Annular vs downleg boosters

[pdq67]

Way back years ago in the dawn of Holley coming alive, they made the old 780 cfm 4150-1 rear jet plate vacuum secondary carb with dog-leg boosters that shortly later, (because I figure pricing), turned into the newer 750 CFM 4160-1,2,3,4, etc.. series vacuum secondary non-rear jet plate carb with straight boosters.

(I hope that I have my -1,-2, -3's correct??)...

I want to say that dog-leg boosters were worth 30 cfm back then vs straight??

pdq67.

PS., I think that Honest Charley used to say that the 780 was the "runniest" carb made back then!

[Troy Patterson]

What would be considered a "cold intake tract' versus a "hot intake tract" in relation to booster type used?

Well, a cold intake track can be anything without an active heat cross-over to a full on race motor with isolated (from coolant passage, heads and block heat) runners and plenum chamber, thermal barrier / dispersant coatings and a reverse flow cooling system (where coolant flows from radiator to heads then block as opposed to radiator to block where the coolant is heated back up some then into the heads).

The cooler the intake track, the less of the fuel is exposed to heat (as there is less heat), the less the lite ends of the fuel is vaporized pre-combustion.
There could potentially be a net loss in power as a result. To counter balance this and take advantage of the cooler intake and the denser intake charge it affords, i.e., make more torque and horsepower, the carburetor needs to be designed accordingly. In simple terms, more atomization.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44

[pdq67]

Please put Smokey's hot air engine together with what Troy says to try to see what's going on here.

And if an engine was burning a pure fuel like Octane, it would be way easier to figure out, but that's not the case because EPA allow's the refineries to blend their fuels with what they have available to hopefully hold price down as well as emissions.

pdq67

[Skeezix]

Why are annular boosters so rarely seen on vacuum secondary carbs?

Are they of less benefit on a VS carb?

[Sandman]

If annular boosters atomise fuel better, would they be better for fuel economy as well.

I'm asking, as I am changing a C3500 454bbc from lpg back to petrol. ATM I have extractors, Offenhauser 360 manifold, and can run eithe 830 annular or 780vs with drop leg booster. It's not for racing. Mainly to tow my race trailer, so looking for best power & fuel economy.

[jet1]

Why are annular boosters so rarely seen on vacuum secondary carbs?

Are they of less benefit on a VS carb?

They cost more to biuld and they do cost some cfm to the carb.

Just a rule of thumb if your buying a carb of the shelf. If you are selecting a smaller sized carb for an engine the down legs will work fine(lots of signal and air speed). If your selecting an oversized carb anulars can/will help(low signal slow air speed).

[Bob Hollinshead]

Why are annular boosters so rarely seen on vacuum secondary carbs?

Are they of less benefit on a VS carb?

The 5.0 mustangs and the 351HO motors along with some 460's had holley vacume secondary carbs with annular boosters on the primary side only, 4180 series carb. I think it was for better mpg and throttle response?

[TurboRay]

.
LINK

C'ya - RAY

[340king]

We dynoed an engine last year with a couple of different carbs. One was a 850 annular booster carb and the other was a standard 750 race prepped carb. The power output was close enough to call it a draw, but the annular booster carb would go rich on the top end. I am pretty sure that we could have tuned around this a little, but it wasn't going to make a ton more HP as it didn't go rich until the last 700 rpm or so.

So the moral of that story is the more sensitive booster may require some work to keep the system from going too rich on the top end. It is certainly doable and most here have fixed that fuel curve issue or had their carb guy do it for them. If you are doing it on your own, then that is one thing to consider with the out of the box fuel curve.

[289nate]

We dynoed an engine last year with a couple of different carbs. One was a 850 annular booster carb and the other was a standard 750 race prepped carb. The power output was close enough to call it a draw, but the annular booster carb would go rich on the top end. I am pretty sure that we could have tuned around this a little, but it wasn't going to make a ton more HP as it didn't go rich until the last 700 rpm or so.

So the moral of that story is the more sensitive booster may require some work to keep the system from going too rich on the top end. It is certainly doable and most here have fixed that fuel curve issue or had their carb guy do it for them. If you are doing it on your own, then that is one thing to consider with the out of the box fuel curve.

How low in the rpm range did you pull them? What did the lower rpm power reveal?

[340king]

There wasn't any significant difference. We were playing with different carbs and fuels. We never really made much more power with any of the stuff we were working with. The fact that it was a 850 and we were comparing it to smaller base carbs with nearly identical results backs up the concept that the annular discharge boosters can hurt the flow compared to the down leg boosters. I think if we would have had a larger carb, we might have made a little more power. The way it was the two we had were very close to the same overall CFM.

Here is the dyno pull with the annular booster carb. You can see the headers glowing at the end of the pull. The timing was verified by previous pulls as the best for overall power. I think we could have made some adjustments to the high speed bleeds to get this closer to what it needed to be. It really only affected the last 500 or so rpm. The massive blow by was from an issue during break in where the primary carb was flooding. It cleared up quite a bit in subsequent pulls. We used a slower sweep for a little smoother data acquisition.

http://www.youtube.com/watch?v=aZNQtj9tpyI

[GARY C]

We dynoed an engine last year with a couple of different carbs. One was a 850 annular booster carb and the other was a standard 750 race prepped carb. The power output was close enough to call it a draw, but the annular booster carb would go rich on the top end. I am pretty sure that we could have tuned around this a little, but it wasn't going to make a ton more HP as it didn't go rich until the last 700 rpm or so.

So the moral of that story is the more sensitive booster may require some work to keep the system from going too rich on the top end. It is certainly doable and most here have fixed that fuel curve issue or had their carb guy do it for them. If you are doing it on your own, then that is one thing to consider with the out of the box fuel curve.

I could be wrong but wouldn't you need a much cooler running intake to see the benefits of a better atomized fuel from the annular booster?

If the intake was coated or valley panned to stay cooler I am thinking the 850 could have been run leaner and showed a gain over the standard set up and possibly not have gone rich??????

[340king]

I would agree if fuel atomization was an issue with the 750 carb. If it wasn't, and we were getting the maximum output from the available air/fuel mixture, then the only difference would be the amount of A/F that was available. Since the 850 carb with annular discharge boosters flows less than one with down leg boosters, the flow and overall A/F availability might have been about the same.

I think we could have played with the high speed air bleeds to see if there was a way to make it keep from going over rich, but the power in the mid band wasn't anything more than the 750 and BSFC looked very close to the 750. As I said it wasn't until the very last 700 rpm, past peak HP, that it started to go rich. So we didn't pursue it. This was a bracket engine and wasn't an all out pursuit of power. The 360 made 480 hp, which was the predicted amount.

[Walter R. Malik]

There ARE "annular discharge downleg" boosters so, they are not necessarily two distinctly different things.

[340king]

They were on these two carbs that I had. Sorry to generalize.