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.

http://www.performanceoriented.com/t...enturi-sizing/

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
Andrew