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Thread: Weber/PMO throttle and venturi sizing equations advice

  1. #1

    Weber/PMO throttle and venturi sizing equations advice

    Hello All,
    I have been finding some very conflicting advice on-line regarding throttle and venturi sizing for Weber carbs on early 911 engines and would really appreciate some back up advice from anyone who really knows what they are talking about. Fundamentally I have found two apparently reputable sources of guidance for carburettor sizing which both refer specifically to early 911 engines. The problem is that they come out with very different answers for the same engine.

    First let me describe the engine in question which is a 2.5 twin plug short stroke that will be going is a period 2.5ST tribute car (but the same principles/issues can be applied to any early 911 engine on carbs). The target car will be driven on track and on road so a compromise specification is required which I normally achieve by building an engine close to the original race engine spec but with milder cams and slightly smaller ports to give it better driveability on the road. So here below is the engine spec -

    Capacity 2518
    Bore/stroke 90/66
    Valves 46/40
    Inlet port size 38mm
    Exit port size 36mm
    Header diameter 36mm ID feeding into 36mm ID headers with twin outlet free flow sports muffler
    Compression 10.5:1
    Ignition twin plug dissy with programmable advance
    Rev limit 8000rpm
    Cams DC40 (or DC60 cams)
    - duration@1 degree 266/249
    - lift 0.474"/0.440"
    Peak power estimated to be 7000-7300rpm

    The first link of note that I used was this one from Performance Orientated which suggests some simple equations to be used to predict optimum venturi and throttle sizing based ion cylinder displacement and peak power RPM.

    The key equations are copied below -

    Throttle bore diameter D = 0.85 X (V X n)^0.5

    Venturi diameter d = 0.8 X D

    D = Throttle bore diameter (mm)
    d = venture diameter (mm)
    V = cylinder displacement (litres)
    n = peak horsepower RPM

    So for this 2.5 engine with DC40 (or DC60) cams, then -

    D = 0.85 X (0.42 X 7000)^0.5 = 0.85 X 54.2 = 46mm ideal throttle size

    d = 0.8 X 46 = 36.8mm ideal venturi size

    Inlet valve diameter = 46mm
    Inlet port diameter = 38mm

    It says that typically there should be a 0.8-0.85 relationship between the 2, so this is spot on at 0.826. (0.85 for very high lift cams and 0.8 for more std cams)

    It does recommend rounding down in size from these theoretical equations to get to the nearest available size.

    The calcs on the previous link seemed to come out with the correct venturi sizes for some known engines right from 2.0S (32mm venturi) up to a 3.6 (42mm). The calcs predictably come out between about 0.5 and and 1.3 above the venturi of choice for each engine. So the logical conclusion was that these equations were a good guide right across the capacity and rev range for 911 engines.

    So these equations would indicate that the ideal is 36mm venturi and 46mm throttles.
    If 36mm venturi are correct for this engine then 46 mm throttles does also sit right in the middle of the recommended range for this size of venturi

    Simple - NOT EXACTLY !

    As ever with the internet here are alternative equations that can be used to calculate optimum venturi size and the answers can be quite different.

    I've also referred to Bruce Andersons well known 'Porsche 911 Performance Handbook' on page 154 where it refers to venturi and throttle sizing for Webers. This uses similar but crucially different equations which I have copied below.

    Optimum venturi size in mm = 20 x ((cylinder capacity/1000) x (peak power rpm/1000))^0.5

    For my 2.5 this comes out as 20x( (420/1000) x (7000/1000))^0.5 = 34.3mm, implying that 34mm chokes would be the optimum size. A very different answer to 36.8mm from the other equation on the previous link.
    The venturi do set the vacuum for the main jet circuit but the throttle diameter sets the vacuum for the idle circuit which can be particularly important for any low speed or part throttle running.

    A slightly conflicting bit of information in Bruces handbook is back on page 141 where it is referring to modifications on a 2.7 engine where it shows a power/rpm graph of some 40IDAs with 32, 34 and 36mm chokes for comparison. The difference is negligible for some reason which Bruce comments on but likewise finds hard to explain.

    So what is the best answer -
    1) 46s with 36 venturi predicted by the equations on the Performance Orientated web link ?
    2) 40s with 34 venturi predicted by Bruces book ?
    3) something in between with 40s to get the idle circuit control better and 36 venturi to give a bit more peak end ?

    I really don't know which system to trust. Certainly the graphs on page 141 of Bruces book are for a car with E cams but the 36mm venturi seems to start showing some upside above 6000rpm when the E cams have pretty much peaked out. With the DC40 or DC60s then there is still a lot further to go.

    I would appreciate any solid engineering advice on this topic and particularly in respect to the best equations/predictions to use for throttle/venture sizing.
    Many thanks

  2. #2
    Im building the same motor and plan on 46PMOs with 36 venturis. Solid engineering should take a second seat to experience in my opinion. Thats why Porsche didnt win the 1968 manufacurers championship, Mr. Piech's computer told him one thing while the mechanics told him another. The nice thing about 911s is so many have been built that it is almost cookbook recipe at this point. The weak point of race motors seems to be rod bearing life. Im asking every builder I know how they deal with that and there are differences of opinions.

  3. #3
    Join Date
    Nov 2004
    Scottsdale, AZ
    +1 Rick.

    No disrespect intended OP but all of your words are typical of an engineer; I nodded off while reading your post. Often times you guys over-complicate things when you need to just ask yourself a simple question:

    "Do I want to know what works best or do I want to theorize about what works best?"

    Unfortunately carbs are still horribly inefficient when compared to programmable EFI. Your quest for the "perfect" induction setup when talking about carbs for a street and track engine is a non starter; the objective is a fool's errand. An attempt to do both will fall far short. (And we have as much experience as anyone with PMO's on small bore race motors.) Lots and lots of time spent on a dyno experimenting and one still ends up with a compromised torque curve.

    With the engine specs you've laid out that's a street motor. Focus on driveability.
    Last edited by Frank Beck; 03-03-2018 at 04:15 AM.

  4. #4
    I'm not quite sure what you are recommending Frank, but I'm certainly looking for 'what works' and not just trying to theorise. I'm trying to make a decision for an actual customer motor and not just blowing smoke. That is exactly why I haven't just trusted the 'theories/equations' and am seeking peoples experience on here, but the problem is that both the 'theories' and the 'experience of what works', is not consistent. If it was then the decision would be easy.
    I did a 2.8 twin plug to an almost similar spec and used 46s with 38mm chokes and think that was best, and I used 40s with 34mm chokes on a single plug high compression 2.4S spec motor and that was really good, but a 2.5 twin plug with DC40/DC60s is just in that grey area inbetween.
    The problem as I see it is that the step up from 40s to 46s is such a big step and therefore involves compromises. Ideally it probably need 36mm chokes with 42mm throttles but this project can't afford to go down that completely bespoke route, so it is back to the best compromise position.

    I've put a similar post on Pelican technical forum ( ) and for a high compression twin plug 2.5 like this one, then on balance it is more in favour of 40s with 34 or 36 chokes rather than 46s with 36mm chokes, so interesting that I think you 2 guys on here seem to support 46s with 36mm chokes. That probably brings it back to 60:40 in favour of 40s with 34 or 36mm chokes.
    I can't change the intended use of the engine and it will be used on track and street, it is just the way it is, and so I need the best compromise position.
    Many thanks for your advice

  5. #5
    I built a 2.5 short stroke motor with Modified S cams, 10.5 cr, 38 mm intake ports, 46 PMOs a few years ago and when we put the car on a dyno the torque curve was remarkably flat. That motor is now in a street only car with MFI. A competitive race motor might have concerns about torque curves the more hp it makes. If you use Webers increase the venturi size if you also use tall secondaries.

  6. #6
    Quote Originally Posted by 66S View Post
    I built a 2.5 short stroke motor with Modified S cams, 10.5 cr, 38 mm intake ports, 46 PMOs a few years ago
    Well that is pretty much identical to this engine I'm building. For the avoidance of doubt I am specing up PMOs, not Webers for this engine. Can you recall the venturi sizing and jetting you used ?
    Did it have any transition issues or significant flat spots lower in the rev range when it was running on the PMOs ? The MFI would of course be quite different in character so not a fair comparison.

  7. #7
    Join Date
    Nov 2004
    Scottsdale, AZ
    There are so many variables you have not mentioned (Altitude range where the car will be mainly used, gearing which could determine the RPM range, whether the driver is super aggressive and likes his foot in it A LOT, etc.) And you haven't nailed down the cams (DC40 and 60 require slightly different tuning.)

    Again, not to come across as too flippant but engine building is a process that involves tailoring to specific conditions, drivers, and individual personalities of a car. Buying the "perfect" carb off a shelf is something a DYI guy might expect but it's not the real world.
    I suggest you buy a set of NEW 46 PMO's and a variety of venturis so you can actually see what works best on the dyno. At least start there. Good luck.
    Last edited by Frank Beck; 03-04-2018 at 03:47 AM.

  8. #8
    And maybe some spare acceleration pump nozzles, or is it just me , and the guy who sold me used PMOs with 3 broken nozzles, that has had trouble installing venturis? They go in with the venturis and imstalling them is a bit tricky. I bought some Weber IDF nozzles from Alfa 1750 on ebay for a decent price, PMO carbs are based on IDF 2 barrel carbs.
    Last edited by 66S; 03-07-2018 at 10:59 AM.

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