Weber Venturi Size - what difference does it make

[matty74]

I was wondering what the difference in venturi size has on performance of my weber carbs.

I have IDA3C set up for a 2.7L motor now you can get different
lengths of venturi from quite tall 70mm to quite short 40mm.

What performance impact does this have on the engine?

Another question I have is related to Manifolds, Motormiester sells 2 types of manifold for the webers standard height and tall.

What benefits are derived from a taller manifold? And could I use a taller manifold with taller venturi what would this achieve?

Located at the bottom of the page
http://www.motormeister.com/suz/carb_access/air_filters.htm

What would be the best combo for a 2.7L street rod?

cheers

Matt

[jluetjen]

What sort of cams are you running?

[jorian]

Don't know the answer to your question but do a search before buying anything from MM. I have 46mm Webers with the talls on my SC.

[Early_S_Man]

Tall manifolds are intended to boost bottom-end torque ... that is why they were utilized on the 906 engine. Tall booster venturis are intended to capture the reversion 'fog' caused by high-overlap, long-duration racing cams, which is why they were used on the 906 engine.

Unless you are running original, early 911S cams or hotter [not '74 - '77 911S CIS engines] and SSI or early heat exchangers ... you don't need the tall booster venturis. Tall manifolds could be useful on engines cammed as mild as 911E or Solex.

PMO has a much better selction of manifolds than MM. Have you not heard of the MM horror stories???

[matty74]

what manifolds can I get from PMO (I'm in Australia I found MM on the internet)

Whats wrong with the MM manifolds? Not like they have any moving parts.

I'm running GE60 cams JE 9.5:1 pistons

[Early_S_Man]

Here is the PMO website:

http://www.pmocarb.com/

[RoninLB]

There is a formula for figuring airflow at max rpm. If over-sized then gas atomization suffers. The better the atomization the better the combustion. Combustion is power.


There are a few formulas around. This is not the Weber or Bosch formula, but almost identical.


2.7 = 2700cc

2700cc / 6 = 450cc each

450cc X 6,500rpm = 2,925,000

2,925,000 / 2,600 = 1,125

Sq.Root 1,125 = 33.5mm venturi

Then some say the venturi should be around 80% of butterfly size.


I went through venturi sizing awhile ago. Larger than necessary venturi also affected acceleration big time. Without readjusting acceleration gas flow it sounded sharper with the 34 than with the 36 vents. Even now there's room for acc flow tuning. I found that the so called seat of pants better feel with 36 is because it runs so lousy below 4k into decent above 4k acceleration instead on more power under 4k with 34 running into less seat of the pants but better above 4k power.

all above is imo and measured with EGTs.

any way a set of vents is cheap. Get both and experiment

[matty74]

so @ 34mm venturies in height or diameter?

[RoninLB]

Originally posted by matty74
so @ 34mm venturies in height or diameter?

this is the main venturi inside dia.
not the auxilary venturi or booster venturi as Warren mentions. PMOs come stock with tall booster vents. Not sure if PMO tall booster vents would fit on Webers?


and I'm not the last word on this. Get both the 34mm and 36mm.

[bede]

From what i have read about MM be careful I am in Australia as well & have tried to get parts from MM with little success.The price they quoted me for freight i could have bought a new set of webers.They also seem very illusive & hard to get in touch with so i gave them a miss.There are a lot of other reputable places in the US. PMO seem to be really helpful even if you are only after advice

[ficke]

Is not the reduce orifice in the carburetor that creates a low pressure zone called the "venturi"?
and the things that bolt to the top of the carburetor to direct air in and hold the "fog" that early S Man refers to called the "velocity stack"?

I am just trying to clarify, I know manfactures use diffrent words some times, I have had british cars

[Steve@Rennsport]

Originally posted by ficke
Is not the reduce orifice in the carburetor that creates a low pressure zone called the "venturi"?
and the things that bolt to the top of the carburetor to direct air in and hold the "fog" that early S Man refers to called the "velocity stack"?

I am just trying to clarify, I know manfactures use diffrent words some times, I have had british cars

Yessir, that is correct on both counts.


A few notes based on many years of engine dyno observations and a clarification,........

The "reversion fog" above the intakes that Warren refers to occurs with long-duration, narrow-lobe center cams such as 906, RSR Sprint, and things like that. You never see it with street-type cams. Running a carbureted or injected 911 race engine on the dyno, one will see the fuel fog form around 4500 RPM and remain until around 5800 depending on camshaft profile. During that time, one will witness an extreme richening of the AFR's as the air being pulled into the motor is enrichened from the airflow reversal and back down into the cylinders. You can watch the torque sag during this phenomenon as well.

Carbureted engines are FAR more susceptable to this than MFI or EFI engines and we were always able to contain this by raising the intake manifolds and lengthening the velocity stacks until it disappeared. Unfortunately, that configuration was too tall to fit into a car,...

We compromised by simply raising the intakes to move the throttle and venturi's as far from the intake valves as was humanly possible.

Header lengths & diameters as well as secondary exhaust lengths also play a BIG role here and thats a complex subject of its own. These REALLY affect AFR's from off-idle to 6000.

Now,......tall auxiliary venturis were not made to control/contain reversion (fog); these were made to help accelerate and initiate fuel flow into the engine when using very large main venturies that generate a weak signal to the float bowls. The 42mm venturies used in the 906-911R's 46mm Webers are a case-in-point. Those needed the tall aux venturies to run in the low & mid RPM range,...

I've barely touched on this subject but I hope this helps a bit,

[911pcars]

Steve, as usual, provides us with some very useful information.

Thanks,
Sherwood

[ficke]

Steve, Once again thanks for the education, Fritz

[randywebb]

two words: direct injection

[Fishcop]

Matty, PMO are very easy to deal with via fax (I've not used their website or email). I also bought some of my weber stuff from PMO through TRE when I was on holiday over there... Pretty sure you can get what you need through Pelican as well... Safer than MM

[cameron.arnott]

Matty I think Bob Whymms in Sydney act as PMO agents out here so might be worth a call to them 1st.

Good luck.

[RoninLB]

"Pretty sure you can get what you need through Pelican as well... Safer than MM "




Yep.. x2

[matty74]

Can PMO rebuild / upgrade my existing webers? The PMO website, sucks not email or contact details a bit basic

[RoninLB]

You have to fax PMO.


I'd call Bieker Engineering to see about completion. Harry & son is real good rebuilders.