Question: is the 46mm secondary venturi different in center hole size, length, etc, than 40mm secondary venturis ?

Yes, I think the only difference is the length. The 46 have the taller or longer ones. These can put in 40 also.
But 40 didn't come with the tall secondary venturis.

The 2 progression holes are start higher in the body than the 40.
Its like they omited the lowest hole on the 40.
Yes, you can control the amount of fuel, but you can't control the vacumm off idel. Thats controled by the vacumm that the engine makes.
The 3.0 dosen't have the proper amount of vacumm to make the 46s work corectly through the transion.

I think rhk has a ignition problem not carbs.
The bigger carbs may just adding to the real problem.

[QUOTE]Originally posted by Porsche Doc
Yes, I think the only difference is the length. The 46 have the taller or longer ones. These can put in 40 also.
But 40 didn't come with the tall secondary venturis.
------ great info

Yes, you can control the amount of fuel, but you can't control the vacumm off idel. Thats controled by the vacumm that the engine makes.
------ so if the same engine, same venturis, and same fuel delivery gas size? then 46 & 40 vacuum and combustion is the same?


The 3.0 dosen't have the proper amount of vacumm to make the 46s work corectly through the transion.
------- what else is there to affect 46 vs 40 idle vacuum than above besides the butterfly velocity restriction? Bigger butterflies = less restriction.

I don't think rhk has a ignition problem not carbs.
-------- me too. He said the 46's were operating fine before his problem developed. Even an alternator circuit could cause detonation problems causing higher EGT's, thus affecting fuel mixtures. Maybe a scope is in order to solve his 3.5k rpm lean?

The bigger carbs may just adding to the real problem.
------- maybe? He did say they were fine before.

[QUOTE]Originally posted by Porsche Doc
Yes, you can control the amount of fuel, but you can't control the vacumm off idel. Thats controled by the vacumm that the engine makes.


------- I forgot about the throttle bore. The 46mm larger throttle bore will diminish the vacuum/velocity.

rhk still has a good idle with the larger 46's. So it seems that the idle jet and idle jet holder are preforming with the bigger bore.

RoninLB; It sounds like you've got a lot of thoughts going at once here and to be honest I'm having a little trouble following them. One of them did jump out at me as not sounding right.

Taking it apart...
True.

Only under WOT. At anything less then WOT, the highest gas speeds will occur at the throttle while the gas speeds will slow down through the rest of the intake system. How much it slows down depends on (and is inversely proportional to) the cross sectional area at that point in the system. The velocity will be slower in a 46 mm carb body then in a 40 mm carb body.

Bottom line: Do I think a 46 mm carb be made to work on a street 3.0? Sure. Will is work as well as a 40 mm carb? Most likely not. It sounds like it may be harder to tune just right, but not impossible. I seem to remember a BA description of a modification to the idle circuit for Weber 46's in his book, but I can't find it now. Anyone else know what I'm talking about?

As far as the availability of the tall secondaries for the 40 mm webers, just call Richard Parr at PMO. While you're talking with him you can also ask him about the issues involved with using 46 mm carbs on a 3.0 street engine.

I talked to Richard at PMO and he said he used to have issues with 46's on a 3.0 until he started adding an additional progression hole. My 46's were bought from PMO and have the extra hole. I think this is the mod BA mentioned.

No. Less gas restriction in general allows higher FLOW CAPACITY, but does NOT create higher flow. Flow is a function of the velocity, restriction, and engine capacity. The bigger carbs only enable one of these 3.

When the piston moves down it creates a certain vacuum. It is a fixed level for any given motor. It must then displace a certain amount of air to fill that vacuum through the carb. A NARROWER carb will result in higher velocity because of this, hence better mixing.

Performance of a motor is related to velocity AND flow, not just flow. Lots of flow capacity (that a 3.0L can't use with 46mm carbs!!!) with no velocity is worthless. So the key again is to balance flow and velocity. Using a carb that supports more CFM than the engine can handle is not just doing nothing, it's worse.

Your contention is that the performance through a 36mm (or whatever) size venturi on a 40 and 46 will ultimately be the same, its NOT.

THink on this: Air flowing through the 40mm carb with a 36mm venturi will be at a higher velocity (because of the smaller diameter) accelerate through the venturi and create a vacuum behind it to draw air out. It then expands out after the venturi to close to the original size of the carb. In the 46mm it has to expand MORE, thus slowing down more. Less velocity.

It is for this same reason race motor designers don't just immediately go to the largest intake port size available for a given head. You must respect velocity.

By that same idea (less 'restriction' is better as you said) why not use 50mm or 60mm carbs? Even more flow right? Your velocity would be so slow that the airflow would stall and be worthless, even with the SAME SIZE venturi...

this makes sense.

I confused myself by expanding the routine on shaving the throttle shafts and butterflies to reduce friction. Even if the gas is speeded up at the same size intake pipes it's the carb atomization function that seems directly affected by the larger carb base, I think. The larger base should allow vacuum to drop enlarging the atomization size. Enlarging the particles is the opposite of what a carb is supposed to do.

At anything less then WOT, the highest gas speeds will occur at the throttle while the gas speeds will slow down through the rest of the intake system.

John's statement still confuses me. If the venturi is narrower than the throttle shouldn't the highest speed happen there?

Anyway, I've been involved in increasing accelerator pump flow and it's atomization. So far it's an ng by only increasing flow. I can watch EGT's fall way too much because of poor atomization. Disturbing the main circuit is not a consideration 'cause it operates beautifully. I need a better idea on this one.

Then rhk comes along with a possible carb issue and I jump on. Not a complete waste of time it seems for any of us.

I could still use an acceleration circuit bright idea.

RoninLB. Yes, in absolute terms you're right. At 95%-99.99% throttle the venturi very well may be the narrowest portion of the intake system and so the highest speed will be there. But then if the throttle are not "throttling" (aka: restricting) the flow of air, then I guess you are functionally at WOT.

I'm sure that the reality is that as airflow transitions from 100% WOT to idle it will transition from a largerly linear flow through a chaotic state to another stable state. Without doing a study of this (and most likely getting a PhD in the process), I can only imagine there is all sort of chaotic stages in the process.

But to us laymen, it's most likely good enough to hypothothize that once you start to close the throttle (past the first 1% or 2%) that the fastest flow is at the throttle plates. Especially since most people spend 80%+ of their time driving on the idle circuits in which case absolute flow capability just doesn't matter.

Bingo

Up-Fixin vol 7, p109 compares 46 Webers to 40 Webers. "We have done a lot of work to modify the transition port design design and have solved this problem quite succesfully."

also
- the 46 has 75% less idle and transistion signal because of larger throttle bores.

YEA! :rolleyes:
Us old guy's have known that for years. :p
GET some 40's :D

Happy Holidays Man SmileWavy

Yup, those differences were what I was referring to in my earlier post (couldn't remember exactly what they were at the time of posting).

-Wayne

Seventy five percent? Wow. I wouldn't have guessed it was that much...

Damn, all you smart people posting here, even a knucklehead like me is begining to understand...

Thanks,

Bob

This is a great, old thread.

I am going to transfer my 3.0 race engine into the street car and would like to keep the EFI system in the race car for a special project. I just rebuilt my 40IDA3Cs and wnat to plop them on the race engine and put that in my Targa.

I have the webers dialed in on hte current 2.7 RS-ism motor with the following:
60 Idles
145 Mains
F26 Emulsions
34 vents

I dont know what power I am gettig on the 2.7, but the 3.0 is doing 240-250 with EFI.

Shall I start with the above or do I definately need bigger mains and idles?

thanks

I seem to recall using 65 idles and 155 mains for my 3.0L with 46mm carbs... I'm thinking that with the incrased airflow and fuel metering signal (ie more vacuum past the ports) of the 40's your current setup should be close.

I think you could start with this and either use an AFM or the old "check the plugs to fine tune it from there.

Resurrecting an old but perhaps useful thread, at least for me.

I have a (new to me) '82 SC with a twin plug, 10.5 compression, 964 cam, with Electromotive HPX ignition, 3.0 engine. I live in Albuquerque, NM at 6K' MSL, but do travel to events in TX and CA coast. We are in the process of cleaning up the engine, which is struggling with the ABQ altitude and I would like to thoroughly clean and rebuild the Weber 40's. Car is not raced but do like 'spirited' driving. The car has headers (no heat exchangers) but we will be installing SSI heat exchanges so I can have heat in the cabin.

What are your size recommendations for:

Venturis
Main jets
Idle jets
Air correction
Emulsion tubes

Thanx,
Harry

The following will get you close, each engine will want to be tested for jetting it wants/needs.

• 34mm venturis with 140 main jets or 36mm venturis with 150 main jets (larger venturis provide more top end power at the expense of mid-range)
• F3 emulsion tubes
• 170 main air correction jets
• 60 idle jets

Be sure your Webers are:

• IDA
• IDAP
• IDT

If you have IDTP then your idle air correction jet needs to be modified from 140 to 100

Jetting is for sea level, reduce main jets by at least 5 for Albuquerque.

Thanx, Paul.
Harry