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Thread: Coanda Effect, Venting and Airfilters

Discuss the Austin Healey Sprite and the MG Midget. Two different but similar cars sometimes referred to collectively as the Spridget.

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    Jedi Knight apbos's Avatar
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    Coanda Effect, Venting and Airfilters

    image.jpg
    My Christmas present that I purchased for my self 4 months ago was installed yesterday with the help of my Uncle from Holland. From now on he will be called the Carb Whisper. Out the box the carb drives the 948 better than the old HS1 ever did. I still have to install the fuel regulator, but so far I am ecstatic! Now I want to dial it in before I move on to other mods (Datsun 5 speed, disc brakes, etc..) So more questions.......

    1. My dissy has a vacuum advance. There is no place to hook up a vacuum advance on my Weber 40 dcoe. I have done some reading and a balance tube ( like that on the stock intake manifold) seems to reduce the Coanda effect on Weber intakes and is a possible attachment site for the vacuum advance. What my question is what size tube? My manifold has 2 ports which can be used for a balanced tube. I have read that a smaller tube will have less fluctuation than a larger tube. The larger tube reduces the Coanda effect to a greater degree allowing fuel to flow more evenly to the cylinders.

    If the mod does not help I can always go to a mechanical sissy, but $$$$$$$.

    2. What air filter(s) would you recommend? I am using 2 inch stacks and have about 3 more inches of clearance.

    3. How do I vent the rocker cover without an attachment to the air filters? The 948 at speed requires a slight vacuum at the rocker to keep the motor from leaking.
    Regards
    Paul
    1949 TR 2000 roadster
    1960 MG MGA
    1-1959 AN5L/9477 AH Bugeye Sprite
    1- 1958 Triumph TR3 TS 28115

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    Yoda
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    Re: Coanda Effect, Venting and Airfilters

    Your car will run fine without the vac advance hooked up. You will just loose the fuel saving advantage that a vacuum advance gives.

    Can you drill and fit a tube to your air filter housing? You would like that the best. Dig around and you will find quite a number of air filters to fit the DCOE...personal preference then.

    Have fun, Paul!

    Kurt.
    65,66 Midget
    57 Berkeley, MGA
    Working on quality rather than quantity

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    Luke Skywalker Gerard's Avatar
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    Re: Coanda Effect, Venting and Airfilters

    Paul,

    I'm not sure how well venting via the air cleaner would be as far as providing enough vacuum for the scroll seal, but here is how I suggested someone do it with a DGV, and he reported it worked extremely well. He nor I never found the vacuum from an air cleaner connection to work that well alone. If your air cleaner solution tunes out not to be sufficient, you could resort to this. I suggested he restrict the orifice to 9/64".

    IMG_1851.jpgIMG_1853.jpg
    Gerard
    http://gerardsgarage.com/
    1960 Bugeye Vintage Racer #50
    1960 Bugeye (partially restored)
    1960 Morris Minor Pickup
    1964 Morris Minor 4 dr Saloon
    1970 Austin Mini (for sale)
    1946 Austin 10


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    Jedi Knight apbos's Avatar
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    Re: Coanda Effect, Venting and Airfilters

    Kurt & Gerard
    The are so many creative people on this forum. From the the big Healey guys https://drive.google.com/file/d/0Bx4...w?pref=2&pli=1 and from the Triumph Guys https://www.ebay.com/itm/ADD-W1-Poli...a27613&vxp=mtr. I even found a picture of a DCOE and also a SU trumpet with a tube welded/soldered on. I think I can find a filter with large enough steel plate to put a tube on.

    i will start playing with the vacuum advance balance tube as it is a cheap mod and can only improve my fuel mileage

    I will post my findings
    Regards
    Paul
    1949 TR 2000 roadster
    1960 MG MGA
    1-1959 AN5L/9477 AH Bugeye Sprite
    1- 1958 Triumph TR3 TS 28115

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    Yoda
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    Re: Coanda Effect, Venting and Airfilters

    Paul, I agree with Gerard that the best setup would be a dedicated PCV as he show's. I imagine a balance tube on a spridget might work since each barrel is feeding two cylinders already but the general consensus among the Lotus guys is that vac advance isn't worth the trouble.

    Kurt.
    65,66 Midget
    57 Berkeley, MGA
    Working on quality rather than quantity

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    Jedi Knight apbos's Avatar
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    Re: Coanda Effect, Venting and Airfilters

    Reading this evening has got me even more confused. If I understand correctly on the 948 at speed the rocker tube should only provide air entry into the block so the tappet draft tube can expel gases out of the block. So I was incorrect in stating a vacuum is required at the valve cover from the air filters at speed. At idle gas might return to the carbs, but this should be very slight and if excessive would be a sign of engine problems. To my thinking an intake manifold pcv attached to the intake and valve cover would pull gases up the tappet draft tube, pulling dirt etc into the block (this would be for the 948) if I am correct a vented oil catch can and no connection to the air filter might all that I require. If I am correct???
    Regards
    Paul
    Last edited by apbos; 12-30-2015 at 07:31 PM.
    1949 TR 2000 roadster
    1960 MG MGA
    1-1959 AN5L/9477 AH Bugeye Sprite
    1- 1958 Triumph TR3 TS 28115

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    Yoda
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    Re: Coanda Effect, Venting and Airfilters

    Paul, most engines use a PCV valve from the crankcase to vacuum at the manifold and a tube from the rocker cover to the air cleaner. The PCV valve is designed to handle most of the engines blow by while a small amount of air is usually sucked in from the air cleaner housing. Under hard acceleration the PCV system is often overwhelmed and the valve cover vent tube then passes excess blow by gasses to the air cleaner housing.

    Kurt.
    65,66 Midget
    57 Berkeley, MGA
    Working on quality rather than quantity

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    Yoda
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    Re: Coanda Effect, Venting and Airfilters

    If I were doing your set up I believe I would go to some type of oil separator from the tappet cover then to a restricted orifice and PCV valve [essentially just a check valve] and that to manifold vacuum. Then I would put also put a tube from the valve cover to the air cleaner. I set my Datsun engined midget up that way but used a cheap auto parts little air cleaner on the valve cover vent tube. I am now getting the smell of engine blow by in the **** pit after hard acceleration so will be hooking that up to the air cleaner. I've gotten rid of the PCV and orifice and am using the take off tube on the HS6 carb now instead. I was getting the same blow by smell with the PCV and now realize that instead of just reringing this engine I should have rebored.

    The DCOE has separate runners for each port so perhaps a Y tube to a nipple on each port would work or I suppose you could try the draft tube and valve cover vent. If you are going to fit a balance tube that would be an ideal place to plumb the PCV to. I guess maybe the vac advance as well. I have a DCOE set up for a Datsun engine and have avoided fitting it just yet because of the headaches you are encountering. When I do fit it I most likely will not worry about vac advance and haven't got round to the crankcase venting problem.

    Kurt.
    65,66 Midget
    57 Berkeley, MGA
    Working on quality rather than quantity

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    Jedi Knight apbos's Avatar
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    Re: Coanda Effect, Venting and Airfilters

    Kurt
    I think you hit it with putting the PCV on the balance tube. I will have to go look through my parts and see if I might have one from an old parts car or find one on eBay that works . Running to the filter can be done with a little fitting.
    The dizzy I will play with. I will try on the balance tube, it will always be "on" and if it does not improve fuel mileage I might have to get a mechanical dizzy. It is a street car so I want to drive the back roads and not fill up extremely frequently.
    I will let you guys know how it works.
    Regards
    Paul
    1949 TR 2000 roadster
    1960 MG MGA
    1-1959 AN5L/9477 AH Bugeye Sprite
    1- 1958 Triumph TR3 TS 28115

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    Jedi Hopeful histerical's Avatar
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    Re: Coanda Effect, Venting and Airfilters

    It is my understanding that the air cleaner is not a source of vacuum but a source if filtered air.

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