Carb ? Progressive vs Std 2 Barrel

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bfreckman
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Carb ? Progressive vs Std 2 Barrel

Post by bfreckman » Wed Feb 08, 2017 8:33 am

I am building a nice, mild 1776 engine and am leaning towards a progressive carb set up, but a friend told me to put a std 2 barrel on instead. This is a street only car with Gilmore AC. It does have good heads (big valves, etc.), a 110 Engle cam, etc. BTW, I am not interested in having a dual carb setup. This in mind, anybody have any input or advice???
Thanks in advance.

Bill in Azle, TX

Ol'fogasaurus
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Re: Carb ? Progressive vs Std 2 Barrel

Post by Ol'fogasaurus » Wed Feb 08, 2017 9:57 am

I had an old Solex (I think it was a Solex but was many years ago so I may have forgotten) 2bbl that I remember as being a progressive 2bbl. It worked very well on several different engines, stock or slightly modified and was very happy with it. My use was on the sand so it was worked hard a lot. I felt bad when the toy it was on was sold. The new owner worked it very hard until he passed the toy on.

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Marc
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Re: Carb ? Progressive vs Std 2 Barrel

Post by Marc » Wed Feb 08, 2017 11:21 am

A single 2-bbl on an ACVW puts the carb about a foot and a half away from the intake valves and presents some major driveability issues on the street, especially in cool/damp conditions...and the heavier the vehicle, the worse it gets.

So, while you might be perfectly happy with one on a lightweight buggy blasting about in the sand on a warm day, in normal street use you'll find it to have an annoying hesitation on acceleration. Overly-rich jetting, a maximized accelerator pump setting, and an intake manifold with as much heatriser action as you can find all help. Hardly any aftermarket exhaust systems provide anywhere near the preheat flow of a stock muffler; extending one of the heatriser pipes to a low-pressure point such as the center of the collector on a 4-into-1 header will help with that.

Overall driveability and fuel economy will be better as a rule with dual carbs, but if you're adamant about not running those the Weber 32/36 progressive (or the Holley or Motorcraft clones) would be the better choice for a mild motor; a larger carb such as a 40IDF or 44IDF will make more power as one would expect but the runner diameter/volume of the manifolds made for those is substantially larger, making the flat spot even worse...OK if all you'll be doing is screaming around at WOT but can be kind of a pain to live with in traffic. If it's usually warm there you could probably get by with the type of manifold that's typically included in most proggy kits (the heatriser system on those is pitiful); if you want it to drive well in cool weather, spring for a manifold that heats the plenum area of the manifold and not just a few inches along the bottom of the runners, such as Redline's "deluxe" model....and consider modifying the exhaust so there'll be net flow through the heatriser passage and not just warming by convection.

http://vwparts.aircooled.net/Deluxe-Cen ... 4.410b.htm

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Re: Carb ? Progressive vs Std 2 Barrel

Post by ainokea » Sat Feb 18, 2017 7:47 pm

Back in the day, there was the Holley 300CFM two barrel carburetor that worked very well on the VW engine as long as an isolated tube manifold was used but not the plenum, and it served me well in winter and summer temperatures in Seattle. It was replaced with the Holley / Weber 32 / 36 DFEV which in my opinion is not superior to the Holley 300. Empi has launched a new two barrel carburetor, the 38Egas. It is not a progressive it is a true two barrel carburetor. I know Empi does not have the best reputation, but their carburetors seem to be gaining in popularity. I'm not sure if Weber offers a similar carburetor you might check them out. A two barrel will probably be much easier to adjust than the progressive.

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Marc
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Re: Carb ? Progressive vs Std 2 Barrel

Post by Marc » Sat Feb 18, 2017 8:40 pm

I had a 300CFM Bug Spray in Seattle too (on a 1679DP) and it was a nightmare here, it would ice up to the point of shutting me down on the highway - on a cool/rainy day I wouldn't leave the house without a propane torch - and fire extinguisher ;)
The solution was to weld on a sheetmetal jacket enclosing the downtubes so they'd receive as much heatriser warmth as possible. Once that problem was dealt with I had no complaints, and bfreckman will probably never have that issue in Arizona....but it's still an old, old design (and so tall that aircleaner clearance can be a problem). Rebuild kits are still around for them though. There's also a 200CFM version which would be too small for a 1775.

Back to the subject of progressive vs non-progressive...although there are a few plenum-style manifolds around for non-progressives, the only thing they're good for as a rule is drag-racing - too much of a bog from low speed to live with in day-to-day driving. As a rule, a non-progressive needs an independent-runner manifold. That old Bug Spray had a little balancing slot between the throttle bores, but with most non-progressive 2-bbls each side of the engine is totally independent of the other so the flow dynamics are like those of dual 1-bbls (but on a VERY tall manifold)...one cylinder draws air, initiating flow, then the second one does after flow is established...then a pause while the other side of the engine goes to work. This results in uneven mixture distribution between the front & rear cylinder on each side at lower RPM, and in the worst case makes it nearly impossible to achieve an idle setting where all four fire. Add the previously-discussed problems from the typical woefully-underachieving heat riser setup and the result's not pretty, at least until you're cruising down the highway.

A progressive MUST use a plenum, but since all four cylinders are drawing from it the flow dynamics are totally different so it's not really a problem.

For a street ride, my vote is still for the progressive if you're going to run a single carb. There are other choices besides the ubiquitous 32/36 Holley/Weber DFV/DGV though. I'm using a 34/34 DMTRA progressive from a Lancia 1800 on my trike and it works surprisingly well; it was just something I had around and this one has no provision for a distributor vacuum advance port (can't even be drilled) so I'm limited to centrifugal-advance only - they do make a model with vacuum though. My mid-engine exhaust system has NO provision for preheat so a good warmup before entering traffic is mandatory in cool weather. Right now I'm at 1849cc with ported stock-valve heads but I plan to take it to 1955cc before Sturgis so it may end up with a single 44IDF (again, just because I have one around) but for freeway driving in 90°+ weather I expect it to be tolerable in a relatively light vehicle...we shall see, I've got 40, 42, and 44DCOEs I can play with as well but would have to buy a manifold - naturally, I stored one for 30 years and never had a use for it so I gave it away last year ;)

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Re: Carb ? Progressive vs Std 2 Barrel

Post by ainokea » Thu Feb 23, 2017 7:22 pm

I checked Weber for a two barrel and yes they do offer one. The 38DGAS/ES.

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Re: Carb ? Progressive vs Std 2 Barrel

Post by ainokea » Thu Feb 23, 2017 7:32 pm

I ran the Holley 300 on an 1835cc engine while living in Seattle and was lucky to not have had the icing problem some others had. I cannot explain why I did not have that problem as I did not do anything extra ordinary or different from a regular build and installation. Of course it took some tinkering to get the jetting right, but that was all. Just plain old dog luck.

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Re: Carb ? Progressive vs Std 2 Barrel

Post by Marc » Thu Feb 23, 2017 8:16 pm

ainokea wrote:
Thu Feb 23, 2017 7:22 pm
I checked Weber for a two barrel and yes they do offer one. The 38DGAS/ES.
You'd want the ES (electric choke) or MS manual. They refer to that as a "synchronous-opening" performance alternative to the 32/36DGV but the parts diagram references "primary" and "secondary" throttle shafts...so I guess it's a non-progressive that's built like a progressive but with different linkage connecting the two shafts.

Anyone know of a site that lists the bolt patterns of the various "rectangular-flange" Weber carbs?

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Re: Carb ? Progressive vs Std 2 Barrel

Post by Marc » Fri Feb 24, 2017 2:13 am

Stumbled across this today, looks like EMPI is making 38DGES clones in China now...http://carcraftstore.com/38mmegassinglecarbkit.aspx

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Re: Carb ? Progressive vs Std 2 Barrel

Post by ainokea » Sat Feb 25, 2017 6:20 pm

Weber offers a two barrel also, 38 DGAS/ES. I would trust the Weber more than the Empi knock off.

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Re: Carb ? Progressive vs Std 2 Barrel

Post by Marc » Sat Feb 25, 2017 11:19 pm

Marc wrote:Anyone know of a site that lists the bolt patterns of the various "rectangular-flange" Weber carbs?
I took some calipers to a PIS Holley/Weber proggy manifold and measured (on center) 1.85" x 3.675" ...47 x 93mm.

Any other contributions to submit? I'd like to make a "sticky" once enough are known.

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