Weber Venturi Size - what difference does it make
 

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

What sort of cams are you running?

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.

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???

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

Here is the PMO website:

http://www.pmocarb.com/

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

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.

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

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
:rolleyes:

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,

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

Thanks,
Sherwood

Steve, Once again thanks for the education, Fritz

two words: direct injection

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 :eek:

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

Good luck.

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




Yep.. x2 :D

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

You have to fax PMO.


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



http://forums.pelicanparts.com/uploa...1156333980.jpg

From what I understand, Richard (the bloke that runs the show) keeps it a small affair and is not particularly computer lierate. But rest assured, if you're after something in particular - PMO will have it.

I know thre is at least one rock show over here that can do webers properly... just can't remember who :(

", Richard (the bloke that runs the show) keeps it a small affair "




he spends a lot of time with satisfying many customers needs and quirks when he calls you back. If he was phone accessible to the masses he'd be broke.


I bought a running set of Weber 40IDA, then traded to nice 40IDS, then said screw it.. the more I knew the more I need PMO. They are unbelievably accurate. Keep in mind that the A/F mix among all 6 should be equal for equal piston power and the strongest running engine. Their differential between the strongest and weakest piston power is in league with high end precision FI, if not tighter.

If you're a carb nut there is no substitute.

all above a prejudicial rant.

Yeah I'm really consider ditching the webers for PMO's on a 2.7 should I go 40 or 46??

Carb size and venturi is a function of max rpm air flow as per cylinder size x6. Venturi should be about 80% of butterfly size.

a 2.7 at 6.5k calls for a 40 with measured 33.5 or 34 actual venturis.

What Ron said :) Not sure what you're plan for the engine is, but if it's not running high revs, then a 2.7 is generally better on 40's. In layman's terms, the volume in each cylinder of a 2.7 does not create enough "suck" to get 46's to do their thing in normal or street conditions.

From memory, both Anderson and Dempsey recommend 46's only on 3.2 litres and above.

You said 'street rod' but you already have fairly aggressive cams - GE60 cams. So I guess the emphasis is on 'rod.'

You also do not have particularly high compression. I guess it is single plugged?

You might want ot back off a bit (go up a level of abstraction) and think about what you really want out of this motor. That (plus what fuel you can get both now and in the future) will determine what to do to the cams/plugs/carbs, depending on your wallet. Or maybe you want to go to a 2.8L !!??

Generally, I'd say 40s tho.

Post what kind of driving you do and the traffic densities you encounter - are you in the middle of Sydney or way out in the outback?

Thanks guys I went with the agressive cams just in case later I wanted to race but yeah 40's are the go.

I mainly do weeekend driving in the car some times country occassionaly to work in the city, and the occassional track day.

I'm going with JE pistons and the plan is to stay single plugged.

Based on that I'd either tinker with the Webers or get PMO's and I'd fax all the info to Richard -- offer to set a time for you to call him since its very LD. He'll prob. call you anyway.

You might want to start a Weber vs. PMO thread, but in summary the PMOs fix every issue with the Webers and are far superior, except for the period-cachet. But they do cost a lot, so ...

yeah, but from what every body says about them, money well spent

I thnk so, that's why I bought them.

But you already have Webers...

So 40's should be good for a stock 3.0L SC motor?

I currently have a bone stock '69T w/ 40 IDA Webers. I know someone who is parting out an SC and I've though of buying the motor (along w/ other bits) and installing it in the '69.

Just curious...if the 40's are to small for a stock 3.0L SC motor I'm going to save my pennies (and benjamin's) and buy a set of PMO's. :)

In my opinion the 40's will be totally fine for what is a fairly mild state of tune (SC) 3 litre - infact I reckon you've still got a little bit up your sleeve if you ever wanted to up the duration of the SC cams. Obviously you'll need to go 34 venturies instead of the current 27. I simply had my 27mm machined to 34mm using a borrowed genuine 34mm as a template.

Yeah I may have webers already, but I'll sell them to buy a set of PMO's and be done with it

keep the webers.. no need to throw money away..

Hi Tim.. funny story.

when I was buying and then trading upscale with my Webers a friend was doing the $ trading as I'm the worst. Needless to say the guy flipped when I threw 1/2 the profit on them at him. He violently opposed the $ so I spent $200 on an Xmas toy for him.

WTF.. carb $ and I got PMOs.. bingo.

otherwise Webers can fine to use and you shouldn't lose $ trading them for PMOs.