the AFR, which is easily correctable, or something more inherant in the engine build, I still can't say without more testing.
          upload_2019-2-12_10-0-44.png
           
          Last edited: Feb 12, 2019
        • Caco2120

          Caco2120 Well-Known Member

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          thanks all! appreciate the explanations! always trying to learn!
           
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          • BigBlockMopar

            BigBlockMopar BigBlockMember

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            Engine (intake) vacuum = Engine Load

            High vacuum - Engine has it easy
            Low vacuum - Engine is working
             
          • MOPAROFFICIAL

            MOPAROFFICIAL Well-Known Member

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            Wow that's a lot of graphs there. Here's how it works in simplistic Layman's ....

            The higher the speed/mph coupled with the least amount of throttle opening/cruise... makes higher vacuum.

            That is why highway mileage is better than around town, you're barely dipping throttle once up to speed.
             
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