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 Idle and transfer slot improvements 
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Joined: Wed May 10, 2017 7:39 pm
Posts: 17
I have a been reading anything and everything I can find on carb tuning operation, emulsion booster upgrades etc. I came across a “discussion” on another board I lurk at, about EFI vs Carbs, currently about 50 pages through 70 something pages of “stuff”, some of it is even about carbs vs efi! Anyway some statements have me wondering, a few comments in regards to carbs low speed operation and the lack of decent fuel atomisation. Especially on the idle and transfer circuit being the down fall of the carburettor. But then being better at high speed due to booster designs etc.

 I have read what I can find in making for a better transfer slot control with restrictors etc but not much on what can be done to improve the atomisation / delivery of these other circuits. Comments in that thread mentioned above about rivers of fuel in the intake manifold may be a little strong but I have seen the fuel trail stains from the progression holes on my side draft webers, and the later models now have extra holes and differently placed to improve the fuel delivery. I got to looking at my holley and started wondering what can be done? It is my understanding that the idle and transfer slot supply fuel at all stages of operation right to WOT, so wouldn’t improvements in the delivery of this fuel improve the performance of the carb across the board?

 Has anyone had some ideas on improvements to this? On the talks about emulsion on the main circuit it appears that air placement and size, therefore the bubble size is a big factor in the quality and timing of the fuel / air mix delivered to the booster, well looking at the idle circuit it appears the fuel is drawn up from, my now relocated IFR to the top of the idle well? Where it meets the air coming from the Idle air bleed, this then travels back down to the idle transfer slot restrictor to the transfer slot and idle mixture screws, hope I have this right?

 Wouldn’t the air bleed coming into the idle well end up coming through in blobs? Or big bubbles, the fuel big bubble mix then has to travel DOWN to the transfer slot. Could improving bubble size at this point improve the “emulsion” to the transfer discharge? Smaller bubbles move more slowly so they wouldn’t be fighting the direction of fuel flow as much.

 Also the transfer slot itself where it discharges to the main throttle area, my thinking is that once it hits the air stream coming around the throttle blade it’s going to be stuck or forced to the edge of the throttle bore where it has to come off sort of like sea spray or it reaches the bottom of the throttle base and “drips” off into the air stream. Can’t put a booster like outlet there obviously, but would a ledge or a shear point like the bottom of the annular discharge booster machined step that protruded a small way into the bore out of the boundary layer of air help atomise it a little better? Even a pin that protruded into the bore a 1/8th in and maybe 1/8th in below the transfer slot for the fuel to travel along? Surface tension might help to draw it out into the higher speed air.

 

Well that’s whats going on in my head today :wft?: what are other peoples thoughts?


Sun Jun 18, 2017 2:36 am
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Joined: Mon May 15, 2017 4:04 pm
Posts: 60
My thought is Holley has had MileADial 1/4 MileADial 2 Stage power valves and reverse idle circuits and likely all kinds of other stuff that did not even make it to market. They even made a Q-Jet Electronic Replacement 4175 style Spread Bore in the late 80's right when EFI was taking over. They had 3 Circuit 4180's on Fords before that and maybe on Mopars. If there was a workable improvement on the Holley 4150/4160 Platform that offered substantial gains on it in over the approximately 61 years its been around I think we would have seen it by now. Electronic Fuel Injection won out pure and simple in the controlled fuel delivery department in my opinion because of its better atomiziation due to higher fuel pressure and self tuning abilities because of O2 sensors. It is fun to try and think of improvements though, I am guilty of it myself. 8)


Sun Jun 18, 2017 4:02 am
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Joined: Mon May 15, 2017 4:04 pm
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PS: I kinda think your last paragraph is what Holley was after with Reverse Idle.


Sun Jun 18, 2017 4:09 am
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Joined: Sun Jun 23, 2013 1:41 pm
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Location: Phila. Penn.
The fuel out of those ports is being drawn to the low pressure areas. Probably only very small gains to be made at the discharge ports themselves. This may be why Holley, Motorcraft and others used a transfer slot instead of a series of holes. The holes should offer more surface per volume. But the fuel is probably going to head directly toward the lowest pressure area - so the action at the slot or hole may not be too important. ?? In other words the most effective distribution improvements probably come from lowering pressure all the way to the cylinder. Timing, compression and exhaust to minimize reversion and on a low rpm motor use cam timing to help lower the pressure even more at intake valve opening.

Another thought on this comes from Shrinker. He wrote about how the manifold walls need to be wet first. I think this has to do with partial pressures as well as gravity, surface tension and temperature differences. This is something akin to condensation of water on cold pipes in a humid building.


Sun Jun 18, 2017 8:27 am
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Quote:
On the talks about emulsion on the main circuit it appears that air placement and size, therefore the bubble size is a big factor in the quality and timing of the fuel / air mix delivered to the booster, well looking at the idle circuit it appears the fuel is drawn up from, my now relocated IFR to the top of the idle well?


As far as the Holley goes emulsion by design isn't there to "emulsify" the fuel, it's there for air correction to to fuel curve that having a booster causes. On anything driven over a wide RPM range of operation at varying loads you DO NOT want to use it to emulsify fuel.

Now for racing, you can get away with a lot of things, some try to add emulsion and sometimes a lot to help atomize fuel to prep for vaporization. In limited RPM ranges it can work, but at a detriment to delivery somewhere else. I believe that proper booster selection will accomplish the task better, then only requiring the correct emulsion for the fuel curve. I've built carbs up to a 2.800 TB x 2.36 venturi, those big ones have a .024 kill bleed and two .020 emulsion bleeds.

Now for idle and transition, I see with different Dominators a difference in idle and transition characteristics due tot the size and location of the t-slots. T-slot jets help, but are there to balance the air bled into the circuit as well. Since I use primarily one vehicle with mufflers and an LM2 I can tell from one to the next a difference in idle and part throttle quality, the carbs with the smallest slots always seem to idle better. It shows on the LM2 as well, when I can adjust the mixture even leaner above 1 Lambda. I'm sure there are also variations where the air velocity past the idle and t-slot are different due to the body shape, but that is tougher to gauge without extensive testing and more precise machining from one to the next. Then add the differences in every engines combustion efficiency...


Sun Jun 18, 2017 10:31 am
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Joined: Mon Oct 21, 2013 12:15 am
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Location: San Diego, CA
I cannot remember which model of Weber I was working on a couple of years back but - these particular carbs were downdraft multi-carb setup on a 1980's Aston Martin Lagonda. Instead of having dedicated, separate idle and transfer discharge, there was​ series of holes that were stacked and staggered slightly up the wall of the venturi. Throttle plate positioning is extremely critical on these carburetors, in order to get circuit timing correct, but once I figured out how they​ worked, were a breeze to tune, and felt like EFI throughout the entire RPM range. Having a good selection of emulsion tubes and jets definitely helped, but the design of the idle/transition/cruise circuits on these carburetors caught my interest. They work quite well once you get them figured out, and I believe these were exclusively built by Weber for Aston Martin.


Sun Jun 18, 2017 11:28 am
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Joined: Mon May 15, 2017 4:04 pm
Posts: 60
Mattax wrote:
The fuel out of those ports is being drawn to the low pressure areas. Probably only very small gains to be made at the discharge ports themselves. This may be why Holley, Motorcraft and others used a transfer slot instead of a series of holes. The holes should offer more surface per volume. But the fuel is probably going to head directly toward the lowest pressure area - so the action at the slot or hole may not be too important. ?? In other words the most effective distribution improvements probably come from lowering pressure all the way to the cylinder. Timing, compression and exhaust to minimize reversion and on a low rpm motor use cam timing to help lower the pressure even more at intake valve opening.

Another thought on this comes from Shrinker. He wrote about how the manifold walls need to be wet first. I think this has to do with partial pressures as well as gravity, surface tension and temperature differences. This is something akin to condensation of water on cold pipes in a humid building.


The problems that I could see with a series of holes instead of transfer slots are the following

1. More costly to produce.

2. More likely to stop up from trash in the fuel.

3. More likely to carbon up.

4. Non Linear operation compared to a slot.

5. Little to No Gains over a Slot.

I maybe all wrong but, it just seems the OEM's would be looking at things from this angle i.e., Cost, Production Ease and Reliability.


Sun Jun 18, 2017 4:55 pm
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Joined: Wed May 10, 2017 7:39 pm
Posts: 17
supervel45 wrote:
My thought is Holley has had MileADial 1/4 MileADial 2 Stage power valves and reverse idle circuits and likely all kinds of other stuff that did not even make it to market. They even made a Q-Jet Electronic Replacement 4175 style Spread Bore in the late 80's right when EFI was taking over. They had 3 Circuit 4180's on Fords before that and maybe on Mopars. If there was a workable improvement on the Holley 4150/4160 Platform that offered substantial gains on it in over the approximately 61 years its been around I think we would have seen it by now. Electronic Fuel Injection won out


I have actually removed 15kgs of EFI wiring piping and injectors to convert back to Carby. These engines are notoriously heavy on fuel so it cant get worse. and i just cant be bothered fighting with it. I have and am using a megajolt controller for ignition. After using one i wont go back to a distributor. I really enjoy tinkering with carbys. I love the science and the physics of it. I will eventually go blow through. I wasnt attempting to beat efi, that thread just got me thinking.

Revvedup, my experience was also with newer weber carbs with the improved transition or progression holes. I found that throttle placement at idle was very tempermental. Adjust it exact to spec and it was perfect, had to use the mixture scews and idle air bypass to ajust, not blade position.

I also found that adjusting idle jets, the norm is to go from a say 60 f8 to a 60 f9 if you wanted leaner, f8 and f9 is different air bleed size, the fuel jet stays the same. I found that no matter what my engine did not like the f9 sized holes. If i adjusted the size up or down on fuel jet size and stayed with f8 it ran better.

I dont think "emulsion" for the sake of emulsion is most important. Im not about to swiss cheese my metering block. I am probably wrong but while the e holes are there to correct the fuel curve i also think that the air and fuel mix must help the flow of fuel through the passages of the carb. My weber idle jets experience just got me looking more at the holley design.

Mark, do you think the transtion slot size make a difference to the way the fuel exited or the signal generated on the opening? Probably a combination of both.


Sun Jun 18, 2017 5:10 pm
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Joined: Wed May 10, 2017 7:39 pm
Posts: 17
supervel45 wrote:
Mattax wrote:
The fuel out of those ports is being drawn to the low pressure areas. Probably only very small gains to be made at the discharge ports themselves. This may be why Holley, Motorcraft and others used a transfer slot instead of a series of holes. The holes should offer more surface per volume. But the fuel is probably going to head directly toward the lowest pressure area - so the action at the slot or hole may not be too important. ?? In other words the most effective distribution improvements probably come from lowering

The problems that I could see with a series of holes instead of transfer slots are the following

1. More costly to produce.

They sure are, but im only playing with 1 carby at a time

2. More likely to stop up from trash in the fuel.

Yep, i had a tank of dirty fuel and it drove me nuts, finer filter with higher flow fixed it.

3. More likely to carbon up.

Never had carbon build up that far back the intake?

4. Non Linear operation compared to a slot.

Not sure about that, if the hole wasnt uncovered it doesnt flow,
Also flow through an orifice increases and decreases with pressure differential so as the blade opens i would imagine the holes already uncovered would decrease slightly as the next hole began flow. From looking around the internut when tuning the weber, people trying to replicate the newer progression hole placement had real dramas if the didnt get all throttle bores identical.

5. Little to No Gains over a Slot.

That weber drove as nicely as some of the efi cars i owned? Not sure i could convert the holley to progression holes. Not sure its needed. Could the transfer slot be improved? Everything can be tinkered with :=D
I maybe all wrong but, it just seems the OEM's would be looking at things from this angle i.e., Cost, Production Ease and Reliability.


Sun Jun 18, 2017 5:23 pm
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Joined: Mon May 15, 2017 4:04 pm
Posts: 60
Don't get me wrong Str8Jacket, I love Holley's Rochesters and Carters but, mostly Holley's.

You can get a $10.00 600VS Holley 1850 and a $20 low buck rebuild and run most any engine from 200 to 500 cubic inch's, and if you have a problem change it with the spare in the trunk in five minutes with a handful of tools on the side of the road if need be. EFI have fun with that. 8)


Sun Jun 18, 2017 5:30 pm
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