Weber Twin 40s on ABF

Discussion in 'Carburettor' started by Kempster, Jan 11, 2013.

  1. Kempster Forum Member

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    Hi all,

    a set of twins 40s have come up locally at a good price, which is got me thinking about the abf in my mk2. I was originally thinking bike carbs. After a bit of initial investigation alot of people recommend twin 45s.

    So calling on experiance can anyone advise me of pros cons etc of running twin 40s on an abf

    cheers
     
  2. fredybender Forum Member

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    I have a Mk1 scirocco with a 9A powerplant with some 40's, and that car is a dream to drive on the streets!


    Taken from this site:http://cnx.org/content/m37431/latest/#id1166616953606

    Determination of the correct venturi sizeThe most common issue with badly tuned Weber DCOE series carburetors is the choice of the correct carburetor. It is commonly (and incorrectly) assumed that 45s will give more power than 40s because of the larger carburetor barrel. However, it is not the barrel size (i.e., 40 or 45) that determines the airflow and therefore potential horsepower, it is the size of the main venturi or choke (22 in Figure 2 and Figure 3). Selection of the correct main venturi size is the first step prior to selecting the carburetor. The size of the venturi is embossed on the inside lip (see Figure 3). Figure 3: A pair of DCOE venturis/chokes. Figure 3 (graphics3.jpg)The purpose of the main venturi is to increase the vacuum acting on the main jet (15 in Figure 2) in order to draw in and atomize the fuel mixture in the most effective manner. The smaller the main venturi, the more effective this action is, but a smaller venturi will inhibit flow. A large venturi may give more power right at the top end of the power band, but will give this at the expense of tractability at lower engine speeds (rpm). Race cars will benefit from this latter compromise, but on a road car drivability is much more important.Figure 4 shows a chart that allows for the correct selection of main venturi size for engines given the engines capacity and the rpm at which it is expected to achieve peak power. The rpm value primarily depends on the choice of cam; however, it is necessary to ensure that the rest of the engine is built to meet the needs of that engine speed. For example, the use of double springs on a pushrod engine or solid (rather than pneumatic) lifters in an overhead cam engine.
    [​IMG]
    Chart showing main venturi sizes for various engine sizes and peak rpm ranges. The red line is for a Formula Vauxhall Lotus, while the blue line is for a Ford crossflow powered Lotus Seven S3. Figure 4 (graphics4.jpg)
    Calculation of the carburetor barrel size Once the correct venturi size has been determined from Figure 4 it is a simple matter to determine which carburetor is required. The ideal barrel size that will accommodate the venturi size selected is calculated according to Equation 1. Table 2 shows a list of the main venturi size available for common DCOE series carburetors.
    graphics5.jpg

    (1)Table 2: The main venturi size available for common DCOE series carburetors.


    DCOE 40 :24-36
    DCOE 45: 28-40
     
    Last edited: Jan 15, 2013
  3. Mike_H Forum Addict

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    It'll be fine on a standard-ish lump. Danster had 40s on a cammed (IIRC) 6A in his Rocco track car.
     
  4. Hotgolf Forum Junkie

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    I , well, my rolling road guy finds the 4 progression hole carbs are better suited to 16v applications, especially if its being used as a road car too.
    My old mk1 was a dream to drive considering the spec.
     
  5. mr hillclimber Club GTI Supporter and Sponsor

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    40's are ok for a std engine as long as it will remain std, with just the carbs and normally a performance exhaust. Dont over choke...33-34mm tops...after that bigger doesn't help as the restricting factor for flow is the axillary venturi, plus the bigger the "hole" relative to what the what the engine will need will reduce air speed and signal strength and thus not give the best fuel metering possible.

    If the engine is going to have head & cam upgrades in the future then go straight to 45's now. You can run down to 34mm chokes & up to 40mm in 45's, so covering a std engine right through to over 200hp in the future.
     
  6. Jon Olds Forum Junkie

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    My mk2 build has 40's at 34mm chokes with a nice head and 260 schricks. Booked for a rollers session early feb so will report on the results, might be of some interest
     
  7. beekeeper Forum Member

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    would be very interested in your results also as i'm just finishing an engine change with similar spec.
    an old brm180 lump with 40's and 34mm chokes.
     
  8. Jon Olds Forum Junkie

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    Results, as below (this was just a shootout);
    idle jets were wrong, were resulting in too weak an AFR initially. (14.5 at 2500, reducing to 12.7 at 4000)
    top end it leaned off again 5500 to 7000, so work to do here.
    max power showed 146hp at 7000, so nothing to crow about.
    peak torque at 5800 was 120ftlb
    Fairly flat torque curve 110ftlbs at 3600 and 100 still at 7000
    next visit
    richer idles, play around with the mains/airs.
    An inter cam adj pulley to get the inlet cam closer, and an adjustable drivebelt one to get the exhaust cam right.
    Hope that's of some interest to somebody
    Jon
     
  9. Grant_mk2_abf New Member

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    Hi I'm doing abf convertion ina mk2 I've bought dellorto twin 45s but looking for a mani
     
  10. Grant_mk2_abf New Member

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    So wot gains will a mk3 abf in a mk2 golf with the 45s
     

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