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Thread: Fuel/Air flow - rocket scientist needed!

  1. #1

    Fuel/Air flow - rocket scientist needed!

    Lauren?

    Sometimes the brain works a little slow here, especially in the winter. When I first bought my car, I had some definate plans for the exhaust and intake if I was going to keep the existing engine. Well, I am keeping it, so one of the first things I went looking for was alternative induction. The setup currently on the car is a 390 Holley four-barrel, vacuum secondary, which is fine. It fits on this manifold:

    As you can see, Offenhauser specifically designed this manifold for a vac secondary carb - the plenums are, indeed, split, so the secondary half really doesn't do anything until they open. From Offy's website, "The quad manifold (competition design) will actually allow the engine to perform with proper balance throughout all stages of RPM resulting from the high rate of velocity in the primary circuit and perfect balance that automatically results when the secondary or acceleration portion takes over."

    Ok... all well and good. In theory that should give reasonable gas mileage, too. But I'm for one to go overboard and do stupid things, like purchasing these:

    Twin 40 DCOE webers on a central manifold by B&N. This kit is still available through a select few manufacturers for well over $2K, and I found this set rebuilt for less than a quarter of that. I had stumbled across the manufacturer's website when I was daydreaming about the build one day (http://www.compperformancegroupstores.c ... FourBarrel for those that want to check it out), and I knew I had to have it.
    What never crossed my mind was that the center manifold was open:

    The wider portion would be where the vac secondaries would be on a regular four-barrel carb. Ah... interesting dilema! I know I'll need a spacer under the unit if it works just to clear the rocker covers. My question to the rocket scientists amoungst us and those that understand fuel/air ratios better than I: Other than the anticipated poorer gas mileage, will this setup actually function as it should on a manifold designed to "split the load" from a four-barrel vac secondary carb? If the setup would fit length-wise on the block, it's possible that I could physically make a partition in the manifold, and have one carb come in a little later than the "primary", but as it is, it will only fit across the block:
    Which, when fitted with small screens on the trumpets instead of the big box filters, is going to look pants when the block is finished and painted!
    So... thoughts?

  2. #2
    ***Euro-Nova Supporter*** Spacenut's Avatar
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    Re: Fuel/Air flow - rocket scientist needed!

    Hi Rick - what a wicked carb set-up! I've seen one or two Bizzarrini 5300 Stradas with this induction setup, which has also been used on Panteras to good effect.

    If the Webers are jetted correctly for the application, you may be surprised how economical they can be. The amount of fuel mixed with the induction air is metered by the progression jet holes in the emulsion tubes, and this is in response to demand, just like the vac secondaries in a 4-bbl. If anything the metering is even more accurate than the vac secondaries, which are less progressive over the whole engine speed/load range.

    The correct mixture for all 6 cylinders is established by the time the fuel/air mix has entered the plenum, so to my mind the various bores in the Offy manifold will merely add some turbulence to the induction path. If your feeling brutal, you can ream out the manifold so that there is just a single large plenum under the carbs. This would maximise the flow, and would replicate a typical single plane performance manifold.

    Having said that, the phenolic spacers you can get to fit under 4-bbl carbs usually have 4 individual circular bores, rather than the large oval hole and two circular ones. I doubt this small constriction would make a lot of difference to anything other than an out-and-out race engine.

    I just thought I'd jump in with that, as I see the rocket scientist hasn't responded yet

    Lauren
    only Pythagoras can save me now!

  3. #3

    Re: Fuel/Air flow - rocket scientist needed!

    I'm not feeling that brutal yet, Lauren. New manifolds are close to $500! My concern is what you can't see going on in the manifold itself. Apparently they are two-stage (and, in two parts - a top half that holds the carb and a bottom half that bolts to the engine). According to Summit Racing, the description of the manifold is this: Offenhauser Dual Port intake manifolds feature completely separate runner systems for the primaries and secondaries. At low-load, the primaries feed the fuel-air charge through the smaller bottom passages at near sonic speed, improving power and efficiency. When the secondaries open, their charge goes through the bigger, cooler upper passages. Then, in turn, it is rammed into the cylinders when it encounters the high-velocity mix from the primaries. The result is better fuel economy and a 15-30 percent power increase across the whole rpm range.
    So that tells me that if I use this dual setup, both passages will be getting equal amounts of fuel, but not necessarily the same airflow. Maybe I'm reading too much into how the fluid dynamics might affect the engine! Oh, and I'll probably mill by own spacer with a simple open-frame design - no restrictions!

  4. #4

    Re: Fuel/Air flow - rocket scientist needed!

    Rick,
    My thoughts would be that it will run reasonably well so long as you get the correct jetting, however a single plane manifold would perform better with what is basically a single entry dump set up that you have. I would go with it as is (plus spacer) whilst you are building the car up and need to move it about at times and just keep an eye out for a single plane manifold over time. The spacer would more than probably be useable on a single plane set up so you wouldn't be wasting time effort and money doing that, if nothing turns up by the time you want it on the road I would get it on a rolling road so you could tune it to suit your style of driving.

    OK Lauren I'll bite why temporarilly?

    PS. still no rocket scientist just a petrolhead.

  5. #5
    ***Euro-Nova Supporter*** Spacenut's Avatar
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    Re: Fuel/Air flow - rocket scientist needed!

    Yes, I agree with BBC if it is a dual-plane manifold. Both the primary and secondary runners must meet up somewhere in there!

    I suspect the primary and secondary runners will still work pretty much as intended anyway, its just that they will be supplied with air/fuel simultaneously, which is equivalent to WOT on the 4-bbl (primaries and secondaries both open). The different runner diameters will speed the airflow from the plenum into the cylinders, with a larger charge mass entering at slower speed through the secondary runners. The same mixing effect will occur.

    An alternative to the plenum-based intake would be a cross ram induction system. Usually posh castings, I've seen custom systems made up from fabricated steel tubing. A V6 is interesting, as you could feed one cylinder bank with one carb and the remaining bank with the other - but that would mean siamesing three runners into one on each side. The cross ram intakes on Hemis look like a big vee shaped block of alloy, but what I am getting at is something like the cross ram Chrysler 383 from the ealy 60s, which used a Carter 4-bbl on a small plenum on the end of each ram pipe.

    http://www.allpar.com/photos/bios/weert ... mmando.jpg

    Replace the Carters with sidedraft Webers and voila!

    Coming back on-topic, I'd try and get a single plane manifold as well

    I'm still waiting to hear from the rocket scientist...

    Lauren
    only Pythagoras can save me now!

  6. #6
    ***Euro-Nova Supporter*** Spacenut's Avatar
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    Re: Fuel/Air flow - rocket scientist needed!

    Sorry BBC - I'm temporarily back on the road with Green Machine. I need to take the old gal off the road again in a week or so the get to the steering rack shims... its a long story, I just haven't had time to write it up yet, or pick up the pictures from the developers!

    All will be revealed soon!!

    Lauren
    only Pythagoras can save me now!

  7. #7

    Re: Fuel/Air flow - rocket scientist needed!

    They do sell the spacer that is wide open, I used it on a 383 that I worked on. It seemed to help out a lot. We actually used two spacers one with a divider and the wide open spacer on his chevelle. This would alow more CFM per cylinder versus the dual plenm manifold only suppling half of your carb. I also have a set of Weber 40's off of a ferarri and they are a handfull.

    Rob

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