Weber size on 3.0

[rhk109]

I've read tons of threads regarding Weber size on a 3.0. Most seem to agree that 40's are better than 46's for the street.

What if 40's or 46's are equipped with the same size venturis? Do they perform equally? If not, what is the difference?

[rhk109]

Bump..

[cstreit]

RHK...

They're better, but still not quite the same... You're still going to not get as much "pressure differential" and typically this will result in larger jets being needed and less smooth throttle transitions... It won't be "binaru" but more peaky compared to the 40.

Is there a particular reason you are considering the 46's? Is it because you already have a set?

If not, the little bit of top end power you might be leaving on the table with a stock motor running 40's will pale in comparison to the lower RPM response differences. 46's are only really useful on a 3.0L when you have really opened it up and need the extra flow at high RPM, even then some may debate this..

[RoninLB]

Originally posted by cstreit

not quite the same...

not get as much "pressure differential"

I always figured the venturi size controlled the "pressure differential? I guess I was wrong?

What else besides the venturi controls the vacuum besides an air by-pass?

[cstreit]

Ron,

It does, but I think it's related to velocity and such...

To make an extreme example, would a 40mm weber with a 36mm choke be the same as one that was 100mm and a 36mm choke?

Yes the 46mm isn't as extreme, but the principal is still there... I can't describe the actual dynamics, but know this from practice...

[RoninLB]

[QUOTE]Originally posted by cstreit

It does, but I think it's related to velocity and such...
------- me thinks the same.
36mm choke = 36 venturi
46carb has a bigger butterfly than a 40carb afaik ?
46carb + 36Venturi/choke = same velocity+ of 40carb + 36Venturi

afaik the bigger butterflies will enable more efficent/ less restriction /greater available vac/ "greater pressure differential"

To make an extreme example, would a 40mm weber with a 36mm choke be the same as one that was 100mm and a 36mm choke?
--------- Yes+. it would have huge butterflies and be, in theory, less restrictive than the 46's + 36Venturi & way less restrictive than 40's + 36Venturi.

Yes the 46mm isn't as extreme, but the principal is still there... I can't describe the actual dynamics, but know this from practice...
------- rhk has a lean condition around 3.5k. Auxilary/secondary Venturis affect the mid range+.

meanwhile.. I've been jerking around with my carbs and have been doing a lot of reading. I could be cross-eyed on this?

[Wayne 962]

No, it wouldn't be the same - there are other characteristics on the 46 that are bigger.

The 40s are the way to go on just about all 3.0s. Run with 34mm venturis for the street...

-Wayne

[RoninLB]

Originally posted by Wayne at Pelican Parts
No, it wouldn't be the same - there are other characteristics on the 46 that are bigger.

ok.. what other characteristics besides changable parts ?

[RoninLB]

Originally posted by Wayne at Pelican Parts
The 40s are the way to go on just about all 3.0s. Run with 34mm venturis for the street...

ok.. so the 34mm venturis would be greater response for the idle circuit. Then once on the main circuit would this also be true?

[jluetjen]

Here's my $0.02 for what it's worth...
I doubt the difference will be felt at WOT or even at the transition from part throttle to WOT. I think the difference will be at idle and small throttle openings.

Originally posted by cstreit

It does, but I think it's related to velocity and such...
------- me thinks the same.
36mm choke = 36 venturi
46carb has a bigger butterfly than a 40carb afaik ?
46carb + 36Venturi/choke = same velocity+ of 40carb + 36Venturi

afaik the bigger butterflies will enable more efficent/ less restriction /greater available vac/ "greater pressure differential"

I would agree that at WOT running this would be true.

To make an extreme example, would a 40mm weber with a 36mm choke be the same as one that was 100mm and a 36mm choke?
--------- Yes+. it would have huge butterflies and be, in theory, less restrictive than the 46's + 36Venturi & way less restrictive than 40's + 36Venturi.

Here's where I think the difference is. Imagine trying to tune the 100 mm carb for idle with the butterflies closed. Just a .01 of wear on the 100 mm butterflies will result in roughly the same change in airflow as .025 wear in the 40 mm carb. (Note that I have't bothered to figure this out exactly, these are "back of an envelope calculations). Now the case with the 46 mm carb isn't as drastic, but still small amounts of variation at closed throttle will result in larger swings in cylinder tuning then would be the case with 40 mm bodies. Since most of us spend 90%+ of our time with the throttles largely closed, I don't think that this difference should be dismissed.

Yes the 46mm isn't as extreme, but the principal is still there... I can't describe the actual dynamics, but know this from practice...
------- rhk has a lean condition around 3.5k. Auxilary/secondary Venturis affect the mid range+.

meanwhile.. I've been jerking around with my carbs and have been doing a lot of reading. I could be cross-eyed on this?

Basically, I suspect that the 46's will be harder to tune the idle settings then 40's will be. Once the throttles are opened 20% or more I doubt that there will be any difference.

[cstreit]

Thanks John,

A much better explanation than I was able to make.

[RoninLB]

Originally posted by jluetjen
. I think the difference will be at idle and small throttle openings.

but still small amounts of variation at closed throttle will result in larger swings in cylinder tuning then would be the case with 40 mm bodies. Since most of us spend 90%+ of our time with the throttles largely closed, I don't think that this difference should be dismissed.

Basically, I suspect that the 46's will be harder to tune the idle settings then 40's will be.

Once the throttles are opened 20% or more I doubt that there will be any difference.

So therefore worn or improperly set larger butterflies would excalaberate a problem.. sounds good

So therefore you buy into my position that a 46mm base& butterflies with 36Venturis is basically the same as 40mm base&butterflies with 36Venturis thru idle&main circuits?

If the last sentence is true? Why would it be harder to tune the idle circuit if no wear or no improperly set-up butterflies ?

The only difference I could imagine is less throttle opening to achieve the same rpm because of greater efficiency. The "efficiency" comes from the larger pipe/butterflies having less air friction.

huh ?

[Porsche Doc]

RoninLB you and Wayne are right in the Idle circuits are different.
The 46s progress on to the mains differently than the 40s.
This is one of the reasions the 40s are better.
rhk109 Send me your 46s and I will send you the 40s.
Put them on and try them If YOU don't think they work better we will trade back!

[pwd72s]

Is this the old "too much carb for the displacement" theory? Back in the stone age, there was a guy in our small town running 6 strombergs atop some trick racing manifold on an otherwise stock 283 chevy. It didn't work well, but it was a hell of a crowd pleaser!

[cstreit]

Ron et. al...

Using the same size choke (lets say 36mm) for two different carb's (46 and 40) is NOT going to yield the same results.


[from Weber tuning guides]

It is easy to make the assumption that biggest is best when selecting a main venturi size, but the purpose of the main venturi is to increase the vacuum acting on the main jet in order to draw in and effectively atomise the fuel mixture. The smaller the main venturi, the more effective this action is, but a smaller venturi will inhibit flow. A large venturi may give more power right at the top end of the power band, but will give this at the expense of lower RPM tractability. Only a circuit racer will benefit from this sort of compromise, on a road car, driveability is much more important. 95 percent of the time, a road engine is nowhere near its peak power, but is near its peak torque for 75 percent of the time. It is much more important therefore to select the main venturi for best driveability, once the venturi size has been selected, then the appropriate carburettor size can be arrived at.

[end of weber plagurize]

If your barrels are too large, the air velocity through the venturis is too low for optimal air/fuel mixing. To get good drivability, your setup will then need bigger idle jets to compensate. You will then be in a constant rich condition at low RPM's because you're adding more fuel to account for worse mixing.

Your air velocity will be lower when you increase the barrel size in the carb, regardless of the venturi size. This will result in a week circuit. This causes the engine to stutter or nosedive when opening the throttle, plus since it's too rich the engine will surge when hot.

.Finally (and I think this is the key) the air velocity and flow an be LOWER in a larger carb with the same size chokes!!!:




So with the vacuum generated on a stock 3.0L, a 46mm Carb is WORSE than a 40mm one with the same size choke!!!

WHAT?!! Right... So they key is to select a carb with the correct CFM for your motor without going too far over and then picking the correct venturi/choke for your application.

[RoninLB]

Originally posted by Porsche Doc

RoninLB you and Wayne are right in the Idle circuits are different.

The 46s progress on to the mains differently than the 40s.

are you saying that Only the butterflies are different ?

How will progress be different with the same size venturis, discounting the difference in secondary venturis ?

[rhk109]

OK, first of all thanks for all the help, support and advice.

Here's what I gather. Different size carbs with the same venturis may have the same vacuum right smack in the center of the venturi. But when the butterflies are moved and the center of the vacuum moves up or down, the vacuum will be greater with the smaller carb.

[Porsche Doc]

Originally posted by RoninLB
are you saying that Only the butterflies are different ?

How will progress be different with the same size venturis, discounting the difference in secondary venturis ?

Yes!
The 46s and 40s are made different.
Yes, the butterflies are different one is 46 mm dia. and the other is 40 mm dia.
The 40's have 3 progression holes in the body.
The 46's have 2 progression holes that are larger in the body.
The 46's 2 progression holes are for larger bore engines with higher vacuums.
That's why they only have 2 holes.
The smaller bore engine just can't make it work right.
That's why they have 3 holes to help with the progression with an engine with less vacuums.
Get a 46 and a 40 and take a look!
Thats why they don't work well on a 3.0.
The smallest engine I put 46s on is a 3.2.
Unless its a race engine then I put them on 2.0s.

[RoninLB]

Originally posted by Porsche Doc

butterflies are different one is 46 mm dia. and the other is 40 mm dia.

The 40's have 3 progression holes
The 46's have 2 progression holes

Get a 46 and a 40 and take a look!

ok, great info.

larger butterflies are great imo

More progression holes mean's more available gas flow. So far I love the option of more available gas flow if I can adjust the progression air/gas emulsion? The fuel emulsion mixture Gas Size of thru the 2or3 holes is the name of the game for combustion at idle, progression off idle, and progression into the mains?

So if I'm still in focus I have to adjust the 46mm idle & off-idle progression emulsion to 40mm Gas Size levels ? I love it.

So on p95 of Tomlinson's book it says that the idle jet holder provides the air for idle jet and progression. If you can control the air you control emulsion gas mix, thus the size. Various size jet holders are available. 46mm can be adjusted to 40mm usage so far?

So if you have an idle and progression problem using 46mm on a suggested 40mm engine you have the option to control any stumbling or bog by changing idle jets & jet holders ?

fwiw.
rhk has a lean condition at 3.5k. rhk has a good idle and progression. The only thing left, If there is a main circuit prob, is the secondary venturi and it's operation in conjuction with the main gas feed.

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

[RoninLB]

[QUOTE]Originally posted by rhk109
Different size carbs with the same venturis may have the same vacuum right smack in the center of the venturi.
-------- as far as I know.

But when the butterflies are moved and the center of the vacuum moves up or down
------ I never heard of that. The area of maximum air velocity is fixed in the main venturi. A larger venturi than needed will slow the velocity. The total available air at max hp is controled by the venturi size. 36mm main venturis should provide enough availabe air for your engine. My 2.7 has 36mm main venturis and the main circuit air generates very beautiful EGT's. EGT's is a crude reflection of combustion. Combustion heat is a reflexion of available hp. Maximum combustion heat within the envelope of CHT/ cylinder head temp is what tuning is all about imo.

, the vacuum will be greater with the smaller carb.
-------- I think your going back to the different size butterflies in the 46mm vs the 40mm carb?

My position all along is that the larger butterflies have less fuel gas restriction than smaller butterflies. Less gas restriction allows the gas to travel at a higher velocity into the combustion chamber. This greater velocity allows better exhaust scavenging and new gas filling into the combustion chamber.

So far it seems like the 46mm carb that's adjusted properly has the potential of greater volumetric efficency/ VE that the 40mm carb, I think.